[TowerTalk] Does Mast Destroy V-Pol UHF Beam Pattern??

[K7GCO@aol.com]

In a message dated 7/28/00 1:15:35 AM Pacific Daylight Time, i4jmy@iol.it 
writes:

<< 
 The mast doesn't necessarily destroy the pattern althought such type of 
 risk effectively exist and depends by multiple variables. 
 Same story apply to a coaxial cable that runs parallel to elements. 
 Therefore the fiberglass mast won't insure at all or always prevent 
 interactions and antenna pattern distortions.
 The best way to overcome the problem is to mount the antenna on a sort 
 of horizontal mast (like when horizontally stacking two antennas) that 
 results normal to elements.
 In this way, shifting the antenna half wavelenght (or more) from the 
 mast and making the coaxial line to follow the boom, the horizontal 
 mast and then the vertical mast, one can be quite sure to really avoid 
 the electrical problems.
 Concering the mechanical point, in the case the antenna weight is 
 consistent, some counterweight opposed to the antenna (why not a second 
 antenna...) could be required to avoid the vertical mast is subdue to a 
 lateral force.
 
 73,
 Mauri I4JMY 
  >>
Mauri is correct that a fiberglss most won't help much with the coax next to 
it if it's the same polarization and the beam.  Mount the beam all in one 
side of the tower with the coax laying on the boom with a reasonable spacing 
from the tower.  Advertisements for 144/450 MHZ antennas often show this and 
it's easy to do there and an progressively more difficult the lower the 
frequency but still mechanically possible with a little mechanical 
ingenuity..  Have a boom that tips at the tower and attach another beam for 
another frequency on the other side for balance.  Use beams with high F/B 
when other than horizontal polarization so they see the least of the tower 
behind it.  This way it's possible to have beams of any or multiuple 
polarizations with very little affect of the tower in particular on vertical 
polarizations.  I have a beam with 6 selectable polarizations that are 
virtually independent of the tower.  On 40-6M I've had great performance with 
vertically polarized yagi's and quads properly installed even with the 
increased ground reflection losses.  There wasn't any additional lift off of 
the vertilcal pattern other than the 180 degree phase shift for about 6 
degrees.  On salt water vertical beams are killer antennas due to the lower 
angle with very low reflection losses on the "Liquid Copper."  They open and 
close the band.  You often need a higher angle from horizontals inbetween.  
K7GCO

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