[TowerTalk] Tall verticals

Tom Rauch W8JI@contesting.com
Thu, 11 May 2000 07:54:00 -0400

Hi Tom

> My TH7 looks like approximately 40 ft of additional
> effective height on top of a 40 ft tower.  If your
> tribander is equivalent, then your tower is at least
> equal to a 115 ft vertical, ignoring what the lower
> beam does to the current distribution.

We should be careful about converting a lumped load into a 
"additional length".

Sometimes that works OK, sometimes it doesn't.

A TH7 may indeed look like an additional 40 ft on your tower on 
160 meters, but it REALLY appears as a capacitor in series with 
some inductance loading the self-impedance of the structure it is 
mounted on. 

On a different frequency, or on a different diameter tower, it could 
appear as a greatly different "additional height". For example, if 
your tower was very fat, the TH7 would have much less effect on 
length. If the tower was thinner, it would "add more height" than 40 

On 80 meters, on your tower, it might add what would be the 
equivalent of 60 feet.

I think the best solution is to model the structure.

73, Tom W8JI

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