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The most conclusive laboratory quality tests done on certified FM transmitters were the field tests conducted by Tim in Bovey using a calibrated Field Intesity Meter. Every discussion on the subject of part 15 FM should include links to Tim’s discoveries.
My less scientific tests using four brands of transmitter, a spectrum analyzer, a car, and numerous portable radios, found that it’s half about transmission, half about reception.
What I mean by that is that you can fiddle with transmitter/ antenna location to achieve slightly better results…
But final results depend on the quality of FM receivers, and they vary wildly.
For example my TECSUN PL-310 will solidly receive weak FM signals for an amazing distance, while other portable FM radios will lose the signal just past the front door.
On average 200-feet is a fair expectation at part 15 field strength, under the best of conditions.