Free-space path loss, EIRP, received power, and link margin from the Friis transmission equation. Useful for first-pass sanity-checks of any RF link before you start adding fading margins, multipath, and rain attenuation.
For an unobstructed free-space path between matched antennas, received power is:
P_RX = P_TX + G_TX + G_RX − FSPL − L_other (all in dB)
where FSPL = 20·log₁₀(4π·d·f / c). Equivalently, in handy log form:
FSPL_dB = 32.45 + 20·log₁₀(d_km) + 20·log₁₀(f_MHz)
The Friis equation assumes free space — no ground reflection, no multipath, no obstructions, perfect polarization match, perfect impedance match. Real-world links almost always lose more than this predicts. Common additions to a complete link budget:
Cable / connector loss · polarization mismatch (typically 0.5–3 dB) · fading margin (Rayleigh / Rician, environment-dependent) · rain / atmospheric attenuation (significant above ~10 GHz) · diffraction loss (Fresnel zone clearance) · body loss for handheld devices.
Use Friis to find the best case, then subtract margins for everything reality will throw at the link.