Repeater Siting Basics: Terrain, Path Profile, Fresnel Clearance, and When a Repeater Earns Its Cost
A repeater solves a path problem by splitting one bad hop into two good ones, but a poorly sited repeater just adds a new point of failure. Siting well starts with the terrain between the sites, not with the radio you plan to buy.
Start with line of sight
At the frequencies used for most SCADA telemetry (VHF, UHF, 900 MHz, and microwave), radio waves travel mostly in straight lines. The practical rule is that the two antennas need a clear path between them, with allowance for the curvature of the earth. Hills, ridgelines, dense tree stands, large buildings, and tanks or facility structures can all block or weaken the path.
Even at lower frequencies where signals diffract over obstacles somewhat, a clear path is the reliable design basis. Unplanned reliance on a signal "bending around" a hill is how links end up with almost no margin.
The path profile
A path profile is a side view of the terrain between two sites, drawn on a distance axis with elevation, plus the antenna heights at each end. Planning tools generate it from elevation data, and field surveys confirm it. A good profile shows:
- The terrain along the straight line between the sites.
- Earth curvature. Over longer paths the earth bulges up in the middle. Designers commonly account for atmospheric refraction using an effective earth radius (the "k-factor" of about 4/3 is a standard planning value), so the bulge appears slightly smaller than geometry alone suggests.
- Clutter. Trees, buildings, and facility equipment add height to the ground line, and tree height can grow.
- The worst obstruction. The point along the path that comes closest to blocking the line of sight governs the design.
Read the profile for the single worst obstruction first. One ridge or one tree line usually decides whether a link is viable, and it also shows you where a repeater might fix it.
Fresnel clearance, conceptually
Radio energy does not travel along an infinitely thin line. It occupies an ellipsoid-shaped region around the direct path, called the Fresnel zone. The first Fresnel zone is the region around the direct path where an alternative path would be no more than half a wavelength longer than the direct signal, so energy there still adds largely in phase, and it is the one that matters most.
The practical lesson: an obstruction does not have to touch the line between the antennas to hurt the link. If it intrudes into the first Fresnel zone, it begins to reduce the received signal. A widely used design guideline is to keep roughly 60 percent of the first Fresnel zone clear of obstructions. Zone size depends on frequency and distance: lower frequencies and longer paths have wider zones, so a 450 MHz or 900 MHz link needs more physical clearance than a microwave link of the same length. Clearance is also widest near the middle of the path, which is why a mid-path obstruction matters so much.
Clearance also changes with time. Trees grow, seasonal foliage returns, and conditions that bend the signal differently from your planning assumption can shrink real clearance. That is one reason a link that tested well in winter can degrade by summer.
Choosing the repeater site
A good repeater location tends to share some characteristics:
- Elevation and visibility. It should see both ends with comfortable clearance, ideally with more clearance to each end than the two ends have to each other.
- Existing infrastructure. Towers, power, and access reduce cost and maintenance burden. A site you cannot reach in bad weather is a poor choice.
- Power. Grid power is simplest, solar adds a battery and sizing exercise, and either way the site needs enough reserve to ride through bad stretches.
- Interference environment. Co-located transmitters and nearby high-power sources can raise the noise floor. A site survey with a spectrum analyzer is worth the effort.
- Legal and access. Landowner agreements, tower rights, and licensing (where required) take time. Build it into the schedule.
- Network topology. A repeater that many sites depend on is a single point of failure. Consider whether the sites behind it have an alternate path, and give a high-dependency repeater better power and monitoring than a spur.
When a repeater earns its cost
A repeater is not free. It adds equipment, power, access, maintenance, and another thing that can fail. It tends to earn its cost when:
- The path is blocked or marginal and cannot be fixed another way.Raising an antenna, moving a mast a few meters, or changing the antenna type are cheaper options and worth trying first.
- One repeater can serve several sites. A well-placed hub that reaches a cluster of remote sites often beats building a taller tower at each.
- The consequence of loss is high. When an outage means lost measurement, a safety concern, or a long drive to a remote location, paying for a reliable path is straightforward.
- The alternatives cost more over time. Satellite or cellular backhaul have recurring costs and their own coverage limits, so compare lifetime costs rather than build costs alone.
And it is often not worth it when a modest antenna upgrade or relocation closes the gap, or when the repeater would sit in a spot nobody can reliably get to.
After building one, measure the result. A repeater improves the path only if both hops have healthy margin, so verify each leg after commissioning and watch it afterward.
Watch both legs of your repeated paths.
Aevus Comms (RF Link Intelligence) helps with the "what happened after we built it" half of this topic: showing the margin on each leg of a repeated path and the trend over seasons, so growth in an obstruction or a slow decline on one hop is visible.
Related Articles
Bands, degradation modes, and redundancy for SCADA radio.
Backup paths so one repeater fault doesn't isolate a group of sites.
The margin each leg of a repeated path needs to stay healthy.
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