4S6TMP.com — FSPL, link margin, range, and Fresnel zone calculator
A link budget is an accounting of all gains and losses in a radio communication link. It determines whether a signal will be successfully received given the transmitter power, antenna gains, path losses, and receiver sensitivity.
Free Space Path Loss (FSPL) is the loss in signal strength due to spreading of the radio wave as it travels through space. It increases with frequency and distance: FSPL(dB) = 20×log10(d) + 20×log10(f) + 32.45 where d is in km and f is in MHz.
Link margin is the difference between received power and receiver sensitivity. A positive margin means the link will work. For reliable links, aim for at least 10 dB margin. For critical links, 20 dB is recommended.
The Fresnel zone is an elliptical region around the direct line of sight between transmitter and receiver. Obstructions in this zone cause diffraction and signal loss. For reliable communication, at least 60% of the first Fresnel zone must be clear of obstacles.
The radius of the first Fresnel zone at a point along the path is: F1 = 17.3 × √(d1 × d2 / (f × d)) where d1 and d2 are distances to each end in km, d is total distance in km, and f is frequency in MHz.
Why does higher frequency have more path loss? Higher frequencies have shorter wavelengths, so the same physical aperture captures less energy. Also, spreading loss is greater for higher frequencies over the same distance.
What is a good link margin? For line-of-sight terrestrial links, 10–20 dB is typical. For satellite links, 6–10 dB is common due to the dynamic nature of the channel. For indoor links, 20+ dB may be needed.