At 12:17 PM 2/24/07, Cqtestk4xs@aol.com wrote:
>
>In a message dated 2/24/2007 4:25:59 A.M. Greenwich Standard Time,
>jimlux@earthlink.net writes:
>
>The best way to answer these questions is to get a copy of HFTA,
>which comes with the ARRL Antenna book. It lets you model the pattern
>of stacked antennas, varying spacing, etc., and factoring in your
>terrain, as well. You just select the kind of antenna (e.g. 5
>element beam),
>
>
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
>But there is a rub to doing that, isn't there? I think the program assumes
>a standard length boom for 3 el and a standard length for a 4 el and so on.
>If one uses the 4 element STeppIR for 10 will it not behave more like a 6
>element in a stack since it has a boom which is what a usual 6
>element has? I
>believe the boom on the SteppIr is around 32 feet and the usual
>6 el 10 meter
>is around 27.
>
>What this means to me is the antenna will "behave" more like a 6 el in
>stacking since the boom length is a primary consideration in
>stacking distance.
Consideration of stacking distance on the boom length comes about
from trying to maximize free space gain by minimizing interactions
when using a yagi design that has fixed element lengths and spacing.
As has been said, one really needs to go back and optimize element
lengths and spacing for the stack as a whole once the the optimal
heights/spacings are determined.
HFTA uses a model a non-interacting antenna. At HF where the spacing
of a stack is driven by achieving a desired pattern over terrain, the
overall system gain from HFTA will be systemically wrong but the
pattern, in particular the low angle pattern, will in general be
accurate unless one tries pathologically close spacings.
So its HFTA first to get the heights/spacings, then back to NEC, YO,
etc. to tweak the lengths and spacings for that specific stack.
73,
Mike K1MK
Michael Keane K1MK
k1mk@alum.mit.edu
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