Military Radio Range Calculator

At the calculator's default antenna heights and radio settings, line-of-sight range (11.65 km, from antenna heights alone, line 17) is smaller than free space range (over 4,000 km, from transmit power, gain, sensitivity, and frequency, line 14), and effective range takes the smaller of the two before applying terrain (line 25). Because line-of-sight is the binding constraint here, antenna height moves effective range while transmitter power, antenna gain, receiver sensitivity, and frequency do not -- that would flip if line-of-sight range were raised well above free space range instead.

Terrain applies a multiplier to whichever of free space range or line-of-sight range is smaller: 0.9 for open/flat terrain down to 0.35 for urban terrain, with heavy forest and mountainous terrain in between (lines 21-22, 26). Urban terrain therefore cuts theoretical range by 65 percent versus open terrain's 10 percent reduction, reflecting how much more signal absorption and multipath interference dense urban structures cause compared to open ground.

Line-of-sight range is 4.12 times the sum of the square roots of each antenna's height in meters, expressed in kilometers (line 17) -- a standard radio-horizon approximation accounting for earth curvature. It depends only on the two antenna heights, not on transmitter power, antenna gain, receiver sensitivity, or frequency, which instead determine the separate free space range figure.

Link margin is link budget -- transmit power plus antenna gain minus receiver sensitivity (line 12) -- minus the actual path loss at the calculated effective range (lines 30-33). A larger positive margin means more headroom before the signal drops below the receiver's sensitivity threshold at that range; receiver sensitivity is the single largest driver of link margin among the radio parameters, since it subtracts directly into both link budget and the effective range calculation.