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[TowerTalk] Wire beam updates and a note on the 12-17 opposing beams

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Subject: [TowerTalk] Wire beam updates and a note on the 12-17 opposing beams
From: L. B. Cebik" < (L. B. Cebik)
Date: Sat, 6 Sep 1997 08:25:11 -0400 (EDT)
I have updated the "Notes on lower-HF wire beams" at my site to include
some interesting design ideas provided to me and used by permission of
Carrol Allen, AA2NN.  He has refined 40-meter reversible Yagis and Moxon
rectangles by using equal elements but loading one with an inductively
reactive length of transmission line.  Actually, there are lines to both
elements, but the driven element sees the line as just another length of
regular transmission line.  The system is similar to that used with the
parasitical delta of K1KP further on in the note, but makes data available
for the Yagi and Moxon configurations.

I have been asked whether it is feasible to place the two antennas in the
same plane using the open-sleeve coupling system for a single feed line.
The answer is yes and no.  Yes, it is possible to develop a set of
dimensions that will produce good performance at a desirable feed
impedance for one frequency within the upper band.  (The lower band is not
affected.)  However, both elements for the upper band are inside the
elements for the lower band, producing a "cage" effect.  The chief problem
created is an extreme narrow bandwidth for desireable characteristics.  In
terms of this design, when the beams face in opposite directions, they
show an operating bandwidth for under 2:1 SWR in excess of the 100 kHz
width of the bands.  When caged (or facing in the same direction, the
operating bandwidth of the upper band beam is far less than the width of
the band.  For building and tuning, this also means that hitting the
precisely needed dimensions is very tricky--and the settings are very
susceptible to the need for change with changes of antenna height below
about 1.5 wl.  Hence, my recommendation for opposing directions, where the
upper band antenna is "uncaged."  (Yes, I know, I watch too many wildlife
documentaries and it is affecting my choice of terminology.)



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