On 1/14/2019 6:32 PM, Wes Stewart wrote:
You do realize that if the common-mode current on the line is due to
asymmetry between the antenna and the line, then choking at the
feed-point reduces the CM current at that point, but just like your
example said, a quarter wave *down* the line you have a CM peak.
Yes, but the choke at the feedpoint prevents that common mode current
coupling to/from the antenna, which is how it gets inside the feedline
as a differential mode signal. And the common mode current on the line
below the choke is that induced by the field from the antenna, not from
any imbalance in the antenna.
If asymmetry isn't an issue then open wire is just fine with a balun
at the tuner and I say this while not being a fan of open wire fed
antennas.
And it's also not an issue if you have no local noise sources, a luxury
that I think you said you have.
I'm also somewhat bemused by the cottage industry that's sprung up in
the last few years to calm the hand wringing about common mode
current. I think there's been a lot of development of cures that are
looking for ailments. In my curmudgeonly opinion of course :-)
If you had local noise you might view things differently. :) FWIW, I
wasn't the first to realize the relationship between common mode current
and RX noise -- I picked up on it from a long treatise that W1HIS wrote
around the time I was doing my work. And I contributed pieces of the
puzzle that he didn't know about, most importantly, the equivalent
circuit of a choke and the importance of a high resistive impedance.
73, Jim K9YC
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