Research On Isoflux Coverage Design Method Of Circularly Polarized Antenna

The development of communication technology has played a vital role in military and civil fields.With the popularity of small satellites and high-altitude aircraft for exploration and remote sensing,low-cost,periodic and miniaturized antenna terminals are favored.In order to provide a more consistent power level for the fixed area of the earth,the antenna with isoflux radiation characteristics is needed to make up for the link loss caused by the curvature of the earth and the near-far effect.Compared with linear polarization antenna,circular polarization antenna is not affected by Faraday rotation and can resist signal fading caused by ionospheric reflection and path interference.In this thesis,the isoflux coverage design method of circularly polarized antenna is studied,mainly including the following work:(1)A miniaturized printed quadrifilar helix antenna with isoflux coverage is designed.By adopting the structure of stepped impedance spiral arm and folded spiral arm,the impedance bandwidth of the antenna is expanded to a great extent,and the size of the antenna is effectively reduced.By setting the pitch and number of turns of the three-stage stepped impedance spiral arm to different values,a better isoflux coverage mode is realized on the basis of 3-d B axial ratio beamwidth.The results show that the antenna achieves an isoflux coverage mode of ± 60° in the frequency range of 1.40 GHz ～ 1.62 GHz,and the 3-d B axial ratio beamwidth reaches more than 200°.It is suitable for applications requiring gain compensation,such as small satellites and high-altitude applications.(2)A small quadrature four feed network with flat output amplitude and phase is designed,which is composed of 3-d B trans directional coupler and 180° differential phase shifter.The main line of 180° differential phase shifter is composed of capacitor and inductor loaded transmission line with stub;The reference line consists of a single transmission line.The phase shifter has the advantages of small size and flat output phase.In this thesis,the theoretical analysis process of phase shifter is given,and the circuit parameters are obtained by using particle swarm optimization algorithm.On this basis,combined with 3-d B trans directional coupler,the final feed network is formed.The measured results show that the designed feed network can provide good circular polarization excitation for the antenna within 18% bandwidth,the amplitude difference between output ports is less than 1 d B,and the phase difference is within 90° ± 5°.The designed feed network is connected with the miniaturized printed quadrifilar helix antenna with isoflux coverage for testing.The measured results are in good agreement with the simulation results.(3)A broadband isoflux circularly polarized antenna with loaded lens is designed.By setting two circular air grooves in the center of the dielectric lens,the radiation characteristics of isoflux are obtained;By loading closed annular grooves at the edge of the dielectric lens,the gain sag versus frequency of antenna from the maximum coverage angle(low elevation on the ground)to the boresight(θ = 0°)in the isoflux mode is improved,which widens the bandwidth of the antenna.The measured results show that the isoflux coverage of the antenna reaches about ± 50° in the frequency range of 4.9 GHz ～ 5.3 GHz,the gain sag fluctuation from the maximum coverage angle to the boresight is less than 3 d B,and the 3-d B axial ratio beamwidth reaches more than 180°,realizing the isoflux coverage and wide-angle circular polarization radiation performance in a wide bandwidth.