What is the formula for free-space path loss (FSPL)?

FSPL (dB) = 20·log₁₀(d) + 20·log₁₀(f) + 32.45, where d is distance in kilometres and f is frequency in MHz. This form is derived directly from the Friis transmission equation and normalises the speed-of-light term into the 32.45 constant. For example, at 900 MHz over 5 km, FSPL works out to 20·log(5) + 20·log(900) + 32.45 = 105.5 dB.

How do you calculate EIRP for a radio link?

EIRP (dBW) = P_TX (dBW) + G_TX (dBi) − L_cable (dB), where P_TX is transmitter output power, G_TX is antenna gain referenced to an isotropic radiator, and L_cable is feedline loss. For example, a 5 W (7 dBW) transmitter feeding a 6 dBi antenna through 1.5 dB of coax loss produces an EIRP of 7 + 6 − 1.5 = 11.5 dBW (41.5 dBm).

How much fade margin should a terrestrial radio link have?

Standard practice calls for 10–20 dB of fade margin on terrestrial links, though the required amount depends on the link reliability objective, terrain, frequency, and path length. ITU-R P.530 provides methods for calculating multipath fading probability, and a demanding 99.999% annual availability target on a 30 km microwave path in a temperate climate may require as much as 35 dB of fade margin or a space/frequency diversity arrangement.

How is receiver sensitivity calculated from noise figure and bandwidth?

Sensitivity (dBm) = −174 + NF + 10·log(BW) + Eb/N0_required, where −174 dBm/Hz is the thermal noise floor at 290 K. For example, a receiver with a 6 dB noise figure, 12.5 kHz bandwidth (10·log(12500) = 40.97 dB), and a 12 dB required Eb/N0 has a sensitivity of −174 + 6 + 40.97 + 12 = −115 dBm.

What Eb/N0 does P25 Phase 1 C4FM require?

P25 Phase 1 C4FM requires approximately 5 dB Eb/N0 at the 5% BER threshold defined in APCO standards. This is notably lower than the theoretical 10.5 dB Eb/N0 requirement for QPSK at BER = 10⁻⁶, though real systems typically add an implementation margin of 1–3 dB on top of the theoretical figure.

Engineering·12 min read·June 17, 2026

📡 RF Link Budget Analysis: From EIRP to Fade Margin

Master the end-to-end RF link budget process: EIRP, free-space path loss, received signal level, fade margin, Eb/N0, and a worked 900 MHz / 5 km example using the Friis transmission equation.

📖Radio Communications — Illustrated Guide (Full Access)

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What Is an RF Link Budget?

An RF link budget is an accounting of all power gains and losses in a radio communication path — from transmitter output to the minimum detectable signal at the receiver. Every terrestrial, airborne, or satellite radio link must be verified with a link budget before deployment. The output tells you whether the received signal exceeds the receiver's sensitivity threshold by a comfortable margin, and by how much.

Topics covered

RF link budgetEIRPfree-space path lossFSPLfade marginFriis equationreceived signal levelreceiver sensitivityEb/N0SNRrain attenuationsystem gain