FSPL / Link Budget Calculator
Calculate free-space path loss, received power and link margin from distance, frequency, antenna gains and losses.
Free-space path loss is an ideal line-of-sight model. It does not account for terrain, multipath, atmospheric absorption, ionospheric propagation or antenna pattern nulls. HF skywave links need a propagation model instead.
Read the link margin
A link budget is most useful when every gain and loss has a clear physical meaning. Start with transmitter output, subtract feedline and other losses, add antenna gain, and then subtract path loss before comparing the result with receiver sensitivity.
Free-space path loss assumes unobstructed propagation. Buildings, terrain, foliage, polarization mismatch and atmospheric effects can add substantial real-world loss, especially outside the idealized conditions of the calculation.
A healthy link margin is more useful than a barely positive theoretical result. For field links, leave margin for installation variation and confirm the result with an actual received-signal measurement.
Use the margin, not just the path loss
A link budget is an accounting exercise: start with transmitter power and add or subtract every significant gain and loss until the expected received level is obtained. Free-space path loss provides a useful baseline, but a real amateur-radio path can include antenna gain, feedline loss, connector loss, atmospheric effects, terrain and other propagation mechanisms. Keep power units consistent and distinguish between dBm, watts and dB gains or losses. A positive antenna gain is not free power; it represents a directional or reference-dependent change in field strength or equivalent radiated power. Likewise, a calculated received level does not guarantee intelligible reception because receiver sensitivity, noise floor, fading and interference still matter. The calculator is most valuable when used to compare scenarios: a longer coax run, a different antenna gain, a lower transmitter power or a change in frequency can then be evaluated on the same basis. For a real path, compare the estimate with measured or reported signal levels and adjust the assumptions rather than forcing the result to match the observation.