R: [TowerTalk] Tall verticals

[Tom Rauch]

> 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?

Sure, model the tower and antennas on a modelling program. It 
isn't necessary to get the antennas modeled exactly, just get them 
close.

Then insert a source near the bottom of the tower, and look at the  
elevation pattern. If the tower is electrically too long, it will show up 
as a high angle radiator. Even in that case, you can still feed an 
insulated four wire cage surrounding the tower as a vertical. 

I quite successfully (even working VS6DO on 160 SSB several 
times from Ohio) used a 140 ft Rohn 55 tower with a full size two 
element forty, and a five-element twenty fifteen feet above it (and 
yes, the 20 meter antenna makes a BIG difference in operation of 
the tower on 160).

That tower was self-resonant just above the middle of the AM BCB. 
You can feed absolutely anything, but the elevation pattern that 
might hurt performance if the tower is self-resonant too far below 
the operating frequency. The tower most certainly does not need to 
be resonant to radiate efficiently, as long as the feed system 
matches the impedances.  

     
73, Tom W8JI
w8ji@contesting.com

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