[TowerTalk] Feedline (choke) question

[Joe Subich, W4TV]

On 2025-09-26 11:04 AM, Brian Beezley wrote:> 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.
Per the Fair-rite data sheets ...

The impedance of a single 1.126" long, 0.40" ID Mix 31 clamp on is
105 Ohms @ 10 MHz, 150 Ohms @ 25 MHz and 221 Ohms at 100 MHz
<https://fair-rite.com/product/round-cable-emi-suppression-cores-2631626402/> 


The 1.26" x 0.55 Mix 31 clamp on is 110/163/261 Ohms
<https://fair-rite.com/product/round-cable-emi-suppression-cores-2631102002/> 


The 2631626402 is equivalent to the 0431167281 clamp on ...
the 26311102002 is equivalent to the 0431164181 clamp on.

One would need at least a dozen clamp on cores to achieve a
significant (> 1K) choking impedance.  One *might* be able
use 1" ID clamp on cores to do two turn chokes but a single
two turn instance would only yield 600-1200 Ohms (10-30 MHz)
so one would still need 5 or 6 of the "large" clamp-on cores
<https://fair-rite.com/product/round-cable-emi-suppression-cores-2631626202/> 


All of this is simply from reading the data sheets and not
verified by measurement.

73,

    ... Joe, W4TV

On 2025-09-26 11:04 AM, Brian Beezley 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
>