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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 15:19:20 2003
At 11:21 AM -0700 8/20/03, Jim Lux wrote:
>At 01:44 PM 8/20/2003 -0400, Chuck Counselman wrote:
>>...I use clamp-around RF current probes to measure common-mode current....
>Excellent idea... What does your probe look like, and is it 
>broadband or tuned?

Actually, I have three RF current probes that I've used for this 
purpose.  All are broadband; one uses a snap-around ferrite toroidal 
core; one uses a continuous ferrite toroidal core [so you must thread 
your cable(s)/wire(s) through it]; and the third is a homebrew, 
electrostatically shielded, 3" x 11" rectangular loop that I place 
beside the subject cable/wire, coplanar with the subject cable/wire, 
so the B-field lines encircling the cable/wire cut perpendicularly 
through the loop.  The voltage induced in the loop is proportional to 
the time-derivative of the flux of B through the loop.

The first two probes are commercially made, surplus, bought on eBay 
for not much money.  They are calibrated.  Each delivers a voltage 
(to a 50-ohm load) which is proportional to the current in the 
subject line, and virtually independent of frequency over a wide 
range.  My homebrew probe is not calibrated, although calibration 
would be easy to do.

The probe with the snap-around ferrite toroidal core was made by 
AILTech; its model number is 94106-4; and I have its manual as a .pdf 
file on my hard disk and will email this file to anyone who asks.

The probe with the continuous ferrite toroidal core was made by 
Pearson Electronics.  Its sensitivity is 0.05 V/A.  I don't recall 
its model number or its frequency range; it's up in my attic, where 
the temperature right now is probably 140 deg F.

The homebrew loop is extremely useful in conjunction with a 
battery-powered portable receiver for quick surveys to find 
common-mode "hot spots."

>I wonder how well a length of iron pipe would work as a choke?

Nowhere near as well as ferrite.  Ditto for steel wool.

-Chuck, W1HIS

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