|[AMRadio] Thermocouple rf ammeter|
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.
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