A Study Of S-band And UHF-band Circularly Polarized Antennas

With the development of wireless communication system,people put forward higher requirements on miniaturization,wide band and circularly polarized of antennas.The circularly polarized microstrip antennas have been widely used in many domains due to their great advantages of low profile,more resistant to interference and easy integration.Meanwhile the research on helix antennas is mature at home and abroad.Helix antennas have appear in various situation depend on circular polarization and wide band.In this paper,a helix antenna of UHF band and a microstrip antenna of S band are designed,which mainly includes:1.Axial mode helix antennas have been studied.First of all,a miniature helix antenna has been designed.We adopt low pitch angle to decrease the height of the helix antenna on basis of favorable electrical property.The ground of shallow cup form and loaded metal disc can adjust the radiation of antenna.In working frequency bands,the VSWR of antenna is less than 1.5.The antenna has proper impedance matching by adjusting matching piece of metal.The axial ratio of the antenna which can radiate right-handed circularly polarized wave in both bands is less than 2d B,and the gain is more than 7d Bic.In order to obtain the double circular polarization characteristics,a double circularly polarized helix antenna has been designed.Two helical coils whose radius is changed are added to original helix antenna.So the helical coils can locate in the center of the antenna.Under the end of helical coils,a metal ground has been introduced which can adjust impedence of the antenna.Thus the antenna gains double circular polarization characteristics.The helix of S band and UHF band have different radius,we can load two helical coils on the same ground,in this way,the helix antenna can work in two bands.2.A broadband circularly polarized microstrip antenna has been studied.First of all,a sequential rotation of microstrip antenna array working in S band is designed.The antenna element consists of multilayer dielectric.Using the four square radiation patches with corner cut,the left-handed cicular polarization is obtained.Meanwhile the parasitic patch above the foursquare can broad the bandwidth of antenna.In the working frequency band,the VSWR of antenna is less than 1.5 and the gain is more than 8d Bic.The feeding network uses Wilkinson divider which can satisfy the sequential rotated feeding.By adopting a sequential rotation technique with 2?2 antenna elements,the antenna array has great axial ratio and gain in the operating band.3.The two antennas in UHF band and S band are combined.By adjusting the distance of two antennas and transforming the loaded metal disc into metal ring,the effect on microstrip antenna will reduce.At the same time,increasing the pitch of helix antenna and the height of ground can improve the radiation characteristics of helix antenna.Finally,the performances of antenna in UHF band and S band are satisfied by means of insistent modulation.