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Using this RF probe to determine actual RF Power

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Electrojosh86

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I found this nifty little probe and decided to build it. It seems to work pretty well and when I connect it to an RF oscillator (working one at that )I get a reading from it. I connect to my digital voltmeter and it shows me a voltage reading. I was wondering how I could use that voltage reading to determine actual RF voltage and/or watt output of the circuit under test.

https://www.electroschematics.com/659/rf-probe-circuit/
 

When you work in RF ,you don't rely on RF voltage
and current measurements,as the phase change is
very fast at these frequencies.
You need to measure reflected and transmitted
rf power(scattering parameters).

If you have an rf probe,you can calibrate it and
can use it to measure power at different points
in a circuit.
 
That is what I though, still works as attended for troubleshooting though and until I get a cheap RF power meter I can use ti to see if amplifiers are actually amplifing. Though only roughly. Now I just need to buy a RF power meter for pretty cheap but I seem to cannot find one so I'll just build one.
 

Since you are interested in only relative measurement
and have a spectrum analyser,,you can do this and
don't need a power meter.
 

A RF power meter is expected to expose an impedance matched input, usually 50 ohm. It can't be used as a probe, has to be connected instead of the regular load. Or use a "through" power meter, e.g. a VWSR bridge or directional coupler together with the regular load.
 

This is a common circuit and has been in use for donkeys years, using thermionic diodes pre 1960. It can be calibrated to be accurate over high audio to 30 MHZ or so. It is a good idea to include a 47K resistor in the probe in series with the DC output. This will reduce the effect of the meter leads acting as aerials, and it won't make much difference to the meter reading, which will be non linear at low voltages (<1.5?).
Frank
 

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