Free Space Path Loss Calculator

The free space path loss calculator allows you to predict the strength of a radio frequency signal emitted by an antenna at any given distance.

Free-space path loss114.203 dBat 5 km and 2,450 MHz

EIRP41.5 dBm14.1254 W equivalent isotropic radiated power

Transmitter power30 dBm1 W

Antenna gain14 dBi25.119× the power an isotropic radiator would put in that direction

Cable and connector loss2.5 dB43.8% of the power lost before it reaches the antenna

EIRP in watts14.1254 W

Received power-64.703 dBm338.6319 pW

Link margin25.297 dBthe link closes, with this much to spare for rain, obstruction and fading

Range at which the margin vanishes92.01 kmpath loss grows 6 dB per doubling of distance, so each 6 dB of margin doubles the reach

Wavelength122.3643 mma quarter-wave element would be 30.5911 mm

Doubling the transmit power44.51 dBmgains 3 dB — the same as 3 dB more antenna gain, and usually far more expensive

The formula

EIRP = transmit power + antenna gain − losses

Antenna gain is free, transmit power is not

EIRP is what a perfectly isotropic radiator would need to emit to match your signal in the direction the antenna points. It is the sum of transmitter power in dBm, antenna gain in dBi, and whatever the cable takes away — and because everything is in decibels, the sum really is an addition.

Antenna gain does not create power; it redistributes it. A 14 dBi antenna puts 25 times more power in its favoured direction by taking it from elsewhere. That is usually exactly what you want, and it is far cheaper than a bigger transmitter: doubling transmit power buys 3 dB, and so does a modestly better antenna at both ends.

Free-space path loss grows as the square of both distance and frequency — 6 dB per doubling of either. The frequency term is why higher bands need more gain for the same range, and the distance term is why every 6 dB of link margin doubles your reach. Note that free space is the best case: real paths add obstruction, ground reflection, foliage and rain, so a margin of 10 dB or more is normal engineering practice rather than caution.