Double periodic composite right/left handed metamaterials
Date of Issue2012
School of Electrical and Electronic Engineering
Electromagnetic metamaterials (MTMs) with simultaneous negative permittivity and permeability, or left-handed materials (LHMs) are a very interesting and eye opening research topic in recent times. The concept was first proposed theoretically by Veselago in 1968. The first experimental realization and verification of LHMs was successfully achieved by incorporating the well-known split-ring resonators (SRRs) and metallic thin-wires. However, this kind of MTMs are bulky and highly resonant and they are more suitable for specific narrow band applications. Recently, an innovative concept of composite right/left handed (CRLH) transmission lines (TLs) is becoming an interesting and promising topic of research. These are well suited for the realization of MTMs offering advantages of non-resonant, more compact, less lossy and wider bandwidth. These CRLH TLs are used in practice to design compact devices, like small leaky-wave antennas (LWAs), compact filters, and many other applications. Most of the CRLH TLs known till now are single periodically loaded with uniform inductance and capacitance, and thus are termed as single periodic composite right/left handed (SP-CRLH) TLs in this thesis. A new transmission line, which is double periodic in nature, is loaded with a pair of alternate capacitances and/or inductances has been proposed and termed as double periodic composite right/left handed (DP-CRLH) TL in this thesis. In comparison to the SP-CRLH TL, the number of degrees of freedom offered by the DP-CRLH TL is doubled. As a result, a new branch can be observed in the dispersion diagram. Apart from the fact that DP-CRLH TL can radiate in the left-handed (LH) region and forward in the right-handed (RH) region (similar to SP-CRLH TLs), DP-CRLH TL promises a new forward leaky-wave (LW) radiation at low frequency below the LH passband with narrow bandwidth. This is an attractive and promising method to design compact LWAs. The new radiation frequency is significantly lower than the LH passband of the SP-CRLH TL with the corresponding similar period. Hence, it is possible to design LWAs which are more compact than the one based on SP-CRLH TLs. The proposed DP-CRLH TLs support three band LW radiations: First one being the new-found LW radiation at very low frequency. Second and third ones are the LH and the conventional RH radiations at higher frequencies. By controlling the values of the loaded components and their period, it is possible to obtain a desirable compact LW radiation behavior. With a suitable selection of the substrate material and geometrical dimensions, it is possible to design super-compact LWAs working at a lower frequency, unbalanced triple-band or balanced dual-band LWAs with frequency-scanning property. The DP-CRLH TL has been analyzed using TL theory. It aims at determining the propagation characteristics obtained from the dispersion behavior using the complex propagation constant (phase and attenuation constants). The new LW radiation is observed at very low frequency below the LH passband. It is noted that TLs double periodically loaded with either alternate capacitances or alternate inductances will contribute to the new LW behavior, although the other components are single periodically loaded. In order to analyze further the radiation characteristics of the proposed DP-CRLH TLs, especially for the microstrip line implementation of DP-CRLH TLs, singular perturbation procedure has been used. Dispersion characteristics are investigated, and the radiation efficiency and radiation angle for the optimum radiation efficiency are numerically estimated. Both the propagation and radiation characteristics of the proposed DP-CRLH TLs have been studied using a number of parametric studies, which include variation of geometrical dimensions, the loaded capacitances and inductances, and the period of the unit cell. The expressions for the cut-off frequencies have been derived for the infinite periodic structures. Also, a qualitative estimation of the cut-off frequencies for the periodic structure has been provided (for three cells). Based on these parametric studies, and suitable modeling of the proposed DP-CRLH TLs, a list of design guidelines has been developed. A few design examples of DP-CRLH TLs are presented. The proposed structures have also been fabricated and measured results are found to be in agreement with the expected results. The thesis concludes by pointing out a few limitations of the proposed DP-CRLH TLs. Multiple unit cells have to be cascaded to get better radiation efficiency, but this increases the geometrical dimensions of the antenna. Another limitation is that it is very difficult to design a wideband LWA at low frequency based on the proposed DP-CRLH TLs, because the boundary of the fast wave region is extremely close to the vertical axis of the dispersion diagram and the fast wave region at low frequency is too narrow. Since the bandwidth of the new LW radiation of the proposed DP-CRLH TLs is narrow, it is hard to design LWAs with frequency-scanning property. Although the bandwidth of the new LW radiation is narrow at very low frequency, the new method to design compact LWAs at a fixed low frequency based on DP-CRLH TLs is still attractive and promising. With further research on the proposed DP-CRLH TLs, it is expected that present limitations can be overcome, and this structure might find applications in more areas of microwave engineering.
DRNTU::Engineering::Electrical and electronic engineering::Electronic apparatus and materials