From: email@example.com (John Crighton)
Subject: Re: RF power measurement (again)
Date: Tue, 19 Nov 2002 00:28:49 GMT
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On Mon, 18 Nov 2002 15:25:34 +0000, Paul Burridge
>I've just stripped a radio control transmitter to try to ascertain its
>output power. This is one of those hand-held Futaba ones with the
>telescopic ariel; runs of 12v battery. total DC current drain was
>220mA on transmit; although that includes a couple of fairly large and
>bright LEDs. Anyway, discounting say 20mA for those, that still
>amounts to DC input power for the whole circuit at 2.4 Watts. Anyway,
>on removing the ariel and soldering a 50 ohm carbon resistor from
>ground to where the ariel was, an oscilloscope showed a pk-pk output
>voltage across the resistor of only 0.25 volts. By my calculation that
>means only 1.25mW output power. Can this possibly be right? It seems
>very low indeed to me. BTW, there does not appear to be a PA stage as
>such on the circuit board at all.
>"I was going to help him up, but then I thumped him."
>Michael Head's plea in mitigation to Weybridge Magistrates, Sept. 1975
with these small hand held transmitters there is no guarantee
that the output impedance is 50 Ohms. There is probably
a fixed or slug tuned inductor connected to the antenna
connection. When you removed the antenna you still
have this inductor in series with your 50 ohm load.
Just for fun lets try a bunch of different dummy loads,
say 10 ohms to 1000 ohms, only this time put a
small trimmer capacitor in series with the load resistor,
say 3 to 30 pF or whatever comes to hand.
The trimmer cap is there to cancel the reactance the
aerial tuning coil.
Antenna connection----trimmer cap----resistor---case ground.
Connect CRO to resistor only.
Now adjust trimmer cap for max output.
If you do not get a peak as you adjust the trimmer, then
try another value for the trimmer cap.
Try a different value of dummy load. More output? Yes! :-)
I am having FUN already.
Remember the CRO shows Pk to Pk volts. Divide by 2.8 for Vrms.
Then calculate power absorbed by the load resistor.
Come back and tell us what you measured. (guessing 50mW minimum)