Start with EIRP
EIRP combines transmitter power, feed-system loss, and antenna gain relative to an isotropic radiator.
EIRP(dBm) = PTX(dBm) - LTX(dB) + GTX(dBi)
Free-Space Path Loss
For distance in kilometers and frequency in megahertz:
FSPL(dB) = 20 log10(d) + 20 log10(f) + 32.44
FSPL is an ideal spreading calculation. It does not include terrain, buildings, foliage, atmospheric loss, polarization mismatch, or multipath fading.
Estimate Received Power
PRX = EIRP - FSPL + GRX - LRX
If EIRP is 35 dBm, FSPL is 100 dB, receive gain is 8 dBi, and receive loss is 2 dB, estimated received power is -59 dBm.
Link Margin
Link margin is received power minus the receiver sensitivity needed for the selected data rate and error performance.
Margin = PRX - sensitivity
A positive margin is not automatically enough. Required margin depends on reliability, fading, interference, movement, weather, and service goals.
A real link design needs site geometry, antenna patterns, line of sight, Fresnel clearance, regulatory limits, and measured or modeled losses.