Jones, Martin R.; (1990) A study of wide bandwidth directional patterns from circular arrays. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Passive radar warning receivers use amplitude comparison direction-finding to determine the angle of arrival of incoming signals. This type of direction-finding system requires wideband azimuth beams that do not change in shape with frequency. The subject of this thesis is the synthesis of wideband beams using a circular array. In the first part of this thesis the excitation of the circular array is analysed using the concept of 'phase modes': the orthogonal terms of a spatial Fourier series. If the variations in phase and amplitude of these modes with frequency are corrected, azimuth patterns formed from these modes are instantaneously wideband. Pattern synthesis uses the principle of linear array equivalence, allowing us to apply low sidelobe techniques developed for linear arrays to the phase modes. The design of the experimental system, operating over the frequency range 8 to 12 GHz, is subsequently presented. The characteristics of the phase modes excited on a four element monopoles array were evaluated, showing that the array could be used to form a low sidelobe beams. The initial beamformer design used a Butler matrix constructed from microwave directional couplers to excite the phase modes. By utilising microstrip compensation networks, the variation in phase and amplitude of these modes with frequency is corrected. Multiple beams are formed from the compensated modes by a second matrix. Both theoretical and measured results showed that the phase and amplitude errors introduced by this complicated network were unacceptable. Multiple beams were not demonstrated in this study. A simplified matrix design was developed to demonstrate a low sidelobe (-28 dB) beam at a single fi*equency. A weighted corporate feed was developed to demonstrate instantaneously wideband pattern synthesis of a single beam. The element excitation required to form a low sidelobe pattern was calculated using phase mode theory. The frequency-dependent element excitation was practically realised using the microstrip networks. Anechoic chamber measurements of the synthesised wideband beam showed that the variation in the -3 dB beamwidth across the band 8 to 12 GHz was less than ±3°. The sidelobe level was below -20 dB. Theoretical calculations limit the frequency bandwidth for this synthesis technique to about one octave. A wideband sin(Nx)/Nsin(x) pattern was also produced to demonstrate the versatility of this synthesis technique.

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