Also known as: scattering, radio scattering

Scattering is the redirection of radio energy into many directions at once when a wave meets a surface or object that is rough, irregular, or comparable in size to its wavelength.¹ Unlike a mirror-like reflection off a smooth plane, scattering spreads a portion of the incident power over a broad angular range. It is a major contributor to multipath in cluttered environments and the mechanism behind rain scatter, tropospheric scatter, and radar returns from precipitation and rough terrain.

A rough or wavelength-scale object redirects incident energy into many directions rather than a single specular beam.

How it works

Whether a surface reflects cleanly or scatters is judged against the wavelength by the Rayleigh roughness criterion: irregularities much smaller than λ look smooth and reflect specularly, while irregularities comparable to or larger than λ scatter diffusely. The same wave that mirrors off a calm building wall at UHF scatters strongly at millimetre wave, because the wall’s texture is now large relative to the wavelength.

Scattering by small particles has a strong frequency dependence:

• Rayleigh regime (particle radius ≪ λ, e.g. drizzle or dust at microwave): scattered power rises steeply with frequency (roughly as the fourth power), so higher bands are hit hardest. This is the same law that makes the sky blue.
• Mie regime (particle size ≈ λ, e.g. raindrops at millimetre wave): scattering is strong and less steeply frequency-dependent, and the redirected power that leaves the path becomes attenuation — the dominant contribution to rain fade.

Variants

Several named propagation modes are scattering in disguise:

• Tropospheric scatter (troposcatter): turbulent refractive-index blobs high in the troposphere scatter a faint fraction of a UHF/SHF beam forward, supporting reliable links of hundreds of kilometres well beyond the horizon.
• Rain scatter: precipitation redirects microwave energy, both attenuating the intended path and creating unintended off-axis paths (exploited by amateurs, and a source of interference in satellite bands).
• Rough-surface scatter: terrain, sea, and foliage scatter contribute the diffuse rays that populate the multipath channel and drive Rayleigh fading.

Relevance to SDR

For land-mobile scanning, the practical face of scattering is clutter multipath: the countless diffuse reflections off buildings, vehicles, and vegetation that combine at the antenna to produce fading and intersymbol interference. There is no clean specular echo to equalise away; the channel is a statistical sum of scattered arrivals, which is exactly why Rayleigh statistics apply in dense urban settings.

Scattering matters more as frequency rises. At the VHF/UHF bands used by P25, DMR, and TETRA it is a moderate multipath contributor; at the millimetre-wave bands of 5G and satellite downlinks it becomes a first-order loss and fade mechanism, driving the need for higher fade margins and adaptive coding. GopherTrunk does not model scattering; it is a channel property that shapes the signal arriving at the receiver, and it appears indirectly as fading and elevated EVM in the decode chain.

Sources

1. Scattering — Wikipedia, on the redirection of waves by rough surfaces and particles, and the Rayleigh and Mie regimes. ↩

See also

• Multipath propagation
• Rain fade
• Atmospheric absorption
• Refraction
• Radio propagation