Broadband Omnidirectional Antennas

Omnidirectional antennas can radiate radio power uniformly in all directions in thehorizontal plane; thus they are widely used for radio broadcasting, satellite communications,radio frequency identification, mobile communications, wireless sensor networks, and aircraftcommuncations. In modern wireless communications, broadband antennas are required for thehigh-speed transmission of a large amount of information. Broadband antennasare also neededfor miniaturization of a wireless commincation system where one antenna can beusedfordifferent purposes. In this thesis, we developseveral broadband omnidirectionalantennas with different polarizations, including circular polarization, horizontal polarization,vertical/horizontal dual-polarization, and45°slant polarization. The major contributions of thisthesis are summirzed as follows.1. A broadband circularly polarized (CP) omnidirectional antenna is proposed based on adirectional CP rectangular loop. The proposed antenna includes four rectangular loopswhich are wrapped around a cylinder. A broadband feeding network is developed to feedthe CP omnidirectional antenna. A conducting cylinder is introduced to improve the CPperformance. The influences of the feeding network, and the conducting cylinder on theaxial ratio (AR) and return loss (RL) are investigated. Experimental results show that thisCP omnidirectional antenna has a10-dB RL bandwidth of45%and a3-dB AR bandwidthof41%. The gain variation in the horizontal plane is less than1.5dB. An eight-elementbroadband omnidirectional CP antenna array is developed for base station applications.2. A broadband CP omnidirectional antenna is developed based on four tilted dipoles. Theantenna consists of four pairs of titled dipoles which are wrapped around a cylinder. Eachpair of titled dipoles includes a driven dipole and a parasitic element which is introducedto enhance the bandwidth. Experimental results show that this CP omnidirectional antennahas a10-dB RL bandwidth of62%and a3-dB AR bandwidth of44%. The gain variationin the horizontal plane is less than1.5dB. An8-element omnidirectional CP antenna arraywith down-tilted beam and a dual CP omnidirectional antenna and its array are alsopresented.3. A broadband horizontally polarized (HP) omnidirectional antenna is proposed. The proposed antenna includes four printed arc dipoles. The four arc dipoles form a circularloop for horizontal polarization in the horizontal plane. Measurement results show that theHP omnidirectionalantenna has a10-dB RL bandwidth of about31%with anomnidirectivity is less than1.5dB. An8-element broadband omnidirectional HP antennaarray is also developed.4. A broadband vertically/horizontally dual-polarized omnidirectional antenna with highisolation is developed. The proposed dual-polarized antenna is a combination of a modifiedmonopole for vertical polarization (VP) and a circular planar loop for horizontalpolarization (HP). The modified monopole is a circular folded patch shorted by four tubeswhile the circular loop consists of four half-wavelength arc dipoles. Experiment shows thatthe dual-polarized omnidirectional antenna achieves a bandwidth of~27%with an isolationof higher than35dB. The gain variation in the horizontal plane is less than2.5dB for VPand1.5dB for HP. An eight-element dual-polarized antenna array is developed for mobilecommunication base stations.5. A broadband45°slant-polarized omnidirectional antenna is proposed. The antenna consistsof four cross-shaped dipoles which are placed around a cylinder for omnidirectionalradiation patterns. The45°slant polarization can be achieved by adjusting the lengths ofthe vertical and horizontal dipoles. Measurment results show that the45°slant-polarizedomnidirectional antenna has a15-dB RL bandwidth of22%. In the horizontal plane, thecross-polarization level is less than-15dB and the gain variation is less than2dB. Aneight-element dual-polarized antenna array is developed for mobile communication basestations.This thesis systematically investigates the operating principles and design methodology ofbroadband omnidirectional antennas with different polarizations, thus enriches the antennatheory and provides technological conditionsfor futurewider applications of omnidirectionalantennas.