[TowerTalk] xm240 install - interaction problems

T A RUSSELL n4kg@juno.com
Sun, 18 Oct 1998 06:40:36 -0600


Re:   Interaction of  tribanders and 40M Beams   de   N4KG

One  approach to reducing interaction of stacked Yagi's
is to rotate one of the booms 90 degrees to other antennas.
This may or may not eliminate interaction problems.
Remember,  BOOMS  are  NOT invisible. 

A 24 ft  Boom, end loaded by 20M elements (or tribander elements)
has a self resonance very close to 40M.  As a result, a TH7 boom
for example, may act as a 40M director, directly above or below your
40M beam, reducing both gain and F/B in the desired directions 
by skewing the antenna pattern upward or downward depending
on the exact resonant frequency of the loaded boom and the
relative antenna placement.

Many low banders have utilized this near resonance on 40M to
match the booms of their tribanders or 3L20 meter beams as
rotary dipoles on 40M.  

With parallel booms, it makes sense to also match the boom
of your tribander (with non-insulated elements) as a 40M 
rotary dipole.  Then it is a simple matter to merely switch from
the 40M beam to the 40M boom dipole to catch stations at
90 degrees to the beam such as SA / Carib when beaming
Europe, or VK / ZL when beaming JA.

de  Tom  N4KG


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