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
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
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.
> Rick N6RK
Searchable Archives: http://www.contesting.com/_topband - Topband Reflector