Topband: Shunt fed towers and common mode chokes

[Tom W8JI]

> My 160 meter shunt fed tower project is essentially done. However, I have 
> an issue with the 80 meter antennas hung off that tower. In a nutshell, 
> the current baluns (ferrite beads) feeding these antennas don't have 
> enough common mode impedance on 160 meters. They heat up, and the SWR of 
> the shunt fed tower changes as they heat up.

I'm assuming the 80 meter antennas are near the tower top, where 160 meter 
voltage is high.  If I was doing what you are doing, I'd use an entirely 
different concept and solution than the standard balun concept.

What you probably want is a self-resonant choke on 160 that has at least 
modest Q, so it has a very high common mode, and to isolate the switching 
box out away from the tower so the switching box and dipoles are independent 
of the tower.

> After some reading, I think what I need are some RG8X toroid baluns, wound 
> on #31 ferrite material. If they are going to replace the existing bead 
> baluns, then they will need to be placed at the feedline/antenna junction. 
> However, if I want to completely isolate the 160 shunt fed tower from the 
> 80 meter feedlines, shouldn't I place the new baluns as close to the 
> tower-mounted antenna switch as possible, and leave the bead baluns in 
> place at the end of the feedline to choke off the 80 meter common mode 
> energy ?

I think so, except for the idea of 31 material. The isolation ideally should 
be at the point where the feedlines exit the tower and go to the external 
antennas, and it has to be a very high impedance without significant 
dissipative losses. You would be much better off with a high-Q material and 
resonating capacitance.

> To add more complications for adding the RG8X toroid balun, my 80 meter 
> antennas are switchable 2 element wire beams, with each element fed with 
> 18' of RG-8. That dimension is critical, as it provides the proper amount 
> of capacitive reactance at the feedpoint to make the element a director.

Well, that complicates things a little bit...but not much. The switch needs 
to be outside the tower. Like this:

ant 1 or 2  = 18 ft feedline (includes balun cable length) =  switch

then from the switch (isolated from tower) you have:

Switch = control and coax through self resonant chokes ==== tower (grounded 
or bypassed  to tower) and then just down to the very base of the tower 
(where it should be grounded again) and out to the world away from the 
tower.

The control and coax choke can be bifilar wound affair (if you can't run 
control through the coax) and self resonant on 160.

73 Tom