[Elecraft] Non-Resonant Antennas and Chokes

[Jim Brown]

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