Repeater Coverage Calculator

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 all cut into it, and a single hill in the wrong place removes the whole path.

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 to 254 Hz) to prevent interference
• EIRP (Effective Isotropic Radiated Power) = TX Power × Antenna Gain expressed as a plain ratio, not in decibels. A 9 dBi antenna is a factor of 7.9, so 50 W becomes 396 W EIRP. The antenna gain calculator does the conversion and takes feedline loss off first
• Standard repeater output: 25 to 50 W into a high-gain collinear or a Yagi

Typical Coverage Radii

Height is the only thing in the horizon formula, so these are worked from height alone, with a mobile whip at 1.5 m. The transmitter power is listed because people always ask, but it changes nothing here: power decides whether you are heard inside the horizon, not where the horizon is.

The last row is the interesting one. An aircraft at altitude with a 5 W handheld beats a mountaintop repeater running ten times the power, because there is nothing in the way. That is the whole lesson of this page: height wins, and it is not close.