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.
| Example path | Approx. midpoint radius | 60 percent clearance |
|---|---|---|
| 1 km at 5.8 GHz | about 3.6 m | about 2.2 m |
| 5 km at 5.8 GHz | about 8 m | about 4.8 m |
| 5 km at 18 GHz | about 4.6 m | about 2.7 m |
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
Survey in more than one season if you can. A path that looks clear over bare winter branches may be blocked in July. If the survey happens in winter, allow for full foliage and growth over the life of the link.
Fixing a marginal path
- Raise the antennas. Taller masts or a higher mounting point on the building are usually the simplest fix.
- Move the endpoints. A different corner of the roof can clear an obstruction entirely.
- Change frequency. A higher band has a narrower Fresnel zone, at the cost of more rain sensitivity and shorter reach.
- Add a relay. Two shorter hops via an intermediate site can get around a blocked path.
- 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
Why do we need 60 percent and not 100 percent clearance?
Keeping about 60 percent of the first zone clear captures nearly all of the benefit. Full clearance is better where practical, and some designers use more margin for critical links.
Can a link work through a window?
Sometimes over short distances, but glass coatings, frames and reflections cause loss and unpredictable results. Outdoor mounting is the reliable choice.