In message , coffelt2
writes

"Major Samwells" wrote in message
...
I'm trying to get some accurate power readings with a low power rig
(5 W) and an RF Ammeter. The trouble is that the ammeter doesn't
read quite up to the highest level of the transmitter and I need to
reduce the power to the meter by about half. I'm thinking a 3 dB
attenuator, but how am I going to find the oddball resistances and
what about resistor wattage? Is there any other way to reduce power
to the meter? Need something simple, quick-- but accurate. Thanks.


I'm not sure there are any simple answers to obtaining "accurate"
power readings
with an "inline" RF Ammeter. (due to variable input and output
impedances and response
of MOST RF Ammeters, and such). However it is relatively easy to
increase the
range of an RF Ammeter by shunting it (paralleling it) with a low
resistance, low
impedance resistor made from a short length of wire. (bailing wire,
iron, fence or
what have you)
The Ammeter will have a very low resistance to begin with, so a
very low resistance
homemade shunt wire resistor can be fairly short, and length determined
by quick
experimentation. Run RF through the meter and take a reading at almost
(or at) the
top end of the scale. Then temporarily jumper the meter with the
homemade shunt
and adjust it's length until the meter reads half scale (if you want to
double the meter's
range), and tighten things up. Done. At less than 5 watts you can do
this hot, with your
fingers. (assuming your final is not a tube type with plate voltage
appearing at the
antenna terminal... YIKES!!!!!
Results will definitely not be accurate, but should be repeatable
for most comparative purposes.

That's my story, and I'm sticking to it.

Old Chief Lynn, W7LTQ

This is exactly how I would do it (in fact, I'm sure I did this about 50
years ago). As you say, a thermocouple RF ammeter will have a very low
resistance between the terminals - and there's no reason why you can't
measure it with a (probably digital) multimeter. You will then have a
good idea of the value of shunt you will need. You can then do the final
adjust-on-test 'make it read half' procedure.
--
Ian