Topband: Shunt fed towers and common mode chokes

[Steve London]

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.

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 ?

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.

Comments ?

73,
Steve, N2IC