[TowerTalk] Test Fixture for Common Mode Chokes

[David Gilbert]

Yeah, I assumed one at the feedpoint.  I was referring to the situation 
where you might want to add another one.  I still think that should go 
at a current maximum if possible.

Dave   AB7E


On 1/25/2026 8:44 PM, Jim Brown wrote:
> 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
>
>
>
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