[TowerTalk] Measuring resonance of a yagi element

[Tom Hellem]

TTers-
To all who participated in this thread:
 I received some valuable information from several contributors and learned
a lot. In fact, there were so many that it will take too long to thank all
personally so just saying thanks to everyone who replied to my post. It
certainly generated some interesting conversation.

Below I am including (with his permission) a pair of replies I received
off-line from Steve, VE6WZ, describing an important property of yagi
antennas that I never noticed before in all the hours of modeling I have
done, and it has been a lot. I thought others might appreciate reading what
he had to say, so here they are:

 On Apr 21, 2020, at 7:02 PM, Tom Hellem <tom.hellem at gmail.com> wrote:
> >
> > I would like to measure the self-resonant frequency of the reflector on a
> > home made 20 meter yagi. It seems obvious the best way would probably be
> to
> > open it up at the center and make it into a split dipole element and hook
> > up my analyzer. However I would like to avoid cutting it and dealing with
> > splicing it back together again. Can anyone suggest another way to
> > accomplish my objective? The reason for doing this is because it is
> > apparent from modeling in EZNEC that the
> > length of the reflector is critical to the performance parameters of the
> > antenna, e.g. gain, F/B, bandwith, etc., and it looks to me like it
> doesn't
> > take much of a change in the length of the element
> >
> > Thanks in advance for any replies.
> >
> > Tom K0SN
> > Missoula, MT
> > _______________________________________________



On Thu, Apr 23, 2020 at 12:01 PM VE6WZ_Steve <ve6wz at shaw.ca> wrote:

> Tom,
> Yes.  there is a MUCH simpler way to do this without climbing the tower.
>
> I have tuned a number of hombrew Yagis and vertical parasitic arrays, and
> used to think I needed to isolate the driver, then measure its resonance.
> That is not required.
>
> Because of mutual coupling, the REAL R (NOT IMPEDANCE) of the driver
> (feedpoint) will show a pronounced dip at the resonant frequency of the
> parasitic element.  This dip in real R is independent of the tuning of the
> driver.  You even can sweep the Yagi from the ground and it should still
> show a pronounced dip.  (it would be preferred to sweep it at the driver,
> or feedline corrected to the driver). Make sure to sweep well above and
> below the central QRG.  (like 13 Mhz to 15 Mhz.)
>
> Look at your antenna model to confirm this. In your antenna model, if you
> do a frequency sweep and look at the output, you will see the dip in the
> real R at the frequency that the reflector is tuned to.  On the tower, you
> want to see this same dip in R at the same frequency.  This effect is a bit
> more complicated on a multi-element Yagi, but on a 3 el Yagi, you will
> still see the dip in the R at both the Reflector tuned QRG and the director
> QRG.
>
> The mistake some make with this analysis is they look at the IMPEDANCE on
> the analyser.  That is wrong.  We see the dip in the real resistance.  ie.
> the R in Z= R+jX. Most analyzers will plot R vs frequency.
>
> 73, de steve ve6wz
>
Steve:

That is indeed useful information. I have done a fair amount of modeling
and never noticed that effect, but I just did a sweep in the model of my
2 el. yagi and you are absolutely correct. The only thing I am not sure
about is, if I put an analyzer (I use the FAVA-4) on the end of a random
length
of feedline coming down my tower from the antenna, and it shows a dip in
the R
at say 13.950, indicating the frequency of resonance of the reflector,
would I see
a pronounced dip in the R at the same frequency  if I measured it again
after changing
the feedline to some other random length?

In your experience, how accurately do you feel EZNEC can predict the
resonant frequency of a tapered element?
I guess if the answer is very accurately, then I probably should not waste
my time measuring it, just build
the antenna to fit the model and go with it

Thanks a million for your reply and input. It's definitely a keeper.

73
Tom

K0SN


  Tom,

Yes.
Just sweeping from the end of the coax in the shack should show the same
dip of R at the correct frequency.
(The *absolute* R *value* (in Ohms), and X *value* will not be correct
because of the feedline transformation, but the *frequency of the R dip
is ONLY a function of the reflector tuning*.)  Try it and see.

In the “early” days of tuning my 2 el 80m Yagis, I did the usual:

1.) Open the driver, then put the analyzer on the reflector element
2.) sweep the reflector (at full height) and measure resonance
3.) Compare to the model, where you have swept the reflector in the model
with the driver open (inset 1 E6 load).
4.) Retune the reflector and test again.

This is entirely un-nesseasry and this is still being used by many as a
method for tuning.  Far too complicated, and also subject to the effect of
the driver tuning.

When you sweep the driven element of any Yagi with the reflector in the
circuit, it will always show a pronounced dip in the REAL R at the
resonance frequency of the parasitic element because of mutual coupling.
This is *completely independent* of the driver tuning.  Think of this as
though at the parasitic resonant frequency, you now have 2 elements in
parallel.  This is like two resistors in parallel.  The resistance will
drop.  It has to because the drive energy is now being shared by 2 elements!

As for the accuracy of EZNEC for tapered elements, this is the benefit of
this tuning method!  It is independent of the specific element
characteristics because you are just tuning for the RELATIVE relationship
between the reflector tuning and the preferred centre SWR QRG.

Look at your model.  Notice the relationship between the *frequency* of* min
R dip*, and *max Gain* and *Max F/B frequency*. In a reflector model,
obviously these are above the min R dip.
The key is that YOU decide what frequency to tune the driver at.  This is
independent of the reflector tuning.  You can tune the driver (min SWR)
wherever you want.  This is just a matter of matching.

1.) If you tune the driver at the Max gain QRG, the feedpoint impedance
will be very low, and the 2:1 SWR bandwidth will be very narrow.
2.) If you tune the driver at the max F/B QRG the impedance will be higher
and the SWR B/W will be more broad.
3.) if you tune the driver well above the min R dip, the SWR bandwidth will
be very broad, but you will be sacrificing gain.

This is the design parameter that you play with to balance max gain, F/B
and SWR bandwidth for a given design with the modeled element spacing and
loading.

What is important is that using the model, you will see what frequency the
driver needs to be tuned for min SWR *relative to the reflector tuning*.
The tuning of the driver is accomplished with a L network, hairpin or gamma
match or whatever will transform the complex impedance to match your
feedline at your chosen QRG.  Tuning the driver is a separate process and
is independent of the reflector tuning.

So, on the tower to do a FINAL tuning of the Yagi:

1. Sweep the driver and find the min R dip
2. Adjust the reflector until it matches your preferred model min R dip
frequency.
3. Based on the model, tune the driver for the desired QRG relative to the
min R dip.  If the analyzer has been calibrated to the feed point, or is AT
the feedpoint, then you can build an L network or hairpin to match based on
the measured R+jX value.

Using this method I have with confidence tuned my 2 el 80m and 2el 40m
Yagis, and also my 2 el 160m parasitic array that uses the shunt fed tower
as a driver.

It is *entirely unnecessary* to do any field testing to “check” for max F/B
or max gain, since this method assures correct tuning.  You know with
confidence that the Yagi is tuned exactly X kHz above the reflector tuning,
which is what your model has shown to yield the parameters you want.

As i said before, the common mistake that guys make is they measure
IMPEDANCE instead of the real R with the analyser.

I explain how i did this in these papers I wrote for my 2 el 80-40m Yagi,
and the 2 el 160m Parasitic array:

80-40m Yagi: <
https://drive.google.com/file/d/1IksvND0LfeSBcH6j7UEWkyjkIvexDIgX/view>
160m 2el parasitic array: <
https://drive.google.com/file/d/1eVklkZtlJW93JIJ9oDqbCyl-Jkik6YEX/view>

73, de steve ve6wz