Line of Sight

Line of sight (LOS) is the direct, unobstructed path between a transmitting and a receiving antenna. At the VHF, UHF, and sub-GHz frequencies used by mesh radios, signals travel in essentially straight lines and are heavily attenuated by hills, buildings, and dense foliage. Achieving good LOS — usually by raising antennas above the surrounding clutter — is the single biggest factor in how far a LoRa link actually reaches, more so than transmit power or antenna gain. A node with a clear shot across a valley can hold a link over tens of kilometres on milliwatts; the same node behind a row of wet cedars may struggle to cross the street.

The Fresnel zone caveat

True radio LOS is more demanding than visual LOS. Radio energy travels not just along the straight ray between antennas but through an elliptical region around it called the Fresnel zone. Obstacles intruding into that zone cause diffraction and destructive interference even when the two antennas can technically "see" each other. The common engineering rule of thumb is to keep at least 60% of the first Fresnel zone clear, with under 20% obstruction strongly preferred. The zone is widest at the midpoint of the path and grows with distance and with lower frequency — which matters for the 868/915 MHz bands most Meshtastic deployments use, where the first Fresnel zone over a multi-kilometre hop can be several metres across at its middle. A link that grazes a rooftop or treeline may work in winter and die in summer when the leaves come in, because foliage that never blocked the visual line now fills the Fresnel ellipse.

Height is leverage

Because the Earth curves and obstacles loom, antenna height is the cheapest performance upgrade in radio. Lifting a node a few metres onto a roof, mast, or hilltop can convert a dead link into a solid one by clearing the Fresnel zone over nearby rooftops and trees, and over long paths it also buys margin against the horizon itself. Before mounting anything, it is worth checking a terrain profile: free path-profiling tools will draw the ground elevation and Fresnel ellipse between two coordinates and show exactly where a hill clips the path. In rolling terrain, a single well-placed relay node on high ground can bridge two pockets that have no direct LOS to each other — which is precisely how a mesh extends coverage, one elevated hop at a time.

Verify links empirically rather than by map alone. Every LoRa node reports RSSI (received signal strength) and SNR (signal-to-noise ratio) for the packets it hears, and a short walk-test with a handheld node tells you in minutes what a propagation model only estimates. LoRa demodulates well below the noise floor, so a link can work at startlingly negative SNR — but a link with no margin fails the day it rains on the foliage or a delivery truck parks in the path. Aim for comfortable margin on a bad day, not a bare connection on a good one. Sub-GHz signals also diffract usefully over sharp obstacles like ridgelines (knife-edge diffraction), which is why a link that looks blocked on paper sometimes works — welcome it when it happens, but never plan on it.

Planning sovereign links

For the sovereignty-minded operator building off-grid communications, LOS discipline is what separates a demo from infrastructure. Survey first: identify the high points you can access — your own roof, a friend's tower, a ridgeline — and treat them as the backbone, letting mesh routing hop around whatever you cannot clear. Match the antenna to the geometry: a higher-gain antenna flattens the radiation pattern, which helps across flat terrain but can overshoot a node in the valley below, so antenna gain should be chosen after the path profile, not before. Keep feedline runs short, since coax loss at UHF eats gain quickly. And remember that a mesh carrying Nostr messages or telemetry only needs each individual hop to be good — five clean 5 km links beat one marginal 25 km shot every time. Get the line of sight right and everything else in the link budget gets easier.