Research On Circularly Polarized Antennas For Confined Space Application

Radio coverage in confined space plays an important role to the reliability of wireless communication,which directly affects the safely operation of the mobile vehicle and its personnel in this particular environment.Circularly polarized waves can provide relatively uniform radio coverage for its features such as anti-multipath fading,anti-polarization mismatch and other characteristics,and there have been engineering cases of circularly polarized antennas acting as transmitting antennas in confined space,such as the Madrid Tunnel in Spain.However,there is no definite conclusion about the coverage of circularly polarized waves in confined space and the research results are little due to the complex boundary of confined space and the variable polarization of the reflected wave.Therefore,it is necessary to study the circularly polarized antenna and its radio coverage in confined space.In this paper,the ray tracing method based on the principle of mirror image is used to study the radio propagation characteristics in rectangular confined space.And then,two kinds of circularly polarized array antennas using bow-tie antennas and cross-oscillator antennas as array elements respectively are proposed.These two array antennas have a small cross section which leads to a low wind resistance and can be used in confined space where wind speed is severe.The main contributions of this paper arc as follows:1.The ray tracing method based on the principle of mirror image which combines the study of the radio coverage and the specific characteristics of the antenna is established.Different polarized wave propagation in confined space is studied.The radio coverage of circularly polarized waves in rectangular confine space are between the radio coverage of horizontally polarized waves and the radio coverage of vertically polarized waves.The reason is analyzed.2.Radio coverage of unidirectional antennas and bidirectional antennas in confined spaces is studied.According to the energy uniformity and system energy efficiency factor,the radio coverage in the long confined space is optimized by changing the lengthwise position of the antenna which is instructive to practical design.3.Using bow-tie antennas placed orthogonally as array elements,a bidirectional low-resistance circularly polarized array antenna is designed.The array antenna operates at 900 MHz and radiates circularly polarized waves with the same sense on both sides,which can be used as a unit of a multi-antenna system for transmitting circularly polarized waves.This array antenna has the advantages of compact structure and convenient installation.4.Using crossed oscillator antennas as array elements,a bidirectional and a unidirectional low-resistance circularly polarized array antenna are designed,fabricated and tested.Both array antennas operate in the 1.7 GHz frequency band that is being promoted by the current railway communication system and can be used in single or multiple antenna systems that transmit circularly polarized waves.