RF Link Budget Calculator

The link budget equation

A link budget sums every gain and loss between transmitter and receiver to predict the power arriving at the receiver input:

All terms are in dB or dBm, so the budget is simple addition and subtraction — which is precisely why the waterfall table above is the heart of the tool: each line is signed and the running total is auditable at a glance.

Link margin and fade margin

The link margin is how much stronger the received signal is than the receiver's sensitivity floor. A positive margin means the link has headroom; a negative margin means it will not close. The required fade margin is the headroom you choose to reserve for signal fluctuations (multipath fading, rain, foliage, movement). This tool compares margin against your required fade margin and returns a clear PASS or FAIL.

Where the numbers come from

Free-space path loss uses the tested Friis-based engine (see the FSPL calculator). Feeder loss can be pulled straight from the coax library — including verified Times Microwave LMR and Gigatronix LBC datasheet figures — so the frequency you enter here drives both FSPL and cable attenuation consistently. Generic RG references remain typical and are flagged amber so you never mistake an estimate for a guaranteed value.

Worked example

With the default inputs, Tx 43 dBm (about 20 W), a 3 dBi antenna and 5 m of LBC240 at 900 MHz give 1.24 dB of cable loss. Adding 0.5 dB of connector loss gives 1.74 dB per feeder and EIRP ≈ 44.26 dBm. Over 1 km at 900 MHz, FSPL ≈ 91.53 dB. With a 3 dBi Rx antenna, the same Rx feeder and no other losses, P_Rx ≈ −46.01 dBm. Against a −100 dBm sensitivity, the margin is ≈ 53.99 dB, exceeding the chosen 10 dB fade reserve. Changing sensitivity to −95 dBm gives ≈ 48.99 dB of margin. The verdict depends on your chosen receiver sensitivity and fade reserve.