[AMRadio] Thermocouple rf ammeter


John Lawson jpl15 at panix.com
Sat Feb 10 10:58:27 EST 2007



On Sat, 10 Feb 2007, A.R.S. -  W5AMI wrote:

>> 
> Thanks Bob.  Well, it looks like I'm missing that.  All I have is the
> meter, by itself in a box.

   Usually in the tiny fine print on the meter face, down at the bottom - 
it will give the meter parameters - "5.0 ma fs" for example - the scale 
says 0 - 50 amperes DC, but it's made to work with an external shunt, 
which , when 50 amps is flowing through it produces a current of 5 mA, 
which goes to the meter.  Many RF Ammeters will have "ext t/c" somewhere 
on the scale plate, or the back.

   Most RF Ammeters have an external device with a wire that the antenna 
current flows through, and a thermocouple in contact with it - the meter 
actually reads the few millivolts that the thermocouple makes.

   If you hook such a meter directly to the feedline, some interesting 
processes will then occur inside it when you transmit - none of which will 
be 'good' for the meter....  ;}

   I do have one 0-1A RF Triplett meter here with a built-in TC, but the 
rest are all external TC-type units.  And, of course, no-one thinks to 
keep the items together, and they slowly drift apart - the meters in one 
box, the TC in another...   etc.


   MFJ makes a cool little crossed-needle dual RF ammeter - I have one here 
in the 450-om ladder line - it works well and does what it's supposed to 
do.   MFJ-835 is the model.


  Cheers

John
KB6SCO
DM09gh




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