When to use: Use to verify an RF link is viable before installation. Accounts for transmit power, cable losses, antenna gains, free space path loss, and receiver sensitivity to calculate RSSI and fade margin. A fade margin ≥ 10 dB is recommended for reliable links. Use for DAS donor links, BDA design, point-to-point radios, and public safety coverage verification. Get FSPL from the Path Loss calculator.
The Link Budget Calculator computes the complete power balance of a radio link from transmitter to receiver using the Friis transmission equation, yielding RSSI, fade margin, and a pass/fail verdict against the receiver's sensitivity threshold. It is the foundational design tool for DAS donor links, BDA installations, point-to-point microwave systems, and public safety coverage verification.
The Friis transmission equation gives received signal strength as: RSSI = EIRP − Path Loss + G_rx − L_rx − L_other, where EIRP = TX Power − TX Cable Loss + TX Antenna Gain, and path loss is calculated using the Free Space Path Loss formula (use the Path Loss Calculator to obtain this value). Fade margin = RSSI − Receiver Sensitivity. A positive fade margin means the link is viable; a fade margin ≥ 10 dB indicates a reliable link under typical conditions.
System Gain = TX Power − Receiver Sensitivity, representing the total power budget available to overcome all path losses and still achieve the minimum sensitivity. For NFPA 1221 public safety systems, the receiver sensitivity target is −95 dBm (DAQ 3.0 quality) and fade margin ≥ 10 dB is recommended. For 18 GHz backhaul links requiring 99.999% availability, fade margins of 35–40 dB are typical to overcome rain fading.
NFPA 1221 Section 9.6 and IFC 510 require −95 dBm minimum RSSI at the portable radio antenna throughout 95% of covered area. TIA-222 (antenna tower standards) and TIA-TSB-88 (land mobile radio) reference the Friis equation framework for link analysis. For licensed point-to-point microwave links under FCC Part 101, a path engineering report including link budget is required as part of the license application. For satellite ground stations, link budgets must account for atmospheric absorption per ITU-R P.676.
The most common error in link budget calculations is omitting additional system losses — body loss (3–5 dB for handheld radios held near the body), polarization mismatch loss (0.5–3 dB), antenna pointing loss, and atmospheric absorption at frequencies above 10 GHz. Always budget at least 3 dB of "other losses" for in-building systems to account for antenna mismatches and cabling imperfections not captured in the cable loss calculation. Receiver sensitivity figures on specification sheets are typically measured in a laboratory under ideal impedance-matched conditions — real-world sensitivity may be 1–3 dB worse. Use the Noise Figure Calculator to verify system sensitivity in cascaded receive chains.
Enter TX Power in dBm and the TX side cable and antenna values. Obtain Free Space Path Loss from the Path Loss Calculator using the operating frequency and link distance. Enter receiver-side antenna gain, cable loss, and receiver sensitivity from the equipment datasheet (−95 dBm for public safety portables, −100 to −110 dBm for sensitive receivers). Set Other Losses to at least 3 dB for in-building systems or 0 for controlled LOS paths. The fade margin result is color-coded: green (≥ 10 dB, pass), orange (0–10 dB, marginal), red (negative, link fails). Use the results to inform BDA gain requirements or antenna placement decisions.
For in-building public safety systems at 800 MHz, a minimum fade margin of 10 dB above the −95 dBm receiver sensitivity threshold is recommended, meaning RSSI should ideally be −85 dBm or better. For outdoor point-to-point links at microwave frequencies, fade margin requirements depend on availability objectives: 99.99% availability requires approximately 30 dB of fade margin at 11 GHz over 10 km, and 99.999% requires approximately 40 dB.
Most modern P25 radios have a manufacturer-specified sensitivity of −116 to −119 dBm at 12 dB SINAD in the 800 MHz band. However, NFPA 1221 and IFC 510 use the DAQ 3.0 (Good Audio Quality) threshold, which corresponds to approximately −95 dBm at the antenna port of the portable when held in a typical body-worn position, accounting for body loss and antenna gain of the portable. Always use −95 dBm as the RSSI target for compliance calculations.
Rearrange the link budget equation: Maximum FSPL = EIRP − Receiver Sensitivity − Margin − Other Losses + G_rx − L_rx. Then use the inverse FSPL formula: d = 10^((FSPL − 32.44 − 20·log10(f_MHz)) / 20) km. This gives the maximum range at which the link meets the minimum fade margin.
System gain is the total power budget from transmitter output to receiver sensitivity threshold: System Gain = TX Power − Receiver Sensitivity. Fade margin is the portion of the system gain remaining after all path losses, antenna gains, and cable losses are accounted for: Fade Margin = RSSI − Receiver Sensitivity. System gain defines the theoretical maximum performance of the equipment pair, while fade margin quantifies how much headroom remains in an actual deployed link.
For narrowband point-to-point links in multipath environments, add a statistical fade margin based on the required availability and the propagation environment. Rayleigh fading (fully scattered multipath) requires 20 dB of additional fade margin to achieve 99% reliability above the median signal level. For in-building systems, the log-distance shadowing model adds 6–10 dB of standard deviation, requiring a location variability margin of 6–10 dB for 90% location probability.
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