RF Link Budget Calculator

The RF link budget applies the Friis transmission equation with explicit accounting for all gains and losses:

• EIRP: EIRP (dBm) = P_t (dBm) + G_t (dBi) − L_Tx (dB)
• Free-space path loss: FSPL (dB) = 20·log₁₀(d_km) + 20·log₁₀(f_GHz) + 92.45
• Received power: P_r (dBm) = EIRP − FSPL + G_r (dBi) − L_Rx (dB)
• Link margin: Margin (dB) = P_r − Sensitivity (dBm)

Expanded: P_r = P_t + G_t − L_Tx − FSPL + G_r − L_Rx. Positive margin = link closes. Negative = link does not close under these assumptions.

References: Friis, H.T. (1946) "A Note on a Simple Transmission Formula," Proc. IRE 34(5), 254–256; ITU-R P.525-4 (2019); Pozar, D.M. (2012) Microwave Engineering 4th ed., §14.1.

IoT uplink: LoRa sensor to gateway at 5 km (915 MHz)

• Tx: +14 dBm sensor, +2 dBi whip, 0 dB cable loss → EIRP = +16 dBm
• FSPL = 20·log₁₀(5000) + 20·log₁₀(915×10⁶) − 147.55 = 74.0 + 179.2 − 147.55 = 105.7 dB
• Rx: +3 dBi gateway antenna, 0.5 dB cable loss, sensitivity = −137 dBm
• P_r = 16 − 105.7 + 3 − 0.5 = −87.2 dBm
• Link margin = −87.2 − (−137) = +49.8 dB — excellent for LoRa SF12

Point-to-point WiFi backhaul at 10 km (5.8 GHz)

• Tx: +23 dBm, +24 dBi dish, 1 dB cable → EIRP = +46 dBm
• FSPL = 20·log₁₀(10000) + 20·log₁₀(5.8×10⁹) − 147.55 = 80 + 195.3 − 147.55 = 127.7 dB
• Rx: +24 dBi dish, 1 dB cable, sensitivity = −90 dBm
• P_r = 46 − 127.7 + 24 − 1 = −58.7 dBm
• Link margin = −58.7 − (−90) = +31.3 dB — adequate for licensed backhaul; add rain fade budget for availability spec

• Free-space line-of-sight — no terrain, buildings, or Fresnel-zone obstruction
• No rain fade — significant above 10 GHz or on long paths; see ITU-R P.838
• No atmospheric absorption — relevant above 10 GHz or above 10 km; see ITU-R P.676
• No multipath fading — add fade margin manually for links subject to Rayleigh or Rician fading
• No polarization mismatch — add up to 3 dB manually for cross-polarized antennas
• No interference floor — receiver sensitivity assumes thermal-noise-limited receiver
• No implementation loss — real receivers have 1–3 dB implementation loss from phase noise, IQ imbalance, ADC quantization; include in Rx loss if known
• Far-field only — FSPL is not valid when the link distance is much less than the antenna aperture squared divided by wavelength
• First-pass planning estimate only — verify with the real link environment, measured path loss, or a propagation model before finalising hardware selection

• Forgetting cable and connector losses: Each connector adds 0.1–0.5 dB; a 10 m LMR-400 cable at 2.4 GHz adds about 1 dB. These stack directly against link margin. Measure or use the cable manufacturer's loss spec.
• Using peak power instead of average power: For pulsed or OFDM signals, dBm typically refers to average power. Peak power can be 6–12 dB higher depending on PAPR. Regulatory EIRP limits are typically peak; link budget uses average. Verify which your Tx spec references.
• Ignoring antenna pointing error: A 24 dBi dish has a narrow beam — 1–2° of misalignment can cost 3–10 dB of gain. The gain figure entered here is the peak gain; pointing loss must be added manually.
• No fade margin for non-LOS or mobile links: Free-space FSPL does not predict fading. For terrestrial links subject to multipath, shadowing, or rain: add 10 dB for moderate availability (99%), or 20–30 dB for high availability (99.9%+).
• Confusing receiver sensitivity with noise floor: Sensitivity already includes the receiver noise figure and the minimum required SNR for demodulation. Adding a noise figure argument again would double-count the impairment. Use sensitivity as quoted in the radio datasheet.