NNTP-Posting-Date: Mon, 18 Nov 2002 15:07:19 -0600
From: Barry Lennox
Subject: Re: RF power measurement (again)
Date: Tue, 19 Nov 2002 10:08:00 +1300
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On Mon, 18 Nov 2002 15:25:34 +0000, Paul Burridge
A bit difficult to do. There are a few problems by just connecting a
50 ohm load. They nearly always use base loading to compensate for the
electrically-short antennas used, so a large proportion of the missing
power will be going into that inductor.
Measure the current drawn with the xtal removed or shorted out, lets
say it's now 70mA. So the power consumed by the transmitter is 220-70
x 12 volts (BTW, you sure it's 12 v, most use 9.6 v?) or 1.8 watts.
Assuming roughly that 20% is used by the oscillator and buffer, and
the PA stage is 60% efficient and you lose about 6dB in the output
network, you will have a radiated power of about 200mW. These figures
are roughly in line with what I have measured.
The only sure way of doing this is by measuring the field strength, at
say 10 metres, and then calculating backwards, takes a bit of test
equipment and maths though.
The PA stage may not like very obvious, most are just a TO-92 or maybe
a little larger "funny" japanese case, transistor.
Remove the three pee's if replying by email.
>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.