What is a good fade margin?

It depends on the availability you need, the climate and the path, so there is no universal figure. What the margin protects against differs with frequency too — rain above about 10 GHz, multipath below it over water or flat terrain. Turning a margin into an availability figure needs a propagation model with its own applicability limits, and that is a separate calculation from this one.

Where do I get the receiver threshold?

From the radio's datasheet, at the modulation and channel bandwidth you are planning for. It differs for every modulation state — the figure for 256QAM is many decibels worse than the one for QPSK — so a single number for a radio is not enough. This tool does not supply thresholds, because inventing them would produce a budget that looks authoritative and is fiction.

Why does path loss depend on frequency if nothing absorbs the signal?

Because free-space loss is not absorption. The energy spreads over an expanding sphere, and a fixed-size receiving antenna intercepts a share of it that depends on the wavelength. A higher frequency means a shorter wavelength, so the same physical aperture captures proportionally less. Nothing is lost — it is spread out.

What is the 32.44 constant?

It is 20·log10(4π/c) adjusted so that distance can be entered in kilometres and frequency in megahertz. It is not a fudge factor, and it takes a different value for other unit combinations — which is why this calculates it from the speed of light rather than typing it in.

Does this account for terrain?

No. It is a clear-air free-space model. If terrain, buildings or the curvature of the Earth intrude into the Fresnel zone, the real loss is higher and can be far higher. That is a geometry question and belongs in the Fresnel zone and clearance calculation.

Why are the two antenna bearings not 180 degrees apart?

Because the meridians converge. The back-azimuth from the far site is the geodesic's own azimuth at that end plus 180, which on a long path differs from the forward bearing plus 180 by a fraction of a degree. With a narrow beam that is enough to matter, so both are shown.

Is anything I enter sent anywhere?

No. Coordinates, frequencies and equipment figures are all calculated in your browser. Nothing is uploaded and nothing is stored.

Microwave Link Budget Calculator

Transmit power in, received level out, with every line of the budget shown so it can be checked against a rack. Free-space path loss, EIRP, RSL and fade margin from figures you supply — including the receiver threshold, which has to come from the radio's specification.

Modelled, under the assumptions you enter This is a clear-air free-space calculation. Rain, multipath, terrain diffraction and atmospheric absorption are separate effects and are not in it. No radio, antenna or threshold figures are supplied — inventing vendor numbers produces a budget that looks authoritative and is fiction.

The path

Enter a distance Work it out from coordinates

Path length (km)

Frequency (MHz)

The radios

Transmit end

Transmit power (dBm)

Feeder & component loss (dB)

Antenna gain (dBi)

Receive end

Antenna gain (dBi)

Feeder & component loss (dB)

Receiver threshold (dBm)

Other losses (dB) — radome, obstruction, ageing

The threshold is the level at which the radio stops meeting its error target, at the modulation you are planning for. It is on the datasheet and it differs for every modulation state — a figure for 256QAM is not the figure for QPSK.

The budget

What the numbers mean

EIRP is what the antenna radiates, referenced to a theoretical isotropic radiator. Transmit power, minus everything lost getting to the antenna, plus the antenna's gain. It is what a regulator limits and what an interference calculation starts from.

Free-space path loss is not really loss. Nothing absorbs the energy — it spreads out over an expanding sphere, so a fixed-size receiving antenna intercepts a smaller share of it the further away it is. That is why it depends on frequency at all: a higher frequency means a shorter wavelength, and a fixed physical aperture captures proportionally less of it.

Received signal level is what actually arrives. Everything in the budget is in decibels, so the whole calculation is addition.

Fade margin is the headroom between the clear-air level and the level at which the link stops working. It is the number the exercise exists to produce, and it is only as good as the threshold you put in.

The formulas, as implemented

The constant 32.44 is not a fudge factor. It is 20·log₁₀(4π/c) adjusted for kilometres and megahertz, and it changes if you change the units — so it is computed from the speed of light rather than typed in.

Two things this does not tell you

Whether the path is clear. A budget assumes free space. If a hill, a building or the curvature of the Earth intrudes into the Fresnel zone, the real loss is higher and possibly far higher. That is a geometry question, and it is the Fresnel zone and clearance calculation.

How often the margin will be eaten. Fade margin is protection against events — rain at higher frequencies, multipath at lower ones over water or flat terrain. How much margin is enough depends on the availability you need, the climate and the path, and turning margin into an availability figure needs a propagation model with its own applicability limits.