Also known as: rain fade, rain attenuation
Rain fade is the attenuation of a radio signal caused by rainfall (and, similarly, snow, hail, and wet foliage) along the propagation path.1 It becomes significant at microwave frequencies above roughly 10 GHz and worsens with frequency, making it the dominant weather impairment for Ku- and Ka-band satellite links, millimetre-wave backhaul, and 5G mm-wave cells. Raindrops both absorb energy and scatter it out of the path, so a heavy downpour can drive an otherwise healthy link into outage.
How it works
Raindrops are dielectric spheres roughly comparable in size to microwave wavelengths, so they interact strongly through two mechanisms:
- Absorption. Water is lossy at microwave frequencies; energy driving the polar water molecules is dissipated as heat.
- Scattering. In the Mie regime, where drop diameter approaches the wavelength, drops scatter a large fraction of the incident energy out of the beam.
Specific attenuation (dB per kilometre) rises with rain rate and with frequency. At C band (~4–6 GHz) it is nearly negligible; at Ku band (~12–18 GHz) a heavy storm costs several dB; at Ka band (~20–30 GHz) and above, the same storm can cost tens of dB — enough to break the link. Because a satellite’s slant path only crosses the rain cell for a limited height, the effective path length through rain, not the whole path, sets the loss. The ITU-R P.618 model combines rain-rate statistics with path geometry to predict outage percentages, and rain also depolarises the signal, degrading systems that reuse frequencies on orthogonal polarisations.
In practice
Because rain is intermittent, links to rain-prone frequencies are designed with a fade margin sized to a target availability (e.g. 99.9% of the year). Two active techniques reduce the required margin:
- Adaptive coding and modulation (ACM): the system drops to a more robust modulation and heavier FEC during a fade, trading data rate for a lower usable SNR so the link stays up.
- Site diversity: two ground stations far enough apart are rarely rained on at once, so the network switches to whichever has the clear sky.
Uplink power control, where the transmitter raises power during rain, is a further remedy.
Relevance to SDR
Rain fade is essentially absent from the VHF/UHF land-mobile bands that trunking scanners monitor — P25, DMR, TETRA, and NXDN sit far below the frequencies where rain matters, so weather has negligible direct effect on their link budget. It becomes a first-order concern only for the microwave and satellite links an SDR hobbyist might also receive: Ku/Ka-band satellite downlinks, DVB-S feeds, and mm-wave systems all show measurable rain fade.
GopherTrunk targets terrestrial land-mobile protocols, so rain fade is not a factor in its decode path; it is included here as the canonical weather-driven attenuation mechanism that shapes the design of any higher-band RF link. Its close cousin is clear-air atmospheric absorption, which sets the baseline loss on which rain fade adds.