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RF Link Budget Calculator

Free RF link budget calculator: enter Tx power, antenna gains, frequency, and distance to get received signal level, link margin, and max range. Covers satellite, terrestrial, and IoT links.

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Formula

Pr=Pt+Gt+Gr−FSPL−Lmisc,FSPL=20log⁡10(4πdfc)P_r = P_t + G_t + G_r - FSPL - L_{misc}, \quad FSPL = 20\log_{10}\left(\frac{4\pi d f}{c}\right)

Reference: Friis, "A Note on a Simple Transmission Formula" (1946)

d— Distance (m)
λ— Wavelength (c/f) (m)
EIRP— Pₜₓ + Gₜₓ − Lₜₓ (dBm)
Pᵣₓ— EIRP − FSPL − L_rain − L_atm − L_pt + Gᵣₓ − Lᵣₓ (dBm)
L_rain— Rain fade (ITU-R P.838) (dB)
L_atm— Atmospheric / gaseous absorption (dB)
L_pt— Antenna pointing / misalignment loss (dB)

How It Works

RF link budget analysis calculates received signal power in wireless systems — telecommunications engineers, satellite system designers, and IoT developers use this to determine if a radio link will close with adequate margin. The Friis transmission equation P_rx = P_tx + G_tx + G_rx - FSPL - L_misc forms the foundation, where FSPL = 20*log10(4*pi*d*f/c) per ITU-R P.525-4.

Free-space path loss increases 6 dB per doubling of distance (inverse-square law) and 6 dB per doubling of frequency. At 2.4 GHz and 1 km, FSPL = 100.0 dB; at 5.8 GHz and 1 km, FSPL = 107.7 dB. This explains why 5 GHz WiFi has shorter range than 2.4 GHz given identical transmit power. According to Skolnik's 'Radar Handbook' (3rd ed.), atmospheric absorption adds 0.01 dB/km at 2 GHz but 0.2 dB/km at 60 GHz (oxygen resonance).

Link margin = P_rx - P_sensitivity represents safety buffer against fading. ITU-R P.530-17 recommends 25-40 dB fade margin for 99.999% availability microwave links. For mobile systems, Rayleigh fading causes 20-30 dB signal variation — LTE systems design for 8-12 dB margin with power control. GPS receivers operate at -130 dBm sensitivity with 25+ dB link margin to ensure global coverage.

Worked Example

Problem: Design a 915 MHz LoRa link for 10 km range with 99% availability in rural terrain.

Solution using ITU-R P.525-4 free-space model:

  1. Transmit power: 20 dBm (100 mW, FCC Part 15.247 limit)
  2. Transmit antenna: 6 dBi omni (elevated on tower)
  3. Receive antenna: 3 dBi (handheld device)
  4. Cable losses: 2 dB total (transmit side LMR-400)
  5. Free-space path loss: FSPL = 20*log10(10000) + 20*log10(915e6) + 20*log10(4*pi/3e8) = 111.7 dB
  6. Additional losses: 6 dB vegetation/diffraction (ITU-R P.833)
  7. Fade margin: 10 dB (for 99% availability per Okumura-Hata)
  8. Required P_rx: 20 + 6 + 3 - 2 - 111.7 - 6 - 10 = -100.7 dBm
  9. LoRa sensitivity at SF12/125kHz: -137 dBm (Semtech SX1276 datasheet)
  10. Link margin: -100.7 - (-137) = 36.3 dB — link closes with substantial margin
At SF7 (sensitivity -123 dBm), margin drops to 22.3 dB but data rate increases from 293 bps to 5.5 kbps.

Practical Tips

  • ✓Design for 10-15 dB link margin minimum for fixed wireless; 20-30 dB for mobile systems subject to multipath fading; 30-40 dB for critical infrastructure (ITU-R P.530)
  • ✓Use ITU-R propagation models appropriate to environment: P.525 (free space), P.1411 (urban), P.833 (vegetation), P.676 (atmospheric), P.838 (rain attenuation)
  • ✓Validate link budget predictions with drive testing or site survey — actual propagation often differs 5-15 dB from models due to local terrain and building effects

Common Mistakes

  • ✗Using free-space path loss for terrestrial links without environmental corrections — add 10-30 dB for urban environments (ITU-R P.1411), 6-15 dB for suburban, 3-6 dB for rural with vegetation per ITU-R P.833
  • ✗Neglecting cable and connector losses — a 30m LMR-400 run at 2.4 GHz loses 3.5 dB; four N connectors add 0.6 dB; total 4.1 dB often omitted from link budgets
  • ✗Confusing antenna gain with EIRP — transmit power + antenna gain = EIRP; regulatory limits (FCC Part 15) typically specify EIRP, not transmit power alone
  • ✗Ignoring frequency-dependent atmospheric absorption — negligible below 10 GHz but critical at 60 GHz (15 dB/km) and 24 GHz (0.2 dB/km) per ITU-R P.676

Frequently Asked Questions

dBm is power referenced to 1 milliwatt: P(dBm) = 10*log10(P_mW). Common values: 0 dBm = 1 mW, 10 dBm = 10 mW, 20 dBm = 100 mW, 30 dBm = 1 W. Receiver sensitivities are typically negative: -100 dBm = 0.1 pW (WiFi), -130 dBm = 0.1 fW (GPS). The dBm scale allows link budget arithmetic by simple addition/subtraction rather than multiplication/division of power levels.
Free-space path loss increases 20*log10(f2/f1) dB when frequency increases from f1 to f2. Doubling frequency adds 6 dB loss. At 1 km: 433 MHz = 92.5 dB FSPL; 915 MHz = 99.2 dB; 2.4 GHz = 107.6 dB; 5.8 GHz = 115.2 dB. This 22.7 dB difference between 433 MHz and 5.8 GHz explains why sub-GHz IoT protocols (LoRa, Sigfox) achieve much longer range than WiFi for the same transmit power.
This calculator provides theoretical free-space baseline per ITU-R P.525. For real environments, add empirical loss factors: Indoor office: +20 to +40 dB (walls, floors); Urban outdoor: +20 to +30 dB (buildings, vehicles); Suburban: +10 to +20 dB; Rural open: +3 to +10 dB (vegetation, terrain). For detailed modeling, use Okumura-Hata (150 MHz-1.5 GHz urban), COST-231 (1.5-2 GHz), or ray-tracing for specific building layouts.
Depends on modulation and bandwidth. WiFi (OFDM, 20 MHz BW): -65 dBm excellent, -75 dBm good, -85 dBm marginal. Cellular LTE: -80 dBm excellent, -100 dBm usable. LoRa (SF12, 125 kHz): -137 dBm sensitivity. GPS: -130 dBm nominal. Bluetooth: -70 dBm excellent, -90 dBm usable. The 60+ dB difference between WiFi and LoRa sensitivity explains the range/throughput tradeoff — LoRa achieves 15 km at 300 bps while WiFi reaches 100m at 100 Mbps.
Antenna gain directly adds to link budget: +3 dBi = doubles range (for constant sensitivity) because 6 dB path loss equals 2x distance. A 24 dBi parabolic dish provides 24 dB more link budget than a 0 dBi omni — equivalent to reducing path loss from 1 km to 60m, or increasing transmit power 250x. High-gain antennas trade coverage area for range: a 24 dBi dish has 10-degree beamwidth requiring precise alignment.
Link budget approach: Available path loss = P_tx + G_tx + G_rx - P_sensitivity - margin. Example: 20 dBm transmit, 2 dBi antennas each side, -137 dBm sensitivity (SF12), 20 dB margin = 20 + 2 + 2 - (-137) - 20 = 141 dB allowable FSPL. Solve FSPL = 20*log10(d) + 20*log10(433e6) - 147.55 = 141 dB for d = 700 km theoretical. Real-world with terrain: 10-30 km rural, 2-5 km suburban, 0.5-2 km urban. The sub-GHz advantage: same calculation at 2.4 GHz yields only 125 km theoretical due to 15 dB higher FSPL.
ITU-R P.530-17 defines fade margin requirements by availability: 99.9% availability: 15-20 dB margin; 99.99%: 25-30 dB; 99.999%: 35-40 dB. Margin accounts for multipath fading, rain attenuation (significant above 10 GHz), equipment aging, and atmospheric variations. For a 10 km, 18 GHz link in temperate climate: 15 dB multipath + 8 dB rain (0.01% exceedance) + 3 dB equipment = 26 dB total margin for 99.99% availability.
Antenna height affects Fresnel zone clearance, not free-space loss directly. First Fresnel zone radius at mid-path: r1 = sqrt(lambda * d/4). For 10 km link at 5.8 GHz: r1 = sqrt(0.052 * 5000) = 16m. If terrain obstructs > 40% of this zone, add 6+ dB diffraction loss. Height determines whether the Fresnel zone is clear — insufficient clearance is the most common cause of link failures in point-to-point systems. Rule of thumb: antenna height should provide r1 clearance above any obstacles at mid-path.
Link margin = P_received - P_sensitivity (total safety buffer). Fade margin is the portion reserved for signal fading events. Example: 30 dB link margin might allocate: 20 dB fade margin (multipath, rain), 5 dB implementation margin (component tolerance, aging), 5 dB interference margin. Fade margin determines availability statistics — 20 dB fade margin with Rayleigh fading yields approximately 99.9% availability per ITU-R P.530. Under-specifying fade margin is the leading cause of intermittent link failures.

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