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.

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.



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?

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?



Yes, quite interesting, since a yagi is _not_ resonant in the design
frequency range, otherwise it couldn't work.

Tom
K0TAR

My, we can sure learn a lot of new things about Yagis from this
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

Tom Ring wrote:
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?



Yes, quite interesting, since a yagi is _not_ resonant in the design
frequency range, otherwise it couldn't work.

Tom
K0TAR

I stand corrected Roy

Roy Lewallen wrote:

My, we can sure learn a lot of new things about Yagis from this
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

Tom Ring wrote:

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?



Yes, quite interesting, since a yagi is _not_ resonant in the design
frequency range, otherwise it couldn't work.

Tom
K0TAR

I should have stated that more clearly. What I meant was, none of the
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

Roy Lewallen wrote:

My, we can sure learn a lot of new things about Yagis from this
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

Tom Ring wrote:

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?



Yes, quite interesting, since a yagi is _not_ resonant in the design
frequency range, otherwise it couldn't work.

Tom
K0TAR

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

Tom Ring wrote:

I should have stated that more clearly. What I meant was, none of the
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

Roy Lewallen wrote:

My, we can sure learn a lot of new things about Yagis from this
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

Tom Ring wrote:

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?



Yes, quite interesting, since a yagi is _not_ resonant in the design
frequency range, otherwise it couldn't work.

Tom
K0TAR

Tom, K0TAR wrote:
"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

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 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.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:

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

Richard Harrison wrote:
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



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