Last Updated on April 17, 2026 by John Berry
A path budget is a decibel account used to predict available radio communication range. It sums all system gains and subtracts all losses between two stations. Engineers often call this a link budget. This calculation determines if a signal remains usable above the receiver threshold. You can view the budget as a “currency pot” for your radio signal to be spent as you see fit and can afford.
Components of the budget
The budget balances your station’s capabilities against the environment. The system value provides your starting balance. This value represents the total loss a signal can endure before failing. You then add antenna gains and subtract feeder losses to find the available path loss. The available path loss determines the path length and excess path losses like diffraction.
Consider a 144 MHz SSB station with a system value of 166 dB. Assume both stations use 12 dBi antennas and 2 dB feeder loss.
The total available path loss (Lp) is 166dB + 2(12dBi -2dB) or 186dBi.
The Free Space Model
A path budget often starts with the Free Space Path Loss (FSPL) model. This model assumes no obstacles and no atmosphere. Solving for d, the path distance in kilometres, in the Free Space Path loss equation gives:
d = 10((Lp – 32.44 – 20log10(f))/20)
Using a total path loss of 186 dB at 144 MHz, the theoretical distance over which communication is possible is approximately 330,000 km.
This distance nearly reaches the Moon. It explains why Earth-Moon-Earth (EME) communication is possible for amateurs. However, paths seldom truly achieve free space conditions.
Terrestrial realities
Practical paths involve the Earth’s surface and atmosphere. The Earth’s curvature eventually blocks the direct line of sight. You must also consider the First Fresnel Zone. This is an elliptical volume around the direct path. Obstacles in this zone cause diffraction and increase loss.
For a 15-metre-high antenna, the ground often intrudes into this zone. This adds significant diffraction loss. A typical terrestrial path may have 60 dB of additional loss from terrain or buildings. This reduces the 186dB budget from a range of 330,000 km to roughly 300 km – a more realistic distance.
A path budget can be calculated for every path and every technology. This helps radio amateurs dimension their systems for different objectives.
Improving things
You can improve your path budget through physical station upgrades. Larger antennas increase dBi gain. High-quality coaxial cables reduce feeder loss. Mounting antennas higher reduces diffraction loss by clearing the Fresnel zone. Every decibel saved allows the signal to travel further or overcome deeper fades.