At 6:17 AM -0700 8/20/03, Jim Lux wrote:
>Hmmm.. is a quarter wave actually how far apart you want them? I
>don't know that putting two chokes at 1/4 wave apart guarantees that
>one is at a high current place in the line....
I never said that it did. However, when you insert a choke at one
point along the line, the common-mode-wave reflection from this choke
causes a current minimum to appear there, so a current maximum
appears one-quarter-wavelength away.
>And how far is a quarter wave, since the propagation velocity of that
>"transmission line" is going to vary somewhat, for the same reasons
>that the Z does.
>...I suspect that empiricism will rule the day here. One probably
>needs to come up with a way to measure the common mode isolation....
Yes. I use clamp-around RF current probes to measure common-mode
current, and a simple bridge to measure the complex impedance of a
common-mode choke. It sure beats working in the dark.
>Let's consider why you're doing all this choking in the first place:
>a) to reduce the perturbations from feedlines on the antenna pattern....
OK, although it's not _my_ primary concern. Maybe for others.
>b) to reduce imbalance....
Yes, although that's a tautology.
>c) so that you don't get common mode induced currents
>coming in from the near field.
I would add
(d) to keep transmitting RF current from flowing into my house and
bothering my telephones, alarm system, audio and video entertainment
systems, computers, etc.; and
(e) to keep QRM and QRN from the electronics etc. in my house from
being conducted out to my antenna.