Varying Antenna Height was: Re: [TowerTalk] SteppIR 4 Element Yagi - Analysis and Thoughts

[Pete Smith <n4zr@contesting.com>]

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
The World HF Contest Station Database
is back, at www.pvrc.org/WCSD/WCSDsearch.htm  

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