A wireless bridge joins two sites with a pair of directional antennas and radios, and it works only if the path is clear: the line between the antennas must clear the ground by the Earth’s bulge plus most of the first Fresnel zone. Over flat ground at 5 GHz that needs masts of about 9.3 m for a 10 km link and about 30.4 m for a 30 km link, so a long link needs tall masts rather than a better antenna. Mount the antennas as high as possible, align them by signal strength, weatherproof every joint and ground the mast.
Part of Networking
Point-to-point radio links between sites: what path clearance a link needs, how to mount and align the antennas, and how far the claimed ranges can be trusted.
A Clear Line Of Sight Is Not Enough, The Zone Around It Must Be Clear Too
The sibling articles give the headline ranges. Network Types says line-of-sight links at 2.4 GHz, 5 GHz or microwave frequencies can span 10-50 km with modest equipment, and the carrier Internet Infrastructure gives up to 30 km with directional antennas and clear line of sight, with 1-30 km and 10-300 Mbps in its table of options. Physical Connections says 5 GHz links can give 100+ Mbps across several kilometres.
What decides whether a given link reaches is the geometry. Radio travels in a straight line, the ground curves away beneath it, and the signal occupies a zone around the line (the first Fresnel zone), not a thread. An obstacle inside that zone, even one that does not touch the line, costs signal. The carrier’s rule of thumb is that for a 10 km link each antenna should be 8-10 m above the terrain between them; the worked example below checks it and shows that it does not scale to longer paths.
What You Need
- Two directional antennas — salvaged wifi dish or panel antennas, the carrier says
- Two radios in bridge mode, or one access point and one client (carrier)
- Masts or buildings high enough for the path
- Outdoor-rated enclosures and cable glands (carrier)
- A ground rod and lightning protection — see Safety
- A map or survey of the path, with the heights of anything between the sites
Method: Build The Link
Step 1: Choose sites and survey the path Find the highest practical point at each end. The carrier notes that signals at 2.4 and 5 GHz do not penetrate hills or dense forest, so a ridge or a stand of trees on the line ends the plan.
Step 2: Check the clearance arithmetic before climbing Use the worked example. If the needed height is more than the mast can give, move a site or add a relay at a high point in between.
Step 3: Mount the antennas as high as possible Keep the feed cable short; put the radio at the antenna if the equipment allows it, since a long cable run costs signal.
Step 4: Configure one end as access point and the other as client, or use bridge mode (carrier).
Step 5: Align by watching signal strength Move one antenna slowly while the other end reports signal strength, then lock it. The carrier says even a degree of misalignment can matter on a long link, so align both ends and check again after the mast has settled in wind.
Step 6: Weatherproof, then test with traffic Seal every outdoor connection, then run a large file transfer both ways and read the error counters (Cable Testing).
Worked Example: Mast Height For A Short And A Long Link
The formulas are general knowledge, not taken from the other articles: the Earth’s bulge at the midpoint is d1 x d2 / (2R), the radius of the first Fresnel zone is the square root of (wavelength x d1 x d2 / D), and a common planning rule keeps 60% of that radius clear. R is taken as 6,371 km, the speed of light as 300,000,000 m/s. Flat ground, equal antenna heights, the obstacle at the midpoint, and no allowance for atmospheric refraction are all assumed.
10 km at 5 GHz. d1 = d2 = 5,000 m, D = 10,000 m. Wavelength = 300,000,000 / 5,000,000,000 = 0.06 m. Bulge = 5,000 x 5,000 / (2 x 6,371,000) = 25,000,000 / 12,742,000 = 1.96 m. Fresnel radius = square root of (0.06 x 5,000 x 5,000 / 10,000) = square root of 150 = 12.25 m; 60% of it is 7.35 m. Needed height above the ground at the midpoint: 1.96 + 7.35 = 9.31 m, about 9.3 m. This agrees with the carrier’s 8-10 m.
10 km at 2.4 GHz. Wavelength = 0.125 m. Fresnel radius = square root of 312.5 = 17.68 m; 60% = 10.61 m. Height: 1.96 + 10.61 = 12.57 m, above the carrier’s range. The rule of thumb fits 5 GHz and understates 2.4 GHz.
30 km at 5 GHz. d1 = d2 = 15,000 m, D = 30,000 m. Bulge = 225,000,000 / 12,742,000 = 17.66 m. Fresnel radius = square root of (0.06 x 15,000 x 15,000 / 30,000) = square root of 450 = 21.21 m; 60% = 12.73 m. Height: 17.66 + 12.73 = 30.39 m, about 30.4 m.
Tripling the distance multiplies the needed height by about 3.3 (30.39 / 9.31). The 30 km figure therefore needs mast or hilltop heights of roughly 30 m, which is why the carrier’s 30 km is a maximum for good sites, not a normal result. A real path over rolling terrain or a hill needs a proper survey, and refraction in the air changes the bulge, so use these as screening numbers.
Common Mistakes
- Planning on the quoted range — the 10-50 km and 30 km figures assume tall, clear sites
- Aiming at the other site’s building instead of its antenna — align on signal strength, not by eye
- Leaving the mast ungrounded — the mast is the tallest conductor around
Safety
Masts, lightning, power lines and radio exposure
Falls from masts and roofs kill; work from a proper platform with a harness and a second person, and never in wind, rain or lightning. A mast or antenna that falls across an overhead power line becomes live: erect masts well away from lines and plan the fall direction. A mast is a lightning attractor and a path into the equipment room: ground it with a ground rod (Ground Rod) and follow Lightning Protection, which requires primary protection at the building entry and keeps the earth resistance under 10 ohms. Do not stand in front of a transmitting dish at close range and do not work on an antenna while its radio is powered; the safe distances are not given in the other articles, so keep well clear.
What’s Next
- Learn next: Routing Protocols — how packets cross the link and onward to other sites
- Combine with: Fiber Optics — the alternative where a fibre path exists