jpl15 at panix.com
Fri May 5 10:42:02 EDT 2006
On Fri, 5 May 2006, Rick Brashear wrote:
> Has anyone made a meter shunt for an RF ammeter? I see no reason it would be
> different than a regular AC or DC meter shunt, is that correct? I have a 5
> amp RF thermocouple meter and would like to extend it to about 25 amps.
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...
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