[ARC5] [Boatanchors] RF Ammeter Design and Construction

[Jim Wiley]

I cannot offer anything in the way of comments about the thermocouple 
issue.  An interesting problem, to be sure.  However, I may be able to 
offer something concerning calibrating RF ammeters that may work for you.

When faced with this problem as I built some RF ammeters using the 
"single wire through a toroid" technique, I found that I could achieve 
reasonable accuracy by measuring the peak to peak RF voltage across a 
measured dummy load, and manipulating the numbers with some math to 
arrive at the amount of RMS voltage across the load.  Since the exact 
resistance of the load was known, it was then a straight-forward ohms 
law calculation to derive the current that was flowing, and from there a 
simple step to calibrate the ammeter accordingly.

By varying the power level fed to the load, I could achieve several 
precise points that could then be marked on the meter.  I then used the 
"meter" software program by Tonne (available on the net) to draw a 
non-linear scale, and presto, instant RF ammeter.

I was able to measure the exact peak voltages across the dummy load by 
using an oscilloscope, taking note of the trace position, then applying 
a variable DC voltage that was measured by an accurate 6-digit ( -hp- 
3478a) digital voltmeter until the DC voltage overlapped the trace 
position, thus giving me the equivalent peak voltage.  A similar test 
for the "negative' side established the peak negative going value, and 
thus the P-P voltage.   One thing that assisted me in this process was 
that I had access to a digital oscilloscope (Tektronix TDS-210) that 
includes digital readouts for various trace parameters.  By using these 
markers, it was easy to set the DC trace deflection to the same point as 
the RF trace deflection, and regardless of
  what the scope thought the voltage was, to use those values together 
with the 3478A DVM to be sure I was actually getting an accurate reading.

By use of this technique I was able to achieve an accuracy on the close 
order of 1 to 2% (at least at the calibration points) which I decided 
was close enough.  Obviously, each meter needed a separate calibration, 
but the technique, once established, was reasonably rapid in execution, 
and future checks for accuracy will be similarly easy.

Hope this helps a bit.


- Jim, KL7CC



On 12/2/2012 9:05 AM, J. Forster wrote:
<snip>
> it is hard to
> calibrate RF Ammeters at RF,
>
> <snip>