Regardless of the diode rectifier used, you have to be careful about the
value of the resistor used across the filter cap and the input impedance
of the DVM. If the DVM loads the circuit too much, you get less than the
peak voltage and calculate less than the true power value.
Most probes are designed to take into account the DVM loading such that
the output voltage is about 0.7 of the peak to give an approximate RMS
value. The Handbook probes, for example, were designed that way so that
the resulting power calculation was simply E^2/50 where E was the voltage
read by the DVM. If I remember right, the load resistor was chosen for an
11 megohm input resistance for the VTVM that used to be the standard.
Of course, if you can confirm that you are actually seeing the peak
voltage, then the calculation is (Ep^2)/100 where Ep is the true peak
voltage across the 50-ohm resistive load.
Fairview, TX 30 mi NE Dallas in Collin county
Amateur Radio W5YR, in the 54th year and it just keeps getting better!
R/C since 1964 - AMA 98452 RVing since 1972
Roy Koeppe wrote:
> "I have an RF probe made per ARRL hand book with a 1N34A diode which is
> good to about 20 volts. I use it for alignments and such. With 20
> RF into a 50 ohm load I have 8 watts carrier out. This is pretty close
> with what I have on the watt meters as well but too low to calibrate
> from. Can I put 10 of these diodes in series and run the RF volts to 200
> volts and expect to get 80 watts on the watt meter? Is this an accurate
> way to calibrate an RF watt meter or is there a better way?"
> I think there is an improved (accuracy) way. You can use a 6AL5 tube
> with its heater tied to its cathode, and its two plates connected in
> parallel. This configuration allows for a full legal power rating. Use
> two caps in the filter, one for RF and the other bigger one for a
> holding factor. Measure the DC voltage output with a VOM or VTVM, and
> convert the reading to RF watts using the standard peak voltage formula.
> Mine works great from QRP to QRO. Accuracy depends on your meter,
> typically 2-percent, etc.
> 73, Roy K6XK Iowa Outback
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