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From: john_c@tpg.com.au (John Crighton)
Newsgroups: sci.electronics.design,sci.electronics.misc
Subject: Re: RF power measurement
Date: Fri, 15 Nov 2002 01:20:13 GMT
Message-ID: <3dd43834.3912158@news.tpg.com.au>
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On Thu, 14 Nov 2002 03:29:50 +0000, Paul Burridge
wrote:
>
>Hi,
>
>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
>power?
>--
>
>Noverint universi per presentes et futuri...
Hello Paul,
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.
For example:
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.
Sneaky!
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
circumstance.
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,
Regards,
John Crighton
Sydney
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