[TowerTalk] Common-mode choke

[Jim Brown]

On 5/19/2014 4:45 AM, Steve Hunt wrote:
> You say you want maximum common-mode impedance at 28 MHz. The best 
> I've achieved is around 15,000 Ohms using 9 turns of RG58 on an 
> FT240-61 core, Mouser part 623-5961003801. Replace the RG58 with a 
> PTFE coax if you want to run QRO

I don't trust any impedance measurements of chokes wound on #61, 
including my own, simply because the Q is so high. Even the slightest 
stray capacitance moves the resonance, and it's the resonance that we 
need. I don't recommend #61 for chokes below 100 MHz, and neither does 
Fair-Rite. Their #61 toroids are listed as inductive parts.

There are part numbers for #31 cores in Appendix One of 
k9yc.com/RFI-Ham.pdf, and specific winding instructions in the Choke 
Cookbook section of that same pdf.

Refer to the online Fair-Rite catalog for part numbers for #43 cores, 
which are a bit less expensive, and a bit better on 10M. You may get 
better pricing from Dexter Magnetics (near Chicago) or Lodestone 
Pacific, simply because they specialize in magnetic materials for 
industrial customers.

73, Jim K9YC