Path Loss
The reduction in power density of a radio signal as it propagates through free space and the environment, increasing with distance and frequency.
Path loss is the attenuation a signal suffers just travelling from transmitter to receiver, and it is the dominant term in any link budget. Even in free space, power density falls with the square of distance — double the range and you lose 6 dB before accounting for anything else. Real environments pile on more: buildings, terrain, foliage, rain, and body blocking.
The part that trips people up is the frequency dependence. Higher-frequency signals don't inherently lose more energy in free space for a given antenna gain, but in practice they're far more easily blocked and absorbed, and the small antennas they pair with capture less energy — so the effective path loss climbs steeply with frequency. That's the core reason mmWave needs dense sites and beamforming while low band reaches for kilometres. RF planners model all this with tools like Okumura-Hata, COST-231, or the 3GPP 38.901 channel models before a single site goes up.
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