[Boatanchors] RF Ammeter Design and Construction

J. Forster jfor at quikus.com
Sun Dec 2 14:18:38 EST 2012

> 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.

Yes. The torroid was acting as a Current Transformer. Since the load on
the secondary is resistive, the voltage across that load should be in
proportion to the current through the core in the single wire.

> 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.

Yes, if the load is perfectly resistive.

> 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.

In actual practice, the core may influence the cal accuracy as the
frequency varies.

> 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.

Yup. This is essentially the same method as using a Tek 7A13 Differential
Comparator. It eliminates any 'scope gain or cal issues.

> 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.

Yes. It's a fine method if the waveforms are sinusoidal, with little
harmonic content, and a VSWR of about 1:1. It would fail on a square wave,
for example, because the ratio of peak-to-RMS is different.



> - Jim, KL7CC
> On 12/2/2012 9:05 AM, J. Forster wrote:
> <snip>
>> it is hard to
>> calibrate RF Ammeters at RF,
>> <snip>

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