Varying Antenna Height was: Re: [TowerTalk] SteppIR 4 Element
Yagi - Analysis and Thoughts
Pete Smith
n4zr at contesting.com
Wed Apr 14 09:32:19 EDT 2004
At 08:13 AM 4/14/04, Bernard wrote:
> I was wondering if you or anyone else on this thread have considered
>a single antenna (like a SteppIR) on a motorized tower where you could raise
>and lower the tower to optimize to the angle of arrival. I haven't seen any
>comments on this subject and I don't know if there are any computer programs
>that take propagation data, etc. and calculate the optimum height. I know
>that many "big guns" spend $1000s for that extra dB or two, but many of us
>don't have the real-estate for multiple towers and/ or antenna stacks.
The program HFTA, bundled with the current edition of the ARRL Antenna
Book, displays statistics for arrival angles derived from thousands of
IONCAP runs modeling propagation over an entire sunspot cycle on all HF
bands. It also calculates a figure of merit for each antenna system
evaluated, which is based on how well the antenna system's pattern matches
the distribution of arrival angles (I explained out this idea, then called
"scoring," in a National Contest Journal article about 4 years ago).
Going from this theoretical construct to an operationally useful matching
of antenna height to arrival angles is tricky. During a single day's
opening on 20, for example, arrival angles may vary by as much as 2 or
3:1. You would find yourself cranking the antenna up and down at least
twice per opening, as angles went from relatively low at the start and end
of the opening to relatively high in the middle of it. There would also be
significant angle variations depending on which part of a continent you
wished to work. For example, the optimum antenna height for working UAs at
1400Z could be a lot different than that required to work G's. And of
course, the "right" height for 20 is probably very different from the
"right" height on 10.
I suspect that on balance you are better off fixing the height of your
antenna where it gives the best compromise match to arrival angles on your
bands of interest, wherever that height is, and then adding other antennas
to deal with other special needs, even if they are just wire dipoles.
Just my two cents.
73, Pete N4ZR
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