[AMRadio] shunt


John Lawson 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 
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




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