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[TowerTalk] 40-2CD Spacing and Interactions with WARC?

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Subject: [TowerTalk] 40-2CD Spacing and Interactions with WARC?
From: (L. B. Cebik)
Date: Thu, 1 May 1997 10:26:53 -0400 (EDT)
I have a model (actually several) of the 40-2CD.  It cannot be considered
fully accurate, because the structure has a complex geometry of wires of
different diameters, and not just in terms of stepped diameter linear
elements.  However, trends in the modeling may be useful as indicators.

The 18 MHz resonance is real, although its precise location in the
spectrum may elude the model.  The operating bandwidth is quite wide--at
least 0.8 MHz in that region.  However, at the lower frequency edge of
the bandwidth region, reactance climbs very rapidly and the pattern goes
to pot.  Resistance in the 70-80 ohm range, with tolerable reactances,
should allow 50-ohm SWRs from 1.5 to 2.0 over the specified region.  The
model was examined at a height of 70' and in free space.

Whether the antenna performs well is a user judgment.  Models show a
guitar body pattern shape (nothing to fret over), but modeled gain and
front-to-back ratio are quite low.  About 3.5-4 dB F-B.  Gain is less than
a dipole by a good margin, according to the model, but gain figures are
suspect.  It is quite likely that even a 2-element 18.1 MHz beam will
outperform the 40-2CD on its 3rd harmonic in clearly detectable ways.
Tracing the resonances of the antenna suggests that the analyses of the
antenna as having a lowered 3rd harmonic resonance by virtue of the
loading are correct.

Until I can establish some reliability to NEC-4 models with complexities
of geometry using different diameter wires, I shall not generally
distribute gain figures for any commercial beam.  However, I believe the
trends noted here are reliable.

I hope this is useful.



L. B. Cebik, W4RNL         /\  /\     *   /  /    /    (Off)(423) 974-7215
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