Repeater Coverage Calculator
Estimate the radio horizon and coverage radius of a ham radio repeater based on antenna height.
Uses line-of-sight radio propagation formula.
How Repeater Coverage Is Calculated
A VHF/UHF repeater extends the range of handheld radios by receiving on one frequency and simultaneously retransmitting on another from a high location. Coverage is primarily determined by radio line-of-sight.
Radio Horizon Formula:
Distance (km) = 4.12 × (√h_repeater + √h_mobile)
Where:
- h_repeater = repeater antenna height above ground in meters
- h_mobile = mobile/portable antenna height in meters
- 4.12 = constant accounting for atmospheric refraction (slightly beyond optical horizon)
Worked Example: Repeater on a 120m tower, mobile station at 1.5m height:
- Distance = 4.12 × (√120 + √1.5)
- Distance = 4.12 × (10.95 + 1.22)
- Distance = 4.12 × 12.17 = 50.1 km radius
This is a theoretical maximum — terrain, buildings, and foliage reduce real-world coverage.
Path Loss Formula (Free Space):
FSPL (dB) = 20×log10(d) + 20×log10(f) + 92.4
where d is in km and f is in GHz.
Practical Coverage Factors:
- 2m band (144 MHz): excellent range, less obstructed by foliage
- 70cm band (440 MHz): more susceptible to building attenuation
- CTCSS tones: repeaters use subaudible tones (67–254 Hz) to prevent interference
- EIRP (Effective Isotropic Radiated Power) = TX Power × Antenna Gain
- Standard repeater output: 25–50W with high-gain Yagi or collinear antenna
Typical Coverage Radii:
- Hilltop 50m, 25W: ~30 km
- Mountain 500m, 50W: ~80–100 km
- Aircraft at 3,000m: ~200 km LOS
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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