How to use the free space path loss and link budget calculator
- Enter the frequency and the distance between the antennas, and choose their units.
- To see the received signal, enter the transmit power and its unit, plus the antenna gains.
- In Advanced options add cable or connector losses, the receiver sensitivity from its data sheet, and the fade margin you want.
- Read the path loss, the received power, the margin above sensitivity and the longest free space range.
Formula
Free space path loss = 20 log10(4 x pi x distance x frequency / c), with c = 299,792,458 m/s
Received power = transmit power - transmit losses + transmit gain - path loss + receive gain - receive losses
Margin = received power - receiver sensitivity
Longest range: the distance whose path loss uses up all the available budget, less the fade margin
Distance is in meters and frequency in hertz inside the formula. Every term except the path loss is a plain sum in decibels, which is why a link budget is easy to do by adding.
A 2.4 GHz hop and a long range radio link
Over 1 kilometer at 2.4 GHz the free space path loss is 100.052 dB. The common shortcut of 20 log10 of the distance in km plus 20 log10 of the frequency in MHz plus 32.44 gives 100.044 dB, which agrees within 0.01 dB.
A 915 MHz link over 5 km with 14 dBm of power, 2 dBi antennas at both ends, 0.5 dB of loss at each end and a receiver sensitivity of -137 dBm has a path loss of 105.656 dB and an EIRP of 15.5 dBm. The received power is -88.656 dBm, a margin of 48.344 dB. With a 10 dB fade margin kept in reserve, the free space range could reach about 413 km, far beyond what the horizon allows.
Why free space is a best case
Free space loss assumes a clear line of sight in empty space. Real paths lose more signal to walls, trees, rain, the ground and misaligned antennas, and the first Fresnel zone needs to be mostly clear. The result is a ceiling for the received power, and the fade margin is the cushion you keep for everything the formula leaves out.
The distance to the horizon calculator shows how far the Earth lets two antennas see each other. The VSWR calculator covers the mismatch at an antenna, and the decibel calculator converts between power and decibels.
Power units
A power in dBm is decibels relative to one milliwatt, so 0 dBm is 1 mW, 20 dBm is 100 mW and 30 dBm is 1 W. Antenna gain in dBi is measured against an ideal radiator that sends equally in every direction. EIRP is the transmit power after losses plus the transmit antenna gain.
Assumptions and limits
- The two antennas have a clear line of sight in empty space, with no obstacles, reflections or atmospheric loss.
- Antenna gains, cable losses and the receiver sensitivity are the values you enter; the page does not look them up.
- The polarization of the antennas is matched and they point at each other.
- Frequency must be above zero and the distance within a range the page can show, from a tiny fraction of a meter to planetary scale.
Frequently asked questions
How do I calculate free space path loss?
Use 20 log10(4 x pi x d x f / c), with d in meters, f in hertz and c the speed of light. For d in km and f in MHz the sum 20 log10(d) + 20 log10(f) + 32.44 gives nearly the same answer.
What is a link budget?
It is the list of gains and losses between a transmitter and a receiver, added in decibels. The margin left above the receiver sensitivity tells you how much extra loss the link can survive.
Why does the loss go up with frequency?
The formula uses the size of the receiving antenna aperture, which shrinks with frequency for a given gain. A higher frequency therefore captures less of the same wave, so the loss rises by 6 dB each time the frequency doubles.
How much fade margin should I keep?
There is no single answer. It depends on weather, obstacles and how reliable the link must be. Some designers add 10 dB or more; the page lets you enter your own value.