LOW SIDE LOBE MICROSTRIP FOLDED DIPOLE ANTENNA ARREY FOR RADAR APPLICATIONS

Abstract

Array antennas due to their ability to achieve high directivity, narrow beams and low side lobe levels are used in modern high performance radar systems. Among array antennas, microstrip arrays posses several unique features relative to other type of antennas such as low profile, light weight, low cost and ease of fabrication. But the design of high gain and low side lobe level microstrip arrays at microwave frequencies is a challenging task as different factors like amplitude and phase accuracies, mutual coupling, spurious feed radiations, errors due to element mismatch and feed network isolation, surface waves excitation, high cross polarization and diffraction effects cause severe degradation in performance of microstrip arrays. Moreover achieving low side lobe requires that tapered amplitude distribution be applied to the array elements. This in turn makes the design of feed network a challenging task. The feed network must provide the power division ratios required to achieve a desired amplitude distribution and in addition should have low insertion loss, broad bandwidth and provide necessary matching. A low side lobe 16x12 microstrip folded dipole arrays is designed in this work. Microstrip symmetric folded dipole antenna element was used in this array because of its shielded feed network, wide band width and low cross polarization. Taylor amplitude distribution was used to obtain the excitation of the array elements. A corporate feed network was used to realize the desired excitations as it provide more control of the feed of individual elements and has large band width. Different types of T junction and inline type power divider having compact sizes and capable of achieving large power division ratios were developed for the corporate feed network design. Extensive simulations using MoM and FEM based EM analysis tools were performed to accurately analyze and optimize the microstrip folded dipole array. This array was fabricated and a measured gain of 28.45 dBi with side lobe levels of -17.89 and -18.79 dB in E and H plane respectively were achieved. The measured results show an excellent agreement with the simulated ones. To improve the side lobe levels a new 16x12 array has been designed with improved feed network in which the amplitude and phase of inline and T junction power dividers was carefully controlled. A simulated side lobe level of -26.78 and -26.5 dB were achieved in this array for a theoretical -30 dB side lobe level.