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
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