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[TowerTalk] Multiband dipoles

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Subject: [TowerTalk] Multiband dipoles
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Date: Thu, 25 Oct 2001 17:00:36 -0600
There are several approaches to making multiband dipoles.

Two dipoles at right angles to each other will have almost
ZERO coupling / interaction.

Three dipoles spaced 60 degrees to each other have slight 
coupling / interaction.

Using traps for several bands reduces efficiency and bandwidth.
One way to minimize this effect is to use 2 dipoles and split
the traps, say 10 and 20M traps plus 80M and 15M traps plus
40M.  Then separate the ends as far as possible.

Force 12 uses 'open sleeve' coupling for their driven elements
on their triband beams.  K9AY published an article on a 3 band
wire dipole for the WARC bands in RF Design several years ago
(Nov 1994).  He suggests conductor spacing (d) according to
this formula:   

Log (d) = 0.54 x Log (D/4)   where D = conductor diameter  

(assuming equal D for each element).

One of my favorite multiband dipoles is simply a 20M dipole
(17 ft each side) fed with ladderline and a balanced tuner.
This antenna has a figure 8 pattern from 10 through 30 M.
Beamwidth varies from 50 degrees on 10M through 90 degrees
on 30M.  30M performance can be improved slightly by using
20 ft on each side with the addition of 4 minor lobes on 10M
and slightly more narrow main lobes on all bands.

An 80M dipole can be used on 80, 30, 17, and 12M fed with
coax since the center feedpoint corresponds to a current maximum
on those bands.  Patterns correspond to a 3/2 Wavelength (WL) 
Long Wire on 30M,  5/2 WL  LW on 17M, and 7/2 WL  LW on 12M.
SWR < 3:1 on 30M, and <2:1 on 17 and 12M.

With ladderline feed, an 80M dipole can be used on all bands
from 80 through 10M.  It will act as various types of long wires
on 30 through 10 meters.  The feedpoint corresponds to voltage
maximums on 40, 20, 15, and 10M and current maximums on
80, 30, 17, and 12M.

Tom  N4KG

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