[TowerTalk] Feedline (choke) question

[Jack Brindle]

The problem is in the way you connect it. If you use a PL-259, then you mix the outer and inner currents at the connector, thus “polluting” the signal going into the radio. But then the same thing will happen if you add a choke with connectors.

The ‘common” connection that we call ground carries all sorts of signals. We dump lots of signals into “ground” in our equipment (thing bypass capacitors). These all come together, and if any are square waves, like the oscillators on microprocessors, then we also dump the harmonics of those. You might argue that these are low level, but I learned in my days at Motorola that the 250th harmonic of a 1.8432 MHz oscillator will kill reception up in the 460 MHz region. I also suspect that lots of our local QRN issues have to do with just these sort of signals. Yes, grounding helps, but the signals are still there and will cause problems. Somehow I have managed to not hear these signals at my station (I have amazingly low noise levels), but I may not be looking for the right thing (or they just may not fall on frequencies of interest to me). 

Noting that basically all words will receive RF signals of all sorts of frequencies (including the coax shield) there is a lot to be filtered out.

Perhaps Brian is right in that the ground really does siphon off signals, but I have learned to place chokes at the antenna _and_ at the shack entry. Both make a difference. Still, I wonder what is mixing at each coax connector.

73,
Jack, W6FB


> On Sep 30, 2025, at 3:29 PM, Brian Beezley <k6sti at att.net> wrote:
> 
> "It just shows current going back and forth between the earth and the coax."
> 
> Dave, I don't know what you mean by back and forth. In my model, the ground wire simply has current. It is about three times that heading toward the transmitter. So it is diverting about 75% of the common-mode current before it reaches the shack. It's not as if the ground wire did nothing.
> 
> I suspect a good CM choke would be more effective than a ground. But if I had access to a decent ground outside my shack and I had CM problems inside, I'd try the ground first, as long as accessing the shield didn't compromise cable integrity. It might just solve the problem.
> 
> Brian
> 
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