Radar performance must first be quantified on the basis of propagation efficiency in various media (air, foliage and ground) with respect to the required detection range. The general principle is that the lower the frequency, the more efficient the propagation of radio waves through the medium. In the presence of obscurants, propagation is most favorable when the RF wavelength is much larger than the particle size composing the propagation medium. This is why radars tend to have much better performance than optical systems through smoke, dust, fog and rain.
However, using a lower frequency dictates a larger antenna for a given angular resolution. As a rule of thumb, the physical dimensions of an antenna are related to the required resolution by the following equation:
θ ≈ 70° λ / w
θ is the half-power antenna beam width (resolution) in degrees,
λ the wavelength and
w the antenna longitudinal dimension.
Using the above equation, an imaging radar at Ka-band (35 GHz) with a two-degree angular resolution would require a 30 cm antenna while a foliage penetrating radar working at UHF (900 MHz) would have a 12-meter antenna to achieve comparable angular resolution.
Applications dictate the radar detection range. A radar intended for airborne surveillance may require a detection range in excess of 1000 km to provide an adequate response time to counter an incoming threat. To assure range performance out to 1000 km under rain conditions, propagation properties would require operating around S-band (2~3 GHz) or even L-band (1~2 GHz). In another application, a police radar used for measuring speed of vehicles would only require a maximum range of 500 m. Given the shorter range required, practically any frequency up to W-band (110 GHz) could be used. In this case, antenna size and component costs are most likely to influence the choice of operating frequency.
Properties of targets must also be considered in selecting the operating frequency as optimal detection is achieved when radar resolution matches the target size. As radar resolution depends on antenna beam widths (azimuth and elevation) and range resolution, the higher frequencies are best suited because they yield small antennas and little fractional bandwidth to achieve range resolution. The fractional bandwidth may be expressed as the percentage of radar signal bandwidth with respect to transmit frequency. In an application to protect flight lines from intrusions by pedestrians and crawling persons, a Ka-band radar is ideal because it allows small size for easy deployment and high range resolution with little fractional bandwidth.
Cost of RF components is another factor to consider in selecting the operating frequency. The higher the frequency, the more expensive are the components. With the recent developments in the telecommunications industry, very affordable components are now found up to 5.8 GHz (C-band). Components at X-band (~ 9 GHz) are now manufactured with high yields and prices have been declining steadily over the last few decades. Components at V and W bands are still expensive because of the manufacturing tolerances required for the short wavelengths (millimeter wave) and of limited demand.
The table below presents the various frequency bands used for radar with their most common applications.
|Frequency Band Designation||Nominal Range||Specific Radar Frequency Assignment (1)||Typical application|
|HF||3 ~ 30 MHz||no specific bands||Over the horizon radar|
|VHF||30 ~ 300 MHz||216 ~225 MHz||Very long range
Ground penetration radar
|UHF||300 ~ 1000 MHz||420 ~ 450 MHz
902 ~ 928 MHz
|Very long range
|L||1.0 ~ 2.0 GHz||1.215 ~ 1.390 GHz||Long range surveillance|
|S||2.0 ~ 4.0 GHz||2.305 ~ 2.385 GHz
2.417 ~ 2.483 GHz
2.700 ~ 3.650 GHz
|Long range surveillance
Air traffic control
|C||4.0 ~ 8.0 GHz||5.250 ~ 5.85 GHz||Air surveillance
Air traffic control
|X||8.0 ~ 12.4 GHz||8.500 ~ 10.55 GHz||Long range ground surveillance
Airborne weather radar
|Ku||12.4 ~ 18 GHz||13.4 ~ 14.0 GHz
15.7 ~ 17.7 GHz
Medium range ground
|K||18 ~ 27 GHz||24.05 ~ 24.25 GHz||Police radar|
|Ka||27 ~ 40 GHz||33.4 ~ 36.0 GHz||Short range ground surveil-
|V||40 ~ 75 GHz||59 ~ 64 GHz||Automotive anti-collision|
|W||75 ~ 110 GHz||76 ~ 81 GHz
92 ~ 100 GHz
Airborne wire detection
Fig 1: Radar frequency bands and applications
(1) NTIA, Office of Spectrum Management, October 2003