How to measure radio signal strength
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.







simulate this circuit &ndash; 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.