How can I measure the power in mW of a radio signal?

Question

I would like to build a small circuit to measure with my oscilloscope the power emitted by the 5.8GHz video transmitters mounted on my drones. On searching, I found this answer.

The question recommends using a 50 ohm dummy resistive load.

Why was that value chosen? Is it random or can the optimal value be calculated?

Personally I will have to measure signals with power lower than one watt.

To have precise measurements and avoid damaging the video transmitter, is that 50 ohm resistor sufficient, or is a more complex circuit necessary as shown here?

Answer 1

You need to use a power sensor.

A rudimentary one, yet quite adequate for your use, takes just a couple of components.

An example design by Herbert Dingfelder – DL5NEG is pictured below. I've also drawn a schematic below, other pictures are Herbert's.

The output voltage vs input power response:

^{simulate this circuit – Schematic created using CircuitLab}

A higher accuracy integrated sensor - probably a bit overkill for your use - could be for example this one built and offered for sale by SV1AFN. Picture from the listing:

You'll need an Arduino to connect to it to read out the power.

The maximum input power of that one, for reasonable accuracy, is around 3mW or -5dBm. To measure higher power, you'll need an attenuator connected in-line.

Answer 2

You're not going to be able to directly measure a 5.8 GHz signal with the oscilloscope you have, it's rated bandwidth is 110 MHz. A multimeter is going to be even worse, probably less than 20 KHz.

Measurements at frequencies that high require special equipment and techniques, even very short wires can be a significant fraction of a wavelength.

What you need is an RF power meter designed for that frequency range (an internet search should turn up meters made for RC hobbyists that will cover the 5.8 GHz) or something like a directional coupler and RF detector rated for that range. This will basically rectify the RF and output a DC voltage that you can measure with a DMM or scope.

You can check this video to get some ideas about how to make measurements at this frequency.