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From: firstname.lastname@example.org (John Crighton)
Subject: Re: RF power measurement
Date: Fri, 15 Nov 2002 01:20:13 GMT
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On Thu, 14 Nov 2002 03:29:50 +0000, Paul Burridge
>I've got several books on RF and they all seem to give slightly
>different variations for the equation for power measurement.
>For example one says Pout = V^2/8R which seems a bit dubious.
>Let's say I get a sine wave of 10v peak-peak on my scope when I
>connect the output of an amp. across a 50 ohm purely resistive load.
>How do I arrive at a figure for a) peak output power and b) RMS output
>Noverint universi per presentes et futuri...
now that you have done some reading it is time to do
some playing with radio transmitters and measuring
output power into loads.
I played with Single Side Band (SSB) High Frequency (HF)
transmitters. It took a while to understand how the various
makers would describe the power output of their transcievers.
Marconi Marine Transocean
400W Peak Envelope Power. ( plus 0 minus 2 dB)
Gee that sounds good.
But that -2 dB knocks the power back a bit. (just under 100W)
This is fine print in the hand book not on the front cover.
Test with a Dummyload/absorbtion power meter and find
that your getting 150 W continuous. How come? Where
has the power gone?
Was only 300W PEP, momentarily untill the Automatic Level Control
(ALC) kicked in milliseconds later turning the power down to a safe
150 W continuous.
So one needs an oscilloscope to measure the peak to peak
voltage across the load during normal speach peaks then
convert the P-P voltage to RMS volts. (Vp-p divide by 2.8 = Vrms)
Then do your P= Vrms squared divided by R.
Then you know what power you are getting under what
150Wcontinuous is a lot different to 400 W PEP in this
case, but 400W PEP looks better in advertising brochures.
Get an oscilloscope and some old HF radio gear and
have a play around. and read the books again.
All Good Fun,
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