jim.thom at telus.net
Wed Apr 12 01:46:27 EDT 2017
Has anybody used a MFJ-854 current meter ?
It will fit over .405 cables like 213 U, but nothing bigger, like 214-U..which
is .425..and has 2 braids. So this device wont fit over cables like LMR-600,
However, it comes with 6 x ranges, and will indicate current as low as 1 ma.
Max scale is 3 A. Apparently, they come calibrated..so if clamped over the
center conductor of coax, feeding a dummy load, you will see 1A with 50w
applied. There is a calibration pot inside to tweak it, if required. The lower
3 scales use the op amp.
I can see where the device would come in handy, like perhaps testing different CM
chokes, and on various freqs. Of course, CM current will vary on the coax, depending
on where you clamp the device. But assuming you clamped at a given place on the coax,
then fed 100w cxr into the ant, measured the CM current, at least you would now
have a baseline current reading. If its a multiband ant, fed with a single coax, then also
measure CM current on each band.
Then perhaps swap, modify, tweak winding spacing, or change the CM choke in question,
then repeat the exact process, clamping at the same place on the coax, and running the same power.
Then you should be able to see if the new CM choke improved or made things worse.
Or on a multiband ant, it might improve on some bands, and be worse on others.
The idea here was if the goal of the CM choke is to minimize coax CM current, why not just measure the
actual CM current ? I can see this device being handy around the shack, and home, since it could be
clamped onto anything and everything, from coax to power cables, to stereo gear, to power supplies,
and anything else you can think of that rf current could ingress into.
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