[TowerTalk] Test Fixture for Common Mode Chokes

[Jim Brown]

On 1/25/2026 3:08 PM, David Gilbert wrote:
> Yes ... but I assume that we want to place the CMC as close to a current 
> maximum as we can, so we kind of need to know where that is.  Am I wrong 
> about that?

In general, no. In the common mode circuit, the feedline, whether coax 
or two-wire, becomes part of the antenna. The number one reason for 
using a choke is to prevent current on the antenna from coupling to the 
antenna, that mostly happens at the feedpoint. We want it to look as 
much as possible like an egg insulator at the frequency(ies) of 
interest. This current consists of both signal and noise. It can, for 
example, fill in the nulls of directional antennas.

W6GJB's contesting trailer, which we use for expeditions to activate 
rare counties in state QSO parties, has a 45 ft repurposed pneumatic 
mast (probably from a TV news truck) on which Glen has rigged a 20M 
yagi, a 40M dipole, and an 80M inverted Vee. We run the 40M dipole on 
15M. The feedpoints of these antennas are within about two feet. With 
chokes, RX bandpass filters, and optimally placed double stubs, we can 
run two K3/KPA500 CW stations on bands related by their second harmonic! 
That is, 80 and 40, and 40 and 20. Remove one of those chokes and we 
cannot. We learned this when Glen replaced one of the feedlines (for 
maintenance) and forgot to install the choke.

In a multi-transmitter environment, "you can't give anything away!"

73, Jim K9YC