Non-line-of-sight propagation

Lore & Background

The term NLOS describes a radio channel where there is no visual line of sight between antennas. Visual line of sight is determined by the human eye's ability to resolve a distant object, but optical wavelengths are much shorter than radio wavelengths—the shortest radio wavelength is about 2000 times longer than the longest optical wavelength. For typical communications frequencies up to about 10 GHz, the difference is on the order of 60,000 times, so visual obstructions do not reliably predict effects on radio propagation.

The influence of an obstruction on an NLOS link ranges from negligible to complete suppression. For example, a cloud passing between a television broadcast antenna and a roof-mounted receiving antenna may make the link NLOS without affecting channel quality, whereas a large building constructed in the path could make reception impossible. Obstructions affect plane waves depending on their size relative to wavelength and their electrical properties. If an obstruction is much smaller than the wavelength, the wave is essentially unaffected; if comparable in size, diffraction and some transmission occur; if many wavelengths in size, the effect depends heavily on the material's electrical properties.

Electrical properties range from perfect conductors to perfect insulators. Good conductors (where the loss tangent is high) substantially reflect incident radio waves with almost the same power, absorbing virtually none. Most materials have mixed properties—for instance, reinforced concrete buildings contain insulating concrete and conducting steel. The loss tangent, given by σ/(ωε₀εᵣ), describes the degree to which a material is a conductor or insulator.

Reader's Guide

NLOS propagation has become increasingly significant in wireless networking because the ability to provide reasonable NLOS coverage greatly improves the marketability and versatility of systems like WLANs and WiMAX in urban environments. The most common method for dealing with NLOS conditions on wireless computer networks is to circumvent the obstruction by placing relays at additional locations, sending content around the obstruction. More advanced schemes use multipath signal propagation, bouncing the radio signal off other nearby objects to reach the receiver. Near-line-of-sight conditions, involving partial obstruction of the innermost Fresnel zone, can usually be addressed with better antennas, but true NLOS typically requires alternative paths or multipath methods. The military uses the related term BLOS (beyond line of sight) for communications linking personnel or systems too distant or too fully obscured by terrain, employing active repeaters, groundwave propagation, tropospheric scatter links, and ionospheric propagation to extend ranges from a few kilometers to a few thousand kilometers.

Did You Know?

• NLOS links may be simplex, duplex, or half-duplex, and under normal conditions all radio links are reciprocal.
• A hot air balloon with multi-wavelength dimensions is unlikely to affect NLOS propagation much because its fabric and hot air are good insulators.
• Low frequency broadcasts at about 200 kHz have a wavelength of 1500 m and are not significantly affected by most average size buildings.
• The loss tangent (σ/(ωε₀εᵣ)) describes whether a material is a conductor or insulator for radio waves.

The Physics Underpinning Every Path

Radio waves, as a form of electromagnetic radiation, behave much like light waves in their fundamental interactions with the environment. They are subject to reflection, refraction, diffraction, absorption, polarization, and scattering—each of which shapes how a signal arrives at a distant receiver. In a vacuum, all electromagnetic waves travel at the speed of light, and the power density of a radiated wave diminishes according to the inverse-square law: double the distance and the power density drops to one-quarter. Because the Earth's atmosphere is thin, radio waves travel very close to the speed of light through it, though variations in density and temperature introduce slight bending over long distances. At typical communication ranges, a transmitting antenna can be approximated as a point source, meaning the electric and magnetic field strengths each halve when the path length doubles. These foundational principles govern every propagation mode, whether a wave bounces off the ionosphere, hugs the ground, or travels in a straight beam between two antennas.

Hugging the Earth: Ground Wave Propagation

At frequencies spanning the MF, LF, and VLF bands—roughly 30 to 3,000 kHz—vertically polarized radio waves can propagate as surface waves that follow the curvature of the Earth. Rather than traveling in a straight line, the wave interacts with the conductive surface of the ground, effectively clinging to it and bending around hills and other obstacles that would block a direct beam. This diffraction-based mechanism allows AM broadcast stations and amateur radio operators to cover their listening areas without requiring a clear line of sight. As the frequency drops further into the VLF and ELF ranges, attenuation with distance decreases dramatically, enabling worldwide communication via ground waves. Perhaps most remarkably, these ultra-low-frequency waves can penetrate significant distances through water and solid earth, a property exploited for communication with submerged submarines and for mine operations where conventional radio links are impossible.

Bouncing Off the Sky: Skywave Propagation

In the medium wave and shortwave frequency ranges (MF and HF bands), radio waves can be refracted by the ionosphere, a layer of charged particles situated high in the atmosphere. When a signal is transmitted at an angle into the sky, the ionized layer bends the wave back toward Earth, potentially delivering it at transcontinental distances far beyond the visual horizon. This mechanism, known as skywave propagation, has been the backbone of international shortwave broadcasting and remains a favorite tool for amateur radio operators who wish to contact counterparts in distant countries. The phenomenon depends on the density and behavior of the ions in the upper atmosphere, making the available propagation conditions variable. Skywave propagation stands in contrast to the more predictable line-of-sight modes used at VHF and above, offering a way to reach receivers thousands of miles away without the need for relay stations or satellite infrastructure.

Beyond the Standard Paths: Niche Propagation Modes

While line-of-sight, ground wave, and skywave propagation cover the majority of practical radio communication, several less common mechanisms fill important gaps in the spectrum. Tropospheric scattering, or troposcatter, exploits irregularities in the lower atmosphere to redirect VHF and UHF signals beyond the geometric horizon. Tropospheric ducting, another VHF phenomenon, occurs when atmospheric temperature and humidity profiles create a waveguide-like channel that traps and guides radio energy over extended distances. Near vertical incidence skywave (NVIS) offers a specialized solution for HF operators who need to communicate within a few hundred miles—distances too short for conventional skywave hops but too long for reliable ground-wave coverage. Together, these mechanisms expand the toolkit available to engineers designing mobile telephone systems, radio navigation networks, radar installations, and point-to-point microwave relay links, ensuring that a viable propagation path exists across nearly every frequency band and geographic scenario.