A good Field Strength Meter (More properly called a Power Density Meter) will have a particular antenna (usually 1M in length, but it may be a loop) and needs to be carefully calibrated before each period of measuring, every day is usually acceptable. It must then be set on a tripod and at a particular height above the ground - usually 2m.
Finally the measuring location needs to be away fromany interfering items (metalwork) and somewhere that cn bee turned to.
The FCC used to demand signal strength readings be conduced to particular specifications and properly recorded, as a Proof of performance. Very few test equipment manufacturers produced kit good enough to do it - Marconi, General Radio and Delta are three that I've used,
but today you can get some smaller models from China, etc, models for as low as a few hundred dollars - if you are prepared to put some work into the operation of them.
The idea that inexperienced listeners can use any old radio to measure signal strength though is a bit preposterous. Field Strengths are very important measurements, used to gauge when a transmitter and antenna system is working correctly; they need accuracy, consistency and professionalism. And need to be compared to a known standard to be meaningful.
