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[TowerTalk] W0IYH Feed line Choke Performance

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Subject: [TowerTalk] W0IYH Feed line Choke Performance
From: (Chuck Counselman)
Date: Wed Aug 20 13:44:43 2003
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.

Sure, "somewhat."

>...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.

-Chuck, W1HIS

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