Microwave Link Lab

One hop, carried through every stage of its design at once — terrain, tower, antennas, budget, availability — so the couplings between them show up while there is still time to change something.

Two things that only appear when the stages sit together Height bought to clear a hill is paid for in feeder loss, at both ends, and can cost more than the fade margin it was bought to protect. And a bigger dish narrows the beam: past a certain diameter, on a tower that moves, the extra gain is more than eaten by the loss from the tower’s own sway. Each stage on its own says the design is fine.

1The path

A single controlling obstacle. For a full terrain profile, and for finding which obstacle controls, use the path profile tool — it is usually not the tallest.

2The tower

Deflection is the pointing error the structure allows at full design wind. It comes from the tower analysis, not from here. A stiff monopole might hold 0.1°; a guyed mast with a large dish high up can be several times that.

3The antennas

The solid line is datasheet gain, which only ever rises. The dashed line is what reaches the link once this tower’s deflection is counted. Where they part company is the whole point of the stage.

4The budget

5Fading — two mechanisms, not one

Vigants-Barnett is a multipath model, not a rain model It describes still, layered air bending the beam into several interfering paths — the fade that happens on calm nights over water. Its terrain and climate factors are about how readily the air layers, not about rainfall. Rain is a completely separate mechanism with its own recommendations (ITU-R P.838 and P.530), and the two are set side by side below rather than muddled together.

The rain rate exceeded for 0.01% of an average year — about 53 minutes — is the reference the whole rain method is built on. It comes from ITU-R P.837 rain-zone maps for your region and there is no sensible default: 42 mm/h is a temperate maritime figure and is wrong for most of the world.

6Look at it in Google Earth

Everything is written at absolute altitude — heights above mean sea level, not above the terrain — so the beam stays where the physics puts it while Google Earth draws its own ground underneath. The file is built in your browser and saved straight to your device; nothing is uploaded.

Why the stages have to be looked at together

Each stage of a microwave design has an owner, a tool and a pass mark. The path survey says the hill is cleared. The structural analysis says the tower holds. The antenna is picked from a datasheet. The link budget closes. Everything passes, and the link still underperforms, because the things that couple the stages are not in any single stage’s remit.

Height is not free

Raising both antennas to clear an obstacle is the standard fix, and on a link with an indoor radio it comes with a bill. Every metre of extra tower is a metre of extra feeder, at both ends. At 18 GHz in elliptical waveguide that is around 0.13 dB per metre — so 28 m of extra height on both towers costs roughly 7 dB.

Seven decibels is not a rounding error. It is the same order as the entire fade margin the extra height was meant to protect. The link that needed 28 m more clearance may be worse off having got it.

This is exactly why split-mount radios — the transceiver mounted at the antenna, with only power and data running down the tower — displaced indoor units. With no RF feeder, height costs nothing in the budget, and the whole coupling disappears.

Gain is not free either

Antenna gain rises with the square of diameter: double the dish, gain up 6 dB. Beamwidth falls with diameter: double the dish, beam half as wide. And loss from a pointing error rises with the square of that error measured in beamwidths.

Put those together. Doubling the diameter adds 6 dB of gain and multiplies the loss from a given tower deflection by four. In decibels, gain grows logarithmically with diameter while pointing loss grows quadratically — so the curves cross, and past the crossing every extra centimetre makes the link worse.

At 18 GHz on a tower holding a quarter of a degree, that crossing is at about 2.8 m. A 6 m dish there has 6.6 dB more gain on its datasheet and delivers nearly 9 dB less into the link. Nothing about the tower changed.

What the datasheet cannot know

An antenna datasheet quotes boresight gain, measured on a range, pointed perfectly. That figure is correct and it is not what the link gets. What the link gets depends on a number that lives in the structural report: how far the tower moves at design wind.

The two documents rarely meet. This lab exists to put them on the same page.

What this does not do

It models one hop with a single controlling obstacle, in clear air, with identical antennas at both ends. It does not do rain fade, terrain diffraction loss, multipath fading statistics beyond the simple exponential, interference, adaptive modulation, or structural analysis of any kind. Deflection is an input here, never an output — that number comes from an engineer with the tower drawings.

Everything is calculated in your browser. Path lengths, frequencies, site data and equipment figures are not uploaded.