Best kept secret around, but since modeling can be a pain on certain
days, noone wants to do ALL the math before bolting in the stacks.
Modeling all the stuff around, including booms and guy wires and some
alternate orientations is a lot of work, and in some cases the number
of segments to represent the entirety exceeds the limits of
"ham-grade" programs.
Speaking for myself, I've NOT found even ONE stack configuration that
did NOT show interaction of the type you describe. Even if the DE is
deleted from the model, the other elements do things.
What's also interesting is to model it with the boom in the model and
one antenna rotated 90 degrees...
Keep preaching.
73, Guy.
----- Original Message -----
From: "Pete Smith" <n4zr@contesting.com>
To: <towertalk@contesting.com>
Sent: Friday, May 30, 2003 10:03 PM
Subject: [TowerTalk] Feeding one antenna in a stack
> I've been doing some modeling of various stack configurations, to
explore
> the effect of various feedline options, and I thought the results
might be
> of interest.
>
> The basic stack is a pair of Force 12 C-3Es at 97 and 69 feet over
average
> ground. At 14.1 MHz, using NEC-2, the stack shows gain of 14.43 dBi
at 11
> degrees takeoff angle, when the two Yagis are fed in phase.
>
> If you feed the bottom antenna only, with no feedline on the top
one, the
> model shows 10.38 dBi at 17 degrees. This result is strikingly
less, and
> at a higher takeoff angle, than the same antenna shows, at that
height,
> without the second antenna in the near field (12.06 dBi at 14
degrees).
>
> Next I simulated a 20-foot 50 ohm feedline connected to the top
> antenna. With that feedline shorted at the far end (for example,
at a
> stack switchbox), the bottom antenna showed a gain of 11.81 dBi at
15
> degrees. With the same length of feedline open at the switchbox
end,
> the bottom antenna showed a gain of 12.24 dBi at a takeoff angle of
13
> degrees.
>
> Now clearly, this result can't be extrapolated to other antennas.
The
> C-3Es have an open cell feed, and are short-boom two-element
designs. What
> it DOES suggest is that it isn't sufficient to consider one antenna
of a
> stack as if it's alone up there, even if you're only feeding that
one
> antenna. What you do with the feedlines of the other antennas can
make a
> real difference.
>
> I'm planning to model some monoband stacks next, and will report
results,
> if there's interest.
>
> 73, Pete N4ZR
> The World HF Contest Station Database was updated 9 May 03.
> Are you current? www.pvrc.org/wcsd/wcsdsearch.htm
>
>
>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers",
"Wireless Weather Stations", and lot's more. Call Toll Free,
1-800-333-9041 with any questions and ask for Sherman, W2FLA.
>
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