RF Link Budget Calculator
Whether a radio path will close: EIRP, free-space loss, received signal and margin, plus the Fresnel clearance that line of sight does not give you.
Transmit side
Path
Receive side
Line of sight is not enough
Radio needs roughly 60% of the first Fresnel zone clear of obstructions, and that zone is far bigger than people expect — nearly 18 metres of clearance at the midpoint of a 10 km link at 2.4 GHz. A path you can see straight down that still does not work is almost always this. The zone widens as frequency falls, so a VHF link over the same ground needs a wider corridor than a microwave one, not a narrower one.
Free-space path loss means free space. No ground, no buildings, no trees, no rain. It is a floor on the loss, never an estimate of it. Real paths over real terrain do worse, sometimes by 20 dB — which is why the margin at the end matters more than the precision of any single term in the middle.
- Aim for 20 dB of margin, not zero. Rain, foliage in leaf, a wet antenna and ordinary fading each cost several decibels, and they arrive together on the worst day of the year.
- Gain is cheaper than power. 3 dB more antenna gain is worth the same as doubling your transmit power, costs nothing to run, and helps on receive as well as transmit. Doubling power only helps one direction.
- Check whether a gain figure is dBi or dBd. They are 2.15 dB apart and manufacturers quote whichever flatters the product.
- The earth bulges. Over 30 km the ground rises about 13 m in the middle even on flat terrain, using the standard 4/3 refraction factor. That adds to the Fresnel clearance you need.
- Beyond the radio horizon this model does not apply at all. Two antennas 10 m up see about 26 km between them; past that you are relying on diffraction or tropospheric propagation, which are far lossier and far less predictable.
How to use
- Enter transmit power, antenna gain and cable loss for each end.
- Set the frequency, distance and any extra loss from rain, trees or walls.
- Enter the receiver sensitivity — it is usually a negative dBm figure.
- Read the margin, then check the Fresnel clearance the path needs.
Frequently asked questions
What is free-space path loss and why is my link worse than it says?
It is the spreading of the signal over distance with nothing in the way at all — no ground, no buildings, no trees, no rain. It is a floor on the loss rather than an estimate of it, so every real path does worse, sometimes by twenty decibels. That is exactly why the margin at the end matters far more than the precision of any single term in the middle.
I have line of sight, so why does the link not work?
Because radio needs roughly 60 percent of the first Fresnel zone clear, not merely an unobstructed straight line. That zone is far bigger than people expect: nearly 18 metres of clearance at the midpoint of a ten kilometre link at 2.4 GHz. A treeline you can see over the top of is very often sitting well inside the zone, and it costs you real signal.
Does a lower frequency need less clearance?
No, more — which surprises almost everyone. The Fresnel zone radius grows as the square root of wavelength, so a VHF link over the same ground needs a wider clear corridor than a microwave one. Low frequencies make up for it by diffracting around obstacles far better, which is a different mechanism from the clear-path calculation here.
How much margin should I design for?
Twenty decibels on anything that has to keep working. Rain, foliage coming into leaf, a wet antenna and ordinary multipath fading each cost several decibels, and they arrive together on the worst day of the year rather than politely spread out. Ten decibels is workable for something you can afford to lose occasionally, and anything under that is a demonstration rather than a link.
Should I add power or antenna gain?
Gain, nearly always. Three decibels more antenna gain is worth the same as doubling transmit power, costs nothing to run, and helps on receive as well as transmit — while doubling power only improves one direction. Gain also narrows the beam, which reduces interference both ways. The catch is that a narrow beam has to be aimed and has to stay aimed in wind.
What is the difference between EIRP and ERP?
Both describe the effective power in the strongest direction. EIRP is measured against an isotropic radiator, ERP against a half-wave dipole, and they differ by exactly 2.15 decibels for the same transmitter. Regulatory limits are written in one or the other and rarely say which in the headline, so read carefully before assuming you are inside a limit.
What happens beyond the radio horizon?
This model stops applying. Two antennas ten metres up see about 26 kilometres between them, allowing for the atmosphere bending signals slightly downward. Past that you are relying on diffraction over the horizon or on tropospheric propagation, both far lossier and far less predictable than free space. If the distance exceeds the horizon figure, raise the antennas rather than trusting the margin.
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