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.
VHF and UHF radio signals travel primarily by line-of-sight. The radio horizon is slightly farther than the geometric horizon due to atmospheric refraction, which bends radio waves slightly downward.
Radio horizon formula: d (km) = 4.12 × √h (meters)
Where h is the antenna height above ground in meters and d is the distance to the radio horizon.
Combined horizon for repeater + mobile: d_total = 4.12 × (√h_repeater + √h_mobile)
Typical mobile antenna height: 1.5–2 m in a vehicle.
Coverage examples:
- Repeater at 30 m height: ~22.5 km radius to a ground-level station
- Repeater at 100 m height: ~41 km radius
- Repeater on a 300 m hill: ~71 km radius
- Mobile at 1.5 m adds another 5 km
Real-world factors: This formula gives best-case line-of-sight coverage. Actual coverage is usually less due to buildings, trees, hills, and other obstructions. Mountainous terrain creates shadows. Tropospheric ducting can occasionally extend VHF coverage hundreds of kilometers, but this is rare and unpredictable.
Increasing coverage: Height is the dominant factor. Every 4× increase in antenna height doubles the coverage radius. This is why hilltop and water-tower repeaters serve such large areas.