title: Investigation of two problems of airborne radar antennas
Creators: name: Niyomjan, Greepol
Publisher: name: University of Liverpool Repository
Year: 2023

description: One of the airborne radar antenna problems is the integration of microwave circuits with waveguide components. As part of this research, an optimum transition from a waveguide to a microstrip using the short transformer technique is proposed. A layer of dielectric substrate is employed as a matching structure between the waveguide and microstrip line structure, which is optimised using the direct search method. It is shown that the transition has an average return loss value of -19.2 dB throughout the X band (8 - 12.5 GHz) with the best predicted return loss value of - 64 dB. Another issue of airborne radar antennas investigated is the high impedance surface (HIS) structure which could be employed to suppress surface effect usually encountered in conventional low profile antennas so that their efficiency such as radiation pattern and gain can be greatly improved. A 3x3 cells simulation model is proposed to investigate performance of high impedance surface (HIS) structure. This simulation model is used to determine resonant frequencies of the test HIS structure. The obtained actual and additional resonant frequencies are 5 GHz and 3.65 GHz respectively. The additional resonant frequency can be used to determine the start of the band gap frequency. Empirical equations for the additional resonant frequency have been successfully derived based on the behaviour ofthe current distribution along the surface ofthe HIS structure. A suspended microstrip fed slot antenna (MSA) on HIS structure is suggested to overcome bidirectional radiation behaviour exhibited in MSA and at the same time improving the gain, radiation pattern and return loss performance. This proposed antenna offers low side lobe, a maximum gain of 10 dB in the desired direction and a maximum gain ofless than -3 dB in the undesired direction. Radiations in both the E and H planes are symmetrical. The accuracy of the effective medium method is enhanced by applying proper boundary conditions. These rules and limits are derived based on the parametric study. A test structure is designed and created to validate the method. A close agreement b~tween the values of reflection phase angle (-90 to 90 degree) and surface impedance from 4.5 to 8 GHz is obtained by using this proposed improved method and numerical simulation. S21 graph and dispersion diagram are plotted to further confirm validity ofthe proposed method. A new improved enhanced effective medium is proposed to evaluate performance ofthe HIS structure with different patch shapes. This method is a combination of the enhanced effective medium method and the Guass' law. To validate the proposed method, the surface impedance values of the circular and octagonal patch HIS structures obtained from both HFSS simulation tool and the presented theory are compared. Close agreement between the results is observed. An industry standard full wave simulation tool (HFSS) is used to optimise the band gap bandwidth of the mushroom-like HIS structure under limited space condition. The optimisation approach is based on the variation of the patch width. It is shown that a fractional bandwidth of 110% can be achieved using our optimum patch width.