The formula
Received power (dBm) = transmit power (dBm) + transmit antenna gain (dBi) - feeder loss + receive antenna gain (dBi) - feeder loss - path loss (dB). Free-space path loss is 32.45 + 20 log10(f in MHz) + 20 log10(d in km). Power in dBm is 10 log10 of the milliwatts, so 1 W is 30 dBm and 5 W is about 37 dBm.
A worked example
A 5 W handheld at 145 MHz (37 dBm), antennas of 0 dBi, over 10 km: the path loss is 95.6 dB, so the received power is 37 - 95.6 = about -59 dBm. A typical FM receiver needs roughly -115 to -120 dBm, so the margin is more than 55 dB. At 435 MHz the loss is 105.2 dB; at 2400 MHz it is 120.1 dB. The margin is what you can spend on hills, buildings, rain, fading and the environment around the receiver.
Common mistakes
Quoting power in watts but gain in dBd, forgetting that dBi equals dBd plus 2.15; counting free space and forgetting terrain (the real loss beyond a hill is far larger); and ignoring noise on HF, where the limit is not the receiver but the noise around you. The coverage map does the sum for you and adds terrain, environment and variability; this guide explains the units.
