Line of sight propagation

Line-of-sight (LOS) propagation is a mode of electromagnetic or acoustic wave transmission in which signals travel directly from a transmitter to a receiver without obstruction. At the frequencies commonly used in modern telecommunications—particularly very high frequency (VHF) and above—signals propagate in straight lines and cannot bend significantly around obstacles or follow the curvature of the Earth. The ability to receive such a signal therefore largely depends on whether the transmitting antenna is visible from the receiving point, subject to the limits of atmospheric refraction and terrain.

Fundamental Principles of Propagation

Electromagnetic waves behave differently depending on their frequency. At lower frequencies, below roughly 3 MHz, radio waves undergo diffraction, allowing ground-wave propagation that follows the Earth’s contour. This enables AM radio broadcasts to reach beyond the horizon. In the shortwave band (approximately 1–30 MHz), waves may be refracted by the ionosphere, producing skywave or skip propagation, which can facilitate global communication.
At frequencies above 30 MHz (VHF, UHF and microwave), diffraction and ionospheric refraction become negligible. Signals at these frequencies therefore propagate in a straight line, and any physical obstruction—such as a mountain, building, or even dense vegetation—may block or significantly attenuate the signal.
The maximum distance over which such a signal can travel is limited by the radio horizon, the geometric boundary where the transmitted ray becomes tangent to the Earth’s surface. Whether a receiver can detect the signal depends on both antenna heights and atmospheric effects.

Impairments to Line-of-Sight Propagation

LOS propagation is easily disturbed by various obstacles or environmental conditions, even when the direct visual path appears unobstructed. Common impairments include:

• Vegetation and weather: Tree branches, heavy rain or snowfall can disrupt microwave transmissions, particularly for low-powered systems.
• Diffraction and scattering: Objects near the direct path may diffract the signal, causing phase distortions or attenuation.
• Fresnel zone obstruction: Optimal propagation requires the first Fresnel zone—a three-dimensional ellipsoidal region between transmitter and receiver—to remain clear of obstacles.
• Ground reflection: Reflected signals may interfere with the direct wave, leading to constructive or destructive interference. Raising antenna height reduces this effect and improves transmission efficiency.

The combined impact of these factors requires careful system design, including site surveys, path profiling and the use of directional antennas.

Influence of Earth’s Curvature and Atmosphere

Because LOS waves travel in straight lines, the Earth’s curvature restricts their range. The Earth bulge denotes the portion of the planet that intrudes into the transmission path over long distances. To determine whether two antennas have LOS, geometric calculations must account for this curvature.
The theoretical vacuum distance to the horizon (in kilometres) from an antenna at height h (metres) is:
distance ≈ 3.57 × √h
For height in feet (distance in miles):
distance ≈ 1.23 × √h
When two antennas communicate, the individual horizons are added to determine the maximum LOS path length.
In practice, radio waves do not travel in perfect straight lines due to atmospheric refraction. Variations in air density with altitude bend waves downward slightly, effectively increasing the Earth’s radius. This is expressed through the k-factor, a multiplier applied to the geometric radius. Under normal conditions k ≈ 4/3, extending the radio horizon by about 15%. In stormy conditions, k may fall below 1, reducing range and occasionally causing phenomena such as ducting or fade-outs.

Mobile Telephone Propagation in Non-LOS Environments

Despite operating at LOS frequencies, mobile phones function effectively in urban areas where buildings and other structures obstruct direct paths. This is possible because of multipath mechanisms such as:

• Diffraction over rooftops and around street corners.
• Reflection from building façades.
• Transmission through walls, windows and indoor structures.
• Multiple propagation paths causing Rayleigh fading, mitigated by digital modulation and error correction.

Mobile networks incorporate several engineering features to address these issues:

• Multiple base stations (cell sites): A handset can typically detect several at once, enabling seamless service through handoffs.
• Sectorised antennas: Instead of a single uniform signal, base stations employ multiple directional antennas to improve signal-to-noise ratio and reduce interference.
• Repeaters and distributed antenna systems: These maintain coverage in tunnels, underground locations and large buildings.
• Digital protocols: Modern systems use sophisticated error detection and correction to maintain signal integrity despite multipath fading.

Certain environments, such as elevators and trains constructed with steel reinforcements, may behave as Faraday cages, blocking signals due to minimal openings relative to the wavelength.

Radio Horizon and Service Range

The radio horizon marks the limit of effective LOS propagation. For tall antennas, increasing height results in a larger coverage radius until the antenna height is sufficient to cover a hemisphere, beyond which no further extension of the horizon is possible.
Although simplified formulas provide a useful estimate of maximum range, real-world performance depends on meteorological conditions, interference, obstacles and local terrain characteristics. Accurate planning therefore requires detailed propagation modelling, including profile analysis and field measurements.

Summary of Propagation Considerations

LOS propagation dominates the behaviour of VHF, UHF and microwave communications. Although conceptually straightforward, its practical implementation is complex due to:

• limitations imposed by Earth’s curvature
• atmospheric refraction
• diffraction and multipath effects
• variable environmental factors
• engineering requirements for antenna placement