[TowerTalk] Testing for unbalanced parallel feedline

[Mark .]

Tom Wrote:

>You really should measure current balance at two points on
>the line by using a clamp-on meter. I know MFJ sells one I
>designed that is calibrated in amperes. It should be OK
>since it is surface mount parts, assuming they haven't
>mucked it up.
>
>Measure current in each conductor by clamping around each
>conductor one at a time and record it. Measure the vector
>sum of currents in BOTH conductors by clamping around both
>conductors at the same time.
>
>Some people think you can do this with light bulbs or
>measuring current in each conductor alone and NOT the vector
>sum where both conductors are clamped inside the probe, but
>you really can't. I'd do this at the tuner and 1/8 to 1/4 wl
>away if possible.
>
>73 Tom W8JI

Thanks, Tom;

Let me be sure I understand this.

1) If the meter's probe is large enough to encircle both conductors, do so 
and measure. The better the balance in current, the closer the meter reading 
will be to zero.

2) If the probe can only encircle one conductor at a time (such as with 
homebrew 600 ohm line), measure both at the exact same distance along the 
line. The better the balance in current, the closer the *difference between 
the two readings* will be to zero.

Now for the fun part. I have modified my field strength meters to have 
female BNC signal input jacks. Can I homebrew a probe from a split ferrite 
bead and a few turns of magnet wire around one half of the bead and plug 
that into my field strength meter? I realize it won't be calibrated, but I 
can perform relative tests. Using such a homebrew probe, if I can 
demonstrate a a meter deflection when measuring current from one conductor, 
then encircle both conductors and measure again, wouldn't I be able to 
verify proper balance if I get little or no meter reading?

I've been wanting to build something like this for checking for RF on the 
outside of coax shields.

Thanks for your help,

--...MARK_N1LO...--