Miniaturized Antennas and Metamaterial-Based Transmission Line Components in Microwave Circuits Applications

This dissertation presents two diversities of miniaturization approaches to the antennas and microwave passive circuit components. The first approach is based on the unique metamaterial transmission line structures. The metamaterial structure or the left-handed structure is an artificial structure that is dispersion engineerable from its constituent parameters. By means of the left-handed transmission lines or the composite right/left-handed (CRLH) transmission lines to replace the conventional microstrip lines, microwave circuit components can be miniaturized via controlling the phase responses at the frequencies of interest, which saves the footprint size. Specifically, this idea was implemented on the dual-band 180°0 and 90° hybrid couplers and both of them demonstrate considerable size reductions in the experiments. On the other hand, the second methodology leading to miniaturization is taking advantage of the slow wave structures. The slow wave structures presented in this dissertation are formed using the capacitive loading periodically. The effective propagation constant beta is enhanced by increasing the effective shunt capacitance in the equivalent circuit model derived from the conventional transmission line theory. The associated guided wavelength is therefore decreased and the same physical structure is capable of operating at lower frequencies. The slow wave structures are employed for compact antenna applications. In particular, the slow wave enhancement factor (SWE), which is defined as the ratio of the loaded to the unloaded propagation constants (beta//beta), is investigated using the loaded unit cell of the equivalent transmission line model and utilized as a design tool for an arbitrary size reduction. It is shown that the SWE agrees very well with miniaturization factor, and therefore load parameters in the circuit model can be readily obtained when a specific size reduction is attempted. Slow wave antennas will be exemplified in the third chapter in this dissertation.;The subject of the second chapter is CRLH-based and miniaturized dual-band hybrid couplers (both the 180° and 90° couplers) and their applications in beam pattern diversity systems as well as to microwave diplexers. To ensure compactness, all possible phase solutions of the CRLH-based transmission lines, which comprise the couplers, are considered and compared in terms of the physical length. This design methodology will be elaborated and given in the second chapter. The focus of the remaining chapter will be the applications of the CRLH-based dual-band couplers as the mode decoupling networks (MDNs) in the beam pattern diversity systems to demonstrate antenna pattern diversity in dual bands, and as the building elements of the microwave diplexers.;The subject of the third chapter is the compact antennas based on the slow wave structures developed by periodically capacitive loading. In this chapter, the design principle of the slow wave antenna with an arbitrary size reduction is proposed by taking advantage of the SWE, which is shown to be equivalent to the miniaturization factor. Two small radiators, the high-frequency (HF) slot-loop antenna and planar inverted F antenna (PIFA), are examples to achieve the desired size reductions. Furthermore, the adverse effects on the impedance bandwidth (VSWR≤2) and radiation efficiency from miniaturization are discussed and improved subsequently. A compact impedance matching circuit derived from the filter design techniques is proposed to alleviate the narrow impedance bandwidth to a degree. By the same token, antennas employing varactor diodes are able to attempt desired size reductions while exhibiting considerable effective bandwidths across the tunable frequency range. A varactor-loaded slot-loop antenna was implemented to illustrate this idea.