Research On Broadband Circularly Polarized Dipole Antennas

With the rapid development of wireless communication technology,people have higher requirements for antenna performance.Circularly polarized antennas are widely used in various wireless coummunication systems because of their unique characteristics.The requirements of wireless communication system for circularly polarized antennas are developing towards broadband,multiband,stable gain,good radiation pattern and low cost.This thesis mainly studies the broadband technology of circularly polarized antenna.The main work of this thesis is divided into three parts:1.A novel broadband circularly polarized crossed dipole antenna is designed.The antenna consists of two orthogonal rectangular dipoles,a coaxial line and a metal reflector.A vacant quarter ring is used to connect the two elements of the dipole,which acts as a 90 degree phase delay line for circular polarization.The antenna is fed in the center by a 50?coaxial line.At the same time,a metal reflector is placed directly below the antenna to obtain its directional radiation characteristics.Four rotationally symmetric rectangular parasitic elements with corner cut and slots are loaded near the crossed dipole on the top side of the substrate,which are used to introduce the coupling to expand the axial ratio bandwidth of circular polarization.The antenna achieves good circular polarization radiation performance in the working frequency band.The measured impedance bandwidth(S₁₁?-10 dB)of approximately 62.7%?1.48-2.83 GHz?and a 3-dB AR bandwidth?AR?3 dB?of approximately 47.2%?1.62-2.62 GHz?.The gain of the antenna is about 7.9±0.5 dBic in the working frequency band.2.A broadband sequential feed circularly polarized crossed dipole antenna is designed.The antenna consists of an unequal power network,four groups of rotating straight dipoles and a metal reflector.The antenna is center fed by a 50?coaxial line.The sequential feeding network is composed of a 270 degree fan-shaped circular ring,a 50?microstrip feeding line and four metal strips.The straight crossed dipole is connected with the sequential feeding network.The four arms of the dipole are fed by 90 degree phase shifts in turn to satisfy the conditions of circular polarization.Slotted elliptical parasitic elements are loaded around the dipole arms in order to expand the axial ratio bandwidth.The final antenna form is simple and easy to process.And it has good circular polarization radiation performance.The simulation results show that the impedance bandwidth?VSWR?2?of about 46.6%?1.30-2.09 GHz?and the 3-dB AR bandwidth of about 45.24%?1.30-2.06 GHz?.The gain of the antenna is stable at 7.8±0.8 dBic.3.A broadband circularly polarized magneto-electric dipole antenna is designed.A pair of rotationally symmetrical L-shaped horizontal patches act as an electric dipole.A pair of rectangular vertical patches and metal reflectors are equivalent to a magnetic dipole.A?-shaped probe feed is used to excite the antenna.The antenna is backed by a metallic cavity with three pairs of gaps to improve the circular polarization radiation performance and expand the axial ratio bandwidth.The simulation results show that the impedance bandwidth and the 3-dB AR bandwidth of the antenna are 63.3%?1.89-3.64 GHz?and 49%?2.19-3.61GHz?respectively.And the gain of the antenna varies from 5.5 to 9 dBic.The antenna can work stably in the mode of left-handed circular polarization in the working frequency band.