Topband: Shunt fed towers and common mode chokes

[Steve London]

On 12/07/2012 08:29 AM, Tom W8JI wrote:

> In this case the original poster has a two-dipole 80-meter antenna
> attached high on a tower. He needs to isolate common mode 160 meter
> tower antenna currents and voltages from that array.
>
> 1.) He clearly stated electrical length (and by definition that also
> means impedance) of the transmission lines to the switch was critical,
> because open circuit reactance of the line tunes the unused element as a
> director.

I could go with higher Vf transmission lines, which would give me a few 
extra feet to play with for winding around toroids, etc.

> 2.) Worse yet, the antenna connection or isolation point is almost
> certainly at a very high voltage point.
>
> Assuming this array is at 100 feet on a typical 130-foot tower,

That's pretty close. It's a 110 foot tower, with a 4 element 40 (linear 
loaded) on top, and a KT-36XA at 75 feet. These two antennas have their 
feedlines isolated from the tower, and the feed methods at the antennas 
are isolated from the booms, so there is no low Z common mode path, and 
neither balun seems to be heating. The two-dipole 80 meter array is at 
105'. Isolating the switching network could be challenging. It's a large 
metal box containing not only relays, but several big air variable caps 
and inductors. Some kind of polycarbonate or other material ?

73,
Steve, N2IC