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Re: [TowerTalk] Force12 EF-320

Subject: Re: [TowerTalk] Force12 EF-320
From: Jim Brown <>
Date: Mon, 8 Aug 2016 10:11:46 -0700
List-post: <">>
On Mon,8/8/2016 9:54 AM, Tom_N2SR via TowerTalk wrote:
With a W0IYH (?), RG-393 (?) with 100 beads, I have a FLAT VSWR across the 
entire band on both antennas.

That's not a great common mode choke because it's primarily inductive rather than resistive. But it doesn't interfere with the operation or matching of the antenna because it doesn't interfere with the 50 ohm impedance of the feedline.

That's important, and our CQP group learned that the hard way. To facilitate fast assembly of the Force 12 C3SS antennas that we use, W6GJB rigged the driven elements with a bifilar-wound common mode choke. The first one he used was wound with THHN, which, as W2FMI noted in a later version of his book, and I confirmed 20 years later, has Zo of about 100 ohms, while the same choke wound with enameled copper has a Zo close to 50 ohms. Tom, N6BT, used some rather interesting and unique matching methods in this series of antennas, and they like the 50 ohm choke, but really do NOT like the 100 ohm choke.

My advice, then, is to 1) use a choke that does not disturb the line impedance and 2) use a choke that provides a common mode Z of at least 3K ohms that is primarily resistive. Use my choke cookbook as a guide.

73, Jim K9YC


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