Free-space path loss
How much the signal spreads out over the distance in clear air: 20 log10(km) + 20 log10(MHz) + 32.44. Doubling the distance costs 6 dB. Rain, foliage and obstructions cost more on top.
Link budget calculator
Check whether a point-to-point link will close before you climb the tower. It runs in your browser: nothing you type is sent anywhere.
- Free-space path loss
- 127.7 dB
- EIRP
- 41.0 dBm
- Expected RSSI
- -63.7 dBm
- Fade margin
- 16.3 dB
- Fresnel zone radius at midpoint
- 11.4 m
Cable and connector loss is the total for both ends, taken off once on the transmit side: EIRP is TX power plus TX gain minus that loss, and RSSI is EIRP minus path loss plus RX gain.
What each figure means
The arithmetic behind each number, so you can check it against your radio's datasheet.
EIRP
The power the antenna radiates towards the far end. Regulators cap it by band and country, so check your local limit before raising TX power.
Expected RSSI
The signal level the far radio should report in clear weather with the antennas aligned. A real link reads a few dB below it.
Fade margin
How far the signal can drop before the radio loses the link at that sensitivity. Use the sensitivity for the modulation rate you want to hold, not the lowest one on the datasheet.
Fresnel zone radius
The radius of the first Fresnel zone at the middle of the path, where it is widest: 8.656 times the square root of km over GHz. Trees, roofs and terrain inside it cost signal even when the far end is in sight.