Capacitive or Inductive reactance?
Michael Young
is capacitive or inductive. Say, if lowest SWR is lower in frequency than
the frequency of lowest reactance, would I expect the reactance to be
capacitive? Or inductive? Would the opposite hold true? Is there a better
way to tell?
Thanks.
Michael.
AA9XI
Richard Harrison
"Lowest SWR ALWAYS occurs at a frequency where the feedline plus antenna
is resonant, ie the input impedance is purely resistive."
Reg is pulling our legs again or there is another possibility. Reg`s
message is dated: Tue, Dec 7 He may have reckoned it was Pearl Harbor
Day from sensing a little "Nip" in the air, and maybe a few from a
bottle, too.
There are non-resonant antennas and non-resonant feedlines which can
give the lowest possible SWR, when they are matched, and that is 1:1,
not zero as Reg well knows. SWR is a ratio of maximum to minimum voltage
or current, either will give the same value in the same situation, in a
feedline. The lowest ratio has a value of one in a "flat" line.
When an antenna is resonant XL=XC and if the feedpoint resistance
matches the characteristic resistance of the feedline, the SWR is 1:1 on
the transmitter feedline.
Best regards, Richard Harrison, KB5WZI
Reg Edwards
impedance is purely resistive. If it doesn't then there's something
the matter with your SWR meter.
But the value of the SWR at its minimum will give no indication of
whether the input resistance is greater than 50 ohms or less than 50
ohms. Intuition might provide some sort of indication. Its either one
or the other.
At frequencies lower than resonance ( minimum SWR ) the reactance
becomes capacitative (negative). At frequencies higher than resonance
the reactance becomes inductive (positive). And this is true at all
higher frequency resonances at which there is a minimum SWR.
To help remember, imagine a simple vertical. At low frequencies where
the height is less than 1/4-wave the input impedance obviously looks
like a capacitance. At frequencies somewhat higher than 1/4-wavlength
the reactance changes to inductive (which as everyone knows can be
tuned out with a series capacitor). The same effect will be observed
in the vicinity of 3/4-waves where another minimum SWR will occur.
At exactly 1/4-wave resonance, zero reactance, the SWR is a minimum.
If there's a good set of ground radials the input resistance will be
less than 50 ohms. If there's a poor ground the input resistance will
be greater than 50 ohms. With a good ground, at some frequency higher
than 1/4-wave it will be exactly 50 ohms. After the series capacitor
has tuned out the +ve reactance the SWR will then be zero and your
transmitter will be very happy.
To observe exactly what happens to the feeder input impedance, R + jX,
as the frequency is swept through a minimum SWR (zero input
reactance), download program DIPOLE1 from my website below. Exactly
the same effect occurs on all types of antennas + feeder. Model a
simple 40m dipole fed with 450-ohm feedline. You can also sweep
feedline length or dipole length to observe similar effects.
--
=================================
A prosperous 2000. Reg, G4FGQ
For free technical radio software go to:-
http://www.btinternet.com/~g4fgq.regp
=================================
Michael Young <nob...@all.xxx> wrote in message
news:s4ormh...@corp.supernews.com...
Reg Edwards
> There are non-resonant antennas and non-resonant feedlines which can
> give the lowest possible SWR, when they are matched, and that is
1:1,
> not zero as Reg well knows. SWR is a ratio of maximum to minimum
voltage
> or current, either will give the same value in the same situation,
in a
> feedline. The lowest ratio has a value of one in a "flat" line.
Rich,
I havn't the foggiest idea what you are waffling about. Do you ? You
do a very poor job of explaining yourself. Typical old wife.
It would be a great pity if you have given Michael the impression
there was anything incorrect with my reply to his query.
Richard Harrison
"Working with an MFJ-259B, I`m wondering if the reactance the meter
reports is capacitive or inductive."
There must be MFJ-259B users who have found convenient ways to determine
which direction the reactance takes.
One might deliberately add one or the other type of reactance and
determine which way the total went.
I am not familiar with the MFJ-259B, but would suggest reading MFJ`s
instructions. Absent these, Contact MFJ for their recommendations.
Reg was correct in saying a resonant circuit should indicate zero
reactance and that an MFJ-259B should indicate zero reactance if it is
working right.
Reg surely didn`t literally mean, "After the series capacitor has tuned
out the +vc reactance the SWR will then be zero and your transmitter
will then be very happy."
Zero SWR?
W6RCecilA
> It would be a great pity if you have given Michael the impression
> there was anything incorrect with my reply to his query.
Well, Reg, this statement of yours contains errors.
> At frequencies lower than resonance ( minimum SWR ) the reactance
> becomes capacitative (negative). At frequencies higher than resonance
> the reactance becomes inductive (positive). And this is true at all
> higher frequency resonances at which there is a minimum SWR.
For instance, consider a 66 ft dipole. The dipole is resonant around 7MHz.
At 16 MHz, which is certainly "a frequency higher than resonance", the
feedpoint impedance is capacitive, not inductive as you indicate above.
Every frequency higher than one resonance is also a frequency lower than
another resonance (for an ordinary dipole).
> You do a very poor job of explaining yourself.
Pot - Kettle, Kettle - Pot.
--
73, Cecil, W6RCA http://www.mindspring.com/~w6rca
Dan Richardson
(Richard Harrison) wrote:
>Michael, AA9XI wrote:
>"Working with an MFJ-259B, I`m wondering if the reactance the meter
>reports is capacitive or inductive."
>
[snip]
>
>Best regards, Richard Harrison, KB5WZI
Richard,
The MFJ 259B's meter displays impedance which, of course, has a
resistive component.
One can by changing the frequency one way or the other and observe
the reading contains capacitive or inductive component then, with a
little math, determine the reactance. This is explained in the user's
manual.
73
Danny, K6MHE
Reg Edwards
Sorry, your example demonstrates nothing.
Someone ought to to tell you that the resonant condition of (dipole +
feedline ), as with any other electrical network, is defined as when
the input impedance of the feedline is purely resistive. It has just
as much to do with feeder length as it has with the antenna. Program
DIPOLE1 allows all resonant frequencies to be determined exactly.
There is an infinite series of resonant points, one at every
1/4-wavelength of the SYSTEM not just the isolated antenna. But only
at the first and odd-numbered 1/4-wave points does the SWR meter
indicate a minimum because only then is the input resistance a low
value.
At the 1/2-wave points the input reactance of a vertical antenna
passes in the opposite direction from +ve back to -ve. BUT THE SWR
METER DOES NOT INDICATE A MINIMUM. It remains stuck at a maximum.
And in my first posting it is quite clear I was referring only to
frequencies in the vicinity of those resonances at which there is a
minimum in the SWR where the reactance changes from -ve to +ve as
frequency.
Did you know that for any passive network, the phase angle of input
impedance versus frequency always has a positive slope, passing
through zero at low impedance resonances and switching from +90
degrees back to -90 degrees at high impedance resonances. See Terman,
Everitt, et al.
--
=================================
Cheers, Reg, G4FGQ
For free technical radio software go to:-
http://www.btinternet.com/~g4fgq.regp
=================================
W6RCecilA <Cecil....@IEEE.org> wrote in message
news:384D2CB4...@IEEE.org...
W6RCecilA
> And in my first posting it is quite clear I was referring only to
> frequencies in the vicinity of those resonances at which there is a
> minimum in the SWR where the reactance changes from -ve to +ve as
> frequency.
Nope, it wasn't clear at all, Reg, because you didn't state your
boundary conditions. You made one of your famous sweeping, all-
encompassing statements that is just not true for all cases. If
it's not true for all cases, it is false, by definition.
I knew what you were talking about but I'm willing to bet most
of the newcomers didn't. Without boundary conditions, your statement
is just another old-wives tale.
Tom Bruhns
Reg Edwards wrote:
>
> Lowest SWR ALWAYS occurs at a frequency where the feedline plus
> antenna is resonant, ie the input reactance is zero and the input
> impedance is purely resistive. If it doesn't then there's something
> the matter with your SWR meter.
I can conceive of situations where that is not true. Quite easy to
visualize as a locus of points on a Smith chart. I'm assuming here a
fixed collection of feedline and antenna elements, swept in frequency
over a limited frequency range (limited at least by the analyzer's
range).
OTOH, if the analyzer is accurate, if there's any question about whether
a certain reactance is inductive or capacitive, one can always add a
known reactance and see whether the analyzer thinks that results in a
larger or smaller net reactance.
Cheers,
Tom
Richard Harrison
"One can by changing the frequency one way or the other and observe the
reading contains capacitive or inductive component then, with a little
Neat! Reg was on the right track, even if I don`t buy his zero SWR. Glad
MFJ`s manual has the explanation of how to determine the direction of
the reactance.
Reg Edwards
reactance and SWR it was obvious to everyone to what frequencies I was
referring. Certainly to the person to whom I had responded. It was
obvious even to YOU.
But you couldn't resist the nitpicking temptation to try
(unsuccessfully as usual) to trip up a foreigner on the false pretext
of hoping to learn something from the ensuing argument. What did you
learn on THIS occasion ? ;o))
--
=================================
Cheers, Reg, G4FGQ
For free technical radio software go to:-
http://www.btinternet.com/~g4fgq.regp
=================================
W6RCecilA <Cecil....@IEEE.org> wrote in message
news:384D511E...@IEEE.org...
Reg Edwards
charts, can you describe a practical situation where lowest SWR versus
frequency occurs at a frequency where the feedline input impedance has
a reactive component. That is, shifting frequency in either direction
causes the SWR meter reading to increase. Non-linear components are
barred.
---
Reg.
Wes Stewart
Harrison) wrote:
>Reg wrote:
>"Lowest SWR ALWAYS occurs at a frequency where the feedline plus antenna
>is resonant, ie the input impedance is purely resistive."
>
>Reg is pulling our legs again or there is another possibility. Reg`s
>message is dated: Tue, Dec 7 He may have reckoned it was Pearl Harbor
>Day from sensing a little "Nip" in the air, and maybe a few from a
>bottle, too.
>
>There are non-resonant antennas and non-resonant feedlines which can
>give the lowest possible SWR, when they are matched, and that is 1:1,
>not zero as Reg well knows. SWR is a ratio of maximum to minimum voltage
>or current, either will give the same value in the same situation, in a
>feedline. The lowest ratio has a value of one in a "flat" line.
>
>When an antenna is resonant XL=XC and if the feedpoint resistance
>matches the characteristic resistance of the feedline, the SWR is 1:1 on
>the transmitter feedline.
Because there is some nitpicking going on in this thread...strictly speaking, on
any practical, i.e. lossy line, there is a reactive component to the line Z0.
With a purely resistive load that matches the real part of the line Z0, the SWR
is not 1:1.
Sorry, just couldn't resist.
73,
Wes N7WS
W6RCecilA
> But you couldn't resist the nitpicking temptation to try
> (unsuccessfully as usual) to trip up a foreigner on the false pretext
> of hoping to learn something from the ensuing argument. What did you
> learn on THIS occasion ? ;o))
I learned that you continue to consider yourself to be omniscient.
Richard Harrison
"Because there is some nitpicking going on in this thread....strictly
speaking, on any practical, i.e. lossy line, there is a reactive
component to the line Z0. With a purely resistive load that matches the
real part of the line Z0, the SWR is not 1:1.
Wes is right. Line loss produces line reactance. If the loss is small,
so is the reactance, but it`s in there.
Like Wes, I was remarking on a small error in something that most would
recognize and accept as picayune. At the time the thought that a
transmitter might be happy with an SWR of zero struck me as egregious.
Now I`m sorry I slammed Reg for it because I don`t think anybody would
have been mislead by the statement and most knew that Reg meant the
reflected energy was zero, not the SWR, which would be 1:1 in that
situation. Like Wes, I found the opportunity to good to resist.
W6RCecilA
>
> Cecil, only W6RCA would find something wrong with too times too is
> fore.
Well Reg, if you are arrogant enough to challenge the group to find
something wrong with your postings, the result seems pretty predictable
to me. Nobody is perfect 100% of the time. I didn't say anything when
I first read the posting. It was only after your challenge to find
something wrong that I went back and found something wrong.
All it would take is for you to admit you left out the word "immediately"
in your statement that for frequencies above the resonant frequency, the
reactive feedpoint impedance of an antenna is inductive. Your statement
was false. The word "'immediately' above the resonant frequency ..."
would have made it true.
W6RCecilA
> Every antenna is resonant on one or more frequencies.
And is hardly ever exactly 50 ohms. But this sounds like a
challenge, Wes, to come up with a truly non-resonant antenna.
On what frequency is an isotropic antenna resonant? :-)
W6RCecilA
> There is nothing inherently resonant about a 50 Ohm resistor and it has a
> bodacious VSWR.
1:1 is bodacious?
> I think you'll find where the combination of the resistive component plus
> the reactive component is equal to 50 Ohms, the lowest VSWR reading will
> be had.
Nope, that's one of Reg's old wives' tales. The lowest SWR occurs
when the reflection coefficient is zero.
Richard Harrison
"I guess if an antenna is providing a truly resistive load, we can say
that it is resonant. This is the case with a quarterwave ground plane
with the radials angled down at about 45 degrees."
There are two issues here involved with the ground plane with angled
radials. First, is resonating the antenna by selecting the lengths of
the vertical and other conducting elements (radials) of the antenna.
Second is angling the radials for a 50-ohm impedance match to the coax.
With horizontal radials the drive point impedance would be about 37
ohms. With the radials at 180 degrees from the upper vertical element,
the drive point impedance would be about 75 ohms.
The in-between angle gives an in-between impedance near 50 ohms at the
resonant frequency where the vertical element is about 1/4-wavelength.
Richard Harrison
"Every antenna is resonant on one or more frequencies."
OK, true but for the nit-picking exceptions. There are rhombics,
terminated vees, Beverage antennas, and a few other terminated
configurations intended to produce a drive point impedance which is
uniform over a wide frequency range.
Michael Young
:-) I didn't have the MFJ manual handy and, by some convoluted logic,
thought I would get a quick, short answer by posting an RTFM question here.
The manual wasn't where I thought I last left it; it wasn't by the PC; it
wasn't by the rig; it wasn't on the workbench; and it wasn't in the satchel
of tools I hauled up to the roof and later hid in the garage. I found it
late last night, after posting, in the ever-growing pile of manuals by my
bedside.
Here's what MFJ has to say: [
NOTE: The MFJ-259B measures reactance, and converts reactance to
capacitance. The MFJ-259B can not determine if the reactance is actually
inductive or capacitive. You can usually determine the type of reactance by
adjusting frequency. If frequency is increased and reactance (X on the
display or Impedance on the meter) decreases, the load is capacitive at the
measurement frequency. If frequency is reduced and reactance decreases, the
load is inductive at the measurement frequency.
] Elsewhere, they say further: [
1.) RESONANT FREQUENCY is where reactance is zero ohms, or in some cases as
close to zero ohms as the MFJ-259B indicates. Since resistance has nothing
to do with resonsance, the resonant frequency is NOT always at the point of
lowest indicated SWR (although they certainly can be the same). The most
desirable load is almost always the load with lowest SWR, even though it may
not necessarily be the point of no reactance (resonance).
]
Also, I was recently reading reprints of Maxwell's Reflection articles, and
ON4UN's book on low-band DX'ing. I couldn't remember which tome said what,
or even the context of what was said. I think there were references to a
possible difference between SWR and feedline+antenna resonance in one or
both also.
> At exactly 1/4-wave resonance, zero reactance, the SWR is a minimum.
I guess this is the point where the most bloody noses were handed out...
From the viewpoint of a transmitter's 50 ohm output, will I still see the
lowest SWR at the frequency where my vertical's impedance is, say, 36.6+0j?
Or at some other frequency where it measures, hypothetically, 32 + 18j? I
guess I'm asking if SWR is meaningful at all when talking about antenna
resonance? Or, more precisely, is antenna resonance important at all when
considering SWR at the transmitter end of the feeder? As I understand
Maxwell, he seems to say losses are lowest when the reflections are in phase
with transmitted power, and that this happens magically with a conjugate
match for lowest SWR. I'm certain he gave elucidation, but it escapes me at
the moment. At any rate, and to totally change the subject, how does this
tie in with antenna resonance? Where does the transmitter's 50 ohm impedance
come into account? In calibrating the meter that measured the SWR?
I now know less than I did even an hour ago... Should I just not bother my
little head, and simply twist the little knobs until the meter hits bottom?
Shaken,
Michael.
AA9XI
MC 10kW Jesus
bodacious VSWR.
the reactive component is equal to 50 Ohms, the lowest VSWR reading will
see that the resistive component could be anything too. I guess if an
antenna is providing a truly resistive load, we can say that it is
resonant. This is the case with a quarter wave ground plane with the
any other antenna that I can think of right now.
MC
Reg Edwards wrote:
> Lowest SWR ALWAYS occurs at a frequency where the feedline plus
> antenna is resonant, ie the input reactance is zero and the input
> impedance is purely resistive. If it doesn't then there's something
> the matter with your SWR meter.
>
> But the value of the SWR at its minimum will give no indication of
> whether the input resistance is greater than 50 ohms or less than 50
> ohms. Intuition might provide some sort of indication. Its either one
> or the other.
>
> At frequencies lower than resonance ( minimum SWR ) the reactance
> becomes capacitative (negative). At frequencies higher than resonance
> the reactance becomes inductive (positive). And this is true at all
> higher frequency resonances at which there is a minimum SWR.
>
> To help remember, imagine a simple vertical. At low frequencies where
> the height is less than 1/4-wave the input impedance obviously looks
> like a capacitance. At frequencies somewhat higher than 1/4-wavlength
> the reactance changes to inductive (which as everyone knows can be
> tuned out with a series capacitor). The same effect will be observed
> in the vicinity of 3/4-waves where another minimum SWR will occur.
>
> At exactly 1/4-wave resonance, zero reactance, the SWR is a minimum.
> If there's a good set of ground radials the input resistance will be
> less than 50 ohms. If there's a poor ground the input resistance will
> be greater than 50 ohms. With a good ground, at some frequency higher
> than 1/4-wave it will be exactly 50 ohms. After the series capacitor
> has tuned out the +ve reactance the SWR will then be zero and your
> transmitter will be very happy.
>
> To observe exactly what happens to the feeder input impedance, R + jX,
> as the frequency is swept through a minimum SWR (zero input
> reactance), download program DIPOLE1 from my website below. Exactly
> the same effect occurs on all types of antennas + feeder. Model a
> simple 40m dipole fed with 450-ohm feedline. You can also sweep
> feedline length or dipole length to observe similar effects.
> --
> =================================
> A prosperous 2000. Reg, G4FGQ
> For free technical radio software go to:-
> http://www.btinternet.com/~g4fgq.regp
> =================================
>
> Michael Young <nob...@all.xxx> wrote in message
> news:s4ormh...@corp.supernews.com...
> > Working with an MFJ-259B, I'm wondering if the reactance the meter
> reports
Wes Stewart
>There is nothing inherently resonant about a 50 Ohm resistor and it has a
>bodacious VSWR.
Oh dear, where to begin.
In the case under discussion, resonance has been (acceptably) defined as the
point on the impedance curve where the reactance is zero. Using this
definition, then a purely resistive 50 (or any other value) ohm resistor is
"resonant."
If, in the usual case, the system impedance is also 50 ohm, then the SWR of a 50
ohm resistive load is 1:1, which I don't believe can be called "bodacious."
>I think you'll find where the combination of the resistive component plus
>the reactive component is equal to 50 Ohms, the lowest VSWR reading will
>be had.
True, as long as the "combination" is a 50 ohm resistor in series with a zero
ohm reactance. (Once again assuming a 50 ohm system)
>Given this reactive component could be anything, I think you can
>see that the resistive component could be anything too.
Wrong.
>I guess if an
>antenna is providing a truly resistive load, we can say that it is
>resonant.
Yes, most of agree with this. See my first comment above.
>This is the case with a quarter wave ground plane with the
>ground radials angled down at about a 45 degrees.
Wrong thinking again. This antenna, as will as any other, be "resonant" on one
(okay, on many harmonically related) frequency. You are trying to develop a
case for an antenna with a particular feedpoint resistance, i.e. 50 ohm and call
that a unique case of "resonance."
>It is not the case with
>any other antenna that I can think of right now.
Every antenna is resonant on one or more frequencies.
Wes N7WS
>
>MC
Reg Edwards
fore.
Reg ;o))
K7ITM
Oh, dear! I do hope we can do this with only a normal antenna and transmission
lines. Not even any caps or inductors, but if you allow them, then it becomes
VERY easy.
OK...may I use a simple model of a dipole near its first resonance, just a
series RLC? I did in any event. If you'd like to give me some specific
complex-impedance-versus-frequency data for a "real" dipole, please feel free.
The dipole model that I used is 73 ohms, 16.2uH, and 100pF. It shows resonance
near 4MHz. The region of interest that I looked at was 3.5MHz to 4.5MHz.
Seems like a reasonable range to me. Now, simply feed that network with a
piece of 50 ohm line, 20 degrees long at 4MHz, and lossless...though of course
it works to make it slightly lossy as well. Now I see two frequencies where
the reactance goes to zero: about 4.14MHz (SWR=2.08:1@50 ohms) and 4.278MHz
(SWR=2.90:1). However, minimum 50-ohm SWR is about 1.46:1 at 3.95MHz, where
the input to the transmission line is somewhat reactive, 64.02-j16.37 ohms.
This is a pretty simple system, no tricks, just an antenna and a short piece of
transmission line. OK?
Cheers,
Tom
Wes Stewart
Harrison) wrote:
>Wes, N7WS wrote:
>"Every antenna is resonant on one or more frequencies."
>
>OK, true but for the nit-picking exceptions. There are rhombics,
>terminated vees, Beverage antennas, and a few other terminated
>configurations intended to produce a drive point impedance which is
>uniform over a wide frequency range.
>
>Best regards, Richard Harrison, KB5WZI
I thought maybe you had me, but now, I think I can wiggle out of this mess.
I did some fiddling around and came up with the following EZNEC example. This
was a file I just happened to have; I don't remember why, when or where it came
from. I just iterated the frequency to see what happens. Changing the
frequency only +/- 10 KHz changes the sign of the feedpoint reactance from plus
to minus. So not only is there a resonance, but it is a very hi Q resonance on
this "broadband, frequency insensitive" antenna.
I guess that this class of terminated-travelling-wave antennas can be viewed as
terminated lossy transmission lines. One can then argue that if the termination
resistance matches the impedance of the line, there are no standing waves and
the "line" then looks infinitely long and cannot be distinguished from a
resistor.
But this is a "lossy" line and back when I was picking nits, I stated that the
load impedance would have to match the reactive line impedance. Therefore,
using a purely resistive load *does not* properly terminate the line so there
are standing waves on the "transmission line" and its input impedance varies
with frequency.
Because one of our stipulations is that resonance occurs where the sign of the
reactance changes from plus to minus, it follows that the rhombic described
below has at least one resonant frequency at ~3.789 MHz.
Frequency = 3.789 MHz.
Wire Loss: Zero
--------------- WIRES ---------------
Wire Conn. --- End 1 (x,y,z : ft) Conn. --- End 2 (x,y,z : ft) Dia(in) Segs
1 W2E1 -377.49, -0.100, 30.000 W5E1 -377.49, 0.100, 30.000 # 12 3
2 W1E1 -377.49, -0.100, 30.000 W3E1 0.000,-184.00, 30.000 # 12 120
3 W2E2 0.000,-184.00, 30.000 W4E1 377.493, -0.001, 30.000 # 12 120
4 W3E2 377.493, -0.001, 30.000 W6E2 377.493, 0.100, 30.000 # 12 3
5 W1E2 -377.49, 0.100, 30.000 W6E1 0.000,184.000, 30.000 # 12 120
6 W5E2 0.000,184.000, 30.000 W4E2 377.493, 0.100, 30.000 # 12 120
-------------- SOURCES --------------
Source Wire Wire #/Pct From End 1 Ampl.(V, A) Phase(Deg.) Type
Seg. Actual (Specified)
1 2 1 / 50.00 ( 1 / 50.00) 1.000 0.000 I
--------------- LOADS ---------------
Load Wire Wire #/Pct From End 1 R (Ohms) X(Ohms)
Seg. Actual (Specified)
1 2 4 / 50.00 ( 4 / 50.00) 600.000 0.000
Ground type is Real, MININEC-type analysis
Conductivity = .0303 S/m Diel. Const. = 20
Whew
Wes N7WS
Brian Kelly
>> of hoping to learn something from the ensuing argument. What did you
>> learn on THIS occasion ? ;o))
>
>I learned that you continue to consider yourself to be omniscient.
>
yourself.
>--
Richard Harrison
"So not only is there a resonance, but it is a very hi Q resonance on
Resonance is an exceptional response of a system having a natural
frequency of susceptibility to a periodic external stimulus of the same
or nearly the same frequency.
I don`t do EZNEC, so I`m unfazed by its results.
I have done many terminated rhombics which exhibited no evidence of
resonance.
Tom Bruhns
K7ITM wrote:
>
> Reg wrote:
>
> >Tom, without confusing the issue with imaginative curves on Smith
> >charts, can you describe a practical situation where lowest SWR versus
> >frequency occurs at a frequency where the feedline input impedance has
> >a reactive component. That is, shifting frequency in either direction
> >causes the SWR meter reading to increase. Non-linear components are
> >barred.
>
> Oh, dear! I do hope we can do this with only a normal antenna and transmission
> lines.
On second thought, there's an even easier and more general way to look
at it.
Consider a normal center-fed dipole-type antenna. It could be just a
dipole, or it could be the driven element in an array. Consider it near
one of its odd-half-wave resonances. The feedpoint impedance shows a
resonance, with zero reactance at resonance, and that's also a point (or
at least extremely close to a point) of lowest SWR with respect to a 50
ohm system. Typically, though, the feedpoint resistance is somewhat
higher than 50 ohms.
Now, feed that with 50 ohm line. The lowest SWR on the line will be at
(or essentially at) the antenna resonance. However, at the feed end of
the line, you will see a reactance unless the line is an odd number of
quarter waves long. Remember, that's the lowest SWR. At many line
lengths, you will see zero reactance at a nearby frequency, but that's
neither where the antenna is resonant nor where the line SWR is
minimum. But remember, we're looking at the feed end of the line, and
DO NOT KNOW where the antenna itself is resonant, at least not by
looking for zero reactance.
IF you can safely assume that the SWR with respect to 50 ohms goes up on
either side of antenna resonance, then antenna resonance is determined
by MINIMUM SWR, NOT by zero reactance! Caveats: antenna fed by 50 ohm
line, SWR measured with respect to 50 ohms. Or substitute other feed
line impedance, so long as you measure SWR with respect to that
impedance.
Cheers,
Tom
Tom Bruhns
Michael Young wrote:
> > At exactly 1/4-wave resonance, zero reactance, the SWR is a minimum.
>
> I guess this is the point where the most bloody noses were handed out...
> From the viewpoint of a transmitter's 50 ohm output, will I still see the
> lowest SWR at the frequency where my vertical's impedance is, say, 36.6+0j?
> Or at some other frequency where it measures, hypothetically, 32 + 18j? I
> guess I'm asking if SWR is meaningful at all when talking about antenna
> resonance? Or, more precisely, is antenna resonance important at all when
> considering SWR at the transmitter end of the feeder?
Please see the posting I just made in this thread. For your specific
example, 36.6+j0 will be SWR=1.366:1 with respect to 50 ohms, and 32+j18
will be SWR=1.87:1 with respect to 50 ohms. Those, presumably, are
antenna feedpoint impedances. If you connect a length of 50 ohm line to
that feedpoint, the SWR on the line will be unchanged along the line,
assuming no loss. BUT!!! Pick the right length line, and that 32+j18
will be transformed to a pure resistance (50*1.87 or 50/1.87 ohms), and
the pure resistance will be transformed to something with reactance.
The lowest SWR will still be the the same frequency...and probably very
close to the ANTENNA FEEDPOINT resonance.
This is why I have a big problem with calling the point of zero
reactance, as seen through a feedline, "resonance." I think that's a
red herring. I can show you specific instances where that "resonance"
is quite broad and in fact where the reactance is capacitive both above
and below it, and never goes inductive, even though the antenna itself
goes through a distinct resonance (at a nearby but somewhat differen
frequency). What sort of "resonance" is that??
(Lowest losses on a line generally occur when there is no reflected
power: when the line is terminated in its characteristic impedance. If
it's not possible to get to zero reflected power, the lowest losses in a
particular system occur when the reflected power is minimized. But you
must qualify even that: since most loss is I^2*R, if the line is very
short compared with 1/4 wave, lowest losses for a given power
transferred may well occur with a much higher load impedance than the
line impedance. If it all makes your head spin, well...maybe you're
just worrying too much about it. But going back to fundamentals and
integrating I^2*R and E^2*G along the line will always tell you how much
power is being dissipated by the line. :-)
Some might say the Smith Chart is obsolete, but to me, it's a wonderful
way to VISUALIZE what's going on with all this.
Cheers,
Tom
Roy Lewallen
probably are also.
The SWR isn't determined by the sum of the resistance and reactance, as
you seem to imply. A load of 50 + j0 results in an SWR of 1:1 on a 50
ohm line, as we all know, but 0 + j50 -- pure reactance -- results in an
infinite SWR. 40 + j10 results in 1.37:1. If you remove the 10 ohms of
reactance, leaving 40 + j0, the SWR drops to 1.25. 30 + j20 gives an SWR
of 2.04:1, but 30 + j0 gives an SWR to 1.67. And so forth.
At an antenna feedpoint, the reactance typically varies much more
rapidly than resistance. Reactance has a much stronger effect on SWR
than resistance does, as illustrated above, so at an antenna feedpoint,
the SWR is minimum at or extremely near to the point where the reactance
is minimum, as Reg said. If you fix the resistance and vary the series
reactance, which is approximately the case for an antenna feedpoint over
a small range, the SWR is always minimum when the reactance is minimum.
Roy Lewallen, W7EL
MC 10kW Jesus wrote:
> There is nothing inherently resonant about a 50 Ohm resistor and it has a
> bodacious VSWR.
> I think you'll find where the combination of the resistive component plus
> the reactive component is equal to 50 Ohms, the lowest VSWR reading will
> be had. Given this reactive component could be anything, I think you can
> see that the resistive component could be anything too. I guess if an
> antenna is providing a truly resistive load, we can say that it is
> resonant. This is the case with a quarter wave ground plane with the
> ground radials angled down at about a 45 degrees. It is not the case with
> any other antenna that I can think of right now.
>
> MC
Peter O. Brackett
It seems that for this thread to converge on an concensus we need an agreed
upon definintion of "resonance".
There seem to be a few slightly different but probably numerically close
definitions being bandied about here.
Even with a simple RLC tuned circuit I have encountered various (but closely
related) definitions of resonance.
If one were to use "analysis" [Theory of complex variables.] and laws of
physics to evaluate the "natural modes" of an antenna (system) one would
obtain a, possibly infinite, set of singularities which are often known as
the "natural modes" of the system. These singular points may be
instantiated by numerical values defining points in the complex plane.
These singularities, or eigenvalues of the system (The "poles" or "natural
modes" of the antenna.) let us call the i'th one z(i) would have numerical
values with real and imaginary parts, say z(i) = x(i) + j r(i). Assume that
the response of the antenna is measured by measuring the current flowing
while applying a zero impedance sinusoidal voltage generator across the
antenna terminals and sweeping it's frequency across a region nearby to one
of these singularities. Say the "first" one.
How then in terms of the resulting current flow do we define the sinusoidal
frequency point of "resonance" with respect to these singularities?
I presume from earlier discussions on this thread that there might be
several defintions of this point! If everyone could agree on the precise
definition of resonance then there could be concensus, otherwise...
Probably Cecil will quote the definition of resonance to us from the IEEE
Dictionary. Maybe we should start there?
Cecil?
Peter AB4BC
Tom Bruhns <tom_b...@agilent.com> wrote in message
news:384EB16E...@agilent.com...
Reg Edwards
newsgroup I have been obliged to confess to a seious slip-up. But no
bones broken I hope.
Reg.
=================================
Brian Kelly wrote ..
Richard Harrison
"He (Reg) is (omniscient), congratulations..."
Well, Reg`s prescription for determining the nature of a reactance, the
magnitude of which is determined by an MFJ instrument was confirmed as
that recommended by the instrument`s manual which finally turned up.
I too say well done, Reg.
Richard Harrison
"There seem to be a few slightly different but probably close
definitions being bandied about here."
A resonant antenna has standing waves but unity power factor when
excited.
Does that correctly cover the situation?
Reg Edwards
> Lowest SWR ALWAYS occurs at a frequency where the feedline plus
> antenna is resonant, ie the input reactance is zero and the input
> impedance is purely resistive.
Michael and Newsgroup,
The above statement is incorrect. My apologies. Replace it with the
following.
Lowest SWR versus frequency on an antenna feedline occurs near a
natural resonant frequency of the antenna. This is only approximately
true due to variation in radiation resistance and other losses over
the frequency band. The effect is more pronounced with low impedance
feedlines and is most pronounced (of course) when the resonant
antenna's input resistance is very near to Zo.
The value of the SWR by itself tells very little of the antenna's
input impedance. It will not even indicate whether Zin is greater or
less than Zo.
When impedance measurements can be made only at the feedline's input,
the antenna's input impedance must be determined by a complex
calculation.
A new program will be available within 24 hours which, for given Zo,
length and attenuation of the line, computes unambiguously what R + jX
of the load must be in order to satisfy given Zin = Rin + jXin of the
line. Should be very useful to owners of these new hand-held R+jX
meters. Effectively provides the Z-meter with a very long test lead.
Has other applications round the shack.
Compare with a Smith chart for convenience, speed and accuracy,
especially with real lossy lines. You'll never go back.
--
=================================
Xmas Greetings, Reg, G4FGQ
Peter O. Brackett
Nice, short, concise, but... Welllllllll...
Unity power factor, I understand. Current and voltage are in phase!
Simple.
But I find the whole notion of waves and especially standing "waves" to be
somewhat arbitrary and unsettling, especially in a "definition" of
resonance.
Question: What exactly is a standing wave, how is it measured, and how do
we know it is present?
I understand that "waves" as such must be calculated and inferred from point
measurements of current and voltage and such "waves" may "travel" or
"stand", but other than the mechanical vibrations of strings and diaphragms,
etc, no one has "seen" them. In fact calculations and measurements of these
things can be quite arbitrary and so I find them to be unsettling.
The common definition of "reflection coefficient" "rho" which measures the
amount of a (voltage) wave reflected from an incident wave by an "object" of
impedance Z is:
rho = (Z - Zr)/(Z + Zr)
Where Zr, the reference impedance, is an arbitrarily chosen value! In
amateur radio practice Zr is traditionally equal to a pure resistance of 50
ohms. But this traditional value of Zr is not necessary and is only so
chosen for convenience. In fact a particular implementation of Zr in
practice is always included as an integral part of the instrument (A bridge
circuit) commonly used to measure rho and VSWR. But apparently, unless I am
seriously mistaken, I am free to choose my own Zr and thus define my own rho
for any Z. In fact forward and reflected waves are nothing more than an
arbitrary bilinear combination (rotation in a vector space) of voltages and
currents.
Now the common definition of VSWR is:
VSWR = (1 + rho)/(1 - rho)
As can be seen rho and VSWR are simply related by a "bilinear"
transformation (Smith chart?) and so...
for any given object (antenna) of impedance Z, I can simply choose an
arbitrary value of Zr to yeild any value of VSWR desired. Anyone with a
modicum of algebraic skills can easily do the manipulations.
And so... I could have a unity power factor, but yet have any desired level
of reflected waves, including zero, depending simply upon the reference
impedance in my VSWR meter. Unsettling to say the least!
Might not a better set of definitions of resonance with respect to antennas
and which eliminates all mention of those arbitrary and unsettling "waves"
be:
To be resonant, an antenna must store electrical energy and simultaneously
dissipate a fraction of that electrical energy as radiant energy, while
exhibiting a unity power factor.
Of ourse when "off resonance" the antenna must store energy and
simultaneously dissipate a fraction of that energy but not exhibit a unity
power factor.
And a "non-resonant" (broadband?) antenna must radiate all of the energy
presented to it while storing none and exhibiting a unity power factor.
A Beverage or terminated rhombic might be examples of the latter.
Is this right?
Peter AB4BC
Richard Harrison <richard...@webtv.net> wrote in message
news:23837-38...@storefull-113.iap.bryant.webtv.net...
ghv...@er.com
Take a rope about 15 feet long and lay it on the ground.
Tie one end of it to the wall. (BTW the floor must be
smooth as to not add friction)
Make a quick start and stop horizontal motion with the
rope.
Watch the sine wave leave the incident side, reflect off
of the wall and come back to you.
Now get someone else to go over to the wall.
And now you make a steady continuous horizontal motion
with the rope and the guy by the wall does the same. If
you both get you frequencies matched (similiar amplitudes
help with the visual impact) you will SEE a standing
wave somewhere between you and the wall. It will be a motionless
sine wave that just sits there as you both are moving the ropes
on both ends.
There are EXACT analagies between physical motions like
this and electrical properties like capacitance/inductance.
Richard Harrison
"Might not a better set of definitions of resonance with respect to
antennas and which eliminates all mention of those arbitrary and
As Peter noted, you can see standing waves in vibrating strings and
other resonant objects.
A standing wave is an interference pattern produved by waves traveling
in opposite directions in the same path. If these are the same frequency
a stationary pattern appears.
In a stringed instrument, incident and reflected mechanical energy meet
along the length of a string to produce a pattern of stationary spots
which alternates with spots of the most intense motion. These patterns
are called standing waves.
Maximum and minimum electrical voltage and current patterns along a wire
are also produced by interference between incident and reflected
traveling waves of electrical energy, if the wire is long enough and the
energy meets a reflection point (discontinuity), or obstruction in its
path.
This phenomenon appears in resonant antennas because all the energy
traveling along an antenna wire has not been exhausted when an open
circuit or short circuit appears in the wire. A reflection must occur
and a standing wave pattern on the wire results.
A resonant antenna has standing waves.
We have coined a term to describe the amplitude of the standing wave
pattern. It is "standing wave ratio" or SWR. This is sometimes written
VSWR to state that it isn`t the powers in the wave that are being
ratioed. The V in VSWR represents voltage. It could have just as well
have been ISWR because current and voltage ratios of maxina and minima
are the same in the interference pattern. In a wave half the energy is
in the E field and half is in the H field. Identical SWR`s are
calculated using either E or I in the SWR calculation.
We have defined SWR as Emax over Emin in the SWR pattern, or the same
thing, Imax over Imin.
SWR can also be expressed in terms of the incident and reflected wave
amplitudes (E1 and E2).
IE1I + IE2I
SWR= ----------------
IE1I - IE2I
That is the max over the min again . If SWR is unity, the formula above
would show that this happens when the absolute value of E2 is zero, i.e.
there is no reflection.
SWR is related to the ratio of the incident wave to the reflected wave,
voltages or currents, by a reflection coefficient. The justification for
this complication is ease and utility of application. Rho, the
reflection coefficient, equals SWR minus one over SWR plus one.
Saying a resonant antenna has standing waves need not involve impedance.
It does say that the antanna has reflected energy as all resonant things
do.
Saying an antenna has standing waves distinguishes it from a resistor
which also has a unity power factor.
Best regards, Richard Harrison, KB5WZi
Donaly
>
> Richard:
>
> Nice, short, concise, but... Welllllllll...
>
> Unity power factor, I understand. Current and voltage are in phase!
> Simple.
>
> But I find the whole notion of waves and especially standing "waves" to be
> somewhat arbitrary and unsettling, especially in a "definition" of
> resonance.
>
> Question: What exactly is a standing wave, how is it measured, and how do
> we know it is present?
I suggest you read the book "Physics of Waves" by William C. Elmore and
Mark A.
Heald. (ISBN 0-486-64926-1) Alternatively, most good undergratuate
physics texts
deal with wave behavior, although maybe not as thoroughly as Elmore and
Heald.
>
> I understand that "waves" as such must be calculated and inferred from point
> measurements of current and voltage and such "waves" may "travel" or
> "stand", but other than the mechanical vibrations of strings and diaphragms,
> etc, no one has "seen" them. In fact calculations and measurements of these
> things can be quite arbitrary and so I find them to be unsettling.
I believe Heinrich Hertz did some good work on demonstrating the
existence of
electromagnetic waves. You may want to check out the Heaviside-Hertz
equations
and puzzle out their implications for yourself. This should be easy,
since you
are oviously aquainted with abstract algebra, thus possessing a
mathematical
sophistication well above that of the average ham.
(Much good sophistry snipped.)
>
> Might not a better set of definitions of resonance with respect to antennas
> and which eliminates all mention of those arbitrary and unsettling "waves"
> be:
>
> To be resonant, an antenna must store electrical energy and simultaneously
> dissipate a fraction of that electrical energy as radiant energy, while
> exhibiting a unity power factor.
>
> Of ourse when "off resonance" the antenna must store energy and
> simultaneously dissipate a fraction of that energy but not exhibit a unity
> power factor.
>
> And a "non-resonant" (broadband?) antenna must radiate all of the energy
> presented to it while storing none and exhibiting a unity power factor.
>
> A Beverage or terminated rhombic might be examples of the latter.
>
> Is this right?
Read the book and decide for yourself.
>
> Peter AB4BC
>
Tom Donaly KA6RUH
Tom Bruhns
Reg Edwards wrote:
>
> Reg Edwards wrote ..
> > Lowest SWR ALWAYS occurs at a frequency where the feedline plus
> > antenna is resonant, ie the input reactance is zero and the input
> > impedance is purely resistive.
> ============================
> Michael and Newsgroup,
>
> The above statement is incorrect. My apologies. Replace it with the
> following.
...
Thanks, Reg. I think we're back in sync now!
For Michael and others following this, I wanted to focus on one topic
that has come up in the thread to make things crystal clear:
At the feedpoint of a halfwave dipole or quarter-wave above a ground
plane, above resonance, reactance goes inductive, and below resonance,
it goes capacitive. But through a quarter wave (or any odd number of
quarter waves) of transmission line, you'll see just the opposite:
capacitive above and inductive below! If the line is an integer number
of halfwaves, the change will be as it is at the antenna.
And at the feedpoint of a fullwave dipole or halfwave above a
groundplane, reactance goes capacitive _above_ and inductive _below_
(the rather high impedance) resonance. Again, an odd number of quarter
waves of line will _invert_ that behavior, just as it does with the
quarter wave.
If line length is in between, you may see _no_ nearby point with zero
reactance.
(I imagine that the program Reg will make available shortly will nicely
take care of ALL of that in a quantitative way for you...likely will
even contain instructions for measuring the electrical length and loss
of the line by testing it with the far end shorted and/or open.)
Cheers,
Tom
Tom Bruhns
ghv...@er.com wrote:
>
> If you want to see standing waves:
> Take a rope about 15 feet long and lay it on the ground.
> Tie one end of it to the wall. (BTW the floor must be
> smooth as to not add friction)
> Make a quick start and stop horizontal motion with the
> rope.
> Watch the sine wave leave the incident side, reflect off
> of the wall and come back to you.
etc.
The ham radio club I got started in 40+ years ago would get 16mm films
from various places, mostly from the Bell phone system, borrowing a
projector from the library. There was a _wonderful_ series of films
that demonstrated transmission line behavior very graphically. They
used mechanical transmission lines. They had a central rod, with cross
pieces at regular intervals of a couple inches. The transmission mode
was torsional, so you could see the waves travelling along by watching
the tips of the rods. They had different impedance lines, made with
shorter and longer cross members, and could terminate the line in a
"short" or "open" or power-absorbing load. It made it very easy to see
the travelling waves and the standing waves. They could be excited with
sinusoids, pulses, steps...
Does anyone know if those training films ever got transferred to video?
I'd really recommend them to anyone wanting to get basic transmission
line understanding.
Cheers,
Tom
W6RCecilA
> Probably Cecil will quote the definition of resonance to us from the IEEE
> Dictionary. Maybe we should start there?
There are six sub-headings under "resonance". There are two pages
of definitions dealing with "resonance" and "resonant".
"resonance - (5)(A)(radio-wave propagation)(in an oscillating system)
The rapid increase or decrease of the oscillation amplitude as the
excitation frequency approaches one of the natural frequencies of
the system. (B)(radio-wave propagation)(of a traveling wave) The
change in amplitude as the frequency of the wave approaches or
coincides with a natural frequency of the medium (for example, a
plasma frequency)."
Peter O. Brackett
Hey thanks for the pointers to good reference material on waves.
Perhaps I was a bit too long in my complaints about a wave description of
resonance, and I have led you to believe that I don't "trust" waves. Sorry
about the "abstract" algebra, it is simply the most direct way to express
the arbitrary nature of SWR meters!
I simply believe that resonance is a widespread phenomena of nature
unrelated to waves and that it is only directly related to the oscillating
storage of energy, hence my "tirade" against waves.
Sorry if I offended any wave worshippers out there. In fact I do believe
in waves.
Various kinds of mechanical and hydraulic waves are readily understood by
humans since they can be physically observed directly with our unaided eyes,
ears and fingers. Electrical waves on the other hand seem to fall victim
to fuzzy thinking due mainly (I believe) to some aspects of the metrology of
wave measurement. Particularily the ubiquitous SWR meter or reflectometer.
Using (abstract) algebra (sorry!) I tried to point out the somewhat
arbitrary nature of the way in which we commonly measure and describe
electrical waves in/on feedlines, systems and antennas in terms of
reflection coefficients and vswr.
In particular, because of the way in which common everyday SWR meters are
designed, implemented, sold, and used in practice, folks tend to forget the
arbitrary nature of vswr and reflection coefficients as usually defined.
I'll make a new posting up the NG further to elaborate on these arbitrary
factors a little further.
Thanks for taking the time to point out those excellent references.
BTW.... what is the value of the reference impedance used in your SWR
meter? Is it written on the meter anywhere, is it in the documentation
anywhere, how accurate is it? Is the value installed in your meter the
correct value for the uses to which you want to put it? etc. etc...
Peter AB4BC
Donaly <dtdo...@pacbell.net> wrote in message
news:384FCB...@pacbell.net...
Donaly
>
> Tom:
>
> Hey thanks for the pointers to good reference material on waves.
>
> Perhaps I was a bit too long in my complaints about a wave description of
> resonance, and I have led you to believe that I don't "trust" waves. Sorry
> about the "abstract" algebra, it is simply the most direct way to express
> the arbitrary nature of SWR meters!
The only thing wrong with abstract algebra is that many people above a
certain
age haven't studied it. Certainly, not many hams.
>
> I simply believe that resonance is a widespread phenomena of nature
> unrelated to waves and that it is only directly related to the oscillating
> storage of energy, hence my "tirade" against waves.
Can you say that a tank circuit has waves? Do waves require dimensions?
>
> Sorry if I offended any wave worshippers out there. In fact I do believe
> in waves.
I'll admit it. I'm a wave hugger.
>
> Various kinds of mechanical and hydraulic waves are readily understood by
> humans since they can be physically observed directly with our unaided eyes,
> ears and fingers. Electrical waves on the other hand seem to fall victim
> to fuzzy thinking due mainly (I believe) to some aspects of the metrology of
> wave measurement. Particularily the ubiquitous SWR meter or reflectometer.
>
> Using (abstract) algebra (sorry!) I tried to point out the somewhat
> arbitrary nature of the way in which we commonly measure and describe
> electrical waves in/on feedlines, systems and antennas in terms of
> reflection coefficients and vswr.
>
> In particular, because of the way in which common everyday SWR meters are
> designed, implemented, sold, and used in practice, folks tend to forget the
> arbitrary nature of vswr and reflection coefficients as usually defined.
>
> I'll make a new posting up the NG further to elaborate on these arbitrary
> factors a little further.
I'm looking forward to reading it.
>
> Thanks for taking the time to point out those excellent references.
>
> BTW.... what is the value of the reference impedance used in your SWR
> meter? Is it written on the meter anywhere, is it in the documentation
> anywhere, how accurate is it? Is the value installed in your meter the
> correct value for the uses to which you want to put it? etc. etc...
>
I use a CIA HF analyzer which uses a reference of 50 ohms. Most of the
trans-
mission lines I use have a characteristic impedance of 50 ohms. I
usually work
out transmission line problems on a Smith chart normalized to 1 ohm. It
works
well enough for who it's for.
Richard Harrison
"I simply believe that resonsnce is a widespread phenomena (sic) of
nature unrelated to waves and that it is only directly related to the
First, resonance exists in an L-C circuit at a frequency where the total
reactance is zero.
Second, resonance is an exceptional response produced by a
susceptibility to stimulus at or near a particular frequency or
frequencies.
Third, A wave is a progressive periodic vibrational disturbance
propagated and progressing through a medium or space without progress of
the particles in the medium or space themselves.
Fourth, the relation between resonance and waves is that of stimulation
and response. The oscillating current in a resomant antenna can result
from a wave sweeping it, or the reverse can occur. A wave in space can
result from the oscillating current in a resonant antenna.
Resonance is not always a requirement but can be very helpful in many
instances.
I would say that seldom will waves not stimulate resonances, and seldom
will resonances not produce waves. Amplitudes and efficiencies are
another matter.
Roy Lewallen
>
> Richard:
>
> Nice, short, concise, but... Welllllllll...
>
> Unity power factor, I understand. Current and voltage are in phase!
> Simple.
>
> But I find the whole notion of waves and especially standing "waves" to be
> somewhat arbitrary and unsettling, especially in a "definition" of
> resonance.
Consider a half wavelength 50 ohm transmission line terminated with a 10
ohm load. Looking into the input of the line, you'll see 10 + j0, a
resonant condition (by definition, since the reactance is zero). Yet
there are standing waves on the line. Resonance and standing waves are
different phenomena.
> Question: What exactly is a standing wave, how is it measured, and how do
> we know it is present?
A standing wave is created by the superposition (sum) of a forward
travelling wave and a reflected, or reverse traveling wave. The
interference of these two waves creates a situation where the
transmission line has fixed locations where the voltage and current are
higher and lower than they would be for a "flat" line (one terminated in
its characteristic impedance, and without standing waves). These fixed
high and low points give the line distribution the appearance of having
non-moving, or standing, waves, even though there are really two waves,
one moving in each direction.
A standing wave can be directly measured with a slotted line and
detector. More commonly, it's indirectly measured with an SWR meter or
return loss bridge. We know that standing waves are present whenever the
SWR is other than 1:1 or the return loss is finite.
> I understand that "waves" as such must be calculated and inferred from point
> measurements of current and voltage and such "waves" may "travel" or
> "stand", but other than the mechanical vibrations of strings and diaphragms,
> etc, no one has "seen" them. In fact calculations and measurements of these
> things can be quite arbitrary and so I find them to be unsettling.
Calculations and measurements are not arbitrary at all. Quantitative
results have been obtained and used daily by engineers for over a
century.
> The common definition of "reflection coefficient" "rho" which measures the
> amount of a (voltage) wave reflected from an incident wave by an "object" of
> impedance Z is:
>
> rho = (Z - Zr)/(Z + Zr)
>
> Where Zr, the reference impedance, is an arbitrarily chosen value! In
> amateur radio practice Zr is traditionally equal to a pure resistance of 50
> ohms. But this traditional value of Zr is not necessary and is only so
> chosen for convenience. In fact a particular implementation of Zr in
> practice is always included as an integral part of the instrument (A bridge
> circuit) commonly used to measure rho and VSWR. But apparently, unless I am
> seriously mistaken, I am free to choose my own Zr and thus define my own rho
> for any Z. In fact forward and reflected waves are nothing more than an
> arbitrary bilinear combination (rotation in a vector space) of voltages and
> currents.
You can arbitrarily choose Zr if you'd like, but if you want to know the
SWR on a transmission line, or calculate the magnitude of the maximum
and minimum voltages or currents (i.e., the magnitude of the standing
wave), you have to use the characteristic impedance of the line for Zr
and the load impedance for Z. Rho is then the reflection coefficient at
the interface between the line and the load, which in turn determines
the SWR on the line.
>. . .
> Might not a better set of definitions of resonance with respect to antennas
> and which eliminates all mention of those arbitrary and unsettling "waves"
> be:
>
> To be resonant, an antenna must store electrical energy and simultaneously
> dissipate a fraction of that electrical energy as radiant energy, while
> exhibiting a unity power factor.
>
> Of ourse when "off resonance" the antenna must store energy and
> simultaneously dissipate a fraction of that energy but not exhibit a unity
> power factor.
>
> And a "non-resonant" (broadband?) antenna must radiate all of the energy
> presented to it while storing none and exhibiting a unity power factor.
>
> A Beverage or terminated rhombic might be examples of the latter.
>
> Is this right?
>
> Peter AB4BC
No. Resonance has nothing to do with what percentage of applied power is
radiated by an antenna. The fraction of the applied power which is
radiated is the efficiency, and that's a function of loss, not
resonance. We've done fine, thank you, with the definition of resonance
which is the frequency at which the reactance is zero. And efficiency as
the fraction of applied power which is radiated. No new definition is
required.
I suggest that you invest in a copy of the _ARRL Antenna Book_ (or visit
your local library for a copy). It has a good basic discussion of
standing waves and SWR. For equations, my personal favorite is
_Reference Data For Radio Engineers_.
Roy Lewallen, W7EL
Reg Edwards
Donaly
>
> Is the CIA to be trusted ?
"Trust" isn't a word one generally associates with spies.
My AEA Engineering Complex Impedance Analyzer, on the other
hand, I've found, can be left alone with my wife for
indefinitely long periods with complete confidence, on my
part, that nothing untoward will happen between them.
Tom Donaly KA6RUH
Reg Edwards
- - - and you don't smoke Havanna cigars.
I am interested in obtaining the AEA Complex Impedance Analyzer.
Is it self-contained, internal battery operated ?
Is it small enough to be held in one hand ?
Does it measure R +/- jX simultaneously, ie, 2 displays.
What is range of magnitude of Z, 1 ohm to 500 ohms ? Or what ?
I am not interested in SWR - just Z versus frequency.
Does manufacturer have a website ?
Would appreciate it if you could find time to answer.
=================================
Xmas Greetings, Reg, G4FGQ
For free technical radio software go to:-
http://www.btinternet.com/~g4fgq.regp
=================================
Donaly <dtdo...@pacbell.net> wrote in message
news:38510C...@pacbell.net...
Tom Bruhns
with an unknown piece of test equipment, would toss it in the rubbish.
:-)
Cheers,
Tom
Donaly wrote:
...
Wes Stewart
wrote:
>Hi Tom,
>
> - - - and you don't smoke Havanna cigars.
>
>I am interested in obtaining the AEA Complex Impedance Analyzer.
>
>Is it self-contained, internal battery operated ?
>
>Is it small enough to be held in one hand ?
>
>Does it measure R +/- jX simultaneously, ie, 2 displays.
>
>What is range of magnitude of Z, 1 ohm to 500 ohms ? Or what ?
>
>I am not interested in SWR - just Z versus frequency.
>
>Does manufacturer have a website ?
Reg:
Look here: http://www.aea-wireless.com/cia.htm
I have trouble with some of the claimed specs. For instance, they claim 40 dB
return loss capability but only 1.1:1 swr capability, I don't lnow how they
can claim 40 dB return loss capability with a SO-239 ("UHF") test port
connector. I guess because it has a digital readout, they get better
"accuracy."
Wes N7WS
Donaly
>
> Hi Tom,
>
> - - - and you don't smoke Havanna cigars.
>
> I am interested in obtaining the AEA Complex Impedance Analyzer.
>
> Is it self-contained, internal battery operated ?
Yes. It does, however, consume batteries at an alarming rate.
>
> Is it small enough to be held in one hand ?
Yes, but it's on the verge of being awkward.
>
> Does it measure R +/- jX simultaneously, ie, 2 displays.
It will give you the numbers simultaneously, but not the plots.
It's easy to switch between screens, however.
>
> What is range of magnitude of Z, 1 ohm to 500 ohms ? Or what ?
There is a screen for the magnitude of Z, which has 3 ranges, 0 - 80, 0
- 200
and 0 - 800.
>
> I am not interested in SWR - just Z versus frequency.
>
> Does manufacturer have a website ?
Yes, but I can't recall what it is. You should know, that, on my unit,
at least,
the sign of jX is often ambiguous. Also, the unit loses accuracy as the
impedance being measured gets farther and farther away from 50 ohms.
Its chief advantage is its graphical nature. For some applications it
seems that having a swept display is an advantage.
>
> Would appreciate it if you could find time to answer.
> =================================
> Xmas Greetings, Reg, G4FGQ
> For free technical radio software go to:-
> http://www.btinternet.com/~g4fgq.regp
> =================================
Tom Donaly KA6RUH
Tom Bruhns
Wes Stewart wrote:
> Look here: http://www.aea-wireless.com/cia.htm
>
> I have trouble with some of the claimed specs. For instance, they claim 40 dB
> return loss capability but only 1.1:1 swr capability, I don't lnow how they
> can claim 40 dB return loss capability with a SO-239 ("UHF") test port
> connector. I guess because it has a digital readout, they get better
> "accuracy."
Hmmm...are you looking at specs beyond what's posted at that URL? What
I see is return loss and SWR measurement ranges, but no claim about
accuracy. And I didn't see anything with 1.1:1 swr...only the range
from 1:1 to 20:1. And...what is the price? Ah, so many questions!
(It does seem like at 54MHz, the unit's upper end, an SO-239/PL-259
junction would result in about 1.1:1 swr. Is that where your number
came from?)
Cheers,
Tom
George T. Baker
that Dr. Grant Bingeman has written and sells, together with a custom
cable to link the computer serial port to the instrument.
You can reach Grant at
and details of the program are found at his website
http://website.mciworld.com/~drb...@mciworld.com
His implementation takes care of the CIA's internal sign ambiguity
problem for reactance. Because of the slow processor in the CIA, his
program (and anyone else's) is slow to obtain and graph data, but the
results are excellent and very useful.
BTW, the CIA can be operated from an external power source when used in
the shack. I have not found that battery consumption is at all out of
line. The instrument is borderline for being something to use held in
your hand, but the large keyboard and display make the size worthwhile.
In summary, I have found the CIA to be far superior to the Autek RF-1
which it replaced. The RF-1 *is* quite small, light and portable, but the
results leave a lot to be desired in my opinion.
72/73, George
Fairview, TX 30 mi NE Dallas in Collin county
Amateur Radio W5YR, in the 54th year and it just keeps getting better!
R/C since 1964 - AMA 98452 RVing since 1972
Gary Coffman
>If you want to see standing waves:
>Take a rope about 15 feet long and lay it on the ground.
>Tie one end of it to the wall. (BTW the floor must be
>smooth as to not add friction)
>Make a quick start and stop horizontal motion with the
>rope.
>Watch the sine wave leave the incident side, reflect off
>of the wall and come back to you.
>And now you make a steady continuous horizontal motion
>with the rope and the guy by the wall does the same. If
>you both get you frequencies matched (similiar amplitudes
>help with the visual impact) you will SEE a standing
>wave somewhere between you and the wall. It will be a motionless
>sine wave that just sits there as you both are moving the ropes
>on both ends.
>There are EXACT analagies between physical motions like
>this and electrical properties like capacitance/inductance.
Except for the minor inconvenience that *there is no rope*.
The waves we're discussing have no rope. Their only physical
reality is the graph paper on which they are drawn. Reading
too much physical reality into the rope analogy is a conceptual
error. If we plot the voltage or current magnitude with respect
to distance on a graph, we get a curve that looks like a wave.
But there is no actual wave in space, just on the map we've
drawn.
Gary
Gary Coffman KE4ZV | You make it |mail to ke...@bellsouth.net
534 Shannon Way | We break it |
Lawrenceville, GA | Guaranteed |
Roy Lewallen
> . . .
> The price according to their QST ad is $399.95 plus shipping and handling of
> $7.50. I always love it when a company that sells through the mail wants to
> charge extra to get the goods to you. "Oh, you want it at _your_ house? That
> will be extra."
It's one of my pet peeves, too, but that's not a bad ratio by dismal
contemporary standards. I just got a solicitation for a Norton software
tools upgrade. ONLY $29.95! (plus $10.95 shipping and handling). And
that's for a CD-ROM.
Roy Lewallen, W7EL
Wes Stewart
Sorry, I misread the SWR measurement range on the web site. Somewhere in the
past however, I saw it specified as 1.1:1 to 20:1. Nevertheless, if they claim
return loss measurement range as 1 to 40 dB, then they still aren't consistant
as 1 dB RL is 17.4:1 SWR, not 20:1.
My point about the connector is that its return loss is nowhere near 40 dB, so
they can't possible (accurately) measure a 40 dB return loss. (1.02 SWR). As
you well know even H-P doesn't do much bettter (if at all) than 40 dB with a
scalar instrument, even with precision 3.5 or 7mm connectors.
The price according to their QST ad is $399.95 plus shipping and handling of
$7.50. I always love it when a company that sells through the mail wants to
charge extra to get the goods to you. "Oh, you want it at _your_ house? That
will be extra."
73,
Wes N7WS
Peter O. Brackett
Hey, my dear wife has done that on numerous occasions!
Heh, heh.
I only let her do it because that is the only excuse I get to buy some NEW
ham gear, heh, heh.
Peter AB4BC
Tom Bruhns <tom_b...@agilent.com> wrote in message
news:385141F6...@agilent.com...
Peter O. Brackett
My man! Thinks after my own heart.
There are no waves, only probabilities.
Regards,
Peter AB4BC
Gary Coffman <ke...@bellsouth.net> wrote in message
news:IclROLDCtD23Kit+=mB104...@4ax.com...
Peter O. Brackett
Hey nice to hear you chime in here on the Internet NG "band", less noisy
than 75 m phone I think.
Nice commentary! Here below are some more remarks of mine to join your
repartee.
Roy Lewallen <w7...@eznec.com> wrote in message
news:3850905F...@eznec.com...
[snip]
> Consider a half wavelength 50 ohm transmission line terminated with a 10
> ohm load. Looking into the input of the line, you'll see 10 + j0, a
> resonant condition (by definition, since the reactance is zero). Yet
> there are standing waves on the line. Resonance and standing waves are
> different phenomena.
[snip]
Roy, how about a 10 Ohm load without a transmission line, is that
"resonant"? It has no reactance?
The basic property of the transmission line that makes it "resonant" is that
it can "store" energy, not that it has zero reactance.
Resonant systems have the capability to "store" energy, non-resonant systems
do not! Simple, but true.
[snip]
>
> > Question: What exactly is a standing wave, how is it measured, and how
do
> > we know it is present?
>
> A standing wave is created by the superposition (sum) of a forward
> travelling wave and a reflected, or reverse traveling wave. The
> interference of these two waves creates a situation where the
> transmission line has fixed locations where the voltage and current are
> higher and lower than they would be for a "flat" line (one terminated in
> its characteristic impedance, and without standing waves). These fixed
> high and low points give the line distribution the appearance of having
> non-moving, or standing, waves, even though there are really two waves,
> one moving in each direction.
[snip]
But common SWR meters have no "spatial' span, they are usually point
devices that simply cannot "see" waves!
[snip]
>
> A standing wave can be directly measured with a slotted line and
> detector. More commonly, it's indirectly measured with an SWR meter or
> return loss bridge. We know that standing waves are present whenever the
> SWR is other than 1:1 or the return loss is finite.
[snip]
Agreed a slotted line or lecher wires at least have the (wavelength) span to
be able to "observe" waves, if they exist, common SWR meters do not!
[snip]
>
> > I understand that "waves" as such must be calculated and inferred from
point
> > measurements of current and voltage and such "waves" may "travel" or
> > "stand", but other than the mechanical vibrations of strings and
diaphragms,
> > etc, no one has "seen" them. In fact calculations and measurements of
these
> > things can be quite arbitrary and so I find them to be unsettling.
>
> Calculations and measurements are not arbitrary at all. Quantitative
> results have been obtained and used daily by engineers for over a
> century.
[snip]
I agree that calculations and measurements are not (necessarily) arbitrary,
aand they allow us to "engineer" wonderful and workable solutions to many
problems, but I maintain that the existence of waves is still nothing more
than an arbitrary mathematical model. Waves, particles, probabilities...
[snip]
>
> You can arbitrarily choose Zr if you'd like, but if you want to know the
> SWR on a transmission line, or calculate the magnitude of the maximum
> and minimum voltages or currents (i.e., the magnitude of the standing
> wave), you have to use the characteristic impedance of the line for Zr
> and the load impedance for Z. Rho is then the reflection coefficient at
> the interface between the line and the load, which in turn determines
> the SWR on the line.
[snip]
What about the case (frequency range) where the transmission line does not
have a (well defined characteristic impedance), for instance say 3 miles of
twisted # 24 guage copper wire over the frequency range of 0 - 10 kHz?
[snip]
> No. Resonance has nothing to do with what percentage of applied power is
> radiated by an antenna. The fraction of the applied power which is
> radiated is the efficiency, and that's a function of loss, not
> resonance. We've done fine, thank you, with the definition of resonance
> which is the frequency at which the reactance is zero. And efficiency as
> the fraction of applied power which is radiated. No new definition is
> required.
>
[snip]
Again I maintain that a pure resistor has no reactance and is not a
"resonant" system.
Resonance must include the phenomena of energy storage, and that can be due
to reactance, or any other means of storing energy. Your earlier example of
a long transmission line which stores energy by virtue of it's transport
latency. Without energy storage one can have no resonance. Roy, above when
you refer to zero reactance, I believe that you are referring to the case of
a system that does indeed comprise reactance, in fact two kinds of
reactance, one capacitive and the other inductive. The universe abounds
with energy storage phenomena many of which are not electrical "reactance".
[snip]
> I suggest that you invest in a copy of the _ARRL Antenna Book_ (or visit
> your local library for a copy). It has a good basic discussion of
> standing waves and SWR. For equations, my personal favorite is
> _Reference Data For Radio Engineers_.
[snip]
Roy, I have a copy of ARRL Antenna Book, Eleventh Edition, Forward by John
Huntoon, no editor mentioned, Copyright 1968 price $2.50! Wow ! That surely
dates me.
In my youth I took all such texts at face value, however now that I am much
older, I have become more sceptical.
Just having fun with SWR meter fans Roy, heh, heh.
Best Regards and Seasons Greetings,
Peter AB4BC
>
> Roy Lewallen, W7EL
Roy Lewallen
>
> Roy Lewallen <w7...@eznec.com> wrote in message
> news:3850905F...@eznec.com...
>
> [snip]
> > Consider a half wavelength 50 ohm transmission line terminated with a 10
> > ohm load. Looking into the input of the line, you'll see 10 + j0, a
> > resonant condition (by definition, since the reactance is zero). Yet
> > there are standing waves on the line. Resonance and standing waves are
> > different phenomena.
> [snip]
>
> Roy, how about a 10 Ohm load without a transmission line, is that
> "resonant"? It has no reactance?
>
> The basic property of the transmission line that makes it "resonant" is that
> it can "store" energy, not that it has zero reactance.
>
> Resonant systems have the capability to "store" energy, non-resonant systems
> do not! Simple, but true.
It's impossible to argue with a certain statement of fact like that. In
my experience, an antenna system is said to be resonant when its
feedpoint resistance is non-reactive. At a single frequency, there's no
way to tell the difference between a 10 ohm resistor and a 10 ohm
resistor in series or parallel with a resonant tank. I don't believe
you'll find anyone who regards a system with reactive feedpoint to be
resonant.
> > A standing wave is created by the superposition (sum) of a forward
> > travelling wave and a reflected, or reverse traveling wave. The
> > interference of these two waves creates a situation where the
> > transmission line has fixed locations where the voltage and current are
> > higher and lower than they would be for a "flat" line (one terminated in
> > its characteristic impedance, and without standing waves). These fixed
> > high and low points give the line distribution the appearance of having
> > non-moving, or standing, waves, even though there are really two waves,
> > one moving in each direction.
>
> [snip]
>
> But common SWR meters have no "spatial' span, they are usually point
> devices that simply cannot "see" waves!
Amazing, isn't it?
> [snip]
>
> >
> > A standing wave can be directly measured with a slotted line and
> > detector. More commonly, it's indirectly measured with an SWR meter or
> > return loss bridge. We know that standing waves are present whenever the
> > SWR is other than 1:1 or the return loss is finite.
>
> [snip]
>
> Agreed a slotted line or lecher wires at least have the (wavelength) span to
> be able to "observe" waves, if they exist, common SWR meters do not!
That's why I said "indirectly" measured. If the line impedance is the
same as for the SWR meter (so that the SWR meter is really measuring the
SWR of the line), with nothing more than the SWR reading, I can tell you
what the maximum and minimum values of the current and voltage will be
on the line. I can also tell you how far apart they are. But since the
SWR meter is a scalar measurement device, I can't tell you where they'll
be on the line. Once one is located, however, I'll know where the rest
will be, and have all the same information you can get with a slotted
line.
> [snip]
>
> I agree that calculations and measurements are not (necessarily) arbitrary,
> aand they allow us to "engineer" wonderful and workable solutions to many
> problems, but I maintain that the existence of waves is still nothing more
> than an arbitrary mathematical model. Waves, particles, probabilities...
Ok. Perhaps you "engineer", but I engineer. The results speak for
themselves.
> [snip]
>
> >
> > You can arbitrarily choose Zr if you'd like, but if you want to know the
> > SWR on a transmission line, or calculate the magnitude of the maximum
> > and minimum voltages or currents (i.e., the magnitude of the standing
> > wave), you have to use the characteristic impedance of the line for Zr
> > and the load impedance for Z. Rho is then the reflection coefficient at
> > the interface between the line and the load, which in turn determines
> > the SWR on the line.
>
> [snip]
>
> What about the case (frequency range) where the transmission line does not
> have a (well defined characteristic impedance), for instance say 3 miles of
> twisted # 24 guage copper wire over the frequency range of 0 - 10 kHz?
Well, at DC, it's undefined of course, but at all other frequencies it's
defined as well as for any other line. In a lossy line like you've
described, the lossless-line simplification isn't valid and more
complete equations have to be used. For a lossy line like that, the
characteristic impedance will be complex, and the SWR and amplitude of
the "standing wave" will decrease as you move away from the generator.
You can find the relevant equations in _Reference Data for Radio
Engineers_. They're use daily by people dealing with telephone lines and
other lossy transmission lines.
> [snip]
>
> > No. Resonance has nothing to do with what percentage of applied power is
> > radiated by an antenna. The fraction of the applied power which is
> > radiated is the efficiency, and that's a function of loss, not
> > resonance. We've done fine, thank you, with the definition of resonance
> > which is the frequency at which the reactance is zero. And efficiency as
> > the fraction of applied power which is radiated. No new definition is
> > required.
> >
> [snip]
>
> Again I maintain that a pure resistor has no reactance and is not a
> "resonant" system.
>
> Resonance must include the phenomena of energy storage, and that can be due
> to reactance, or any other means of storing energy. Your earlier example of
> a long transmission line which stores energy by virtue of it's transport
> latency. Without energy storage one can have no resonance. Roy, above when
> you refer to zero reactance, I believe that you are referring to the case of
> a system that does indeed comprise reactance, in fact two kinds of
> reactance, one capacitive and the other inductive. The universe abounds
> with energy storage phenomena many of which are not electrical "reactance".
Suit yourself. So add one picoohm of inductive reactance and one picoohm
of capacitive reactance in series with the ten ohm resistor. Now it's
resonant, and we're both happy, aren't we?
>
> [snip]
>
> > I suggest that you invest in a copy of the _ARRL Antenna Book_ (or visit
> > your local library for a copy). It has a good basic discussion of
> > standing waves and SWR. For equations, my personal favorite is
> > _Reference Data For Radio Engineers_.
>
> [snip]
>
> Roy, I have a copy of ARRL Antenna Book, Eleventh Edition, Forward by John
> Huntoon, no editor mentioned, Copyright 1968 price $2.50! Wow ! That surely
> dates me.
The ARRL Antenna Book was greatly improved when it was completely
re-edited by Jerry Hall some years ago. I believe it was the 14th or
15th Edition. Older editions have the same name, but are a quite
different book. Anyone with an older version is missing out on a really
good book.
> In my youth I took all such texts at face value, however now that I am much
> older, I have become more sceptical.
>
> Just having fun with SWR meter fans Roy, heh, heh.
>
> Best Regards and Seasons Greetings,
>
> Peter AB4BC
>
> >
> > Roy Lewallen, W7EL
There are others on this newsgroup who enjoy arguing semantic points and
deep philosophy, but I don't. I believe I've offered as much as I
constructively can on the topic.
Roy Lewallen, W7EL
Richard Harrison
"For a lossy line like that, the characteristic impedance will be
complex, and the SWR and amplitude of the "standing wave" will decrease
as you move away from the generator."
Just a nit picking minute. You didn`t mean that. SWR is produced by
reflection and always is greatest at the discontinuity (mismatched
load).
Wes Stewart
>There are others on this newsgroup who enjoy arguing semantic points and
>deep philosophy, but I don't. I believe I've offered as much as I
>constructively can on the topic.
>
>Roy Lewallen, W7EL
A role model for us all. I wish I possessed such self-restraint.
Wes Stewart, N7WS
Gary Coffman
>>
>> Roy Lewallen <w7...@eznec.com> wrote in message
>> news:3850905F...@eznec.com...
>>
>> [snip]
>> > Consider a half wavelength 50 ohm transmission line terminated with a 10
>> > ohm load. Looking into the input of the line, you'll see 10 + j0, a
>> > resonant condition (by definition, since the reactance is zero). Yet
>> > there are standing waves on the line. Resonance and standing waves are
>> > different phenomena.
>> [snip]
>>
>> Roy, how about a 10 Ohm load without a transmission line, is that
>> "resonant"? It has no reactance?
>>
>> The basic property of the transmission line that makes it "resonant" is that
>> it can "store" energy, not that it has zero reactance.
>>
>> Resonant systems have the capability to "store" energy, non-resonant systems
>> do not! Simple, but true.
>
>It's impossible to argue with a certain statement of fact like that. In
>my experience, an antenna system is said to be resonant when its
>feedpoint resistance is non-reactive. At a single frequency, there's no
>way to tell the difference between a 10 ohm resistor and a 10 ohm
>resistor in series or parallel with a resonant tank. I don't believe
>you'll find anyone who regards a system with reactive feedpoint to be
>resonant.
I doubt anyone would consider a load with a reactive feedpoint
resonant. However, I also doubt that anyone would consider a
10 ohm composition resistor resonant simply because it exhibits
no reactance over a span of freqencies. As Peter says, a resonant
system dynamically stores energy. Having a non-reactive feedpoint
is one necessary condition of resonance, but not the only one.
klo...@toad.net
"Peter O. Brackett" <ab...@ix.netcom.com> wrote:
>
> The basic property of the transmission line that makes it "resonant"
is that
> it can "store" energy, not that it has zero reactance.
>
Peter,
There seems to be some floundering without a basic definition. From the
ARRL Antenna Book, p. 16-2 (17th edition): "antenna resonance is defined
as the frequency at which the input impedance at the antenna terminals
is purely resistive." If you want to extend this to a system consisting
of transmission line and antenna, system resonance would be the
frequency at which the input impedance at the transmission line
terminals is purely resistive. This definition is backed up in an
antenna book by Joseph Carr K4??? and in Kraus's Electromagnetics where
the resonance points in a Smith chart plot (p. 672) of complex impedance
occur where at the reactance is zero.
> Resonant systems have the capability to "store" energy, non-resonant
systems
> do not! Simple, but true.
>
Both resonant and non-resonant systems store energy. At resonance, the
inductive reactance exactly equals (and cancels) the capacitive
reactance. The energy in a system is a constant and passes back and
forth from inductance to capacitance. Away from the resonant freq, the
maximum energy stored in either the inductance/capacitance exceeds the
maximum energy stored in the capacitance/inductance. The excess energy
is supplied by the source during one part of the cycle and returned to
it in another part. From Fields and Waves in Modern Radio, by Ramo &
Whinnery.
73,
Bill N3WK
Sent via Deja.com http://www.deja.com/
Before you buy.
Richard Harrison
"The basic property of the transmission line that makes it "resonant" is
A storage battery stores energy. Never saw a battery exhibiting
resonance on its own though.
The basic property of the transmission line that makes it resonant is
the same as that of an organ pipe. It has a resonant length and its
energy is not all absorbed at the end but is reflected to and fro when
excited.
Richard Harrison
"The excess energy is supplied by the source during one part of the
---")
And so to get rid of the excess current and loss, we have power factor
correction (zero reactance) in the electric power business.
Gray Frierson Haertig
Gary Coffman wrote:
>
> I doubt anyone would consider a load with a reactive feedpoint
> resonant. However, I also doubt that anyone would consider a
> 10 ohm composition resistor resonant simply because it exhibits
> no reactance over a span of freqencies. As Peter says, a resonant
> system dynamically stores energy. Having a non-reactive feedpoint
> is one necessary condition of resonance, but not the only one.
>
> Gary
> Gary Coffman KE4ZV | You make it |mail to ke...@bellsouth.net
> 534 Shannon Way | We break it |
> Lawrenceville, GA | Guaranteed |
Being able to store energy may or may not be a requirement for
resonance, but certainly being resonant is not a requirement for storing
energy. All the circuits we think of as being able to be resonant,
parallel and series tanks, antennas, etc. store energy just as well off
resonance as they at resonance.
If it somehow offends one to say a resistor isn't resonant, because it
doesn't store energy, so be it. But it seems to me that the simple
resistor is just a trivial and not very interesting case of resonance.
Certainly, deciding whether a resistor is resonant or not adds very
little to understanding what resonance is.
The essence of resonance is that a resonant circuit, be it electrical,
mechanical, acoustic, or hydraulic, doesn't return any energy to the
energy source that feeds it, i.e., it is non-reactive.
Gray
--
Telecommunications Engineering
Gray Frierson Haertig & Assoc.
820 North River Street, Suite 100
Portland, Oregon 97227
503-282-2989
503-282-3181 FAX
g...@haertig.com
Peter O. Brackett
Thanks for your insights and inputs.
I don't mean to be picayune, but here's my point.
I maintain that it is misleading to say that the absence of reactance is
the main indicator of resonance.
Surely it is the presence of reactance or of "inertia" and "stiffness" or
"energy storage" that really defines "resonance".
Purely resistive devices or "dampers" cannot be "resonant".
Roy, consider a guitar string, which is clearly a "resonant" system,
compared to say a jar of molasses. One can be made to "vibrate" at it's
resonant frequency, while the other "just sits there". One is resonant and
the other is not.
I believe it is the "single frequency" view of narrow band enthusiasts like
amateur radio operators that leads to such "specific" definitions of
resonance as "absence of reactance". Surely most antennas, except perhaps
the so called "travelling wave" antennas do have consideable reactance.
Seasons Greetings,
Peter AB4BC
Roy Lewallen <w7...@eznec.com> wrote in message
news:38538270...@eznec.com...
Ian Jackson
<g...@haertig.com> writes
'to resound'. Organ pipes do this. Is the 'resistive' definition (while
precise) not a bit artificial?
Ian.
--
Ian Jackson
Peter O. Brackett
Exactly! I like the Ramo-Whinery definition.
This whole tread got started by an assertion of Reg's (Since refuted by
him.) that resonance could be detected by SWR measurements. I think we have
just about beaten this subject to death, and Reg has admitted his keyboard
dyslexia.
Heh, heh.
Seasons Greetings,
Peter AB4BC
<klo...@toad.net> wrote in message news:830tq7$ptq$1...@nnrp1.deja.com...
> In article <82uo02$m9v$1...@nntp1.atl.mindspring.net>,
> "Peter O. Brackett" <ab...@ix.netcom.com> wrote:
> >
> > The basic property of the transmission line that makes it "resonant"
> is that
> > it can "store" energy, not that it has zero reactance.
> >
>
> Peter,
> There seems to be some floundering without a basic definition. From the
> ARRL Antenna Book, p. 16-2 (17th edition): "antenna resonance is defined
> as the frequency at which the input impedance at the antenna terminals
> is purely resistive." If you want to extend this to a system consisting
> of transmission line and antenna, system resonance would be the
> frequency at which the input impedance at the transmission line
> terminals is purely resistive. This definition is backed up in an
> antenna book by Joseph Carr K4??? and in Kraus's Electromagnetics where
> the resonance points in a Smith chart plot (p. 672) of complex impedance
> occur where at the reactance is zero.
>
> > Resonant systems have the capability to "store" energy, non-resonant
> systems
> > do not! Simple, but true.
> >
>
> Both resonant and non-resonant systems store energy. At resonance, the
> inductive reactance exactly equals (and cancels) the capacitive
> reactance. The energy in a system is a constant and passes back and
> forth from inductance to capacitance. Away from the resonant freq, the
> maximum energy stored in either the inductance/capacitance exceeds the
> maximum energy stored in the capacitance/inductance. The excess energy
> is supplied by the source during one part of the cycle and returned to
Richard Harrison
"The essence of resonance is that a resonant circuit, be it electrical,
that feeds it, i.e., it is non-reactive."
Very true for one essence. Another characteristic is "simple harmonic
motion". They can oscillate when stimulated.
klo...@toad.net
"Peter O. Brackett" <ab...@ix.netcom.com> wrote:
> Bill:
>
> Exactly! I like the Ramo-Whinery definition.
>
Peter,
I have to admit that when I saw the word "Exactly" I wanted to shout
"error! error! inconsistent!" like one of those computers on Star Trek.
However, after reading some of your more recent posts, it seems you're
really talking about a resonating object like an antenna (which could be
at its resonance freq, above resonance, or below resonance) as opposed
to a non-resonating object like a resistor, a dog turd or a Mounds bar.
Is that the distinction? 73,
Peter O. Brackett
Right. I'm glad you see it that way, heh, heh. I could not have stated a
better analogy myself. The dog turd is definitely not "resonant".
Repugnant maybe but not Resonant!
Yeah, here's another way of stating my point.
Just because when driven, or measured, at one particular steady state
frequency, the capacitive reactance "cancels" inductive reactance in an
antenna, this does not indicate a lack of reactance! It only signifies that
at that particular frequency inductive reactane equals capacitive reactance.
The dang thing is still "vibrating" strongly and storing lots of energy.
Most antennas have very large amounts of (equal and opposite) reactance at
resonance. The intrinsic inductance and capacitance of the antenna is
easily calculated from first physical principles and by a simple appication
of that irrational number pi together with the rational 2, and the
frequency, one can easily deduce the reactances. The "reactance" and the
intrinsic ability to store energy in those reactances does not dissappear at
the particular frequency called the resonant frequency.
In fact, in practical, rather than theoretical antennas, the exact frequency
at which capacitive reactance and inductive reactance equal each other and
"dissappear" may not even be the "true" resonant frequency, i.e. the
so-called natural frequency or frequency of natural vibration. Usually, due
to secondary effects such as resistance, skin effect, etc...the natural
frequency is slightly different from the frequency at which capacitive
reactance equals inductive reactance. This latter distinction is picayune,
and maybe insignificant in most cases, but true none-the-less.
Have we broken the NG "record" yet for length of a "thread"?
Heh, heh.
Seasons Greetings,
Peter AB4BC
<klo...@toad.net> wrote in message news:831q2i$d0h$1...@nnrp1.deja.com...
Gary Coffman
>Gary Coffman wrote:
>
>>
>> I doubt anyone would consider a load with a reactive feedpoint
>> resonant. However, I also doubt that anyone would consider a
>> 10 ohm composition resistor resonant simply because it exhibits
>> no reactance over a span of freqencies. As Peter says, a resonant
>> system dynamically stores energy. Having a non-reactive feedpoint
>> is one necessary condition of resonance, but not the only one.
>
>resonance, but certainly being resonant is not a requirement for storing
>energy. All the circuits we think of as being able to be resonant,
>parallel and series tanks, antennas, etc. store energy just as well off
>resonance as they at resonance.
>
>If it somehow offends one to say a resistor isn't resonant, because it
>doesn't store energy, so be it. But it seems to me that the simple
>resistor is just a trivial and not very interesting case of resonance.
>Certainly, deciding whether a resistor is resonant or not adds very
>
>The essence of resonance is that a resonant circuit, be it electrical,
>energy source that feeds it, i.e., it is non-reactive.
The essence of a resonant circuit is that it is frequency selective.
At some frequency, the circulating energy it stores will exhibit an
amplitude maximum (or minimum depending on circuit topology).
That will be its resonant frequency. It so happens that its feedpoint
will also be purely resistive at that frequency, *but not at others*.
Both conditions are required in order to say that the circuit is
resonant. A composition resistor doesn't meet either condition
because it neither stores energy nor exhibits frequency selectivity.
klo...@toad.net
"Peter O. Brackett" <ab...@ix.netcom.com> wrote:
> In fact, in practical, rather than theoretical antennas, the exact
frequency
> at which capacitive reactance and inductive reactance equal each other
and
> "dissappear" may not even be the "true" resonant frequency, i.e. the
> so-called natural frequency or frequency of natural vibration.
Usually, due
> to secondary effects such as resistance, skin effect, etc...the
natural
> frequency is slightly different from the frequency at which capacitive
> reactance equals inductive reactance. This latter distinction is
picayune,
> and maybe insignificant in most cases, but true none-the-less.
Peter,
Based on the definition of resonance, I might argue that the frequency
at which the capacitive reactance and inductive reactance equal each
other and disappear is the "true" resonant frequency.
> Have we broken the NG "record" yet for length of a "thread"?
>
> Heh, heh.
>
Nah, not even close. Try searching on "47 kW mobile" or something like
it. That went on forever - it was the thread that would not die.
73 es MC es HNY,