Study On Compact Wideband Planar Antennas

Antennas may be generally classified into two types:resonant type or non-resonant type according to their operating mechanism. Resonant type antennas are usually compact in size but their bandwidth is always narrow. Non-resonant type antennas have a wide operating bandwidth whereas is large in size. In the traditional resonant antenna designs, only the first resonant mode is utilized in the purpose of size miniaturization and design simplification. However, this single resonance nature leads directly to the narrow operating bandwidth. The main objective of this thesis is to develop new bandwidth enhancement techniques for traditional resonant antennas with a single radiator, especially for those with planar configurations, e. g., patch antennas, narrow slot antennas and planar dipoles. In this study, the bandwidth enhancement of the planar resonant antennas is realized by using two different approaches, e. g., impedance matching and multiple-mode-resonance concept. In the first part, the author developed a miniaturization technique of the impedance matching circuit for planar resonant antennas. In the second part, the author successfully developed several planar antennas (narrow slot antennas and printed dipole antennas) with 3:1 or larger bandwidth (VSWR<2.0), using the multiple-resonance concept.Impedance matching is a traditional technique in circuit design. An impedance matching network could be easily designed by using the matching theory after extracting the electrical parameters of a single-mode antenna. The radiating structure needs not to be altered by using this technique, thus the radiation pattern will remain stable after adding the matching network. The main shortcoming is the enlargement in antenna size because of the impedance matching circuit. To solve this, two compact implementations are proposed in this research. One is suitable for patch antennas; the other one is for slot antennas. After adding the proposed matching circuit, the antenna bandwidth can be multiplied with no significant size enlargement.Multiple-mode-resonance is a unique feature of the distributed resonators (radiators). Only one resonant mode is used in traditional resonant antennas, thus the antenna bandwidth is hard to be improved. The reported wideband antennas using the multiple-resonance concept are complicated in structure. The design procedure depends on cut-and-try method without clear rules or formulas due to the unclear radiation mechanism. In this study, the multiple-resonance technique is applied to two basic radiators (magnetic and electric dipoles) to broaden their operating bandwidths. As validated by the numerical prediction and experiment, the bandwidths of the traditional narrow slot antenna (magnetic dipole) and printed dipole can be extended to be 3:1 or even wider with proper feeding schemes and referenced port impedance. Those successful designs indicate that the multiple-resonance technique makes great improvement on the bandwidth of the resonant antennas whereas antenna miniaturization is retained.