Topband: Shunt fed towers and common mode chokes

[Jim Brown]

On 12/6/2012 12:39 PM, Steve London wrote:
> 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. 

Right.  5K Ohms is a good rule of thumb for choking Z to prevent noise 
coupling from feedline to antenna, and if the antenna is reasonably 
close to balance, is also enough from the point of view of dissipation. 
A very good choke for 80 and 160 would be 16 bifilar turns of #12 on a 
#31 2.4-in o.d. core (that is, 32 turns total), connected as a parallel 
wire transmission line. Tightly spaced enameled wire will yield Zo of 
about 50 ohms, THHN will be closer to 100 ohms.  Either presents a 
relatively small discontinuity (both because it's a small mismatch and 
because it's pretty short as a fraction of a wavelength).

There's measured data for chokes like these, and for a good range of 
others wound with RG8X and RG8 around multiple cores on my website, 
along with considerable tutorial material about how they work and 
dissipation considerations. In short, chokes overheat because their 
choking Z is too low to keep common mode current low. An important 
element of this is the common mode voltage, which is directly related to 
antenna balance, and also to the common mode length of the feedline 
(that is, a resonant length that establishes a high common mode voltage).

http://audiosystemsgroup.com/RFI-Ham.pdf

I strongly disagree with Tom about #31 -- it is an excellent suppression 
material for the HF bands, especially on 160 and 80, when used properly.

73, Jim K9YC