RF Link Budget Calculator
Calculate EIRP and received signal power for an RF link using free-space path loss.
Inputs
Received Power
-70.03dBm
EIRP
39.00dBm
Free-Space Path Loss
114.03dB
Step by step
Values used
Transmit Power = 30 dBm; Transmit Antenna Gain = 10 dBi; Cable Loss (Tx side) = 1 dB; Link Distance = 5 km; Frequency = 2.40 GHz; Receive Antenna Gain = 5 dBi
EIRP
EIRP = Tx_power + antenna_gain − cable_loss
Free-space path loss
FSPL(dB) = 20·log10(distance_km) + 20·log10(freq_GHz) + 92.45
Received power
received = EIRP − FSPL + Rx_gain
Received Power
= -70.03 dBm
EIRP
= 39.00 dBm
Free-Space Path Loss
= 114.03 dB
How it works
This RF link budget first calculates EIRP (Effective Isotropic Radiated Power) — the transmit power actually radiated toward the receiver after antenna gain and cable loss: EIRP = Tx_power + antenna_gain − cable_loss. It then subtracts the free-space path loss (FSPL), which grows with both distance and frequency, and adds the receiving antenna's gain to arrive at the final received power: received = EIRP − FSPL + Rx_gain. This is the standard method for estimating link feasibility for any RF system — WiFi, point-to-point microwave, or satellite.
Formulas
EIRP
EIRP = Tx_power + antenna_gain − cable_loss
Free-space path loss
FSPL(dB) = 20·log10(distance_km) + 20·log10(freq_GHz) + 92.45
Received power
received = EIRP − FSPL + Rx_gain
Frequently Asked Questions
Why is EIRP calculated separately before applying path loss?
EIRP represents the actual effective power leaving the system in the direction of the antenna's main beam, which is what actually propagates through space and experiences path loss — it's more meaningful than raw transmitter output power alone, since antenna gain focuses that power directionally.
Why does path loss increase with frequency at the same distance?
Higher-frequency waves have shorter wavelengths, and for a fixed antenna size/gain, isotropic path loss increases with frequency — the FSPL formula's 20·log10(freq) term captures this, meaning a link at 5 GHz suffers roughly 6.5 dB more path loss than the same link at 2.4 GHz.
Is free-space path loss realistic for terrestrial links?
FSPL models an idealized, obstruction-free path and is a reasonable approximation for clear line-of-sight links (satellite, point-to-point microwave with a clean Fresnel zone), but real terrestrial links often experience additional loss from terrain, foliage, and multipath not captured by this simple model.
How does this differ from the microwave link calculator?
This calculator produces a full end-to-end received power estimate including antenna gains and cable loss. The Microwave Link calculator focuses specifically on the FSPL component itself, useful when you just need the path loss figure to plug into a separate link budget.