Polarization Reconfigurable Omnidirectional Antennas

Reconfigurable antennas are becoming one of the key technologies in the next generation wireless communication systems; so this topic is a research hotspot in the field of antenna theory and technology. Polarization reconfiguration is one of the forms of reconfigurable antennas. Since omnidirectional antennas cannot implement the reconfiguration of the radiation pattern, it is very important for the omnidirectional antenna to realize polarization reconfiguration. Therefore it is necessary to investigate polarization reconfigurable omnidirectional antennas. At the same time, it is difficult to realize polarization reconfigurable omnidirectional antennas because omnidirectional antennas with different polarizations have different antenna structures. In modern wireless mobile communications, omnidirectional antennas have been used in point-to-multipoint applications. Whereas by employing technology of polarization reconfigurable antennas, it can realize frequency reuse, improve the capacity of the communication system and mitigate the signal loss caused by multipath fading. Thus the combination of omnidirectional antennas and polarization reconfguration will be the direction of future communication systems and have broad applications in terms of the integration of antenna functions and reducing the number of antennas. In this paper, we studied the reconfigurable omnidirectional antenna with various forms of dual-polarization including vertical/horizontal polarization, ±45°slant polarization, circular polarization. A broadband vertically/horizontally dual polarized omnidirectional antenna with polarization diversity technique improving diversity gain is also studied. The author’s major contributions are as follows:1. A vertical/horizontal polarization reconfigurable omnidirectional antenna is proposed. The proposed polarization reconfigurable antenna is comprised of four double dipoles as radiating elements that are placed around a hollow cylinder, feeding network, diodes and a control circuit board. The operating principle of the double dipole antenna for vertical/horizontal polarization omnidirectional antenna is explained. A DC bias circuit is designed to control the polarization states of the antenna and an antenna prototype is fabricated. Experimental results show the antenna has good omnidirectional radiation characteristics and can realize the vertical/horizontal reconfiguration.2. A ±45°slant polarization reconfigurable omnidirectional antenna is developed. The slant polarization reconfigurable antenna consists of four cross type dipoles that placed around a cylinder, broadband feeding network, diodes and a control circuit board. The mechanism of the cross dipole antenna for +45°slantly polarized or-45°slantly polarized omnidirectional antenna is explained. The design process of a DC bias circuit for the control of the polarization states is described. Measurment results of an antenna prototype show the antenna has good ±45°slant polarization omnidirectional radiation characteristics in 1.9–2.2 GHz and can realize +45 ° or-45 ° slantly polarized reconfiguration.3. A circular polarization reconfigurable omnidirectional antenna based on leaning dipole is proposed. The proposed antenna is composed of four multiplication sign dipoles that are placed around a cylinder, broadband feeding network, diodes and a control circuit board. The mechanism of the multiplication dipole antenna for left-handed circularly polarized(LHCP) or right-handed circularly polarized(RHCP) omnidirectional antenna is explained. The design process of a DC bias circuit employing 16 PIN diodes for the control of polarization states is described. Experimental results of a fabricated antenna show the antenna has good circularly polarized omnidirectional radiation characteristics in 1.9–2.2 GHz and can realize LHCP/RHCP reconfiguration.4. Circular polarization(CP) reconfigurable omnidirectional antennas based on circular/rectangular loops are investigated. The CP reconfigurable antenna consists of four circular/rectangular loops that are placed around a cylinder, feeding network, diodes, control circuit board and a metal reflector. The operating principle of the circular/rectangular loop antenna for LHCP or RHCP omnidirectional radiation is explained. The design process of a DC bias circuit using 8 PIN diodes for the control of the polarization states is demonstrated. Experimental results show the antennas have good circularly polarized omnidirectional radiation characteristics in 2.4–2.48 GHz and can realize LHCP/RHCP reconfiguration.5. A broadband vertically/horizontally dual-polarized omnidirectional antenna is proposed. The proposed dual-polarized antenna consists of four vertical dipoles for vertical polarization and four horizontal dipoles for horizontal polarization, which are printed around a hollow cylinder for omnidirectional radiation. Two broadband feeding networks are employed to excite the vertical dipoles and the horizontal dipoles, respectively. Simulation and measurement results show that the dual-polarized omnidirectional antenna achieves a bandwidth of ~30%(1.7–2.3 GHz) with an isolation of >25 d B and has good broadband dual-polarized omnidirectional radiation characteristics. The correlation coefficient of antenna pattern is much less than 0.5.As a result of the comprehensive investigation above, the key technologies for polarization reconfigurable omnidirectional antenna are developed. These antenna technologies can be applied to omnidirectional antennas to provide multifunctions and improve the capacity and the reliability for modern wireless communication systems.