# how to calculate the power of an RF signal?

Discussion in 'Wireless & RF Design' started by bagath ch, Aug 6, 2014.

1. ### bagath ch Thread Starter New Member

Aug 6, 2014
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I want to find out the output power of an RF power amplifier at a frequency of 437 MHz can anybody suggest a device for this purpose?

can i use an digital oscilloscope for this?

Thankyou

Feb 24, 2006
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3. ### inwo Well-Known Member

Nov 7, 2013
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I've been working on a similar problem with a 1mhz device. ~5-7 watts.

Non of my watt-meters were in range or gave reasonable results.

Not a high degree of accuracy, I'm sure, but got me a ball park figure to compare.

I connected an appropriate load resistor. Then measured the time it took to go from 25-75C.

I then experimentally matched the change using an easily measured dc source.

There were no comments on this method in my other thread. Hope to get some input here.

It seems to match measuring ppvolts and ppamps on scope and converting to rms.

And also measuring ppvolts across known non-inductive load.

4. ### alfacliff Well-Known Member

Dec 13, 2013
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why not use a dummy load (non inductive resistor) designed for the frequency? 437 mhz requires a bit more care to eliminate swr than at 1 mhz, and use your digital scope if the frequency response is enough to measure the rms voltage across the resistor? P=Esquared divided by R. or hind a local ham who has a watt meter.

5. ### Papabravo Expert

Feb 24, 2006
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2,591
Mt first attempt at making a dummy load was a miserable failure. I put it on a network analyzer and saw the inductive effects of a length of wire almost immediately (10 MHz. or so). By the time it got to 50 MHz it was heading for capacitor land on the Smith chart.

6. ### inwo Well-Known Member

Nov 7, 2013
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How much power?
What impedance cable?

+1 on ham.
I have a 100-1000mhz wattmeter.

7. ### alfacliff Well-Known Member

Dec 13, 2013
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the ARRL handbook shows how to make dummy loads. there is a lot to length of wires, type of resistors and such.

Aug 8, 2011
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BR-549 likes this.
9. ### BR-549 AAC Fanatic!

Sep 22, 2013
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If your scope is calibrated, and has the bandwidth, and you have a clean, symmetrical sine wave,...you can follow alfacliff's advice. For that high of F, I would barrow a bird and then mark my swr meter for pwr for reference.

10. ### iconic.digital New Member

Aug 28, 2014
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Hi Guys,

I've been working on a new project configuring the Motorola digital radios into a new UFH band. I'm new to this but have had some advice from a radio hire company called Wall to Wall Communications. Their site is http://www.walltowallcomms.co.uk. Can someone give me some advice about adjusting my symmetrical sine wave as it is not clean at the moment and I want to increase the performance of the radio over a wider range. Thanks guys.