Page 211 - Radar Technology Encyclopedia

201 frequency bands, radar frequency band, X-band

sizes for field deployment. Under smooth-surface conditions,
Table F7 VHF radars can take advantage of ground or sea-bounce mul-
tipath constructive interference effects to extend the detection
Standard Radar-Frequency Letter Bands
range to nearly twice the free-space range, but then must con-
Band Nominal fre- Specific frequency ranges for tend with the resulting nulls in coverage due to the accompa-
letter quency range radar based on ITU assign- nying destructive interference effects. VHF is also a region of
ments for Region 2 the spectrum suited to the detection of low-RCS targets, in
that in this region, so-called stealth RCS reduction techniques
HF 3–30 MHz No special radar bands
are least effective. VHF radars can be relatively inexpensive
assigned
solutions to wide-area air surveillance problems, and have
VHF 30–300 MHz 138–144 and 216–225 MHz seen service in this role. In general, however, radar use of this
RF region carries with it major compromises in terms of
UHF 300–1000 MHz 420–450 and 890–942 MHz
achieving uninterrupted spatial coverage and good target res-
L 1–2 GHz 1.215–1.4 GHz olution. PCH
S 2–4 GHz 2.3–2.5 and 2.7–3.7 GHz The ultra-high-frequency (UHF) region covers a range
from 300 to 1000 MHz, and the wavelengths (1 to 0.1m)
C 4–8 GHz 5.25–5.925 GHz
make it suitable for use in physically constrained platforms,
X 8–12 GHz 8.5–10.68 GHz such as in airborne early warning (AEW) aircraft. Beam-
widths can be made reasonably compatible with aircraft vec-
K u 12–18 GHz 13.4–14.0 and 15.7–17.7 GHz
toring requirements, and radars at these frequencies are little
K 18–27 GHz 24.05–24.25 GHz affected by atmospheric attenuation and rain. PCH
K a 27–40 GHz 33.4–36.0 GHz L-band covers from 1.0 to 2.0 GHz, and its wavelengths
make it suitable for higher resolution radars associated with
V 40–75 GHz 59–64 GHz
airspace surveillance and en route air traffic control. Weather
W 75–110 GHz 76–81 and 92–100 GHz effects, although more severe than those suffered by radars at
lower frequencies, are still of minor consequence for ranges
mm 110–300 GHz 126–142, 144–149, 231–235,
out to 400 km. L-band radars have application in surface-
238–248 GHz
based, seaborne, airborne (AEW), and space radar roles and
The designation mm is derived from millimeter-wave may lend themselves to uses where 3D, multifunctions
radar and is also used to refer to radar frequencies above 40 (search and track) are required. PCH
GHz and occasionally also to K -band. PCH, DKB S-band, 2.0 to 4.0 GHz, holds many advantages for medium-
a
Ref.: IEEE Standard 521-1984; Skolnik (1990), p. 1.14 range radar applications. Weather radars at S-band provide
The high-frequency (HF) region of the radar spectrum accurate data on rainfall rate, and the superior beamwidths
extends from 3 to 30 MHz, and as a consequence of the rela- achievable with moderate-sized antennas make this frequency
tively long wavelengths associated with this band (10 to band suitable for multifunction radars and specialized track-
100m), the RF hardware is physically large, and very large ing/instrumentation radars as well. Most of the airport sur-
antenna structures are required to obtain narrow beams for veillance radars (ASR) operate at S-band, as do many military
high angular resolution. Early use of this frequency band, in search radar whose requirements include accurate target des-
the Chain Home network of air-surveillance radars, was made ignation to subsidiary radar-directed fire control systems.
by the British before World War II because in 1938 HF was PCH
the highest frequency for which high-power RF components At C-band (4.0 to 8.0 GHz), the effects of atmospheric atten-
were available. An advantage of HF lies in the fact that at uation and weather become serious impediments to its use for
these frequencies, ionospheric refraction occurs, extending long-range search. C-band represents a middle ground
the potential detection range far beyond the radar line-of- between S-band and X-band and shares the advantages, as
sight. This over-the-horizon (OTH) feature makes HF attrac- well as the disadvantages, of each. Given sufficient power-
tive for surveillance of large areas of the earth's surface, usu- aperture product and clear-air conditions, C-band can func-
ally, however, at the price of good target resolution. PCH tion well in the range instrumentation radar role, and, in addi-
The very-high-frequency (VHF) region shares many of the tion, is probably the lowest frequency at which a precision
characteristics of HF, including high occupancy (by nonradar multifunction (target acquisition, track, and missile guidance
communications), potentially high external noise interfer- support) can be seriously entertained. PCH
ence, and relative immunity to weather effects. Since the The X-band extends from 8.0 to 12.5 GHz, making this fre-
wavelengths at VHF are ten times shorter than HF, high- quency regime suitable for high-resolution radar applications
power transmitters, along with antennas yielding acceptable such as weapon-system fire control (either as separate search
angular resolution, are more readily achievable in practical and track radars, or in 3D multifunction radars), precision