Abstract
In recent years, the defected ground structure (DGS) has found numerous applications in the design and improvement of microwave circuits including amplifiers, filters, couplers and oscillators. DGS has the attractive benefits of band rejection, alleviation of the difficulties of fabricating high impedance microstrip lines and size reduction, owing to its inherent resonance, high characteristic impedance and high permittivity features. However, in the design of many DGS patterns, there exists a small centre slot that would be difficult to fabricate with inexpensive PCB fabrication techniques if steep attenuation slope for good filtering applications is desired. Furthermore, current DGS patterns would require either a large etched area, or very fine centre slot width to achieve resonance for low microwave frequency operations. Following from knowledge of the limitations of current DGS patterns, the research activities in this thesis focus on the design of novel DGS patterns that alleviate fabrication difficulty and achieve lower resonance in a compact size. Two novel DGS patterns are proposed for the first time: one without the narrow centre slot, so as to provide ease in fabrication, and the other for achieving low resonance in a compact size that is easily fabricated with conventional PCB fabrication techniques. The simulation and experimental work in this research are based upon analysis using fundamental physical principles. Empirical results are reported, which show that the first novel DGS pattern achieved the same frequency response as a dumb-bell shaped DGS pattern with a centre width almost five times larger than the centre slot width of the latter. In addition, the frequency response of the novel DGS pattern was shown to be less susceptible to fabrication errors in its centre width. Measured results of the second novel DGS pattern showed a 1.32 GHz reduction in resonant frequency compared to a dumb-bell shaped DGS pattern with a slot width of 0.3 mm. In contrast, the smallest dimension in the second novel DGS pattern was only 0.5 mm. When compared to a DGS pattern with enlarged etched dimensions to achieve the same lower resonance, the second novel DGS pattern was shown to be 50% smaller in total etched area. The second novel DGS pattern exhibited a symmetric frequency response that is suitable for band-stop filter applications at low microwave frequencies. This symmetry in the frequency response overcomes the difficulty of an asymmetric frequency response for a DGS pattern with enlarged etched dimensions, that is reported in this thesis, and therefore limits the use of such a pattern as band-stop filter. Finally, a potential application of DGS to a two-section impedance transformer was investigated and confirmed by practical measurement. The high characteristic impedance of 141.4 required for the second quarter-wave section, which corresponds to a line width of 8.54 μm on a conventional microstrip, was easily fabricated with PCB fabrication technique when DGS was implemented on the impedance transformer. The fabricated DGS transformer exhibited a return loss >15 dB with a bandwidth of 800 MHz.