Electronic – How to measure radio signal strength

antennaradioRFsignaltransmission

I am building several types of radio antennas, and I would like to test which is the most effective. I am wondering: What is the best way to measure the strength of the received signal?

It is at ~151 MHz and 2 watt transmission power, and I will be testing over a distance of at least a couple of hundred feet. The results don't have to be calibrated, only correct relative to one another. Any ideas?

Edit as per comment request:

I am transmitting from a handheld unit which is capable of 2, 4, and 8 watt transmission power, but I am using the 2 watt setting to comply with MURS regulations.

The ideal measurement device would be low cost or home made, such as the one described in the answer below.

Thanks in advance.

Best Answer

All you need is a tuned whip or dipole, a matched tuned amplifier with "hot-carrier" or Schottky diode, 1k ~10k load and DMM to measure square law power. Then use Friis Loss calculator to compare results with distance.

Two rooftops worked for me at work in '76 when I measured my antenna polar pattern with a turntable to plot polar gain and thus beamwidth. I had enough Tx power so that an Rx amp was not necessary.

schematic

simulate this circuit – Schematic created using CircuitLab

A half-wave dipole should be fed using a balanced transmission line matching its typical 65–70 Ω input impedance.

You do not have to tune the Rx antenna since you are only interested in relative gain.

But if desired to measure matched frequency and impedance tuning, I used a directional coupler or RF splitter to measure the reflected power using the same diode detector for null voltage. Then nearby motion reflections from people and static reflective walls will affect the result within 10 wavelengths or so declining with distance. I used a sweep generator to find resonant point then trimmed antenna at desired f. A ground plane and vertical or horizontal polarization are considerations depending on what you want to test.

In the field, for calibrated measurements, I have used omni-frequency calibrated biconical dipole antenna in an open field site with quasi-peak measuring spectrum analyzers.

If your background RF levels are greater than your received signal you can also use a radio with AGC to record levels or an SDR or use an LC tank at 131 151 MHz with Zc= 10% of R load e.g. near C~11pF 100nH 1k.

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