# Mountain and Complex Terrain ## Mountain and Complex Terrain Propagation Mountain and highly variable terrain introduces propagation challenges - and opportunities - that differ fundamentally from flat-land or urban planning. Terrain masking is the dominant factor, but ridge placement can turn a liability into an asset. ### Terrain Masking Terrain masking is the most significant propagation factor in mountains. When terrain lies between the transmitter and receiver, path loss increases dramatically - **diffraction over ridges adds 10 - 30+ dB** compared to free-space loss at the same distance. Before planning a mountain link, verify line-of-sight using a terrain profile tool (HeyWhatsThat, Radio Mobile, or SPLAT!). If the path crosses terrain, budget for the additional diffraction loss. ### Knife-Edge Diffraction When a signal diffracts over a sharp ridge, it bends into the shadow zone on the far side. This diffraction is calculable using **Fresnel zone analysis** - the geometry of the ridge height relative to the first Fresnel zone determines how much loss (or occasionally gain) results. Tools like Radio Mobile model knife-edge diffraction accurately. A sharp, isolated ridge causes less diffraction loss than a broad rounded hill, which blocks a larger portion of the Fresnel zone. ### Ridge-Mounted Repeaters A repeater placed on a ridgeline provides coverage to **both** the illuminated side and the shadow side of the ridge simultaneously. This is the key insight for mountain mesh design: **place repeaters on ridges, not in valleys.** A valley node can communicate well within the valley, but a ridgeline node covers the entire valley plus the next valley over, multiplying coverage dramatically per node. ### Valley Isolation Nodes in valleys can communicate well within the valley but are effectively isolated from nodes in adjacent valleys without a ridge or mountain repeater bridging them. This creates natural "valley clusters" in mountain mesh networks - each valley segment is connected internally but disconnected from neighbors unless ridge nodes exist. Planning a mountain mesh means identifying which valleys need coverage, then finding the ridgelines that can serve multiple valleys with a single node. ### Elevation vs. Range (Rule of Thumb) At approximately 45°N latitude, for an antenna at height *h* meters above local terrain: > **Radio horizon distance ≈ 4.1 × √h km**
Height Above Valley FloorRadio Horizon
10 m~13 km
50 m~29 km
200 m~58 km
500 m~92 km
A repeater at 200 m above the valley floor sees a radio horizon of approximately 58 km - potentially covering an entire mountain region from a single node. ### Emergency Drone / Hilltop Mesh Extension Some communities deploy temporary mesh nodes on hilltops via drones during emergencies to bridge isolated valleys. The same capability works for large outdoor events in terrain-constrained areas. A drone-carried LoRa node at 100 - 200 m AGL can bridge two otherwise-isolated valleys for the duration of its battery, providing emergency communications coverage without requiring permanent infrastructure. Portable battery-powered repeater kits carried to hilltops on foot are another practical approach for planned events or disaster response. ### Snow Effects on Antennas Snow has low RF absorption at 915 MHz and does not significantly affect propagation. However, **heavy wet snow buildup on antennas can detune them**: - Use vertically-polarized antennas with a drip point design to shed snow - Avoid horizontal radial elements in heavy-snow environments - Wet snow accumulation on a horizontal radial can shift resonant frequency measurably, reducing antenna efficiency and SWR For high-altitude winter deployments, sleeved dipoles and verticals with a tapered/drip-capable profile outperform flat-panel or horizontal-element designs.