Radio Distance Calculator

How far will a radio signal actually reach? The answer depends on which limit you hit first: the curve of the Earth, or the point where the signal fades below what the receiver can hear. This calculator covers both, because most tools only cover one. Use the horizon mode for line-of-sight range from antenna heights, the height solver to work backward from a target distance, and the link budget mode for free-space range from transmitter power, antenna gain, receiver sensitivity, and frequency.

Quick Answer

How to use the radio distance calculator

Each tab answers a different planning question. Radio horizon takes two antenna heights and adds their individual horizons. Required height takes a target distance and one fixed antenna, and solves for the other. Link budget range ignores terrain entirely and asks how far the signal stays decodable in empty space.

All three tabs come pre-filled with the worked examples used here, so you can press Calculate first and watch the numbers land before changing anything. Heights are in meters and distances in kilometers throughout. If you do not know your site-to-site distance yet, measure it first with the air distance calculator, then come back and ask whether your antennas can cover it.

The radio horizon formula: two horizons added together

Each antenna sees out to its own horizon, and the two horizons add: d = k × (√h₁ + √h₂), with heights in meters and distance in kilometers.

The factor k is 3.57 for pure geometry, from d = √(2Rh) with the true Earth radius. It becomes 4.12 under standard atmospheric refraction, where the atmosphere bends waves slightly toward the ground and stretches the effective Earth radius by about 4/3. That bending is real physics, not a fudge: the ITU-R P.453 recommendation on the radio refractive index is the reference behind the 4/3 model, and its refractivity data is what propagation engineers use when conditions differ from standard.

The table below is the building block for every horizon answer here: one antenna, alone, at common heights. A link range is just two rows of this table added together. For a single observer rather than a two-antenna link, the horizon distance calculator covers the visual version of the same geometry.

Worked examples, verified in the calculator

All three examples below are the pre-filled defaults on their tabs, so the numbers below are the numbers the tool returns.

Example 1: repeater to handheld (horizon mode)

A repeater antenna sits 50 meters up; a handheld radio is held about 10 meters above the valley floor (a hill plus arm height). With standard refraction selected, the calculator returns a transmitter horizon of 29.13 km, a receiver horizon of 13.03 km, and a combined line-of-sight range of 42.16 km (26.20 miles). Switch to geometric and the same link drops to 36.53 km, which is the size of the gift the atmosphere gives you on an average day.

The split matters for upgrade decisions: the tall antenna is doing most of the work, so raising the 10 m end buys range faster per meter than raising the 50 m end, because horizon grows with the square root of height.

Example 2: how high for 30 km (height solver)

You need a 30 km link and the far end is fixed at 10 meters. The solver computes (30 ÷ 4.12 − √10)² = 16.97 m (55.7 ft) for your antenna. Raise the known end and the required height drops, which the tool shows in its breakdown line; under the geometric factor the same target needs 27.47 m, a full 10 m more tower.

Example 3: 5 W handheld at 446 MHz (link budget mode)

Enter 5 W transmit power, 6 dBi combined antenna gain, −100 dBm receiver sensitivity, and 446 MHz. The calculator converts 5 W to 36.99 dBm, finds a path loss budget of 142.99 dB, and inverts Friis to get 755.20 km of free-space range. That number looks absurd until you remember it assumes empty space with zero obstacles: it is the signal-strength ceiling, not a coverage promise. Compare a Wi-Fi flavored setup (0.1 W, 3 dBi combined gain, −85 dBm sensitivity, 2400 MHz) and the same tab returns 2.50 km, which shows how fast the ceiling falls as power drops and frequency climbs.

Solving backward: how tall does the other antenna need to be?

The Required height tab is the horizon formula rearranged: h = (d ÷ k − √h_known)². It exists because the forward question (how far?) is usually the wrong one during planning; the real question is how much tower to buy.

Two behaviors are worth knowing. If the known antenna already covers the target on its own horizon, the answer is 0 m with an explanation, not a negative height. And the solver accepts a known height of 0, which models a receiver sitting at ground level at the far end. One asymmetry to note: in horizon mode the transmitter height must be above zero, while the receiver may be zero; in the solver, either role can be the fixed end by swapping which site you call your own.

Link budget mode: when signal strength, not the horizon, sets the limit

Link budget mode answers the second question, how far the signal stays loud enough to hear, using the Friis transmission equation: distance = (wavelength ÷ (4 × π)) × 10^(path loss ÷ 20), where the path loss budget is transmit power in dBm plus antenna gain in dBi minus receiver sensitivity in dBm.

The mode follows the same free-space method the ITU-R P.525 recommendation for free-space attenuation formalizes: loss grows with the square of distance and the square of frequency, and nothing else absorbs the signal. Two honest details about this implementation. Wavelength is computed as 300 ÷ frequency in MHz, using the rounded speed of light (300,000 km/s rather than 299,792 km/s), which shifts results by about 0.07 percent. And there is no fade margin built in: the output is the distance at which the signal arrives exactly at sensitivity, with nothing spare. For a link you intend to rely on, hold back a margin yourself, and use the link margin calculator to check how much headroom a specific planned path keeps. Antenna gain is the other input people guess at; if you are building a dipole, the dipole antenna calculator helps you estimate what its dimensions give you.

What competing calculators miss

The search results split into two camps that never meet. Naming the split is the fastest way to see what a complete tool has to do.

• Horizon tools stop at geometry. The leading line-of-sight calculators compute the radio horizon well, and the best of them explains the 4/3 Earth model properly, but they cannot say anything about signal strength. One leading tool states outright that it does not predict signal strength, which depends on power, frequency, and gain.
• Link budget tools ignore the Earth. Engineering sites such as Pasternack run the Friis math competently, but their tool answers a different question (received power at a distance you supply) and none of them checks whether the Earth itself blocks the path.
• Single-height tools cannot plan a link. Several radio horizon calculators take one antenna height and return its horizon. A link has two ends, and the useful answer is the sum, ideally with the per-antenna split shown so you can see which end limits you.
• Nobody else solves backward. Within the horizon camp, no competitor we found offers a reverse solver: you cannot ask how high an antenna must be to reach 30 km without rearranging the formula yourself.
• Simplified formulas drop frequency. One popular broadcast calculator uses a square-root shortcut with no frequency term at all, so it cannot distinguish a 146 MHz link from a 446 MHz one. The link budget tab here keeps frequency, power, gain, and sensitivity all in play.

Planning a real link: why the field number is shorter

Both modes are ceilings. The horizon mode assumes a smooth Earth; the link budget mode assumes empty space. Real paths have terrain, tree lines that grow a meter a year, rooftops, and rain, and the effective k-factor itself drops below 1 in some weather, shrinking the radio horizon under the geometric value.

Three habits keep the calculator honest in practice. First, use heights above the surrounding terrain, not above sea level and not the tower’s brochure height: broadcast engineers formalize this as height above average terrain, and the FCC maintains a HAAT calculator for exactly that adjustment. Second, keep the path clear of obstacles in the middle, not just at the ends; a ridge at the midpoint intrudes into the path even when both antennas can see over their local ground. Third, treat the answer as a maximum to stay comfortably under, then verify with a field test before committing to tower work. These are theoretical planning estimates, not a substitute for a professional RF site survey.

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