| With the rapid development of a variety of wireless broadband communi-cation systems, the antenna, which is one of the most significant components of communication systems, faces many growing demands such as wideband, multi-frequencies and so on. Supported by the National Natural Science Foun-dation of China, this thesis is devoted to:(1) The dual-wideband magneto-electric dipole antenna for 2G/3G/LTE/WLAN applications; (2) Miniaturiza-tion and planarization for wideband multi-frequency magneto-electric dipole antenna; (3) High isolation and high gain for wideband multi-frequency an-tenna; (4) Miniaturization and high gain for wideband multi-frequency antenna at low cost. The main contents and some innovations are listed below:1. A novel dual-wideband double-layer magneto-electric dipole unidirec-tional antenna with a modified horned reflector for 2G/3G/LTE applications is proposed. Firstly, a double-layer electric dipole structure is presented to pro-vide a dual-wideband feature. In addition, to reduce the size of the antenna and improve impedance matching, a new feeding structure designed with inverted U-shaped and tapered line is introduced. Finally, a modified horn-shaped re-flector, instead of a ground plane, is employed to achieve stable and high gains. The antenna prototype can achieve a bandwidth of 24.4%(0.78GHz-1.01 GHz) with a stable gain of 7.2±0.6 dBi for the lower band, and a bandwidth of 67.3% (1.38GHz-2.78GHz) with a gain of 7.5±0.8 dBi for the upper band, covering all the frequency bands for 2G/3G/LTE systems. In order to analyze the an-tenna structure effectively, we first propose a simplified equivalent circuit of the double-layer ME dipole antenna model, and then derive its equivalent for-mula to guide the design.2. Based on the single-fed cross-ME-dipole concept, a novel dual-wideband complementary patch antenna with a dual-layer cross-ME-dipole structure is proposed for 2G/3G/LTE/WLAN applications. In order to provide dualwide-band and complementary characteristics, an inverted U-shaped feeding struc-ture is introduced to feed dual-layer cross-ME-dipole patches. In addition, hook-shaped patches are placed on the corner of upper-layer electric dipoles to broaden the impedance bandwidth of upper frequency band. Moreover, a box-shaped reflector is employed to attain high and stable gain. The prototype antenna can offer an impedance bandwidth of 51.3%(1.68 GHz-2.84 GHz) with a stable gain of 8.7±1.5 dBi, and an impedance bandwidth of 11.4%(5.31GHz-5.9 GHz) with a gain of 8.2±1.1 dBi for the lower and upper bands, respec-tively. Besides, nearly symmetrical E-and H-plane patterns are also obtained. The measured cross-polarization levels in E-and H-planes are generally below-20 dB while the front-to-back ratios are larger than 18 dB. Hence, it can be suitable for 2G/3G/LTE/WLAN base-station communications in China.3. A planar-printed dual-wideband U-shaped magneto-electric dipole om-nidirectional antenna for WWAN/LTE applications is proposed. Firstly, a U-shaped electric dipole structure is presented to provide a dual-wideband feature by changing the surface-current distributions. In addition, in order to reduce antenna size and improve impedance matching, a new feeding structure de-signed with inverted U-shaped tapered line and meandering T-shaped line is in-troduced. Finally, instead of a conventional vertical ground plane, a small-size one is printed on the reverse side of the substrate to achieve stable gains and om-nidirectional radiation patterns. The antenna prototype can attain a bandwidth of 35.8%(0.78 GHz-1.12GHz) with a stable gain of 3±0.5 dBi for the lower band, and a bandwidth of 50.5%(1.66GHz-2.78GHz) with a gain of 3.8±0.6 dBi for the upper band, covering the frequency bands granted for WWAN/LTE systems. To the best of our knowledge, it is the first real-sense planar magneto-electric dipole antenna proposed. It can be easily fabricated at low cost and thus is promising for commercial WWAN/LTE communication.4. A novel differentially-driven dual-polarized dual-wideband magneto-electric dipole patch antenna with high isolation is proposed for 2G/3G/LTE applications. In order to generate dual-polarization and dual-wideband charac-teristics, a pair of bi-orthogonal dual-layerη-shaped tapered line feeding struc-tures are utilized to feed the two pairs of dual-layer U-shaped patches, respec-tively. Besides, a horned reflector is introduced to improve radiation patterns and provide stable gain. The prototype antenna can achieve a bandwidth of 25.7%(0.78GHz-1.01GHz) with a stable gain of 7.8±0.7 dBi for the lower band, and a bandwidth of 45.7%(1.69GHz-2.69GHz) with a gain of 9.5±1.1 dBi for the upper band. Input isolation exceeding 30 dB has been obtained in the wide bandwidth. Thus, it can be potentially used as a base station antenna for 2G/3G/LTE networks.5. Unlike previous band-notched UWB antennas with low gain and com-plex structures, a novel unidirectional dual-band-notched UWB antenna with high gain is proposed. In order to provide dual-band-notched feature, a U-shaped slot etched in the dipole radiating patch and a pair of capacitively loaded loops close to the feed line are employed simultaneously. The former mainly serves as an interference suppressor against WiMAX system (3.3GHz-3.8GHz), while the latter chiefly offers interference immunity to WLAN system (5.15GHz-5.825GHz). In addition, to meet the requirements in security and efficiency, a modified horned copper reflector is utilized to generate favourable directional-ity and high gain. The prototype antenna can offer an impedance bandwidth of 113%(3GHz-10.8GHz) with notched frequency bands at 3.2GHz-3.9GHz and 5.1GHz-5.9GHz, as well as a stable gain of 11±1.5dBi for SWR<2.6. A low cost U-shaped bow-tie electric dipole antenna with a modified horned reflector is presented for UWB applications. Instead of a horizontal planar electric dipole, a U-shaped bow-tie electric dipole structure with two notch slots is utilized to provide wideband nature, since it can evidently change the current distribution and be equivalent to radiation element. In addition, to reduce the size of the antenna and achieve excellent impedance matching, an inverted U-shaped feeding structure, which adopts perforation and tapered line, is introduced to the design. Moreover, in order to improve the stable gains and symmetrical radiation patterns, a dual-layer horned reflector is employed to fit the special structure scheme. To the best of our knowledge, the proposed antenna is the first U-shaped bow-tie magneto-electric dipole unidirectional an-tenna for UWB applications. The proposed antenna can achieve a stable gain of 10±1.2 dBi and an impedance bandwidth of 118% for SWR≤2 from 2.81 to 10.92 GHz. In addition, symmetrical E- and H-plane unidirectional radiation patterns, low cross polarization and high front-to-back ratio, a wide impedance bandwidth as well as absolute bandwidth which covers the entire frequency band for UWB communications can also be attained.The thesis can be conducted a comprehensive investigation and research on the operation principle and design methods for the multi-frequency wideband antennas. In addition, new design theories and methods for the wideband multi-frequency antennas are also provided.
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