Design of Novel Waveguide Filters and Filtering Antennas

Abstract

In the wireless communication transceivers and receivers, bandpass filters and antennas are basic components. For example, an RF front-end includes at least a bandpass filter and an antenna for receiving or transmitting the needed signals which occupy a certain frequency range. Many researchers have made substantial contributions to the improvement of the performances of the bandpass filters and antennas. Numerous novel structures and design methods were proposed to flourish these two research areas. As we all know, the transmission zeroes benefit the bandpass filters with large rejection and high selectivity. They are widely introduced to improve the out-of-band performance of bandpass filters. Therefore, in this thesis, the zero techniques (ZTs), which are to introduce transmission zeroes for bandpass filters or induce radiation zeroes, i.e. radiation nulls, for antennas, are proposed. Several novel bandpass filters and a filtering antenna are presented in this thesis. These two basic components are designed by different proposed methods, then fabricated and measured. The proposed design methods have been summarized and verified.

First of all, a novel design of E-plane filter in rectangular waveguide, called electromagnetically induced transparency (EMIT) filter, is proposed. The transmission zero is realized by the EMIT. This EMIT filter is implemented by a rectangular ring (RR) on one side of the substrate and two strip lines resonators (SLRs) on the opposite side, which brings in electrically induced transparency (EIT) and magnetically induced transparency (MIT) by the electrical and magnetic couplings between the RR and SLRs, respectively. Transmission zeroes are induced by the EIT and MIT. Properties of high selectivity, low insertion loss, flat group delay, and small size of only about 0.2 λg, are realized by this design method. A sample has been fabricated and tested. The measured central frequency is 33.77 GHz with a 3-dB fractional bandwidth of 9.4 % and the minimum insertion loss in the passband is 0.5 dB. Using the proposed miniature EMIT filter as an application, a diplexer is further designed with compact sizes and low insertion losses. The measured minimum insertion losses are 0.9 and 0.5 dB in the lower and upper passbands centered at 31.79 and 34.74 GHz with 3-dB relative bandwidths of 7.2% and 8.6%, respectively. Good agreements between the simulated and measured results are obtained both in the EMIT filter and the EMIT diplexer.

Secondly, a novel compact E-plane waveguide filter using multiple resonators is presented, which has multiple transmission zeroes. The transmission zeroes are directly generated by the resonators loaded in a waveguide which is a highpass transmission line. The proposed filter is less than 0.15 λg in length of a waveguide. The key of the proposed method is to design the locations of transmission zeroes. Four transmission zeroes, of which two locate in upper stopband while the other two are in lower stopband, are produced by four resonators. The couplings between the resonators and locations of the transmission zeroes are carefully analyzed and designed, which enhance the selectivity as well as the out-of-band performance. Theoretical studies and experimental investigations are conducted to demonstrate the proposed design. The good agreement between the simulation results and measurement results is achieved.

Thirdly, a concept of transmission zero resonator pair (TZRP) is proposed to realize another novel technique. The transmission zeroes are generated directly by the TZRP. Based on the proposed TZRP, a new method to induce a passband is demonstrated, by which two different types of bandpass filters are designed. The TZRP is structured by a pair of resonators with different resonant frequencies, which lead not only to two transmission zeroes but also to a transmission pole between them. Passband filters can then be built by designing the proximity coupling between the proposed TZRPs, as the TZRP works as a basic resonant element as usual. The passband of these filters can be flexibly controlled by sizes of the resonators of TZRPs, which determine the locations of the transmission zeroes and poles. By carefully allocating the transmission zeroes, high selectivity and large out-of-band rejection can be realized. This design method is applied to design filters in two different transmission media, namely, microstrip line and rectangular waveguide. Simulated and measured results demonstrate the effectiveness of this new approach of bandpass filter design. The designed filters have the properties of small size, easy fabrication, low cost, and low loss. This kind of bandpass filter with a single passband can be extended to multiple passband bandpass filters by carefully designing multiple pairs of the TZRPs and the proximity couplings between the TZRPs.

Then, based on the TZRPs, a novel dual-band bandpass waveguide E-plane filter using three pairs of resonators is presented. In this configuration, two pairs of TZRPs are on the top of the substrate and the last pair is at the bottom. The length of the proposed filter is about 0.14 λgL or 0.17 λgH (λgL and λgH are standard waveguide wavelengths of the first and second passbands, respectively). Two passbands are designed flexibly with high selectivity and low insertion loss. A sample of the proposed dual-band filter is fabricated and tested. Good agreement between the simulation and measurement results is achieved. This kind of dual-band waveguide filter has verified the extension capability mentioned above and has benefits of low cost and mass production.

Finally, a new inducing radiation null technique is presented in designing a filtering antenna. The design method by using the filtering-radiating patch (FRP) is demonstrated. The so-called FRP is a structure of a rectangular patch etched with slots. It inherits the radiation performance of conventional patch antennas, and more importantly, introduces filtering feature with a radiation null (zero) at either the upper or lower band-edge of radiation efficiency curve. The frequencies of radiation nulls are easy to control. Based on the FRP, a novel filtering antenna is proposed. Two radiation nulls are realized at both band-edges of the antenna efficiency curve, leading to a sharp band skirt and good selectivity in the boresight gain response. The locations of the two radiation nulls can be flexibly controlled by the lengths of slots. A prototype is fabricated and tested. The measured results show an impedance matching bandwidth of 7% with a center frequency of 5.24 GHz, two radiation nulls at 4.7 and 5.85 GHz, respectively, a realized gain of 6.6 dBi, the cross-polarization rejection larger than 23.4 dB, and the front-to-back ratio better than 15 dB. The presented method demonstrates the capability of not only achieving good filtering-radiating performances but also possessing very simple structures by only etching slots on the patch of a conventional microstrip antenna.

Date of Award	23 May 2018
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Kwok Wa LEUNG (Supervisor) & Quan XUE (Co-supervisor)