What Is Ku-band?

Ku-band, also written as Ku band, is the segment of the microwave spectrum spanning 12 to 18 GHz under the IEEE letter designations for radar frequency bands, placed immediately below the K-band in that nomenclature. The designation derives from the German word kurz, meaning short, with the "u" marking the sub-band that sits under K. Wavelengths in this range run from about 1.7 to 2.5 centimeters, short enough that antennas of modest physical size produce narrow beams and high gain, yet long enough to avoid the severe atmospheric absorption that limits the bands above it.

The split of the original K-band into Ku and Ka was driven by physics rather than convention. A water vapor absorption resonance near 22 GHz makes the center of the K-band unattractive for terrestrial and satellite links, so system designers work below it in Ku or above it in Ka. That decision, made early in the development of commercial satellite communications, gave Ku-band a durable role as the workhorse spectrum for fixed satellite service, direct-to-home broadcasting, and mobile satellite terminals.

Spectrum Allocation

Allocations within Ku-band are set by the International Telecommunication Union and vary by ITU region. The satellite industry applies the Ku label more loosely than the radar standard does, stretching it down to about 10.7 GHz so that a single name covers the whole fixed satellite service downlink range, which straddles the 12 GHz boundary. On that broader usage, satellite downlinks occupy 10.7 to 12.75 GHz and uplinks occupy 13.75 to 14.5 GHz, with a further uplink segment near 12.75 to 13.25 GHz. In ITU Region 2, which covers the Americas, the 11.7 to 12.2 GHz slice is assigned to the fixed satellite service while 12.2 to 12.7 GHz carries the broadcasting satellite service used by direct-to-home television. The detailed band-by-band allocation tables maintained by the National Radio Astronomy Observatory show how radio astronomy, radiolocation, and terrestrial fixed services are interleaved with the satellite assignments across the same frequencies.

Propagation and Rain Fade

At centimeter wavelengths, raindrops are comparable in size to a meaningful fraction of the wavelength, so precipitation scatters and absorbs Ku-band signals. The resulting rain fade is the dominant link impairment, and it can remove several decibels of margin during heavy convective storms, with worse statistics in tropical climates than in temperate ones. Ku-band suffers less fade than Ka-band but noticeably more than C-band, which is why C-band retained a foothold in equatorial regions long after Ku-band dominated elsewhere. Modern systems manage the problem with uplink power control, site diversity, and adaptive coding and modulation, which trades throughput for robustness as conditions change. Link budget models normally specify availability as a percentage of the worst month rather than an annual average, since fade events cluster seasonally.

Payloads and Ground Terminals

The short wavelength allows compact apertures, and a 60 to 90 centimeter dish delivers enough gain for a consumer terminal, compared with the multi-meter reflectors that C-band requires. Spacecraft payloads pair traveling-wave tube amplifiers with shaped or spot-beam reflectors to concentrate power over service areas. NASA's Tracking and Data Relay Satellite constellation uses Ku-band single access service through steerable tri-band reflector antennas to relay high-rate data from spacecraft in low Earth orbit, including the International Space Station. Electronically steered flat-panel arrays have since extended Ku-band service to aircraft and ships, and surveys of satellite communications for non-terrestrial networks describe how these terminals are being folded into 3GPP mobile architectures.

Applications

Ku-band has applications across a range of fields, including:

• Direct-to-home television broadcasting and satellite radio distribution
• Very small aperture terminal networks for enterprise and banking connectivity
• In-flight and maritime broadband service
• Spacecraft telemetry and high-rate data relay
• Weather and airborne precipitation radar
• Backhaul for cellular sites in remote terrain