Subject: Re: RF power measurement (again)
Date: Mon, 18 Nov 2002 17:04:58 -0800
X-Mailer: Mozilla 4.78 [en] (Win98; U)
John Crighton wrote:
> On Mon, 18 Nov 2002 15:25:34 +0000, Paul Burridge
> >Hi all,
> >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
> Hello Paul,
> 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)
> Have Fun,
> John Crighton
Can we assume that the bandwidth of the scope, probe and whatever other
involved in the connection to the transmitter are not affecting the
A 50 ohm coax connected to the 50 ohm scope input will remove some of the
With circuits such as this, optimized for a particular antenna, it might be
useful (relatable to real world performance) to mesure field strength.
Just for the sake of curiosity, what are you gonna do with the number when
you get it?
Bunch of stuff For Sale and Wanted at the link below.
Tek 2465, TFT projector, ham radio, 30pS pulser
Tektronix Concept Books...