Repeater Coverage Radius Estimator
Estimate the free-space coverage radius of a radio repeater from its EIRP, the receiver sensitivity, the frequency and a required fade margin. It inverts the free-space path-loss equation to turn a link budget into a range.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the repeater EIRP in dBm and the receiver sensitivity in dBm (usually negative).
- Enter the operating frequency in MHz, plus the receiver antenna gain (dBi) and any required fade margin (dB) you want to reserve.
- Read the maximum allowable path loss and the resulting coverage radius, and remember it is an optimistic free-space upper bound.
How it works
First the budget gives the maximum path loss the link can stand: max path loss = EIRP + Rx gain − Rx sensitivity − required margin. Then that loss is fed back into the free-space path-loss formula FSPL(dB) = 32.44 + 20·log10(d_km) + 20·log10(f_MHz), solved for distance: d_km = 10^((maxPL − 32.44 − 20·log10(f_MHz)) / 20). Because free-space assumes an unobstructed line of sight, the answer is the best case; terrain and clutter always cut it down.
Worked example
Worked example. With EIRP = 50 dBm, sensitivity = −110 dBm, f = 450 MHz, Rx gain = 3 dBi and a 10 dB margin: max path loss = 50 + 3 − (−110) − 10 = 153 dB. Then d = 10^((153 − 32.44 − 53.064) / 20) = 10^3.375 ≈ 2,370 km as a free-space upper bound — real VHF/UHF coverage would be a small fraction of this over terrain.
Common mistakes
- Reading the free-space radius as real coverage — over terrain and clutter the usable range is far shorter.
- Using the wrong frequency units: the 32.44 constant is for distance in km and frequency in MHz.
- Omitting a fade margin, which overstates the reliable coverage radius.
Frequently asked questions
Why is the coverage radius so large?
Because free-space loss assumes a clear line of sight with no obstructions. It is the theoretical maximum; hills, buildings and vegetation add loss that dramatically reduces the real service radius.
What is EIRP?
Effective isotropic radiated power — the transmit power plus the transmit antenna gain minus feeder losses, all in dB, expressed relative to an isotropic radiator. It already bundles the transmitter side of the link budget.
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- Two-Ray Ground Reflection Loss Calculator
- Fresnel Zone Clearance Calculator
- Radio Horizon Distance Calculator
- Link Margin Calculator
- Thermal Noise Floor Calculator
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Tip: Enter any known values to calculate the remaining results.
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