Topband: Tuning a 2el parasitic array

[Guy Olinger K2AV]

With the era of cheap VNA's it does seem time to move forward. Especially
with wire yagi's, you can get VNA's with smartphone style 5V USB charged
batteries that talk to a tablet running supplied software, communicating
with bluetooth. You can put the VNA right at the feed, and pull it up in
the air so ground effects are representative and can be watched with
changing elevation. None of the complications of the significant cable
between VNA and the feedpoint.

Zowie, is that way less work than the good ole days. Now I use my lab
calibrated FIM41 field strength meter to do before and after documentation.
And from time to make sure it's still performing by staying at the
benchmark. Not for making adjustments. As Steve mentioned, too much motion
for these old bones.

73, Guy K2AV

On Sun, Jun 21, 2020 at 3:14 PM VE6WZ_Steve <ve6wz at shaw.ca> wrote:

> Hi Rick,
> Yes you can field test for max F/B, but that is far from ”simple” and easy
> to do.  I have done it. Many times over the last 22 years.
>
> With a Yagi on the tower I used both an external source as well as an
> external RX in the field and tried to tune for max F/B.
> Using an external source I also plotted real-time polar plots of the Yagi
> pattern.
> However...In practice, here is how it goes:
> To check F/B….we rotate Yagi forward…record measurement, back to shack,
> rotate Yagi to back, record measurement.  Then guess if we need the
> parasitic longer or shorter… Climb the tower adjust element (or adjust
> lumped load at the element). test again. Guess again….longer maybe? Climb
> tower…adjust.  Meaure-rotate-measure again. Maybe we getting closer to
> optimum? shorter, test…longer test. Are we yet at the maximum F/B or can we
> get a bit more? Climb tower…readjust…etc. etc..  This is especially tedious
> and difficult with a narrow-band shortened low-band Yagi.
> I speak from experience. This method is a lot of work and can take many
> iterations and you will still not be sure.…been there done that!
> The other issue is aberrant ground reflections and wave angle
> considerations depending on your distance from the tower, Yagi height etc.
> My field measured polar plots often showed some funny things and pattern
> distortions.
>
> The method I describe in my video is MUCH easier, and leaves no doubt that
> the antenna is tuned as per your model. I have used it to tune both my
> 80-40M Yagis as well as the 160m parasitic array and subsequent on air test
> confirm they are optimally tuned.  In all cases tuning was quick and easy.
> I honestly cant imagine doing it by field strength testing!
> Understanding how the SWR bandwidth becomes more narrow as you approach
> the max gain point, and comparing this to your model, you can
> even “dial-in” the tuning more to be exactly where you want it.
>
> 73, de steve ve6wz
>
>
> > You should be able to tune up any 2 element array, whether parasitic
> > or driven, by simply putting a signal source behind it and adjusting
> > for max F/B.  Or putting the signal source in front and adjusting for
> > maximum gain.  No analyzer required.  Then,  you can substitute fixed
> components for the variable L's and C's if desired.  This method works even
> if the driven element is a shunt fed grounded tower.  Just use
> > the feed as it was when the tower was just a single vertical.
> >
> > After doing that, you can get out the analyzer and
> > measure the drive impedance of the phasing network and design a
> > simple matching network to go between it and the transmitter.
> >
> > BTW, parasitic arrays seem simpler, but driven arrays (especially
> > 2 element ones) have better F/B ratio ), and broader bandwidth,
> > AFAIK.  YMMV.
> >
> > 73
> > Rick N6RK
>
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