Microwave Path Profile

The ground along a hop, the beam over it, and which obstacle actually controls the link — which is usually not the tallest one.

The tallest hill is rarely the problem Both of the things that eat clearance — the Fresnel radius and the Earth's bulge — are largest at mid-path and fall to nothing at the towers. A 42 m hill halfway along a 20 km hop can fail a link that a 46 m hill two kilometres out does not touch. Looking for the highest ground finds the wrong obstacle.

The hop

Sixty per cent of the first Fresnel zone is a widely used planning threshold. It is a convention, not a law — some operators plan to 100% of F1, others accept less on short hops.

The ground

Obstaclekm from AHeight (m)

Heights are above sea level. Nothing here leaves your browser.

The profile

What height would clear it

Solved against every point on the profile rather than against the worst one, because raising an antenna changes which point is worst.

Look at it in Google Earth

Needs the coordinates from the Fetch terrain tab, whether or not you fetch anything with them — the export is geometry, and geometry needs positions. Everything is written at absolute altitude, so the beam stays where the physics puts it rather than being draped over Google Earth’s own terrain. The file is built in your browser and saved straight to your device.

Why the middle costs more

Two separate things take clearance away from a beam, and both of them peak at mid-path.

The first Fresnel zone has a radius proportional to √(d₁·d₂/D). At either tower one of those distances is zero, so the zone has no width at all. Halfway along, it is at its widest — at 7 GHz over 20 km, about 14.6 m.

The Earth's bulge is proportional to d₁·d₂, which again is zero at the ends and largest in the middle. Over the same hop, and under standard atmosphere, it lifts the apparent ground by about 5.9 m at mid-path.

Add them and mid-path terrain is penalised roughly 20 m harder than terrain near a tower on this hop. That is why a survey that reports the highest point on the route is answering a question nobody asked.

The K factor is the one to stress

The atmosphere bends radio slightly downward, which effectively flattens the Earth — K = 4/3 is the usual temperate-climate planning value. It is a starting point, not a property of the site.

What fails marginal paths is sub-refractive conditions, where K drops below 1 and the effective Earth becomes rounder than the real one. A hop that clears comfortably at K = 1.33 can lose most of its Fresnel clearance at K = 0.6. Checking the profile at both is a two-second exercise that is worth doing before anything is ordered.

Where the terrain comes from, and what that costs

Typed and pasted profiles are arithmetic on your own machine and disclose nothing.

Fetching terrain is different, and it is the only feature on this site that sends anything anywhere. It sends the coordinates of each sample point to a public elevation service so it can look them up. You are shown exactly what would go, to whom, before anything is sent, and nothing happens until you press the button.

The data that comes back is coarse. An elevation model averages over its cell, so summits come back lower than they are — measured against published heights this source returned Ben Nevis 12 m low and Snowdon 52 m low. That error makes a path look clearer than it is, which is the direction you cannot afford to be wrong in. It is good for finding where the problem is. It is not good enough to sign off a path.

Frequencies, antenna heights, site names and everything else you type stay in your browser under every mode, including while terrain is being fetched.