R: [TowerTalk] Tall verticals

[Pete Smith]

At 09:59 PM 5/10/00 +0200, Maurizio Panicara wrote:
>
>It's practically impossible to prevent the beam from (top) loading the
>tower.
>Even isolating the yagi antenna from the tower, its coaxial cable will run
>along the tower and will consequeltly follow the current distribution of the
>antenna.
>The yagi, isolated or not from the tower, will consistently load the
>vertical structure as much as the horizontal antenna is approaching (or
>close) to an high voltage point of the vertical radiator.

Interesting.  My tower is 30m of Rohn 25 with a Force 12 C-3 and an EF-240S
(total about 1 square meter of wind area) at the top.  I'm planning to add
another C-3 (.5 sq. meter) side-mounted at the 20-meter height point on the
tower to stack on 20-10.  

I would also like to shunt feed the tower on 160.  Is there any way to
ascertain analytically whether the electrical length of the tower or the
loading at 20 meters above ground will cause serious complications so far
as the impedance to be matched, and the height of the shunt-feed point?
I've looked at ON4UN's book for guidance on  estimating the loading effect
of antennas on a tower, and have followed this discussion, but don't know
whether the bottom line is that the shunt feed is just a bad idea, or
whether it can be done practically.  I lack the engineering skills to do
the analysis.

73, Pete Smith N4ZR
n4zr@contesting.com 



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