How to Measure a 2M Yagi Impedance?
Paul (Home) News
I have done quite a bit of numerical analysis of VHF Yagi's with usually
very good agreement between theory and performance at least for gain and
pattern. One of the things that seems more difficult to correctly predict
however is the input impedance, and thus to design some sort of a match
without some sort of subsequent trial and error. In an effort to explore the
differences further I have been looking at ways of measuring the impedance
directly rather than indirectly by vswr with the matching in place. I have
made a number of different return loss bridges, and even tried the technique
described in an old Ham Radio Article where you take two VSWR readings with
and without an added series resistance. All of course making allowances for
coax length etc. The problem I have is all of them give, in some cases
wildly, different answers even when used on the same antenna. So my
questions are:
What sort of accuracy can I expect from these sorts of methods?
Is there a better way (which doesn't involve large sums of money) to measure
antenna impedance at say 146Mhz?
Thanks
Paul VK3DIP
Reg Edwards
"Paul (Home) News" wrote
===============================
Paul,
Attempts to accurately determine antenna input impedance, using an
inherently ambiguous, innacurate SWR meter at the transmitter end of a line
of uncertain length and velocity, are doomed to failure.
*Never* expect to obtain numbers worthy of serious engineering application.
There are far too many uncertainties of unknown magnitudes.
The only way of obtaining an accurate measurement is to climb a ladder
taking with you an R plus or minus jX hand-held impedance bridge. Can you
borrow one ?
But why do you wish to know antenna input impedance when you are aleady
quite happy with using an inaccurate SWR meter to fiddle a 1-to-1 SWR at the
transmitter end.
The ultimate objective, of course, is just to obtain a 50-ohm load for the
transmitter regardless of what the SWR and antenna impedance might be.
----
Reg, G4FGQ
David.Shrader
Align the antenna so that the reflector is 'down' and the last director
is 'up'. Ground effects are minimized due to the F/B of the antenna. The
antenna is radiating straight 'up'.
Next take a 1 wavelength, allowing for velocity factor, coax line and
connect it to the antenna feedpoint.
Finally, beg, borrow, requisition, pilfer, rustle, etc., an antenna
analyzer similar to the MFJ 259B. Connect it to the other end of the 1
wavelength coax.
Select the function to read impedance. Dial in your frequency and read
the impedance.
A one minute job once the antenna, coax and meter are at hand.
+ + +
Reg Edwards
"David.Shrader" <Deaco...@comcast.net> wrote in message
news:rqZAc.57026$Hg2.5940@attbi_s04...
> The input impedance can be measured reasonably well at ground level.
>
In principle it can be done. But to determine antenna impedance from
measurements (NOT SWR) made at the transmitter end of the line it is
necessary ACCURATELY to know the Length, Impedance Zo, Attenuation and
Velocity Factor of the line.
Line attenuation need not be known with high accuracy. But, obviously, with
a longish line the information available about what is going on at the far
end becomes degraded. And to rely on measurements made with an ordinary,
ambiguous, inaccurate swr meter, as you have already found, is asking for
trouble.
Measure input impedance, R+jX, of the line. Then use program ZL_ZIN to
calculate what the antenna input impedance, R+jX, must be to provide such an
input impedance.
I havn't looked at the program recently but I think an ambiguity can arise
if the whole number of half-wavelengths in the line is not known. So it may
help to have some idea about what's going on in the process. But you'll know
when you get the right answer.
Program ZL_ZIN can be downloaded in a few seconds from website below and run
immediately from its desktop icon. It's easy enough to use when the input
data is known.
----
...........................................................
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
...........................................................
J. McLaughlin
Green Bank, I worked on a 400 MHz feed for the 85 foot dish that
consisted of two dipoles fed in phase above a reflector. Since this was
for radio astronomy measurement, it was desired to have a good match at
two frequencies 60 MHz apart to use with a receiver having a 30 MHz IF.
("signal" came in both images.) I worked on the stubs with the antenna
feed pointing away from me in a very large room while measuring with a
GR admittance bridge. The bridge is adjusted for a null while listening
through headphones.
While thinking about the next adjustment, I heard the sound in the
headphones go out of null and then back. Eventually I traced the
in-and-out to secretaries walking back and fourth around a table to
collate a report in a room way across the empty lab and on the other
side of a thin wall. Energy coming back to an antenna changes its
apparent Z.
Quick-like-a-little-bunny, as we say in these parts, the feed was
put on the top of a step ladder outdoors pointing at the sky. When
adjusted while up on the ladder, it worked as expected when placed at
the focus. But that is another story.
73, Mac N8TT
--
J. Mc Laughlin - Michigan USA
Home: J...@Power-Net.Net
"David.Shrader" <Deaco...@comcast.net> wrote in message
news:rqZAc.57026$Hg2.5940@attbi_s04...
Bob Bob
Gordon VK2ZAB (and others) published some time ago a complex Z bridge
thing for VHF/UHF. It uses transmission lines for tuned elements and is
band specific.
Try http://www.vhfdx.oz-hams.org/measurements.html
And do other google searches using Gordons callsign
Cheers Bob VK2YQA
David.Shrader
Reg Edwards wrote:
> "David.Shrader" <Deaco...@comcast.net> wrote in message
> news:rqZAc.57026$Hg2.5940@attbi_s04...
>
>>The input impedance can be measured reasonably well at ground level.
>>
>
> ==================================
>
> In principle it can be done. But to determine antenna impedance from
> measurements (NOT SWR) made at the transmitter end of the line it is
> necessary ACCURATELY to know the Length, Impedance Zo, Attenuation and
> Velocity Factor of the line.
SNIP Reg, I did not specify the transmitter end of the line. I stated 1
wavelength from the antenna. In this case, 1 wavelength, the line
impedance is not a factor, but the velocity factor is.
Line attenuation at 1 wavelength at 146 MHz is generally negligible. I
did not use the SWR meter. I stated use an antenna analyzer.
I think the confusion arises because your reply is to my post and not
the original post.
DD
Reg Edwards
following your response with mine and including a comment of yours caused a
little confusion. Sorry!
I agree your method will work. The problem, a practical one, is obtaining a
COAXIAL line length exactly an integral number of 1/2-wavelengths long.
There's no way of proving it exept by climbing a ladder and disconnecting
the line from the antenna.
And it is an exact 1/2-wavelength long at ONE frequency only. But it is
required to make measurements over a whole band of frequencies. To shift to
other frequencies
involves calculations taking Zo into account. But Zo is not accurately
known. So then you have to measure line Zo. And so on.
And you have to know exactly what you are doing because the 259B does not
provide the sign of jX in R+jX.
But as I said before, all you want to know is whether or not the transmitter
is loaded with 50 ohms. To hell with SWR and antenna input impedance. ;o)
----
Reg, G4FGQ
Jerry Martes
I'm going to make one of those impedance "meters". I sure appreciate
having guys like you do all the research work for me. Thanks again.
Jerry
"Bob Bob" <bcn...@optusnet.com.au> wrote in message
news:nnoeq1-...@p400bob.lse.com.au...
Roy Lewallen
measurements. That's a viable method if the imedance of the antenna is
close to the characteristic of the line. If it's not, you'll find that
even a surprisingly small line loss -- one that you'd normally consider
negligible, can seriously skew your results. The calculation is
straightforward presuming you know the loss -- I'm sure Reg's program
would be adequate. Do some what-ifs with various antenna impedances and
you'll see what I mean. The effect gets worse as the antenna and
transmission line impedances get more different, and as the line gets
longer. That is, a one wavelength line will have more effect than a half
wavelength one.
Also, line length becomes more and more critical as the impedance of the
antenna and transmission line become more different and as the line gets
longer. Again, a little experimentation with the calculations will
illustrate what to expect.
Even if you carefully account for the transformation of the connecting
line or don't use any line at all, you have to be aware of common mode
currents and how your test setup differs from your normal rig
connection. And finally, even with a perfect lab setup, you'll find that
good impedance measurements can be hard to make with amateur equipment.
Before you get carried away, make some measurements on the bench with
your meter and using good quality loads, or at least RC combinations
using chip resistors and capacitors or ones with extremely short leads.
Make impedances similar to ones you hope to measure. If you can get
values which are accurate enough to suit you, go to the next step and
measure the same loads through a transmission line as has been
suggested, and see if you're able to extract the actual load value from
the measured value with sufficient accuracy.
If you get that far, you've partially answered your question about what
kind of accuracy to expect, and you're ready to start figuring out how
to deal with common mode currents.
Decent antenna impedance measurements aren't simple to make, even at HF.
They're more difficult at VHF and above.
Roy Lewallen, W7EL
Dave Shrader
> Dave, I was replying to the original questioner. But by immediately
> following your response with mine and including a comment of yours caused a
> little confusion. Sorry!
>
> I agree your method will work. The problem, a practical one, is obtaining a
> COAXIAL line length exactly an integral number of 1/2-wavelengths long.
The MFJ 259B will measure the length of the line for you before test.
Leave it open circuited and connect the other end to the MFJ. It finds
the 1/2 wavelength frequency.
> There's no way of proving it exept by climbing a ladder and disconnecting
> the line from the antenna.
A bench setup with a 1 wavelength line does not require climbing and
measuring at the tower.
>
> And it is an exact 1/2-wavelength long at ONE frequency only. But it is
> required to make measurements over a whole band of frequencies. To shift to
> other frequencies
> involves calculations taking Zo into account. But Zo is not accurately
> known. So then you have to measure line Zo. And so on.
Agree it is a one frequency measurement.
>
> And you have to know exactly what you are doing because the 259B does not
> provide the sign of jX in R+jX.
But, the sign of X is very easy to determine. Increase frequency
'slightly' on the 259B and observe absolute value of X. If X increases
it is inductive. If X decreases it is capacitive. BTW the Or is a value
at antenna resonance only at one frequency. But, you a very well aware
of that. The comment is made for other readers.
>
> But as I said before, all you want to know is whether or not the transmitter
> is loaded with 50 ohms. To hell with SWR and antenna input impedance. ;o)
> ----
> Reg, G4FGQ
There is one possibility remaining. If the Yagi is to be tuned for
MAXIMUM gain, and that is the objective, then Ro will be the lowest
value at resonance. Most Ham yagis are not tuned for optimum gain as we
all know.
There is WAY WAY too much emphasis among today's hams regarding low
VSWR. My 75/80 doublet has a VSWR approaching 30:1 and works just FB!!
The problem is the absence of output tuning in most of the rigs
available today. Oh! for my OLD VIKING II or my Drake 4C. <grin>
Gor Bless and you have my permission to celebrate the USA Father's Day.
>
Reg Edwards
Roy, your caution is well placed.
As I've said before, most amateurs and professionals (it seems from these
walls) suffer from delusions of accuracy.
Their delusions are seldom frustrated by things going wrong after some
tedious, supposedly highly accurate, design work has been done. They
congratulate themselves on their success and sometimes follow up by writing
learned papers incorporating 6 places of decimals about it.
But in engineering reallity, especially with Radio, things work simply
because any bloody thing will work after a fashion. And if the transmitter
is loaded with an actual but unknown impedance between 30 and 80 ohms, such
that it works, they continue to remain oblivious to their delusions.
I relate this, certainly not to cast ridicule, but with the inention of
enhancing the underlying beauty of this intriguing hobby of ours.
Perhaps 'suffer from' are the wrong words.
In the UK it is Father's Day. So I am about to pour myself another glass
from a bottle of special reserve port, a thoughtful present from a loving
'doter'.
----
73 and 88, Reg, G4FGQ
"Roy Lewallen" <w7...@eznec.com> wrote in message
news:10dag90...@corp.supernews.com...
Ian White, G3SEK
>If the Yagi is to be tuned for MAXIMUM gain, and that is the objective,
>then Ro will be the lowest value at resonance.
That's an interesting assertion. Do you have further evidence for it?
--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek
Dave Shrader
maximum radiation which requires maximum current which requires lowest
radiation resistance. Twenty years ago, or so, Ro of 15 to 20 ohms was
common in high gain Yagis wher Gamma matching was used to raise the
impedance to approximately 50 ohms. A slight reduction in gain allows Ro
of close to 50 ohms.
Kraus, Antennas, McGraw-Hill 1950, Chapter 11 provides the analysis for
a simple 2 element 'Yagi' type array. In written terms, the driving
point, feed point, resistance, ignoring losses, is the radiation
resistance of the driven element minus the ratio of the mutual impedance
to the self impedance of the parasitic elements. Far field gain is
maximized by a term where the input power is divided by the net
impedance of the driven element minus the net impedance contributed by
the parasitic elements.
Conclusion, maximum gain, in any configuration [3 element, 4 element,
etc.], requires lowest Rr produced by highest mutual coupling.
I'm not arguing that more gain is produced by the longest boom or the
most elements. What I am stating is that for any configuration the gain
for that configuration is MAXIMIZED when the Rr is minimized.
Tom Ring
> Dave Shrader wrote:
>
>> If the Yagi is to be tuned for MAXIMUM gain, and that is the
>> objective, then Ro will be the lowest value at resonance.
>
>
> That's an interesting assertion. Do you have further evidence for it?
>
>
Yes, quite interesting, since a yagi is _not_ resonant in the design
frequency range, otherwise it couldn't work.
Tom
K0TAR
Roy Lewallen
newsgroup. Unfortunately, they're not true.
I have a very high confidence in the ability of EZNEC to accurately
model Yagi antennas. This is due to feedback from several professional
customers who have analyzed Yagis with EZNEC and tested the actual
antennas on test ranges.
Let's take the EZNEC example file NBSYagi.EZ.
If you change the driven element (wire 2) length from 2 * 54.875" to 2 *
54.56", you'll find that the feedpoint impedance is 11.53 - j0.0752 ohms
-- it's resonant, and it's certainly functioning as a Yagi. The pattern
and gain are nearly identical to the original NBS design.
Now, change the director (wire 3) length from 2 * 54.313" to 2 * 56".
This drops the gain from 9.68 dBi to 8.66 dBi, and lowers the feedpoint
resistance from 11.53 ohms to 7.849 ohms. The point of maximum gain is
obviously not the point of minimum feedpoint resistance.
Anyone having an explanation for why the gain should be greatest when
the feedpoint resistance is minimum and why a Yagi can't work when
resonant should examine their explanations carefully in order to uncover
the flaws that are obviously present in the explanations.
Roy Lewallen, W7EL
Dave Shrader
Tom Ring
elements of a yagi are resonant, except perhaps the driven element. My
point was that the elements except the driven one(s) must be above or
below resonance, or the yagi isn't a yagi.
I have also seen a commercial yagi with the driven element longer than
the reflector, so it likely wasn't remotely near resonance. It was also
a very poorly performing commercial yagi, but that's a different matter.
tom
K0TAR
Tom Ring
group, no one responds. And it as a group you are all, even Roy,
obviously subject to this. No one bothered to even think about what I
originally said, or try to see the tongue in cheek.
I guess if you can't argue, it's no fun. I don't blame you all for
that, but it is interesting to observe. And sad.
tom
K0TAR
Richard Harrison
"What I meant was, none of the elements of a yagi are resonant, except
perhaps the driven element."
That`s usually right. The reflector is lengthened and directors are
shortened to conveniently produce phase relations which determine
reinforcement or repression in directions as desired.
However, this is not the only way. Commercial broadcast curtain antenna
arrays use parasitic elements which have the same length as the driven
elements in some instances. Short-circuit stubs repalace drive lines in
the parasitic elements, and these are adjusted for the desired phasing
instead of adjusting element lengths.
Best regards, Richard Harrison, KB5WZI
Richard Harrison
"Is there a better way (which doesn`t involve large sums of money) to
Use a line of any number of 1/2-wavelengths to connect the antenna to a
VHF admittance or impedance bridge complete with signal source and
bridge detector (VHF receiver). Measure away and record your results.
I agree with most of G4FGQ`s response. You can expect the antenna`s
environment to affect its performance and impedance. I suggest the
transmission line which is a minimum integral number of 1/2-wavelengths
as required to connect your bridge to the antenna as an alternative to
Reg`s ladder. A 1/2-wave line repeats the impedance connected to its
end.
Tom Ring
> That`s usually right. The reflector is lengthened and directors are
> shortened to conveniently produce phase relations which determine
> reinforcement or repression in directions as desired.
>
> However, this is not the only way. Commercial broadcast curtain antenna
> arrays use parasitic elements which have the same length as the driven
> elements in some instances. Short-circuit stubs repalace drive lines in
> the parasitic elements, and these are adjusted for the desired phasing
> instead of adjusting element lengths.
>
That's a nice trick. Of course that still means they aren't resonant
since you just displaced the "center" of the element. Seems a good way
for a broadcaster to be able to adjust the pattern if needed after
construction.
I seem to remember an HF wire antenna project that used that method to
go from driven plus reflector to driven plus director to get a
reversible beam. I also remember a set of 5 slopers that were in the
ARRL antenna book or handbook that could be steered.
Oh well, way off topic here now. cul
Tom
K0TAR
Cecil Moore
> Paul, VK3DIP wrote:
> "Is there a better way (which doesn`t involve large sums of money) to
> measure antenna impedance at say 146 MHz?"
>
> Use a line of any number of 1/2-wavelengths to connect the antenna to a
> VHF admittance or impedance bridge complete with signal source and
> bridge detector (VHF receiver). Measure away and record your results.
I've been out of town and not following this thread. Here's what I do
for HF - knowing the length, VF, and attenuation factor of ladder-line.
Trim the laddder-line until the impedance looking into the ladder-line
is purely resistive. Draw the corresponding SWR circle on a Smith Chart.
Using the line-attenuation factor, draw an SWR circle outside of that
one. The antenna feedpoint impedance lies on that outside SWR circle.
Calculate the exact electrical length of the length of ladder-line
being used and use the Smith Chart to track from the purely resistive
feedpoint impedance back to the antenna feedpoint impedance on the
largest SWR circle.
Of course, the accuracy of the final indirect measurement depends upon
the accuracy of all the parameters used in the calculation. My accuracy
has always been good enough for what I needed.
I've never done it with coax but I assume the same principles apply.
--
73, Cecil http://www.qsl.net/w5dxp
-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
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Richard Harrison
"Of course that still means thery aren`t resonant aince you just
displaced the "center" of the element."
Kraus describes adjustment of the phase between driven and parasitic
elements on page 320 of his 1950 edition of "Antennas":
"The parasitic element may have a fixed length of 1/2 wavelength, the
tuning being accomplished by inserting a lumped reactance in series with
the antenna at its center point."
In my case, the "lumped reactance" was a tuned stub adjusted to the
desired phase difference between parasitic and driven elements as
indicated by an RCA WM-30A phase monitor.
Richard Harrison
"Trim the ladder-line until the impedance looking into the ladder-line
is purely resistive."
Sure. The line is purely resistive at resonant lengths where the power
factor is one. No reactance. A 1/2-wave is a resonant length.
Charlie Wright, an A.D. Ring and Accociates engineer used to drive our
German engineers crazy, telling them that slopes on the autobahn used
coble stones because they didn`t know how to pour concrete on an
incline.
Charlie also got to a group using an RCA WM-30A phase monitor to tune
parasiitic elements in a curtain array. Most medium-wave directional
stations at the time used a WM-30A as a phase monitor, just as shortwave
stations used them for tune-up.
Charlie had used the monitor for years and knew it had an underated
resistor which sometimes failed. The group had upended the chassis and
Charlie offered to help troubleshoot. The Germans acquiesced.
Charlie asked for voltage measurements from unrelated parts of the
circuit, took out his slide rule and feigned a few calculations. Then,
Charlie pointed to the defective resistor and said: "Change that one."
The crowd shook its collective heads but complied. The monitor
miraculously sprang to life again. Charlie chuckled to himself as he
left the incredulous crowd.
J. McLaughlin
My 999 story: A major automobile manufacturer tasked their German
branch to design a new transmission for a "sporty" car. Prototype
arrived at the proving grounds and looked anemic. Transmission was
placed into prototype car. Everyone went to see the first use. Driver
wound up the engine to red line, and loud 9 9 9 was heard as clutch was
engaged and shrapnel was produced. US engineers turned to German
engineers and said: "We told you how it would be used - now believe
us."
An antenna system was used to send data back for analysis. 73 Mac
N8TT
--
J. Mc Laughlin - Michigan USA
Home: J...@Power-Net.Net
"Richard Harrison" <richard...@webtv.net> wrote in message
news:19023-40...@storefull-3258.bay.webtv.net...
> Cecil, W5DXP wrote:
<snip>
Tdonaly
>
>About what I expected. If someone states something truthfull in this
>group, no one responds. And it as a group you are all, even Roy,
>obviously subject to this. No one bothered to even think about what I
>originally said, or try to see the tongue in cheek.
>
>I guess if you can't argue, it's no fun. I don't blame you all for
>that, but it is interesting to observe. And sad.
>
>tom
>K0TAR
>
You can think of it this way, or more probably, you need to
work on your communication skills.
73,
Tom Donaly, KA6RUH
Tom Ring
>
> You can think of it this way, or more probably, you need to
> work on your communication skills.
> 73,
> Tom Donaly, KA6RUH
>
>
here like to argue. And while I may need to work on my comm skills, it
doesn't change that fact at all. Like I said, sad.
tom
K0TAR
Tdonaly
>Message-id: <40da179e$0$439$5cbe...@news.real-time.com>
What's wrong with wanting to argue? Argument, sometimes even violent argument,
has
been a hallmark of Western science for a long time. People who take everything
at face value, without question or disagreement, end up believing the strangest
things.
73,
Tom Donaly, KA6RUH
Tom Ring
>
> What's wrong with wanting to argue? Argument, sometimes even violent argument,
> has
> been a hallmark of Western science for a long time. People who take everything
>
> at face value, without question or disagreement, end up believing the strangest
> things.
> 73,
> Tom Donaly, KA6RUH
>
Arguing endlessly on the same old subjects knowing the opposition won't
budge is what annoys me. And that's what goes on here very often. I
have no issues with a discussion that actually goes someplace.
tom
K0TAR
Cecil Moore
> What's wrong with wanting to argue?
What' wrong indeed? Arguments are the cornerstone of logic. My dictionary
says a definition of "argument" is "3. a process of reasoning".
Ian White, G3SEK
>Ian White, G3SEK wrote:
>> Dave Shrader wrote:
>>
>>> If the Yagi is to be tuned for MAXIMUM gain, and that is the
>>>objective, then Ro will be the lowest value at resonance.
>> That's an interesting assertion. Do you have further evidence for
>>it?
>>
(Apologies for the delay in replying to this, Dave. I've been away from
the computer for two weeks.)
>I've been away from Yagis for many years. But, maximum gain requires
>maximum radiation which requires maximum current which requires lowest
>radiation resistance. Twenty years ago, or so, Ro of 15 to 20 ohms was
>common in high gain Yagis wher Gamma matching was used to raise the
>impedance to approximately 50 ohms. A slight reduction in gain allows
>Ro of close to 50 ohms.
>
>Kraus, Antennas, McGraw-Hill 1950, Chapter 11 provides the analysis for
>a simple 2 element 'Yagi' type array. In written terms, the driving
>point, feed point, resistance, ignoring losses, is the radiation
>resistance of the driven element minus the ratio of the mutual
>impedance to the self impedance of the parasitic elements. Far field
>gain is maximized by a term where the input power is divided by the net
>impedance of the driven element minus the net impedance contributed by
>the parasitic elements.
>
>Conclusion, maximum gain, in any configuration [3 element, 4 element,
>etc.], requires lowest Rr produced by highest mutual coupling.
>
This is stretching a simplified theoretical case, way beyond the point
where it ceases to apply.
I agree that the maximum *theoretical* gain - ignoring losses - is
achieved when the element currents are as high as possible, and the
feedpoint resistance is as low as possible. This also requires that the
element spacing is as close as possible... which leads to the
interesting conclusion that a compact beam should have more gain than a
full-sized one!
In practice, of course, this doesn't happen. The reason is that losses
can *never* be ignored in this particular problem. As the element
currents rise and the feedpoint impedance drops, the I^2*R losses in the
elements and the matching losses to 50R rapidly overtake any theoretical
increase in gain.
This means that high-gain beams with deliberately high element currents
are only a theoretical curiosity. The underlying theory has a valid
place in academic textbooks such as Kraus, but it isn't relevant to
practical antenna engineering. (Even superconducting elements and
matching circuits wouldn't make such antennas practical.)
Also, it isn't correct to apply generalizations about 2- and 3-element
yagis to a long, multi-element yagi. In particular, the first 2 or 3
elements of a long yagi cannot be considered in isolation from all the
other elements.
It is true that gain optimization in multi-element yagis tends to reduce
the feedpoint impedance towards 15-20R, but this is a remote side-effect
of all the other design parameters. A low feedpoint impedance certainly
isn't a desirable design aim in itself, because it leads to significant
matching losses and a reduction in the SWR bandwidth.
Numerous designers have found that when they are getting close to a
gain-optimized design, it is usually possible to raise the feed
impedance back towards 50R by inserting an additional first director
with a very close spacing ahead of the driven element. (This technique
may have been developed after you ceased to take a close interest in
yagi design, Dave.)
The close-spaced first director is mostly an impedance-changing device,
and it has relatively few side-effects on the overall gain and pattern.
With a multi-element yagi, it is usually possible to take out most of
these side-effects in the next round of optimization. The result is a
yagi that can be fed directly from 50R coax (through a balun) which
eliminates matching losses and greatly improves the SWR bandwidth. If
the re-optimization is done well, any decrease in gain is almost
undetectable in simulation, and completely undetectable on the air.
Tom Ring
our older beams, only a couple years, seemed low in gain. We
ScothBrighted the elements, Al welding rod, as I remember, with a hobby
brass driven element and T match, and got 3 or 4 10th's more on a 17
foot beam I designed for EME than I had on the 1st range test.
Sorry about the looong sentence.
tom
K0TAR