The Fresnel zone and line of sight

A wireless link needs more than a clear sightline between two antennas. Radio energy travels in a football-shaped region around the direct line, called the Fresnel zone, and obstructions inside that region weaken the signal even if you can see the far antenna. The usual rule is to keep at least 60 percent of the first Fresnel zone clear, after allowing for the curve of the earth on longer paths.

Visual line of sight vs radio line of sight

Standing on a roof with binoculars, you may see the far building clearly. That is visual line of sight. Radio line of sight is stricter. A radio wave spreads as it travels, and parts of it reflect or bend around objects near the path. When those parts arrive at the receiver out of step with the direct signal, they partly cancel it. A roof edge, a tree line or a parapet that sits just below the sightline can cost a lot of signal.

Links are sometimes described as:

• Line of sight (LOS): clear sightline and clear Fresnel zone
• Near line of sight: clear sightline but some Fresnel zone obstruction
• Non line of sight: the sightline itself is blocked

Some radios can work in near or non line of sight conditions, especially at lower frequencies, but with reduced capacity and margin. For a link you rely on, design for true radio line of sight.

What the Fresnel zone looks like

Picture an elongated ellipse stretched between the two antennas. It is narrow at each end and widest at the midpoint. The first Fresnel zone carries most of the energy that matters for the link, which is why design rules focus on it.

Its size depends on two things:

• Path length. Longer links have wider zones.
• Frequency. Lower frequencies have wider zones. A 5 GHz link needs more clearance than an 80 GHz link of the same length.

Estimating the radius

A common approximation for the radius of the first Fresnel zone at the midpoint of the path is:

radius (metres) ≈ 8.66 × √(path length in km ÷ frequency in GHz)

For example, a 5 km link at 5.8 GHz gives about 8.66 × √(5 ÷ 5.8), or roughly 8 metres at the midpoint. Keeping 60 percent clear means nothing should come within about 5 metres of the direct line at the middle of the path. Closer to either end, the zone narrows. Path planning software does this calculation along the whole profile and is the right tool for real designs; the formula is useful for a quick sense of scale.

Checking clearance at the obstruction, not just the midpoint

Obstructions are rarely at the exact middle of a path. The radius at any point along the link can be estimated with a more general form of the same formula:

radius (metres) ≈ 17.32 × √((d1 × d2) ÷ (frequency in GHz × path length in km))

where d1 and d2 are the distances in kilometres from the point to each end. Take the same 5 km link at 5.8 GHz and a tree line 1 km from site B. Then d1 is 4 km and d2 is 1 km, and the radius works out to about 6.4 metres, so 60 percent clearance is about 3.9 metres. If both antennas are mounted at the same height over flat ground, the sightline at the tree line is at that height too. A tree line 5 metres below the sightline passes today, but a couple of metres of growth would put it inside the zone. On a path this short the earth bulge adds little; on longer paths, planning software adds it to the profile.

The curve of the earth

On longer links, the earth itself rises into the path. Radio waves bend slightly in the atmosphere, which designers allow for with an effective earth radius factor, but on paths of several kilometres the earth bulge at the midpoint can still add a significant amount to the height you need. On short campus links it is usually negligible. Path design software includes it automatically.

Common obstructions to look for

• Trees, including future growth and summer leaves that are absent in a winter survey
• Rooftop equipment, parapets, penthouses and other antennas
• New buildings planned or under construction along the path
• Construction cranes, which can appear for months at a time
• Terrain rises, berms and stockpiles at industrial sites
• Large reflective surfaces such as water or flat roofs near the path, which can cause fading

Step by step: a path check

Before any equipment is ordered, a path check confirms that the link is buildable. A practical sequence, and what to record at each step:

1. Fix both endpoints. Record GPS coordinates, ground elevation and the candidate mounting height at each end, with photographs of the mounting location.
2. Draw the profile. Use path planning software to plot terrain and the sightline between the two points. Record the path length and the frequency band being considered.
3. Add known obstructions. Enter buildings, tree lines and structures with their estimated heights and distances along the path. Note where heights are estimated rather than measured.
4. Confirm on site. From each end, look towards the other with binoculars and a compass or mapping app. Photograph the view along the azimuth. Look for anything the maps missed, such as new rooftop units, cranes or stockpiles.
5. Check clearance at every obstruction. Compare each obstruction's height with the sightline minus 60 percent of the Fresnel radius at that point. Record the clearance, or the shortfall.
6. Decide and document. If the path clears, record the mounting heights needed and any margin. If it does not, record which of the fixes below is proposed.

Keep this record with the link documentation. If the link degrades years later, the first question is whether something has grown or been built into the path, and the original profile answers it quickly.

Fixing a marginal path

1. Raise the antennas. Taller masts or a higher mounting point on the building are usually the simplest fix.
2. Move the endpoints. A different corner of the roof can clear an obstruction entirely.
3. Change frequency. A higher band has a narrower Fresnel zone, at the cost of more rain sensitivity and shorter reach.
4. Add a relay. Two shorter hops via an intermediate site can get around a blocked path.
5. Accept reduced capacity. For non-critical links, a near line of sight design may be acceptable if tested and monitored.

Raising antennas usually means taller mounts on roofs or towers, which brings structural and fall protection questions. See anchor points for fall protection for the safety side.

How ehZee can help

A path survey is the first step of our point-to-point wireless service: we check sightlines, Fresnel clearance and mounting options at both ends before choosing equipment. For background on how links are built, see point-to-point wireless links explained.

Common questions