What 9 dBi Actually Looks Like
Antenna gain visualized against real Cascade-to-Puget-Sound terrain
TL;DR
Those simplified antenna diagrams with two fat ovals? They're lying to you. A high-gain omni with a 15° vertical beamwidth covers far more terrain than those diagrams imply — the narrow beam shoots out across the landscape as a thin wedge that interacts with terrain in ways the simplified view completely hides. This page overlays a geometrically correct beam pattern on real USGS elevation data from a site in the Cascades heading west to the Puget Sound, so you can see what the coverage actually looks like against terrain.
You've seen the image: two roughly equal ellipses sitting side by side, one labeled "9 dBi" — as if a high-gain omni just squishes the vertical pattern into a slightly flatter oval. That diagram gets the concept right (gain concentrates energy by compressing the vertical beamwidth) but the scale catastrophically wrong.
At 9 dBi, the main lobe concentrates power at 7.9× the density of an isotropic radiator — by squeezing the vertical pattern into a narrow disk. A 15° vertical beamwidth means all that energy is packed into just 4.2% of the full vertical circle.
From the antenna site at 47.809024, -121.727227 (elevation ~1,153 m in the Cascades), with a 30 m tower, the antenna sits at roughly 1,183 m ASL. With a 15° beamwidth pointed horizontally:
This visualization uses real USGS elevation data sampled along a due-west transect from the antenna site across the Cascades foothills, Snohomish lowlands, Puget Sound, the Kitsap Peninsula, and out to Hood Canal — roughly 76 km total. The beam is drawn with correct geometry: ray angles are computed in true (distance, elevation) space and then projected into the exaggerated view alongside the terrain.
HOW TO READ THIS
The orange wedge is the antenna's main lobe — to scale. The vertical exaggeration applies equally to terrain and beam, preserving the true angular relationship. Drag the sliders to change parameters and watch where the lower beam edge meets the ground. Try dropping the exaggeration to 1× to see just how thin the beam really is.
NOTE ON VERTICAL EXAGGERATION
The default view uses 15× vertical exaggeration so you can actually see the terrain features. Drag the exaggeration slider down to 1× to see the true aspect ratio — the beam becomes an almost invisible sliver and the Cascades look like a gentle bump. That's the honest reality of a 15° beam across 76 km.