Surface-penetrating radar presents the system designer with significant restrictions on the types of antennas that can be used. The propagation path consists in general of a lossy, inhomogeneous dielectric, which, in addition to being occasionally anisotropic, exhibits a frequency dependent attenuation and hence acts as a lowpass filter. The upper frequency of operation of the system, and hence the antenna, is therefore limited by the properties of the material. The need to obtain a high value of range resolution requires the antenna to exhibit ultra-wide bandwidth, and in the case of impulsive radar systems, linear phase response. The requirement for wide band width and the limitations in upper frequency are mutually conflicting and hence a design compromise is adopted whereby antennas are designed to operate over some portion of the frequency range 10 MHz to 5 GHz depending on the resolution and range specified. The requirement for portability for the operator means that it is nor mal to use electrically small antennas, which consequently results generally in a low gain and associated broad polar radiation patterns. The classes of antennas that can be used are therefore limited, and the following factors have to be considered in the selection of a suitable design; large fractional bandwidth, low time sidelobes and in the case of separate transmit and receive antennas, low crosscoupling levels. The interaction of the reactive field of the antenna with the dielectric material and its effect on antenna radiation pattern characteristics must also be considered.

Inspec keywords: dielectric materials; transmitting antennas; radar antennas; ground penetrating radar; antenna radiation patterns; microwave antennas; UHF antennas; receiving antennas

Other keywords: antenna reactive field; impulsive radar system; inhomogeneous dielectric material; receiving antenna; linear phase response; surface-penetrating radar; crosscoupling level; range resolution; frequency 10 MHz to 5 GHz; lossy; low time sidelobe; fractional bandwidth; lowpass filter; frequency dependent attenuation; ultrawide bandwidth; transmitting antenna; polar antenna radiation pattern

Subjects: Radar equipment, systems and applications