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.
Estimated Coverage Radius
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