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
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