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Link Budget Calculator

Calculate RF link budget from transmit power, gains, and losses. Free online ham radio calculator with instant, accurate results.

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Inputs that matter
TX Power, TX Antenna Gain, RX Antenna Gain, Path Loss, and 2 more
Output to expect
Link Budget Calculator
  • Check the units and required inputs before comparing results.
  • Keep the assumptions with a copied result so you can reproduce the calculation later.
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Reference & details

How it works

Received Power Equation

The fundamental link budget sums transmit power, antenna gains, and subtracts all losses including free-space path loss, cable attenuation, and connector losses.

Received_Power(dBm) = Tx_Power - Tx_Losses + Tx_Antenna_Gain - Path_Loss + Rx_Antenna_Gain - Rx_Losses

Free-Space Path Loss

FSPL depends on frequency and distance. Higher frequencies and longer paths increase loss exponentially.

FSPL(dB) = 20×log₁₀(d_km) + 20×log₁₀(f_MHz) + 32.44

Link Margin

Compare received power to receiver sensitivity plus required SNR. Positive margin indicates a reliable link; 10 dB margin is typical for analog FM, 3 dB for digital modes.

Margin(dB) = Received_Power - (Rx_Sensitivity + Required_SNR)

Updated: July 2026

Example Scenarios

A 5W HT on 146.52 MHz accessing a repeater 15 km away with rubber duck antenna.

Tx Power: 37 dBm (5W)Tx Antenna Gain: 0 dBi (rubber duck)Cable Loss: 1 dBDistance: 15 kmRx Sensitivity: -117 dBm

→ FSPL: ~103 dB; received: ~-67 dBm; margin: ~50 dB — strong access

Working an FM satellite at 400 km altitude with 25W and a 7-element Yagi (10 dBi).

Tx Power: 44 dBm (25W)Tx Antenna: 10 dBi YagiPath Distance: ~2,000 km (slant range)Frequency: 435 MHz

→ FSPL: ~129 dB; received at satellite: ~-75 dBm; viable for FM satellite uplink

NVIS operation on 80m at 300 km using 100W and a dipole at 15 ft height.

Tx Power: 50 dBm (100W)Antenna Gain: 0 dBi (dipole)Distance: 300 kmFrequency: 3.8 MHzGround Loss: 10 dB (NVIS absorption)

→ Received: ~-85 dBm; NVIS reliable for regional emergency comms

Common Mistakes to Avoid

Confusing dBi and dBd antenna gain ratings

dBd is gain relative to a dipole (dBi = dBd + 2.15). Mixing units causes 2 dB errors. Always convert to dBi for link budget calculations.

Ignoring coax loss at UHF and above

RG-58 loses ~6 dB per 30 m at 440 MHz. Use low-loss LMR-400 or place the radio at the antenna mast for VHF/UHF weak-signal work.

Using free-space path loss for HF ground-wave paths

HF ground wave and NVIS paths have additional ground absorption and ionospheric effects. FSPL alone overestimates received signal on HF.

FAQ

A minimum of 10 dB margin above receiver sensitivity ensures reliable analog FM copy with fading margin. Digital modes like FT8 can decode with 0 dB margin due to coding gain.

Add obstruction losses: 10–20 dB for one wall, 20–30 dB for terrain blockage. Use propagation models (Hata, Longley-Rice) for VHF/UHF over irregular terrain.

High SWR causes reflected power that reduces effective radiated power. A 3:1 SWR wastes ~25% of power (~1.25 dB loss). Match antennas to keep SWR below 2:1.

Cross-polarization (vertical TX, horizontal RX) adds 20 dB loss. Satellite and weak-signal work requires matching polarization or using circular polarization.

Use 12 dB SINAD sensitivity for analog FM (typically -117 dBm for VHF rigs). For digital modes, use the manufacturer's MDS (minimum discernible signal) or published decode threshold.

About Link Budget Calculator

An RF link budget accounts for every gain and loss between your transmitter and receiver to predict whether a signal will be copyable. Ham radio operators use link budgets for VHF/UHF repeater planning, satellite contacts, and weak-signal DX work.