Linear Amps and SWR
Richard Smith
compared to when it is turned off ?
My theory is that the AMP is possibly introducing harmonics into the coax,
and being as the antenna is not tuned for them they are being reflected and
causing a higher SWR.
Would a low pass filter help or would it just be better to purchase an
antenna matcher ?
Thanks,
Richard.
Bill Eitner
>
> Could somebody tell me why my SWR is higher when my linear AMP is on,
> compared to when it is turned off ?
Yes.
> My theory is that the AMP is possibly introducing harmonics into the coax,
> and being as the antenna is not tuned for them they are being reflected and
> causing a higher SWR.
That's some of it. The other part is that SWR meters
use diodes. Diodes are non-linear devices, therefore
the accuracy of the instrument changes as the voltage
through it increases. In other words, the reading will
naturally increase to a certain degree when the amp is
on.
> Would a low pass filter help or would it just be better to purchase an
> antenna matcher ?
If the difference is not extreme, I wouldn't worry
about it. In other words, if the SWR with the amp
off is low (1.5:1 or lower as measured at the out-
put of the amp), and the SWR with the amp on is below
say 2.5:1, there's nothing to worry about. An
antenna matcher won't do any good at all since the
antenna is already properly tuned, and a low pass
filter is just a mask--it doesn't make the amp more
stable (instability is the cause of the high harmonic
and parasitic content).
If the difference is extreme (amp off SWR is 1.5 or
lower and amp on SWR is in the red), there are two
possibilities: the antenna is breaking down when
high power is applied, or, the amp is very unstable.
The latter is more likely if the amp produces 500
peak watts or less. The best way to check the
antenna is by substitution. Sub in another amp
that is known to be stable, or an antenna that is
known to be able to handle the power of the amp in
question. If it turns out to be amplifier insta-
bility, you can mask it to a degree with a low-pass
filter, or (a better solution), have a good amp tech
re-optimize the amp for increased stability.
> Thanks,
You're welcome. I hope this helps.
> Richard.
--
-------------------------------------------------------------------
-=[Bill Eitner]=-
Link to A.M. Tutorial, rec.radio.cb FAQ
and The Dark Side:
http://kd6tas.conk.com
-------------------------------------------------------------------
TheGame
buy a meter that will allow you to read swr with amp on .. Such as a bird
meter...
"Richard Smith" <rsm...@planeteer.com> wrote in message
news:Wkn85.19407$qS3....@tor-nn1.netcom.ca...
> Could somebody tell me why my SWR is higher when my linear AMP is on,
> compared to when it is turned off ?
>
> and being as the antenna is not tuned for them they are being reflected
and
> causing a higher SWR.
>
> antenna matcher ?
>
>
> Richard.
>
>
Hamm4fun
>
>Richard Smith wrote:
>>
>> Could somebody tell me why my SWR is higher when my linear AMP is on,
>> compared to when it is turned off ?
>
>
>> My theory is that the AMP is possibly introducing harmonics into the coax,
>> and being as the antenna is not tuned for them they are being reflected and
>> causing a higher SWR.
>
> use diodes. Diodes are non-linear devices, therefore
> the accuracy of the instrument changes as the voltage
> through it increases. In other words, the reading will
> naturally increase to a certain degree when the amp is
> on.
>
>> antenna matcher ?
>
Most of these amps have nothing for harmonic suppresion and it is pretty normal
to find the situation you have for both mentioned reasons, harmonics and meter
sensitivity. Most amps can also stand some improved output impedance matching.
A circuit that will get rid of harmonics to a large degree and improve
impedance matching can be made will one coil and one variable capacitor making
a simple L network. The ARRL handbook is the source for how -to.
Professor
That's the problem with every amp I've ever hooked up. That's the main
reason they get so hot.
You could use a matcher to correct this situation. It will help the
reflections problem and keep the amp running cooler as well.
--
||||||||||||||||||||||||||||||||||||||||||||||||||||||
"Professor"
http://personal.kwom.com/bgriffey/
||||||||||||||||||||||||||||||||||||||||||||||||||||||
"Richard Smith" <rsm...@planeteer.com> wrote in message
news:Wkn85.19407$qS3....@tor-nn1.netcom.ca...
> Could somebody tell me why my SWR is higher when my linear AMP is on,
> compared to when it is turned off ?
>
> My theory is that the AMP is possibly introducing harmonics into the coax,
> and being as the antenna is not tuned for them they are being reflected
and
> causing a higher SWR.
>
> Would a low pass filter help or would it just be better to purchase an
> antenna matcher ?
>
> Thanks,
>
> Richard.
>
>
'Doc
Richard,
The simple answer is that the output impedance of the amplifier
is not 50 ohms. There could be a number of reasons for that, and
not knowing what the amplifier is, I don't have the slightest idea
which one it could be.
'Doc
TheGame
this would not happen. Everyone is always quick to blame the amplifier. try
a "Bird" or coaxial dynamics meter with a dual line section take 2 slugs 1
for reading forward power the other for reading reflect power. Turn the amp
off check the swr turn it on and check the swr the ratio will still be the
same.
"Professor" <bgri...@kwom.com> wrote in message
news:wTo85.689$Q7.3...@news-east.usenetserver.com...
Richard Smith
>check your swr with the amp off its that simple, unless you can afford to
>buy a meter that will allow you to read swr with amp on .. Such as a bird
>meter...
I have a radio shack SWR meter that can handle up to 2000 watts. I guess it
is only usable for measuring power with the amp on, and the SWR with the amp
off. By the way, I only have a 100 watt amp.
Richard.
Richard Smith
With the amp off, Channel 40 SWR is 1.05:1 and Channel 1 is 1.5:1.
With the amp on, Channel 40 SWR is 1.5:1 and channel 1 is 2.2:1.
(I generally hang around 38 LSB, so the above fortunately works out well for
me !)
Unfortunately I am at the end of the adjustment range of my K-40, so that is
as good as I can get my SWR. I believe that the antenna is not breaking
down, as it has a 500 watt rating and the amp I have is only rated at 100
watts. Also, signal reports from other CBers are very good with the amp on.
(I would imagine breaking down of the antenna would cause distortion).
My radio shack SWR meter can handle up to 2000 watts, so I guess it may be a
case of it only being good for measuring power with the amp on, and SWR with
the amp off.
Thanks very much for the information Bill.
Richard.
Richard Smith
local CB shops, and even if it doesn't help I may use it in another
installation.
Thanks,
Richard.
Professor wrote in message ...
Richard Smith
kc2elo
calibrate the meter with just the radio and just switch the amp on
without adjusting for the increased forward power then it will throw
the reading off.
not to make you sound like an idiot but just trying to cover all bases
here.
Sent via Deja.com http://www.deja.com/
Before you buy.
Programbo5
>Could somebody tell me why my SWR is higher when my linear AMP is on,
>compared to when it is turned off ?
>
Check your SWR with your amp off....When you turn it on you get all sorts of
stuff going on that produces false readings
I hope I did`nt brain my damage.
Professor
well. You should easily be able to reduce your moderately bad match to flat!
--
||||||||||||||||||||||||||||||||||||||||||||||||||||||
"Professor"
http://personal.kwom.com/bgriffey/
||||||||||||||||||||||||||||||||||||||||||||||||||||||
"Richard Smith" <rsm...@planeteer.com> wrote in message
news:1tp85.19439$qS3....@tor-nn1.netcom.ca...
Professor
Depending on which side of the amp you have the SWR meter on... you are
either seeing the amps input or output impedance.
--
||||||||||||||||||||||||||||||||||||||||||||||||||||||
"Professor"
http://personal.kwom.com/bgriffey/
||||||||||||||||||||||||||||||||||||||||||||||||||||||
"Programbo5" <progr...@aol.comnospam> wrote in message
news:20000704153019...@ng-co1.aol.com...
TheGame
after the amp you cannot tell what the amps impedance is with a swr meter ..
And you are supposed to be a designer of amplifers, it shows you dont have a
clue
"Professor" <bgri...@kwom.com> wrote in message
news:e2r85.2155$Q7.1...@news-east.usenetserver.com...
-=PEEKABOO=-
> > >You could use a matcher to correct this situation. It will help the
> > >reflections problem and keep the amp running cooler as well.
This will not fix the problem with the output shunt on an amp unless there
is not enough capacitance.
--
-=PEAKABOO=-
"Trying not to choke,
On that 10 ohm smoke"
Check out http://www.geocities.com/watergate_rollin
-=PEEKABOO=-
accurate reading, amp design can make a meter read a high SWR. This could be
due to the output tuning of the amp. This seams to really show up if the
antenna is reactant.
--
-=PEAKABOO=-
"Trying not to choke,
On that 10 ohm smoke"
Check out http://www.geocities.com/watergate_rollin
"Richard Smith" <rsm...@planeteer.com> wrote in message
news:2tp85.19441$qS3....@tor-nn1.netcom.ca...
> TheGame wrote in message ...
> >check your swr with the amp off its that simple, unless you can afford to
> >buy a meter that will allow you to read swr with amp on .. Such as a bird
> >meter...
>
>
> I have a radio shack SWR meter that can handle up to 2000 watts. I guess
it
> is only usable for measuring power with the amp on, and the SWR with the
amp
Richard Smith
Richard.
Richard Smith
Bill Eitner
>
> Bill,
>
> With the amp off, Channel 40 SWR is 1.05:1 and Channel 1 is 1.5:1.
>
> With the amp on, Channel 40 SWR is 1.5:1 and channel 1 is 2.2:1.
For daily use, that's plenty good enough.
> (I generally hang around 38 LSB, so the above fortunately works out well for
> me !)
> Unfortunately I am at the end of the adjustment range of my K-40, so that is
> as good as I can get my SWR.
There are longer whips available, but getting the SWR
down even lower won't produce a noticeable benefit.
> I believe that the antenna is not breaking
> down, as it has a 500 watt rating and the amp I have is only rated at 100
> watts.
I agree.
I have a K-40 load and whip here that was subjected
to 800 watts in daily use for 10 years. After 10 years
it finally broke down. There is a small hole burned
into the dielectric that you can see when you look into
the connector end of the load. Now, anything over 50
peak watts sends the SWR into the red. Under 50 peak
watts the antenna still works properly.
> Also, signal reports from other CBers are very good with the amp on.
> (I would imagine breaking down of the antenna would cause distortion).
The easiest way to detect antenna breakdown is to vary
the output power while monitoring SWR. At some point
there will be an arc-over and the SWR reading will pop
up into the red. If that doesn't happen, the antenna
is handling the power without a problem.
> My radio shack SWR meter can handle up to 2000 watts, so I guess it may be a
> case of it only being good for measuring power with the amp on, and SWR with
> the amp off.
Radio Shack meters are only good for relative power
readings. In other words, comparing one radio, amp,
or system to another without looking for specific
wattage values. As for SWR, Radio Shack meters are
typical non-linear, diode driven meters. So, as the
power through it increases, so will its sensitivity
and accuracy.
> Thanks very much for the information Bill.
You're welcome.
Bill Eitner
> to find the situation you have for both mentioned reasons, harmonics and meter
> sensitivity. Most amps can also stand some improved output impedance matching.
> A circuit that will get rid of harmonics to a large degree and improve
> impedance matching can be made will one coil and one variable capacitor making
> a simple L network. The ARRL handbook is the source for how -to.
The secondary of the output transformer and the output
load cap make up the simple L network you speak of.
I find that the impedance is usually pretty close and
can be varied to optimization by changing the output
transformer turns ratio and changing the value of the
output load cap. Then, any increase in SWR with the
amp on is mainly due to instability, antenna breakdown,
or increased meter sensitivity.
-=PEEKABOO=-
output power of your amp.
--
-=PEAKABOO=-
"Trying not to choke,
On that 10 ohm smoke"
Check out http://www.geocities.com/watergate_rollin
"Richard Smith" <rsm...@planeteer.com> wrote in message
news:1tp85.19439$qS3....@tor-nn1.netcom.ca...
> I may pick one up and try it. They seem to be going fairly cheap at the
> local CB shops, and even if it doesn't help I may use it in another
> installation.
>
> Thanks,
>
> Richard.
>
> Professor wrote in message ...
> >You are seeing that because your amp has a poor match...
> >That's the problem with every amp I've ever hooked up. That's the main
> >reason they get so hot.
Hamm4fun
What do you mean instability? I usually equate this with the amp breaking into
oscillation. I am familar with what you are doing but find that an L network
gets rid of the harmonics much better than your method as in taking the third
harmonic from 20db down to 60db down. If you build it into the amp it has the
dsiadvantage of making it a one band amp unless you add a lot of complicated
and expensive switching. This makes it even more difficult to convince the FCC
it is not a CB amp.
Garden Hose
the amps it reduces harmonics and is a 2 pole low pass filter .and they are
50 ohm amps .. breaker
.. hamm4fun is correct
"Hamm4fun" <hamm...@aol.com> wrote in message
news:20000706030235...@ng-mb1.aol.com...
Bill Eitner
> What do you mean instability? I usually equate this with the amp breaking into
> oscillation.
Oscillation is instability taken to the final level.
What I mean by instability is that the amp hasn't
broken into oscillation but is symptomatic, and if
the instability were to become worse the amp would
begin to oscillate. It's not super easy to explain,
but I'm sure you've seen it. At one end there's a
perfectly stable amp whose spurious content doesn't
change whether it's running into a dummy load or a
reactive antenna that's only 5 feet away from it.
At the other end is an amp that oscillates even when
running into a dummy load. In between those two states
is what I see as some level of instability. In other
words, most amps aren't rock stable regardless of the
load and the density of the RF field they are working
in; but, on the other hand, they're not oscillating
either. I see instability as having a varying degree.
In other words, I don't see amps in either one or the
other of two states (stable or oscillating).
> I am familar with what you are doing but find that an L network
> gets rid of the harmonics much better than your method as in taking the third
> harmonic from 20db down to 60db down.
My method involves more than just tuning the output
circuit.
> If you build it into the amp it has the
> dsiadvantage of making it a one band amp unless you add a lot of complicated
> and expensive switching. This makes it even more difficult to convince the FCC
> it is not a CB amp.
I see your point and the effectiveness of the method
you mention. My point is that if one were to strive
for increased stability an additional tuned L network
wouldn't be necessary.
I'm sure you've seen this for yourself, but I'll mention
it for the benefit of the other readers. If you take a
typical solid-state CB amp and run it into a dummy load
it shows a proper impedance match to the load and its
spectral purity is acceptable. Then, you take the same
amp and run it into a typical mobile antenna. In most
cases the spurious content increases and there's an SWR
reading that's seemingly indicative of an impedance mis-
match. Now you're at a crossroad. You can add an L net-
work (if you can fit it inside the amp) or you can take
steps to increase the amplifier's stability. Personally,
I prefer the latter because it can always be done and
you see benefits in more areas. That's all I was getting
at.
73.
Bill Eitner
My point that adding a tuned output circuit to an unstable amp
is masking another problem is still correct. Making the amp
as stable as possible should always be the first priority.
Then, if necessary or desired, a tuned output circuit can be
added.
In their broad-band state, many of the larger solid-state amps
aren't an optimum match to a purely resistive 50 ohm load at
27 MHz. That's why Dave and some of the other amp builders
add tuned output circuits. They aren't doing it to mask an
instability problem. Impedance match and instability are two
separate issues.
That's where this thread started. The original author asked
if adding a low-pass filter or an antenna tuner to the output of
his small amp was the proper approach to lowering the SWR. I
said that since the SWR was low with the amp off the antenna is
tuned and the antenna tuner wouldn't do any good, and that the
low-pass filter would be masking the problem rather than solving it.
I still believe what I said to be correct. With his amp the
impedance match is good. The increased SWR reading is because
of the increase in meter sensitivity with increased power
through the meter due to the use of diodes in the meter and
some instability in the amp. Running the amp into a dummy load
will verify that. If the SWR is high when running into a dummy
load, the amp is very unstable or is a poor match to a 50 ohm
load.
--
> > >--
> > >-------------------------------------------------------------------
> > > -=[Bill Eitner]=-
> > >
> > >Link to A.M. Tutorial, rec.radio.cb FAQ
> > >and The Dark Side:
> > >http://kd6tas.conk.com
> > >-------------------------------------------------------------------
> > >
> > >
> > >
> > >
> > >
> > >
> >
> > What do you mean instability? I usually equate this with the amp breaking
> into
> > oscillation. I am familar with what you are doing but find that an L
> network
> > gets rid of the harmonics much better than your method as in taking the
> third
> > harmonic from 20db down to 60db down. If you build it into the amp it has
> the
> > dsiadvantage of making it a one band amp unless you add a lot of
> complicated
> > and expensive switching. This makes it even more difficult to convince the
> FCC
> > it is not a CB amp.
--
Frizz
'Doc
TheGame wrote:
>
> bullshit, if you have a meter that is capable of giving you correct reading
> this would not happen. Everyone is always quick to blame the amplifier. try
> a "Bird" or coaxial dynamics meter with a dual line section take 2 slugs 1
> for reading forward power the other for reading reflect power. Turn the amp
> off check the swr turn it on and check the swr the ratio will still be the
> same.
> "Professor" <bgri...@kwom.com> wrote in message
-------clipped----------
I wouldn't bet on it...
'Doc
TheGame
power of amp is inline, transistor or tube .. 10 forward watts /.05
reflect, 100 forward/ .5 reflect.. 1000 forward/ 5 reflect .. 10000 forward/
50 reflect. We are checking the load impedance that ratio should not change
with more power being applied.
"'Doc" <w5...@oio.net> wrote in message news:3964EC46...@oio.net...
-=PEEKABOO=-
feedback before you move to the next step, then I believe the negative
feedback ought to be added as a precaution, now I know that in the some of
the amps with a separate drive stage it is very hard to get them clean and
stable without using negative feedback, but if you try real hard and get it
as good as possible and then add the feedback what you get is a good, stable
amplifier. I do recommend output filtering to reduce harmonics but if it is
done right and not over driven the harmonics are really not that bad.
--
-=PEAKABOO=-
"Trying not to choke,
On that 10 ohm smoke"
Check out http://www.geocities.com/watergate_rollin
"Bill Eitner" <kd6...@freewwweb.com> wrote in message
news:3964C609...@freewwweb.com...
-=PEEKABOO=-
change on some meters.
--
-=PEAKABOO=-
"Trying not to choke,
On that 10 ohm smoke"
Check out http://www.geocities.com/watergate_rollin
"TheGame" <the...@hotmail.om> wrote in message
news:Js895.90$m_5....@news2.voicenet.com...
Bill Eitner
in an RF dense environment (a nearby mobile antenna as the
load) is alot to ask unless by stable you mean not oscillating.
I agree that output filtering is nice as long as it's under-
stood that the amp will no longer be broad-banded.
--
-=PEEKABOO=- <tappin...@hotmail.com> wrote in article
<8k3b6f$1m04f$1...@ID-26551.news.cis.dfn.de>...
> Bill I have to agree, the amp should be stable even without negative
> feedback before you move to the next step, then I believe the negative
> feedback ought to be added as a precaution, now I know that in the some
of
> the amps with a separate drive stage it is very hard to get them clean
and
> stable without using negative feedback, but if you try real hard and get
it
> as good as possible and then add the feedback what you get is a good,
stable
> amplifier. I do recommend output filtering to reduce harmonics but if it
is
> done right and not over driven the harmonics are really not that bad.
> -=PEAKABOO=-
> "Trying not to choke,
> On that 10 ohm smoke"
> Check out http://www.geocities.com/watergate_rollin
>
>
>
'Doc
TheGame wrote:
>
> bullshit, if you have a meter that is capable of giving you correct reading
> this would not happen. Everyone is always quick to blame the amplifier. try
> a "Bird" or coaxial dynamics meter with a dual line section take 2 slugs 1
> for reading forward power the other for reading reflect power. Turn the amp
> off check the swr turn it on and check the swr the ratio will still be the
> same.
-------------clipped------------
People tend to blame the amplifier because that's typically
the reason with a lot of the "CB" amplifiers. Hey, don't blame
me, blame the people who make 'cheap' amplifiers. By the way,
that 'cheap' doesn't mean price, it means quality.
'Doc
Hamm4fun
certainly will not mask it either. This type of problem means something is
broken or there was just poor enginnering from the beginning. Somebody said
something against using negative feedback to fix stability problems. I dont
understand what is wrong with this. Negative feedback is the time proven method
of stabilizing an amp. This makes perfect sense because most instability is
caused by positive feedback. Of course if the amp is just poorly constucted or
designed these factors should be taken care of first by reducing the amount of
postitive feedback through proper component orientation, shielding, grounding,
bonding, ect.
Handy Andy
On Tue, 4 Jul 2000 11:09:21 -0400, while the modem was pondering it's
navel, "Richard Smith" <rsm...@planeteer.com> dissertated the
following:
>Could somebody tell me why my SWR is higher when my linear AMP is on,
>compared to when it is turned off ?
It's simple, a series of events...power fron the source, is being
applied to the input of a complex impedance.
In order to meet the proper impedance of both, the output
impedance of the source [CB radio] and the input impedance
of the amp need to be equal.
You don't always get that.
>My theory is that the AMP is possibly introducing harmonics into the coax,
>and being as the antenna is not tuned for them they are being reflected and
>causing a higher SWR.
A likely cause, but then - has the radio itself, been optimized to use
the amp?
Most amps can't tolerate a wildly varying power - at least they don't
like it.
Most CB radios pass thru 4 watts, with up to 16 to 22 watts total PEP
power when audio [while you talk] is also applied.
Thats a considerable range of dynamic power that the amp has to
accommodate and compensate for.
Thats why I recommened that you have the radio re-tuned to reduce this
power problem to help the amp meet your needs.
Sure it may reduce power below stock levels when you don't use the
Amp, but you need to make some sacrifices to make this system work.
>Would a low pass filter help or would it just be better to purchase an
>antenna matcher ?
I'd use both. Pair them together at the radio or the amps output, and
Swap them around, tuner to amp, filter at output, then vicea-versa to
see how SWR is affected, and use the best combination.
>
>Thanks,
>
> Richard.
>
>
Regards!
.:+> Andy <+: ( ( (
. ) ) )
... You know you're a VERSATILE CB'er when ( ( (
... you're linear functions as a Bun Warmer /|/|/|/|/|
... Coffee heater and Hamburger Grill / / / / /||
... on a cold winters night! ||||||||||/|
... [ == .. ]/
Professor
agree the amp should be unconditionally stable before that is undertaken.
--
||||||||||||||||||||||||||||||||||||||||||||||||||||||
"Professor"
http://personal.kwom.com/bgriffey/
||||||||||||||||||||||||||||||||||||||||||||||||||||||
"-=PEEKABOO=-" <tappin...@hotmail.com> wrote in message
news:8k3b6f$1m04f$1...@ID-26551.news.cis.dfn.de...
Okie
"Bill Eitner" <kd6...@netzero.net> wrote in message
news:01bfe7c9$e9cf8f80$d52cd1d1@eniac...
Bill Eitner
>
> Is this the same Bill Eitner of Broken Bow, OK?
No. I'm 25 miles south of San Francisco.
Bill Eitner
unconditionally stable without feedback.
--
Bill Eitner
>
> OK, I never said that a filter or L network would fix a stability problem.
I never said you did.
> It
> certainly will not mask it either.
In many cases, mild to moderate instability shows up as
an increase in the spurious content. The increase in
the spurious content shows up as an increase in SWR when
the load is a tuned circuit (like an antenna). A low-
pass filter or a built-in tuned output circuit will
lower the SWR reading by filtering some of the spurious
content. I see that as a masking effect.
> This type of problem means something is
> broken or there was just poor enginnering from the beginning.
I more-or-less agree. A mislead attempt to produce
maximum output at any cost equates to poor engineering.
> Somebody said
> something against using negative feedback to fix stability problems.
That certainly wasn't me.
> I dont
> understand what is wrong with this.
Neither do I.
> Negative feedback is the time proven method
> of stabilizing an amp.
I couldn't agree more.
> This makes perfect sense because most instability is
> caused by positive feedback. Of course if the amp is just poorly constucted or
> designed these factors should be taken care of first by reducing the amount of
> postitive feedback through proper component orientation, shielding, grounding,
> bonding, ect.
Many CB amp designers/builders don't understand the
concept of phase shift or the resulting positive
feedback.
73.
Bill Eitner
explanation.
There are four good reasons why the SWR will change with
increased output power: non-linear devices in the SWR meter;
instability in the amp; antenna breakdown; and, an impedance
mismatch.
--
-=PEEKABOO=- wrote:
>
> This is true, but the output tuning of an amp can make this "appear" to
> change on some meters.
> --
> -=PEAKABOO=-
> "Trying not to choke,
> On that 10 ohm smoke"
> Check out http://www.geocities.com/watergate_rollin
>
> "TheGame" <the...@hotmail.om> wrote in message
> news:Js895.90$m_5....@news2.voicenet.com...
> > i would bet on it ... the swr ratio remains the same regardless of what
> > power of amp is inline, transistor or tube .. 10 forward watts /.05
> > reflect, 100 forward/ .5 reflect.. 1000 forward/ 5 reflect .. 10000
> forward/
> > 50 reflect. We are checking the load impedance that ratio should not
> change
> > with more power being applied.
> >
> >
> > "'Doc" <w5...@oio.net> wrote in message news:3964EC46...@oio.net...
> > > TheGame wrote:
> > > >
> > > > bullshit, if you have a meter that is capable of giving you correct
> > reading
> > > > this would not happen. Everyone is always quick to blame the
> amplifier.
> > try
> > > > a "Bird" or coaxial dynamics meter with a dual line section take 2
> slugs
> > 1
> > > > for reading forward power the other for reading reflect power. Turn
> the
> > amp
> > > > off check the swr turn it on and check the swr the ratio will still be
> > the
> > > > same.
> > > > "Professor" <bgri...@kwom.com> wrote in message
> > >
> > > -------clipped----------
> > >
> > > I wouldn't bet on it...
> > > 'Doc
> >
> >
--
Bill Eitner
I don't have time to point it out in a point-by-point rebuttal.
--
--
Handy Andy
No, I didn't miss it, it just hasn't been covered in this thread.
And it is a big problem when you have a system the customer doesn't
want to change, or budge on, easily.
It's too easy to blame the part added in, when the radio is merely
amplifying the problem that the amp is not designed to handle.
Er, excuse me, handles too easily, but not the way the
customer wants it done.
On Sat, 08 Jul 2000 00:37:48 -0700, while the modem was pondering it's
navel, Bill Eitner <kd6...@freewwweb.com> dissertated the following:
>I think you (Andy) missed it right off the bat. Unfortunately,
>I don't have time to point it out in a point-by-point rebuttal.
Regards!
:+> Andy <+:
.
... You CERTIANLY are a CB'er when... .----------------.
... your idea of a GOOD TIME is | .----. |0| | ||\
... answering a lady breakers 10-36! | | 19 | .-. .-.| |
... | `----' (0) (0)| |
... `----------------' |
... \_______________\|
Professor
load angles. I probably should have said "good stability"... and then you
incorporate the feedback to gain level.
--
||||||||||||||||||||||||||||||||||||||||||||||||||||||
"Professor"
http://personal.kwom.com/bgriffey/
||||||||||||||||||||||||||||||||||||||||||||||||||||||
"Bill Eitner" <kd6...@freewwweb.com> wrote in message
news:3966C8C0...@freewwweb.com...
Bill Nelson
: I'm glad you didn't get sucked into agreeing with an aggressive
: explanation.
: There are four good reasons why the SWR will change with
: increased output power: non-linear devices in the SWR meter;
Negated by the level adjustment in the SWR meter.
: instability in the amp; antenna breakdown; and, an impedance
: mismatch.
Amp instability could change the harmonic content - so the indicated
SWR could change. You are also correct about an antenna breakdown.
However, a change in the impedance of the output of the amplifier (what
this whole discussion is about) will NOT affect the SWR reading.
--
Bill Nelson (bi...@peak.org)
TheGame
"Bill Nelson" <bi...@spock.peak.org> wrote in message news:8k90oa$el5
> However, a change in the impedance of the output of the amplifier (what
> this whole discussion is about) will NOT affect the SWR reading.
> Bill Nelson (bi...@peak.org)
>
TheGame
different amp with a different output impedance you would have to retune
your antenna since now your swr is different ...
"Bill Nelson" <bi...@spock.peak.org> wrote in message news:8k90oa$el5>
However, a change in the impedance of the output of the amplifier (what
> this whole discussion is about) will NOT affect the SWR reading.
>
> --
> Bill Nelson (bi...@peak.org)
>
Hamm4fun
>Bill, finally a man with some sense has spoken...
>
>> However, a change in the impedance of the output of the amplifier (what
>> this whole discussion is about) will NOT affect the SWR reading.
>
>>
>
>
>
>
>
>
>
>
I know it does not effect the actual SWR, but I am not so sure that output
impedance of the amp will not effect the reading.If the carpenters and painters
get through tommorow I may try a little experiment to verify this .
Garden Hose
Bill Eitner
>
> Bill Eitner <kd6...@freewwweb.com> wrote:
> : I'm glad you didn't get sucked into agreeing with an aggressive
> : explanation.
>
> : There are four good reasons why the SWR will change with
> : increased output power: non-linear devices in the SWR meter;
>
> Negated by the level adjustment in the SWR meter.
Nope. You'll notice many of the better SWR
instruments have more than one scale and
instructions that describe how to use the
different scales based on level of transmitter
output power. The trend is that the needle
deflection in the reverse (aka reflected or
SWR) position will increase with increased
TX power. The higher power scales compensate
for that.
> : instability in the amp; antenna breakdown; and, an impedance
> : mismatch.
>
> Amp instability could change the harmonic content - so the indicated
> SWR could change. You are also correct about an antenna breakdown.
>
> However, a change in the impedance of the output of the amplifier (what
> this whole discussion is about) will NOT affect the SWR reading.
I don't know what you base that statement on,
but it's not true. The degree of impedance
mismatch is directly related to the amount
of reflected power regardless of whether it's
a load that doesn't match the source, or a
source that doesn't match the load.
> --
> Bill Nelson (bi...@peak.org)
TheGame
high SWR... ??
If the input impedance is high will it make the swr high on the output?
"Bill Eitner" <kd6...@freewwweb.com> wrote in message
Bill Eitner
>
> So Bill if i miss tune an Ac amplifer you will see it on the other end as a
> high SWR... ??
> If the input impedance is high will it make the swr high on the output?
Nope. I was wrong. I just did the
experiment you mentioned. Mistuning
the amp didn't change the SWR. It
doesn't make sense but it's true.
You'd figure a mismatch in either
direction would change the SWR but
it doesn't. It's another case of
SWR meters not being as useful as
they're cracked up to be.
--
> "Bill Eitner" <kd6...@freewwweb.com> wrote in message
> > I don't know what you base that statement on,
> > but it's not true. The degree of impedance
> > mismatch is directly related to the amount
> > of reflected power regardless of whether it's
> > a load that doesn't match the source, or a
> > source that doesn't match the load.
The Game
dog the swr meter it is not supposed show you this. As i said try a bird
meter as well " yes, i know doc he may have to calculate the swr from the
forward/reflect readings.
Thats why it makes me laugh when people say ... ohhh those amps have high
swr... how the f.. would they even know...
"Bill Eitner" <kd6...@freewwweb.com> wrote in message
news:396A5A19...@freewwweb.com...
Professor
the transmission line... no more... no less. That's why even though your
match shown on the SWR meter can be peachy... the amp can get hotter than
hell and have a terrible match to the feedline/antenna. This results in a
very poor power transfer out to the antenna...
In this instance... most of the power from the amp is being reflected back
into the amp and is lost heating the sink.
--
||||||||||||||||||||||||||||||||||||||||||||||||||||||
"Professor"
http://personal.kwom.com/bgriffey/
||||||||||||||||||||||||||||||||||||||||||||||||||||||
"Bill Eitner" <kd6...@freewwweb.com> wrote in message
news:396A5A19...@freewwweb.com...
Professor
between the set and the amp. It will then be reading the transmission line
vs. the amp input impedance. This is a useful reading when evaluating the
input impedance of the amp and how well it's matched to get the power into
it !
--
||||||||||||||||||||||||||||||||||||||||||||||||||||||
"Professor"
http://personal.kwom.com/bgriffey/
||||||||||||||||||||||||||||||||||||||||||||||||||||||
"The Game" <the...@yahoo.com> wrote in message
news:xita5.281$Hh4....@news3.voicenet.com...
The Game
Bill Eitner
>
> No... it makes perfect sense. The SWR meter just checks the antenna against
> the transmission line... no more... no less.
What happens when there is no transmission line?
In other words, the SWR meter is connected directly
to the output of the amp, and the feedline and
antenna constitute the load. Using your analogy
the amp is the feedline and the coax and antenna
is the antenna. You'd figure one is being checked
against the other but it's not. For example, a
25 ohm source feeding a 50 ohm load shows up as a
1:1 SWR when it should show up a 2:1 SWR.
> That's why even though your
> match shown on the SWR meter can be peachy... the amp can get hotter than
> hell and have a terrible match to the feedline/antenna. This results in a
> very poor power transfer out to the antenna...
> In this instance... most of the power from the amp is being reflected back
> into the amp and is lost heating the sink.
What you've listed here are consequences, but
your explanation still doesn't explain why an
SWR meter can't detect an impedance mismatch
when it's the source that doesn't match the
load.
> --
>
> ||||||||||||||||||||||||||||||||||||||||||||||||||||||
> "Professor"
> http://personal.kwom.com/bgriffey/
> ||||||||||||||||||||||||||||||||||||||||||||||||||||||
>
> "Bill Eitner" <kd6...@freewwweb.com> wrote in message
> news:396A5A19...@freewwweb.com...
> > Nope. I was wrong. I just did the
> > experiment you mentioned. Mistuning
> > the amp didn't change the SWR. It
> > doesn't make sense but it's true.
> > You'd figure a mismatch in either
> > direction would change the SWR but
> > it doesn't. It's another case of
> > SWR meters not being as useful as
> > they're cracked up to be.
Hamm4fun
I tried the test this afternoon using a Bird 43. I match my rdio into a 100 ohm
load then put on a 18 ft cable terminated with 50 ohms. SWR was 1:1. Then set
up the radio to match 25 ohms and repeated. There did seem to be an ever so
slight increase in the SWR reading. If the effect is there at all it is not
much. Perhaps other SWR meters are effected more. Would be interesing to find
out. I have a couple of other meters of the cheap variety. They did not seem to
be working very well(they have been on the shelf for 6 years) so I could not
test them. If anyone else would like to try this I made up the 25 ohm load with
2 50 ohm dummies and a T connector and the 100 ohm load out of a couple of 2
watt resistors in paralell.
George Warner
> Professor wrote:
> >
> > No... it makes perfect sense. The SWR meter just checks the antenna
> > against the transmission line... no more... no less.
>
> What happens when there is no transmission line?
> ...your explanation still doesn't explain why an
> SWR meter can't detect an impedance mismatch
> when it's the source that doesn't match the load.
The SWR meter must be built for a particular characteristic
impedance. It samples both voltage and current to perform a
directional reading. In order for the reverse voltage and out
of phase reverse current to cancel out of the forward reading,
the meter expects a fixed relationship between the voltage and
current -- the characteristic impedance.
Thus, an SWR meter measures the "SWR" for one particular characteristic
impedance even if no transmission line is employed. It measures the
"SWR" against a predetermined impedance; not the transmission line
impedance or transmitter output impedance.
Hamm4fun
>Hamm, save your time it will not change ...
>"Hamm4fun" <hamm...@aol.com> wrote in message
>news:20000710005031...@ng-md1.aol.com...
>> >
>> >Bill, finally a man with some sense has spoken...
>> >
>> >"Bill Nelson" <bi...@spock.peak.org> wrote in message news:8k90oa$el5
>> >> this whole discussion is about) will NOT affect the SWR reading.
>> >
>> >>
>> >
>> >
>> >
>> >
>> >
>> >
>> >
>> >
>>
>> I know it does not effect the actual SWR, but I am not so sure that output
>> impedance of the amp will not effect the reading.If the carpenters and
>painters
>> get through tommorow I may try a little experiment to verify this .
>
>
>
>
>
>
>
>
Your right , it didn't change. but now I know.
Hamm4fun
>The SWR meter must be built for a particular characteristic
>impedance. It samples both voltage and current to perform a
>directional reading. In order for the reverse voltage and out
>of phase reverse current to cancel out of the forward reading,
>the meter expects a fixed relationship between the voltage and
>current -- the characteristic impedance.
>
>Thus, an SWR meter measures the "SWR" for one particular characteristic
>impedance even if no transmission line is employed. It measures the
>"SWR" against a predetermined impedance; not the transmission line
>impedance or transmitter output impedance.
>
>
>
>
>
>
This has pretty much been my understanding too. However you can't help but
wonder whether or not placing the meter in an environment which it was not
designed(source impedance of other than 50 ohms) would effect the reading. I
worked with some Westinghouse engineers a while back that insisted on placing a
10 db pad on the output of a signal generator to insure the output impedance
was 50 ohms. They checked an alignment I had previously done without the pad.
They found no error though. They attributed this to the fact the the generator
was a good match to but but stated that it may not have been and I should use
the pad just in case. (sounds like a KOTEX commercial).
Bill Nelson
:>
:> No... it makes perfect sense. The SWR meter just checks the antenna against
:> the transmission line... no more... no less.
: What happens when there is no transmission line?
: In other words, the SWR meter is connected directly
: to the output of the amp, and the feedline and
: antenna constitute the load. Using your analogy
: the amp is the feedline and the coax and antenna
: is the antenna. You'd figure one is being checked
: against the other but it's not. For example, a
: 25 ohm source feeding a 50 ohm load shows up as a
: 1:1 SWR when it should show up a 2:1 SWR.
Keep in mind that the SWR is the ratio between the power fed to the
antenna and the power reflected by the antenna (instead of being absorbed
by the antenna).
It does not matter if there is a feedline present or not. You can connect
the SWR meter directly to the transmitter and antenna. The SWR meter has
a short length of transmission line in it. The meter looks at the junction
at the antenna side for the voltage/current measurements. So the short
length of transmission line in the meter is sufficient to "isolate" the
transmitter.
If you could figure out a way to measure the RF currents without inserting
a meter between the transmitter and antenna, then you would not have a
transmission line to cause any mismatch. The mismatch would be between
the transmitter output and the antenna. THEN, mistuning the output would
affect the SWR.
--
Bill Nelson (bi...@peak.org)
Bill Eitner
> > >
> > > No... it makes perfect sense. The SWR meter just checks the antenna
> > > against the transmission line... no more... no less.
> >
> > What happens when there is no transmission line?
> > SWR meter can't detect an impedance mismatch
> > when it's the source that doesn't match the load.
>
> impedance. It samples both voltage and current to perform a
> directional reading. In order for the reverse voltage and out
> of phase reverse current to cancel out of the forward reading,
> the meter expects a fixed relationship between the voltage and
> current -- the characteristic impedance.
>
> Thus, an SWR meter measures the "SWR" for one particular characteristic
> impedance even if no transmission line is employed. It measures the
> "SWR" against a predetermined impedance; not the transmission line
> impedance or transmitter output impedance.
You got it.
Bill Nelson e-mailed me with an equally sensible
argument.
One thing I asked him that I'll know ask you now:
(Hi George--long time no argue); what tool would
you use to determine source (generator) impedance?
Bill Eitner
>
> Bill Eitner <kd6...@freewwweb.com> wrote:
> :>
> :> No... it makes perfect sense. The SWR meter just checks the antenna against
> :> the transmission line... no more... no less.
>
> : What happens when there is no transmission line?
> : to the output of the amp, and the feedline and
> : antenna constitute the load. Using your analogy
> : the amp is the feedline and the coax and antenna
> : is the antenna. You'd figure one is being checked
> : against the other but it's not. For example, a
> : 25 ohm source feeding a 50 ohm load shows up as a
> : 1:1 SWR when it should show up a 2:1 SWR.
>
> Keep in mind that the SWR is the ratio between the power fed to the
> antenna and the power reflected by the antenna (instead of being absorbed
> by the antenna).
This is what I didn't get the first time.
The source impedance is irrelevant. The source
is just a source. In other words, all the source
does is supply *some* amount of power--it's efficiency
won't effect the SWR meter reading.
I get it now.
The next question is what should guys like me do to
actually measure the source impedance?
Professor
meter from amp to antenna and getting a low reading is only part of that
process. The other part involves matching the amps output impedance to the
line/antenna. That is accomplished by taking the amps output and hooking a
dummy load to it via a piece of 50 Ohm cable. Then I use a scope to measure
the voltage across the load... and adjust amplifier components to get a
maximum reading. Now you know that your amps output is as close to the 50
Ohms impedance as possible.
Remember that SWR is a "transmission line phenomenon" and has really no
meaning when the line is very short... although you still have to have an
impedance match to couple the power correctly. Remember also that the SWR
meter has a short transmission line inside that has the usual 50 Ohm
characteristic impedance associated with it.
So... to answer your question about hooking an SWR meter and load directly
to the amps output...
This still will not show anything about the amps output impedance because
now the SWR meter is the transmission line(instead of the coax) feeding the
load. You will still not be able to evaluate the amps output impedance that
way...
--
||||||||||||||||||||||||||||||||||||||||||||||||||||||
"Professor"
http://personal.kwom.com/bgriffey/
||||||||||||||||||||||||||||||||||||||||||||||||||||||
"Bill Eitner" <kd6...@freewwweb.com> wrote in message
news:396A6509...@freewwweb.com...
> Professor wrote:
> >
> > No... it makes perfect sense. The SWR meter just checks the antenna
against
> > the transmission line... no more... no less.
>
> What happens when there is no transmission line?
> In other words, the SWR meter is connected directly
> to the output of the amp, and the feedline and
> antenna constitute the load. Using your analogy
> the amp is the feedline and the coax and antenna
> is the antenna. You'd figure one is being checked
> against the other but it's not. For example, a
> 25 ohm source feeding a 50 ohm load shows up as a
> 1:1 SWR when it should show up a 2:1 SWR.
>
> > That's why even though your
> > match shown on the SWR meter can be peachy... the amp can get hotter
than
> > hell and have a terrible match to the feedline/antenna. This results in
a
> > very poor power transfer out to the antenna...
> > In this instance... most of the power from the amp is being reflected
back
> > into the amp and is lost heating the sink.
>
> What you've listed here are consequences, but
> your explanation still doesn't explain why an
> SWR meter can't detect an impedance mismatch
> when it's the source that doesn't match the
> load.
>
Hamm4fun
>: What happens when there is no transmission line?
>: In other words, the SWR meter is connected directly
>: to the output of the amp, and the feedline and
>: antenna constitute the load. Using your analogy
>: the amp is the feedline and the coax and antenna
>: is the antenna. You'd figure one is being checked
>: against the other but it's not. For example, a
>: 25 ohm source feeding a 50 ohm load shows up as a
>: 1:1 SWR when it should show up a 2:1 SWR.
>
>antenna and the power reflected by the antenna (instead of being absorbed
>by the antenna).
>
>the SWR meter directly to the transmitter and antenna. The SWR meter has
>a short length of transmission line in it. The meter looks at the junction
>at the antenna side for the voltage/current measurements. So the short
>length of transmission line in the meter is sufficient to "isolate" the
>transmitter.
>
>If you could figure out a way to measure the RF currents without inserting
>a meter between the transmitter and antenna, then you would not have a
>transmission line to cause any mismatch. The mismatch would be between
>the transmitter output and the antenna. THEN, mistuning the output would
>affect the SWR.
>
>--
>Bill Nelson (bi...@peak.org)
>
>
>
>
>
>
>
There is always a transmission line. The SWR meter is a transmission line.
Bill Nelson
: The source impedance is irrelevant. The source
: does is supply *some* amount of power--it's efficiency
: won't effect the SWR meter reading.
: I get it now.
: The next question is what should guys like me do to
: actually measure the source impedance?
I sent you e-mail with a suggestion. Feel free to post it to this
group, if you wish.
--
Bill Nelson (bi...@peak.org)
Bill Nelson
:>
:>If you could figure out a way to measure the RF currents without inserting
:>a meter between the transmitter and antenna, then you would not have a
:>transmission line to cause any mismatch. The mismatch would be between
:>the transmitter output and the antenna. THEN, mistuning the output would
:>affect the SWR.
:>--
:>Bill Nelson (bi...@peak.org)
: There is always a transmission line. The SWR meter is a transmission line.
So what does this have to do with what I wrote? What if you don't use
an SWR meter?
--
Bill Nelson (bi...@peak.org)
Professor
--
Prozac
air wire dirt shit piss what ever.
--
Visit:
www.bayshorenetwork.com
and you too my get 10-8 some day!
"Professor" <bgri...@kwom.com> wrote in message
news:w_Xa5.3044$sR.1...@east3.usenetserver.com...
: Correct...
:
:
:
:
:
George Warner
> ...what tool would
> you use to determine source (generator) impedance?
A direct measurement is almost impossible for a transmitter.
I would create a tunable matching circuit and a good 50 ohm
dummy load. Attach this to the radio and transmit into it.
Tune the matching circuit for maximum voltage (power) on
the 50 ohm load. Then remove the matching circuit/load and
measure that with an antenna analyzer or network analyzer
(don't forget to stop transmitting first ;-D ).
Its impedance will be the complex conjugate of the transmitter
output impedance.
Bill Eitner
I figured you could disconnect the tuned circuit
from the final stage and couple into it using an
antenna analyzer configured like a grid dip meter
(or a network analyzer). The problem with that
method might be that you'd end up measuring the
output impedance of the device(s) in the final
stage (the input impedance to the matching net-
work) rather than the output impedance of the
matching network. It would probably be better
to do it the way you describe.
Thanks.