Gain and efficiency enhancement of the miniaturized split ring based antennas
Small antennas are very attractive due to their compactness and lightness. However, the price one has to pay for having a small antenna, such as significant reduction in radiation efficiency, gain and bandwidth, makes them less practical and less useful. This thesis demonstrates how one can miniaturize an antenna by artificially inducing high-permeability in the antenna such that less efficiency and gain will be sacrificed. The improvement in efficiency is achieved by reducing the losses in the antenna; and the improvement in gain is achieved by increasing both efficiency and directivity, which are realized by reducing the losses in the antenna and replacing a conventional ground plane with a U-shaped ground plane, respectively. The high-permeability is induced by employing metallic loops (e.g. split ring resonators (SRRs)) and properly arranging them with respect to the magnetic field. First, split ring loaded dielectric resonator antennas are studied. It is shown that loading a conventional dielectric resonator antenna (DRA) with SRRs can indeed miniaturize the antenna. The miniaturization capability varies with SRR parameters and arrangements: the larger the SRR area, the higher the miniaturization capability. The arrangement of a single SRR allows for the most miniaturization, compared to other arrangements, but the others can provide wider bandwidths. One of the examples shows that loading a very low-permittivity (ε_r=2.2) based DRA with three layers of single SRR is equivalent to increasing the permittivity of the dielectric to ε_r=976. The miniaturized antenna also has a reasonably good efficiency (80.85%) and gain (6.69 dBi), despite its small dimensions (0.124λ_0×0.062λ_0×0.002λ_0). In order to further miniaturize the SRR loaded DRAs, two types of metallization, namely U-shaped and split-ring-shaped metallizations, are applied to the surfaces of the loaded DRAs. It is found that both metallizations have little effect on a single-element loaded DRA, but can effectively miniaturize the unloaded DRA, since similar to SRRs, both metallizations are also capable of inducing permeability. Therefore, a probe-fed flat-wire open ring antenna, essentially just the split-ring-shaped metallization, is also studied. Good antenna performance (efficiency of 86.36% and gain of 7.94 dBi) is also observed with such a miniaturized antenna (dimensions: 0.113λ_0×0.057λ_0×0.011λ_0). In order to further miniaturize the open ring antenna, a two-turn flat-wire open ring with different slit combinations is investigated. By placing the slit of the outer ring at the bottom and the slit of the inner ring at the top, the antenna can be most effectively miniaturized.