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Link budget calculator

A link budget adds every gain and subtracts every loss between transmitter and receiver, in decibels, to predict the signal level arriving at the far end. Compare that level with the receiver’s sensitivity and the difference is the fade margin — the headroom that decides whether the link survives rain, fading and interference.

dBm

= 100 mW

dB
dBi
dB

Free space, obstructions, everything between the antennas. Get the free-space part from the FSPL tool.

dBi
dB
dBm
RX signal
−91.5 dBm
= 708 fW at the receiver input
Fade margin
−1.5 dB
below sensitivity — the link does not close
EIRP
27.0 dBm
what leaves the TX antenna
−150−100−50050sensitivity −90.0 dBm20.0radio19.0TX cable27.0TX ant−93.0path loss−91.0RX ant−91.5RX inmargin −1.5 dBlevel diagram · left to right along the chainsignal level (dBm)EIRP 27.0 dBm

Each dot is the signal level in dBm after one stage of the chain — radio, TX cable, TX antenna, path, RX antenna, receiver input — so the line drops at every loss and steps up at every gain. The amber dashed line is the receiver sensitivity, and the bracket on the right measures the fade margin between it and the arriving signal.

Uses your path-loss number as given — it does not model terrain itself.

How it works

  1. 01

    Start at the transmitter, in dBm

    Put the whole chain in decibels. Working in dB turns the multiplications and divisions of linear power into additions and subtractions, so the entire link collapses into one line of arithmetic. A 20 dBm (100 mW) radio and a 43 dBm (20 W) radio differ by 23 dB and nothing else.

  2. 02

    Subtract losses and add gains in chain order

    Feedline and connector loss comes off first, then the antenna gain goes on: PTX − Lcable + GTX is the EIRP leaving the mast. Do the same in reverse at the far end. Typical magnitudes: 0.5–3 dB of coax between radio and antenna, 8 dBi for a small panel, about 25 dBi for a 40 cm dish at 5.8 GHz.

  3. 03

    Subtract the path loss

    This is the dominant term and the one worth getting right. It starts at free-space loss — about 120 dB for 10 km at 2.4 GHz — and grows with terrain diffraction, building and foliage penetration and rain. Every other term in the budget is a few dB; the path is a hundred or more.

  4. 04

    Compare with the receiver sensitivity

    The difference between the arriving level and the sensitivity figure is the fade margin: Margin = PRX − Psensitivity. Planners normally want 10–20 dB, because rain, multipath fading, antenna sway and interference all eat into it over time. A link with 3 dB of margin on a clear day drops out on a wet one; below 0 dB it never closes at all.

Formulas

Received signal level
PRX = PTX − LTX cable + GTX − Lpath + GRX − LRX cable
  • PRX — received power at the receiver input, dBm
  • PTX — transmitter output power, dBm
  • LTX cable, LRX cable — feedline and connector loss, dB
  • GTX, GRX — antenna gains, dBi
  • Lpath — total loss between the two antennas, dB
Fade margin
Margin = PRX − Psensitivity
  • Psensitivity — level the receiver needs for the wanted data rate, dBm
  • Margin — headroom in dB; 10–20 dB is the usual planning target
EIRP (the transmit half of the chain)
EIRP = PTX − LTX cable + GTX
  • EIRP — equivalent isotropically radiated power, dBm
  • the level that leaves the antenna, before any path loss

Worked example

A 20 dBm radio, 8 dBi and 2 dBi antennas, 120 dB of path loss
  1. EIRP = 20 − 1 + 8 = 27 dBm
  2. PRX = 20 − 1 + 8 − 120 + 2 − 0.5 = −91.5 dBm
  3. Margin = −91.5 − (−90) = −1.5 dB
  4. → this link does not close: it is 1.5 dB short before any fading
  5. → add antenna gain or height, or shorten the hop

FAQ

What is a good fade margin?
10–20 dB is the usual target for a terrestrial link, and 20 dB or more where rain fade matters (roughly above 10 GHz) or where the link must hold 99.99% availability. Under 10 dB a link works on calm, dry days and drops out on the rest. Under 0 dB it never closes.
What is the difference between EIRP and transmitter power?
Transmitter power is what the radio produces at its connector. EIRP is what the antenna appears to radiate in its main beam: EIRP = PTX − Lcable + GTX. A 20 dBm radio with 1 dB of cable and an 8 dBi antenna reaches 27 dBm EIRP — five times the power in the beam direction, from the same radio.
Why is a link budget written in dB?
Because a link spans an enormous range: a 20 dBm transmitter and a −90 dBm receiver sit eleven orders of magnitude apart in power. Decibels compress that into two-digit numbers and turn every multiplication into an addition, so the whole chain becomes a sum you can check in your head.
Can I just increase transmitter power?
Only up to the regulatory limit, which caps radiated power (EIRP), not the radio: 20 dBm EIRP in the EU 2.4 GHz band, 36 dBm for US point-to-multipoint. Power also helps one direction only — a louder base station does not fix the handset’s uplink. Antenna gain and antenna height improve both ends at once.

The path-loss line is where budgets go wrong. Waveshed computes it over real terrain — free, in your browser.

Get the real path loss →

Related guides

Sources & further reading

Related tools

Free-space path loss calculator

Signal loss over distance for any frequency, from the FSPL formula.

EIRP calculator

Radiated power from TX power, cable loss and antenna gain — with regulatory limits.

Noise floor calculator

Thermal noise (kTB), receiver noise figure and sensitivity.

LoRa / Meshtastic range calculator

Maximum link distance from spreading factor, TX power, antennas and heights: free space, ground and horizon limits.