Good info, John and something to think about. Shrader (Electronic
Communications 3rd Edition) seems to think a shunt is doable. However,
there is no mention in the test of how the shunt (coil of wire) would be
affected by radiation. A shunt would certainly limit the current
available to the thermocouple, but would the results be contaminated by
external RF? I think you're right, it would be a risky setup.
I want to measure the antenna current from a broadcast transmitter (1
KW) into a dummy load, of course. I realize a 5 amp thermocouple meter
should suffice, but I wanted a little headroom in the event of a load
disaster that could send the current soaring and destroy the meter.
Your suggestion of a sampling device would certainly do the trick and
probably with much better accuracy, however, I want to set it up as it
would have been in a broadcast station situation. Since I'll be running
much less than 1000 watts I'm sure, after rethinking it, the 5 amp RF
meter will do fine.
Thanks for the info and comments.
Rick/K5IZ
John Lawson wrote:
I think there might be a couple of things to consider in such a
scenario.
One is - for most 'regular' meter shunts, they are simply a very
low-ohm calibrated power resistor. And generally the drop accross them
at full-rated current (50 amps, say) is measured in the millivolt
range: 25 - 20 - 100 - 300 etc.
RF Ameters, on the other hand, use a thermocouple element which is
heated by a small resistance wire through which the RF passes - there
is no 'drop' measured electrically, rather the temperature rise of the
resistance element is used - so the 'cuurent' meter is actually
reading a few MVDC coming from the TC junction in the "shunt" (which
it's not).
If you insert another actual resistance-type shunt around a
thermocouple unit, it would have to still drop enough RF to provide
the full-scale EMF accross the heating wire. So if the the meter is 5
Arf full-scale, your shunt must drop a 5th at 25 Arf.
(HEY!!! Just what are you doing on the air that need 25 fargin' AMPS
of antenna current...?) ;}
The other thing I'm wondering is - what effect all that Stuff (in the
feedline, basically) is going to have on SWR - production of harmonics
and spurs - etc.
I'd be tempted to just build a simple pickup like the MFJ
Balanced-line ammeter has, and use a DC opamp driver to provide
full-scale readings in the ammeter of your choice.
Just my 200 millidollar...
Cheers
John KB6SCO
