Line-of-sight propagation

Lore & Background

Line-of-sight propagation is the dominant mode for frequencies above 30 MHz, including VHF and higher bands. At these frequencies, diffraction and ionospheric refraction are not significant in the lower atmosphere, so the signal behaves similarly to light: it can be blocked by any obstruction. The radio horizon marks the farthest point of direct propagation, and its distance depends on antenna heights and atmospheric refraction. The curvature of the Earth must be accounted for when calculating line-of-sight paths, and the effective Earth radius is often increased by a factor of about 4/3 due to atmospheric refraction, which bends radio waves downward. Impairments include objects within the first Fresnel zone, reflected radiation from ground or water, and atmospheric conditions such as heavy rain or snow, especially for low-powered microwave transmitters.

Reader's Guide

Line-of-sight propagation is fundamental to many modern communication systems, including broadcast FM radio, television, and mobile telephone networks. Although mobile phones use frequencies in the line-of-sight range, they function in cities through a combination of effects: propagation over rooftops, diffraction into street canyons, multipath reflection along streets, diffraction through windows, and attenuated passage through walls. Base stations are positioned on rooftops or hilltops, and use sectorized antennas to improve signal-to-noise ratio. Rapid handoff between base stations and digital error correction help maintain connections. The radio horizon can be extended by raising antennas, and the effective range is increased by about 15% under normal atmospheric conditions due to refraction. Earth bulge limits direct communication over long distances, but repeaters can relay signals beyond the horizon. The concept remains essential for planning microwave links, cellular networks, and other terrestrial wireless systems.

Did You Know?

• At frequencies above 30 MHz, any obstruction between transmitter and receiver will block the signal, similar to visible light.
• The first Fresnel zone should be free of obstructions for best propagation.
• Under normal weather conditions, the effective Earth radius is increased by a factor of about 4/3 due to atmospheric refraction.
• Mobile phones in cities rely on diffraction, multipath reflection, and attenuated passage through walls to maintain connections.