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 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
