How a microwave link budget works

Everything in decibels, so the whole calculation is addition — and a column that adds up is a column you can check.

The shape of it

A link budget is a single column of numbers that starts at the transmitter and ends at the receiver. Every line is a gain or a loss in decibels, and because decibels turn multiplication into addition, the whole thing is a sum you can check by reading down the page.

1. Transmit power, in dBm.
2. Minus the losses between the radio and the antenna — jumpers, feeder run, connectors, circulator, radome.
3. Plus the transmit antenna gain. Everything to here is the EIRP.
4. Minus the free-space path loss, which is nearly always the largest number on the page.
5. Plus the receive antenna gain, minus the losses at that end.

What is left is the received signal level. Subtract the receiver's threshold and you have the fade margin, which is what the exercise was for.

Free-space loss is not loss

Nothing absorbs the signal in free space. The energy spreads over an expanding sphere, and a receiving antenna of fixed physical size intercepts a smaller share of that sphere the further away it is.

This explains something that otherwise looks wrong: free-space loss depends on frequency, even though empty space cannot care what frequency you use. It depends on frequency because the antenna does. A higher frequency means a shorter wavelength, so a fixed physical aperture is a smaller collector in wavelength terms. Nothing is lost; it is spread out, and you are catching proportionally less of it.

The practical consequence: doubling the distance costs 6 dB, and so does doubling the frequency. Both are the same factor of four in received power.

The constant is not a fudge factor

The usual expression is 20·log10(d) + 20·log10(f) + 32.44, and that last term looks like something someone measured. It is not.

It is 20·log₁₀(4π/c) with the unit conversions folded in for kilometres and megahertz. Use miles and gigahertz and it is a different number. Anything that prints 32.44 without saying which units it belongs to is inviting a mistake, which is why it is worth computing from the speed of light rather than typing in.

Where the numbers come from

Transmit power and receiver threshold come from the radio's datasheet. The threshold is the level at which the radio stops meeting its error target, and it is different for every modulation state — the figure for 256QAM is many decibels worse than the one for QPSK. A single “sensitivity” figure for a radio is not enough to build a budget with.

Antenna gain comes from the antenna's datasheet at the frequency in question. Aperture theory gives a good planning estimate; the measured figure is the real one.

Feeder loss is per metre at your frequency, times the run, plus the connectors. It is easy to underestimate: a long run at a high frequency can be several decibels each end, and those are decibels you paid for at the transmitter.

What fade margin protects against

Fade margin is headroom, and what eats it depends almost entirely on frequency.

Below about 10 GHz, the usual threat is multipath: a second copy of the signal arriving via a reflection — off water, off flat ground, off an inversion layer — slightly out of step with the direct one and partially cancelling it. It is worst on long paths over water or flat terrain, and worst at night when the atmosphere layers.

Above about 10 GHz, rain dominates. Water droplets absorb and scatter energy, and the attenuation rises steeply with both frequency and rain rate. At 38 GHz a heavy downpour can take tens of decibels, which is why high-frequency links are short.

These are different mechanisms with different statistics, and turning margin into an availability figure means picking a model appropriate to the one that dominates your path.

What a budget cannot tell you

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

Whether the margin is enough. That depends on the availability you need, the climate, the path length and the terrain. A margin that is generous on a short urban hop may be inadequate on a long path over water.

Anything about interference. A budget describes one link in isolation. Whether another transmitter raises the noise floor at your receiver is a different question with a different calculation.

A sanity check worth doing

Before trusting any budget, check that the lines actually sum to the stated result. It sounds trivial and it catches real errors — a loss entered as a gain, a figure double-counted at both ends, a feeder run included twice. A budget whose column does not add up is telling you something, and it is usually about the budget rather than the link.

Frequently asked questions

Why does free-space loss depend on frequency?

Because the receiving antenna does, not because space does. Energy spreads over an expanding sphere and a fixed physical aperture intercepts a share that depends on wavelength. Higher frequency means shorter wavelength, so the same dish is a smaller collector in wavelength terms.

What is the 32.44 constant?

20·log10(4π/c) with unit conversions folded in for kilometres and megahertz. It takes a different value for other units, so a tool that prints it without saying which units it belongs to is inviting a mistake.

What is a good fade margin?

There is no universal figure. It depends on the availability target, the climate, the path length and the terrain, and on whether multipath or rain is the dominant threat — which depends on frequency. Anyone quoting a single number without those is guessing.

Where do I get the receiver threshold?

The radio's datasheet, at the modulation and channel bandwidth you are planning for. It differs for every modulation state, so one figure per radio is not enough.

Does doubling the antenna size help as much as doubling power?

More, usually. Doubling the transmit power is 3 dB. Doubling a dish diameter is about 6 dB, and it applies at both ends if you do both — and unlike power it costs nothing to run.

Does a link budget account for terrain?

No. It is free space by construction. Terrain, buildings and the Earth's own curvature are geometry, and belong in a Fresnel clearance calculation.