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[TowerTalk] "Double" V-Beam Question

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Subject: [TowerTalk] "Double" V-Beam Question
From: (L. B. Cebik)
Date: Wed, 19 Mar 1997 07:34:35 -0500 (EST)

Your proposal for a set of Vees, 4 wl long (560' legs) at 40 meters, each
50 degrees spread, is intriguing.  So I did a preliminary model of the 4
wires and tried various feeds at 7.15 MHz.  Although the model and the
data need refinement, here are the initial results.

First, let's set up 4 wires, each 560' long.  I left 1' center space to
create a feed wire segment to go between any pair of wires I wanted.  The
center line splits the wires into pairs.  So from "top" to "bottom" they
are 75 degrees above center line, 25 degrees above, 25 degrees below, and
75 degrees below.  This let's me speak in +/- terms about the results.
since you did not specify a height, I put the wires at 50' up.

If I feed wires 2 and 3, I get a main lobe on the centerline, with a rear
lobe, of course.  If I feed wires 1 and 2, I get a main lobe 50 degrees
off center through the middle of the fed Vee.  Same for wires 3 and 4, but
50 degrees the other way.

The patterns are almost completely independent, as if the unused wires
were not there.  For all 3 combinations on 40, gain is 9.5 dBi at an
elevation angle of 17 degrees.  The patterns are very wide, shaped like a
spreading tulip in both lobes.  The forward, rear difference is about 2.3
dB.  The patterns give a low TO angle and at least 7 dBi across the
"flower" surface so that switching among the 3 combination gives very good
coverage. Feedpoint Z about 450-1000 ohms.

You can also feed wires 1 and 4 and get a roughly N-S pattern (if we think
of the centerline as E-W), of about 6.5 dBi with just a little tilt toward
the wire side--a good fill pattern in case the other 3 patterns do not
quite reach a desired signal.  The elevation angle for this connection is
21 degrees.

If you connect wires 1 and 2 and wires 3 and 4 and feed that junction, you
get a roughly diamond shaped pattern at 19 degrees elevation with a peak
gain of about 7 dBi--working now almost like an omnidirectional vertical.

These are very preliminary results, and only for 40.  But they suggest
that the 4 wl on 40 idea is feasible.  The TO angle is surprisingly low
and apt for DX.  I suggest that you place a relay box at the spot where
the wires come in together.  You can use remote switching:  the simplest I
can think of is a separate switch and relays for each side of the line.
Then you can use Switch A to select wires 1 or 2 and switch B to select
wires 3 or 4.  You can add a further position to each switch to connect
1-2 together and 3-4 together, if the diamond-omnidirectional pattern is
of interest.  If you only want the individual Vee patterns, you can cancel
the reactance at the box and have a matched 450-ohm feeder--or use the
shack ATU to do all the work.  At least on 40 meters.

As I say, these are very rough models, with no runs on 160, 80, or 30.
But the 40 meter patterns give you gain that is 2-3 times a half square at
a comparably low angle or equivalent gain to a 2-element Yagi at the same
height, but with a much lower TO angle--and multi-band capability to boot.

Hope these notes are useful.



L. B. Cebik, W4RNL         /\  /\     *   /  /    /    (Off)(423) 974-7215
1434 High Mesa Drive      /  \/  \/\     ----/\---     (Hm) (423) 938-6335
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