In this dissertation, a few novel resonant structures based on the defected ground structure (DGS) have been presented, and their applicability to microwave filter has been theoretically and experimentally verified through simulation, fabrication and experiment. Resonators/filters have played important roles in a wide variety of RF/microwave applications. Emerging and challenging applications such as the latest radars and wireless communications demand new RF/microwave resonators/filters meeting much more stringent requirements - higher performance, smaller size, lighter weight, easier fabrication, and so on. Therefore, there have been many efforts to invent, apply and improve many new resonant concepts and components in order to overcome such difficult requirements, and the forbidden bandgap (FBG) structures such as electromagnetic bandgap (EBG) structures and DGSs are one of the results of the numerous challenges and attempts. However, the FBG structures also have the intrinsic drawbacks and difficulties of application. Due to too many parameters that should be considered, a main disadvantage of the EBG structures is the difficulty in its modeling, which gives rise to the emergence of the DGS. Although the DGS consists of one single unit cell and is easier to model than the EBG structure, it also needs the help of the EM simulator for the quantitative design. Thus, in spite of a few design parameters, the DGS configuration that can be easily controlled is preferred. In addition, due to the inherent limitations derived from the ground etching, the DGS specially suffers from the low quality factor and back-radiation loss.
In summary, this work is the result of efforts to surmount these drawbacks of the conventional DGS and to devise the simple and compact resonators with higher performance as follows.
Firstly, the resonant ground structure (RGS) based on a cavity-backed DGS is presented. The substrate-filled metallic cavity (SFMC) which covers a ground-defected pattern can improve the quality factor significantly and remove the back-radiation without increasing the size. The RGSs with dumbbell-shaped and spiral-shaped defects are fabricated, measured and compared with the DGSs having the same defects respectively. The quality factor of the RGS with a spiral-shaped defect is larger than that of the DGS with the same defect by a factor of 7.4. It may be expected that the RGS has several times higher quality factor than the conventional DGS depending on the defect shape. In other words, the low quality factor and back radiation of the existing DGSs can be easily surmounted through the concept of the RGS/SFMC. Moreover, the simple design method for the compact triple-band resonator has been validated using three different resonators: defected microstrip structure (DMS), split-ring resonator (SRR), and RGS.
Secondly, the modified spiral-shaped DGS (MS-DGS) is described in detail. The conventional spiral-shaped DGS (CS-DGS) is one of the representative DGSs with the highest quality factor. The MS-DGS is made by symmetrically dividing the CS-DGS into two pieces and forming a middle lane. Therefore, the current is coupled to the ground-defected pattern at the resonant frequencies and it flows along the middle lane at the non-resonant frequencies without coupling. On the contrary, the current at all frequencies in the CS-DGS flows along the ground-defected pattern because the transversely formed defect directly impedes the current flow. As a result, in spite of the same defect size, the MS-DGS has higher quality factor than the CS-DGS by a factor of 6.04. Moreover, in case of the asymmetric MS-DGS, two asymmetric defects divided by the middle lane generate a dual-band resonance which features a similar bandwidth and high quality factor.
Thirdly, the optimum bandstop filter (BSF) using the DGS-embedded stub is presented. The proposed DGS-embedded stub resonator consists of the shunt open-circuited stub and the embedded dumbbell-shaped DGS, which is placed at the stub-connected point in a T-junction. In the frequency range where the harmonic stopband is generated by the stub, the embedded DGS prevents the current from being coupled to the stub. Therefore, the current does not recognize the stub and just flows along the main line. On the other hand, in the low frequency range, the slow-wave effect of the embedded DGS reduces the stub length. Therefore, compared with the optimum BSF using the conventional stubs, the optimum BSF using the DGS-embedded stubs has about the 50 % upper passband extension and about the 15.0 % size reduction. It is noteworthy that the DGS embedded in the stub resonator can remove the harmonic stopband, while the DGS has been conventionally used to suppress undesirable or unwanted passbands so far.
Fourthly, the compact resonant slit structure (RSS) for the rectangular waveguide BSF is presented. Though the EBG structure for the rectangular waveguide has been presented, the DGS is not feasible in the rectangular waveguide which features the non-grounded structure. However, the newly introduced RSS has many comparable characteristics as well as configuration which is reminiscent of the dumbbell-shaped DGS, and thus can be modeled as a series branch of parallel RLC resonant circuit. Depending on how to install the RSS, various responses such as narrow-band, wide-band, and dual-band can be achievable. In addition, the RSS does not have the drawbacks of the DGS such as low quality factor and back-radiation loss. Owing to the compact RSS, it is possible to implement a much smaller BSF compared with the conventional BSF based on a shunt branch series-resonant circuit. The compact rectangular waveguide BSF using the RSS is designed, fabricated and measured by the equivalent circuit-based design method.
To conclude, the research work presented here is a theoretical and experimental study on the new resonant structures based on the DGS, which could impart much flexibility in the design of future RF/microwave resonator/filter applications.