k2av at contesting.com
Wed Apr 12 09:17:45 EDT 2017
I have a much smaller non-powered version of this instrument, the MFJ-853,
that cost about half the price of the 854. The 853 works 300 ma, 1 amp and
3 amps. It has been quite adequate for seeing in what conductor, 100 watts
of 1.83 MHz is inducing current.
The 854 should be able to chase common mode current induced from a QRP TRX
running half power. That will allow you to use a portable rig plus an
induction loop to directly put power on the shield of a piece of coax and
then go see how much loss you ACTUALLY accomplish with your CM blockers and
other tricks, and then experiment to see what locations for your grounds
and CM blockers ACTUALLY reduce the current the most.
What I still see in the 854 is the cheeepy-cheeepy clamp on top. The clamp
is a coax clamp which is made to click closed ONCE and be left in place
forever. This was nearly impossible to open and reclose in the field. I
finally cut the nibs off the close catch and use a rubber band to hold it
closed. This one will be no different. Just cut off the catch nibs on the
clamp housing and leave a small rubber band around it. Loop back side of
clamp to front side of clamp, twist once and loop rubber band around bottom
of the meter. Klutzy, but quite effective and rubber bands are cheeep,
cheeep, cheeep and everywhere.
These RF ammeters are good for seeing low band induction into dB-sucking
miscellaneous conductors in the area of your 160m inverted L, wondering why
you can't get it to tune no matter what you do. (It's because 3/4 of your
current is in the miscellaneous conductor and you AREN'T tuning it.)
It is good for TX common mode current on feedlines, all of them, not just
the feedline going to the TX antenna.
For trying to measure RX feedline common mode noising up RX, one has to be
indirect and induce TX on some frequency to see what is happening.
Gimme a buck for everyone to whom I have described that who said "That's
too complicated" and you understand why everyone does not already have one
The idea of common mode current, that is TX RF current on **anything**
other than the TX antenna per se, as a dB-sucking pox on your TX signal,
just really has not caught on yet. IF it had, everyone would have one of
Most folks still think that as the SWR goes down, performance goes up. I
would remind folks that the first SWR meter I ever saw was on a CB set.
Crystal 807 transmitters and Johnson Valiants, Rangers, Adventurers,
Vikings, etc. were dip and increase load and redip. SWR? What was SWR?
Something CB ops did because they didn't know how to tune transmitters.
Things are so very much more complicated than SWR. Getting into defeating
common mode current TX and RX would be a really good step up from SWR uber
73, Guy K2AV
On Wed, Apr 12, 2017 at 1:46 AM, Jim Thomson <jim.thom at telus.net> wrote:
> Has anybody used a MFJ-854 current meter ?
> It will fit over .405 cables like 213 U, but nothing bigger, like
> is .425..and has 2 braids. So this device wont fit over cables like
> heliax etc.
> However, it comes with 6 x ranges, and will indicate current as low as 1
> Max scale is 3 A. Apparently, they come calibrated..so if clamped over
> 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
> Or on a multiband ant, it might improve on some bands, and be worse on
> 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.
> Jim VE7RF
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