| Beidou second-generation(Beidou II)navigation system is a navigation system which is independent researched and developmented in China.It has three frequency bands(1207.14±10.23MHz,1268.52±10.23MHz and 1561.098±2.046MHz)and its polarization mode is right-handed circular polarization.Antenna,as the compass navigation system terminal device,responsible for transmitting and receiving electromagnetic signals.Its performance directly affects the communication quality of the whole system.Therefore,it is very important to study the wide-band circularly polarized antenna which can cover all frequency bands of the Beidou ? navigation system and with good performance.In the design of the navigation antenna,in order to improve the antenna performance,the method of changing the antenna structure can be used,and the method of loading the external structure to optimize its design can also be used.Because appling the electromagnetic bandgap structure(EBG)to the antenna can reduce the surface wave in the edge of the substrate side and rear lobe level,increase the antenna front-to-rear ratio,improve the antenna pattern,improve the antenna gain,therefore,in this paper,the design methods of the Beidou ? navigation antenna and the EBG structure applied to the antenna were studied.The main works and achievements include the following points:1.An EBG structure worked at Beidou II bands was designed.The effect of multi-frequency stop-band characteristic was achieved by the method of grooving on the patch of the mushroom-type EBG structure,and the structure was tested and verified by the suspension microstrip line method.The results show that the EBG structure has three different stop-bands,which are 1.15?1.29GHz,1.38?1.45GHz and 1.5?1.655GHz.The first stop band and the third stop band together covered all the three bands of Beidou II navigation system,completed the expected requirements.2.A circular polarization microstrip antenna with coplanar waveguide feeding was designed.The wideband circular polarization was realized by loading the cross-section and the T-shaped slot on the rectangular wide slot antenna.After theoretical calculation and a large number of parameters debugging,the performance of the antenna is as follows:center frequency is 1.4GHz,impedance bandwidth is 630MHz,the relative bandwidth of the impedance is about 45%;3dB axis bandwidth is about 450MHz,3dB axis relative bandwidth is about 32%;maximum gain is 2.04dBi;cross-polarization discrimination rate is about-35dB.And the antenna covers all three bands of the Beidou? navigation system.And ultimately meet the design requirements.3.A wide-band circularly polarized antenna feeding by coupling gap was designed.First,a 90-degree phase-shifted broadband phase-shifting network was designed,and then two layers of circular radiation patch were fed through the gap coupling.The center frequency of the antenna is 1.35GHz,the impedance bandwidth is about 550MHz,the axial ratio is less than 3dB in the 1.16GHz to 1.62GHz frequency band,the maximum gain is 5.4dBi,the cross-polarization discrimination rate is about-20dB,it successfully achieved the wide-band circular polarization characteristics,and covers all three bands of the Beidou ? navigation system.4.Loading the EBG structure which has been designed beforehand to the two broadband circular polarization antennas,by adjusting the distance between antenna and the EBG structure to find the best loading position.Due to the frequency bandgap characteristics of the EBG structure,the reverse electromagnetic waves can not be transmissioned in the antenna,thus reducing the energy loss,thereby enhancing the performance of the antenna.Finally,the impedance bandwidthes of the two antennas are increased by 50MHz and 100 MHz,respectively,the gain are increased by 1 dBi and 1.6 dBi,respectively,and the directional patterns are improved by loading the EBG structure.In this paper,the theoretical design method was combined with the optimization design of the electromagnetic simulation software,and the physical processing and testing were carried out to verify the design result.The tested results show that the two antennas both achieved the wideband circular polarization in all the three frequency bands of the Beidou ? navigation system.The antenna bandwidth is increased,the gain of the antenna is improved,and the directional patterns of the antenna are improved by loading EBG structure. |
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