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Re: [TowerTalk] Common-mode choke

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Common-mode choke
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Mon, 19 May 2014 07:08:20 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
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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