[SOLVED] measuring RF output with a meter... maybe

[KEØBRS]

I was attempting to measure the output of a small FM "microphone transmitter" I made that works in the regular FM broadcast band (around 108MHz). I attached a 50Ω load to the antenna output, then using an analog meter measured the AC voltage.

The pictures are crappy -- I was holding the camera in one hand and probes in the other. At best, the readings were generally 12V.

The meter is set to AC on the 50 volt scale. The photos shows the needles still around 5V because my grip wasn't great. But I think you get the point.

Question: is this generally a reasonable homebrew method of measuring output? Should I add some diodes and make a different load attachment before I take a reading? The meter also read 9dB at around 12V, but does that mean anything at all? And how does a thermos work?

No, I don't have an oscilloscope... I have a DMM and this analog thingee.
And yes, I know... I should get one.
:-D
Cheers,
KEØBRS

 

[Dan Mills]

Generally that will not give a meaningful result as the meter will tend to pick up all sort of random RF under these conditions, remember at 100MHz the wavelength is just 3M, and your test leads are a significant fraction of that.

See here for some discussion of the right way to do this : http://n5ese.com/rfprobe1.htm

Regards, Dan.

 

[KEØBRS]

Thanks Dan! And I put one together... a bit more ghetto than I normally would make things, but it works great. I appreciate that.
Pete
KEØBRS

- - - Updated - - -

Final Results:
Using my new RF Probe attached to my DMM and a 50 ohm dummy load on the antenna, I now read 1.540V (set on DC Volts). This is the RMS Equivalent Voltage.

My handy calculator tells me this battery operated FM transmitter's output is equal to 47.4mW (or 16.8dBm).

Nice, considering it was cobbled together on a perf board with surplus parts.

Gracias Dan!