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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