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The selection of a radars operating frequency is generally the result of a trade-off analysis thatconsiders desired detection range, weather and clutter environment, available aperture size,properties of targets of interest, and cost of RF components.

Radar performance must rst be quantied on the basis of propagation efciency in variousmedia (air, foliage and ground) with respect to the required detection range. The general prin-ciple is that the lower the frequency, the more efcient the propagation of radio waves throughthe medium. In the presence of obscurants, propagation is most favorable when the RF wave-length is much larger than the particle size composing the propagation medium. This is whyradars tend to have much better performance than optical systems through smoke, dust, fogand rain.

However, using a lower frequency dictates a larger antenna for a given angular resolution. As arule of thumb, the physical dimensions of an antenna are related to the required resolution bythe following equation.

70 / w

where is the half-power antenna beam width (resolution) in degrees,

the wavelength andw the antenna longitudinal dimension.

Using the above equation, an imaging radar at Ka-band (35 GHz) with a two-degree angularresolution 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 resolution.

Applications dictate the radar detection range. A radar intended for airborne surveillance mayrequire a detection range in excess of 1000 km to provide an adequate response time to coun-

ter 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 wouldonly require a maximum range of 500 m. Given the shorter range required, practically any fre -quency up to W-band (110 GHz) could be used. In this case, antenna size and component costsare most likely to inuence the choice of operating frequency.

Properties of targets must also be considered in selecting the operating frequency as optimal de-tection is achieved when radar resolution matches the target size. As radar resolution dependson antenna beam widths (azimuth and elevation) and range resolution, the higher frequenciesare best suited because they yield small antennas and little fractional bandwidth to achieverange resolution. The fractional bandwidth may be expressed as the percentage of radar signal

bandwidth with respect to transmit frequency. In an application to protect ight lines from in-trusions by pedestrians and crawling persons, a Ka-band radar is ideal because it allows smallsize 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. Thehigher the frequency, the more expensive are the components. With the recent developmentsin 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 priceshave been declining steadily over the last few decades. Components at V and W bands are stillexpensive because of the manufacturing tolerances required for the short wavelengths (mil-limeter wave) and of limited demand.

RADAR SPECTRAL BANDS TECHNICANOTES

4PierrePoitevin

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