Repeater Coverage Calculator

Repeater range at VHF/UHF is dominated by the radio horizon — the line-of-sight distance limited by Earth's curvature — rather than by transmit power alone, which is why height matters more than watts. This calculator computes each antenna's horizon distance with d = 4.12 × √h (km, h in meters), a standard formula that already bakes in the 4/3-Earth-radius correction for atmospheric refraction bending the signal path slightly beyond true geometric line of sight, then adds the repeater's and the mobile's horizon distances together for the total clear-path radio horizon. That theoretical horizon then gets scaled down by a terrain factor (1.0 for flat/open ground down to 0.35 for dense urban or mountainous terrain) to reflect real-world obstruction, giving the "effective coverage radius" the calculator highlights, along with the circular coverage area in km² and mi².

Separately, it estimates the actual signal strength a mobile station would see at the edge of that coverage circle: it converts repeater power and terrain-adjusted distance into free-space path loss (20log₁₀(d) + 20log₁₀(f) + 32.44), then compares the resulting received signal against a typical FM receiver's -116 dBm sensitivity threshold for 12 dB SINAD to report a signal margin. Because FM repeater systems are typically limited by the weaker of the two directions, remember this only checks the repeater-to-mobile downlink — the mobile's own transmit power (often just a few watts from a handheld) may not reach back to the repeater even where downlink signal looks strong, so real two-way range is often set by the uplink, not this calculator's coverage circle.