Broadband Antennas And Omnidirectional Antennas For Wireless Communication Applications

With the rapid developments of electronics and integration, the modern communication devices become more compact, intelligent and broadband. Being a critical equipment of wireless communication system, antenna featuring broadband characteristic is in great demand. For the application of covering large service area, omnidirectional radiation pattern is usually needed for antenna in its operating plane. This thesis does some research on the broadband antennas and omnidirectional antennas for wireless communication system. The author's major contributions are summarized as following:The design and realization of a reconfigurable cage, with broadband impedance and radiation bandwidth, are presented. The cage antenna can operate at VHF/UHF bands. Compared to the traditional cage antenna, this cage antenna has adjustable strips and any angle of the stretch of the strips can be controlled, which leads to a versatile pattern reconfiguration. PIN diodes are used and placed in the strips to control the length of the strips, which leads to a versatile frequency reconfiguration.A low cost printed dipole for tri-band TD-SCDMA base station is proposed. The antenna which employs a planar reflector is composed of two tapered arms fed by an integrated balun and loaded with a lumped element (a 0.6 pF capacitor), leading to a broad impedance bandwidth and stable gains. The capacitor and the balun comprise the adjustable input impedance matching network. Measured result shows that the antenna provides an impedance bandwidth of 40%. Good radiation performances and gains over the operating band are observed.The X-band conformal slot array antennas with omnidirectional radiation performance are investigated, which are based on the substrate integrated waveguide (SIW) technology. Basically, the conformal array antenna comprises four by three longitudinal slots as radiation elements. A cylindrical conformal slot array SIW antenna excited by a 4-way power divider with horizontal polarization is studied. In additional, in order to simplify the antenna, polarization rotated SIW antennas without any feeding network exhibiting horizontally omnidirectional performance are proposed. The SIW cavity radome can rotate the vertically polarized electric field radiated from the dipole/monopole inside into the horizontally one. And the 90-degree polarization rotation in X-band is realized by orthogonally placed slots etched on the cavity.