rickbras at airmail.net
Fri May 5 12:13:44 EDT 2006
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.
John Lawson wrote:
> I think there might be a couple of things to consider in such a
> 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...
> John KB6SCO
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