[TowerTalk] Feedline (choke) question

[Jack Brindle]

Why sever the connector? If we are talking about RG-213 size coax, that is .405 inches in diameter. A PL-259 shell is 3/4” in diameter. Thus you need a bit over 1 9/16” of space to be able to thread the cable through a toroid of suitable size. A 3” die toroid might do it, but a 4” core certainly will. It will give better results with fewer turns to boot. This is a much better solution that beads or coiling cable, which was pushed by Hy-Gain, Bushcraft and others back in the day, but now proven to be ineffective.

The 4” cores are slightly expensive, but available. Our friend KF7P will sell you one in mix 31 for $28.95.

73,
Jack, W6FB


> On Sep 26, 2025, at 10:04 AM, Brian Beezley <k6sti at att.net> wrote:
> 
> On 9/25/2025 8:27 PM, David Gilbert wrote:
> 
> 
> "3.  A common mode choke, like any other inductor, isn't going to have much effect at a current minimum, so placing common mode chokes at current maximum(s) along the coax seems like it would be good additional protection ... and they aren't necessarily located at either end of the coax.  Note that this would be the current maximums for the common mode current, not necessarily for the differential current.  It would probably require a field probe to find those points if they exist."
> 
> 
> Dave, I don't think it would be hard to locate suitable points with an impedance probe or RF ammeter. But adding a choke normally requires severing the coax and installing connectors. I wonder if anyone has looked into using several large split ferrite beads clamped over the coax for this application. Even if their impedance was less than that of of a good toroidal choke, they might solve the problem. I know that CM chokes used to be made this way, but I don't know their impedance properties.
> 
> Brian
> 
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