| With rapid development of wireless communication technology, higher performance and smaller volume of devices are needed for communication system. This not only require that the device can operation in multi frequency bands for multi-communication system, but also need the ability of processing information smarter. Miniaturization of device reflects human’s pursuit, and is easier to accept and use by people. As the front-end device of wireless communication system, higher performance of antenna is need. So, multi-band operation and low-profile design are the main goals of antenna. With the specific needs of wireless communication system and the design goal of multi-band operation and low-profile characteristic, dual-band low profile omni-directional antenna, multi-band directional antenna and compact multi antenna system are studied and analyzed in this paper. The author’s major contributions are outlined as follows:1. Multi-band low-profile omni-directional antenna is introduced in this paper. In order to meet the demand of low profile of omni-directional antenna, a dual-band low-profile microstrip monopolar antenna, which is based on zero order resonance, is proposed. The antenna is based on a circular patch antenna that is concentrically surrounded with a set of mushroom elements. Zero order resonance is formed with mushroom element, which can resonant at the sub-wavelength frequency band. Meanwhile, this antenna radiates a similar pattern to a vertical monopole antenna with TM02 mode of circular patch and annular ring. Fabricated on a dielectric substrate, the antenna has a low profile(about 0.02λ at zeroth order resonant frequency). Proposed antenna can operation at two frequency bands, and obtained results show that there is a deep null in the broadside in the elevation radiation patterns and a nearly omni-directional pattern in the azimuth plane.2. Multi-band low-profile directional antenna is studied in this paper. Three kinds of multi-band low-profile directional antenna are proposed with different design methods. First, low-profile antenna is designed with EBG plane, which is equivalent to perfect magnetic conductor. Second, based on the traditional rectangular patch antenna, a conductive disk with SRR slot is placed between the patch and ground, so a multi-band characteristic is achieved. With different kinds of SRR, dual-band and tri-band low-profile antenna are proposed. Finally, a low-profile antenna constituted with shorted patch and square patch is proposed for dual-band application. A SIR microstrip is applied for coupling feed, and U shaped reflector is used for improving forward radiation. Coupling capacitance of feed and inductance of shorting probe cancel each other, so the bandwidth of high frequency band is improved. Dual-band low-profile antenna has impendence bandwidth of 1.6% at low frequency band and 60% at high frequency band.3. Compact multi antenna system is studied in this paper. Compact omni-directional MIMO antenna and directional MIMO antenna are proposed for the need of LTE communication system. Omni-directional MIMO antenna is composed of asymmetric biconical unit and six printed dipole, while VP unit can work at two frequency bands and HP can work at high frequency band. HP unit is nested between two cones of VP unit, so the whole antenna structure is compact. The isolation between VP and HP unit is better than 25 dB, which meets the requirement of MIMO system. So the proposed antenna not only can operate for GSM900, CDMA800, WCDMA, etc. with single VP element, but also can operate for LTE systems with the VP and HP elements. Directional MIMO antenna is composed of two shorted patch antenna with slot coupling. The antenna is fed with H shaped slot, and the distance between the units is only 4mm. Defected shorted wall can change the shape of shorted wall and the current on the ground, and the isolation between the unit is improved.
|
PDF Full Text Request