Design Of Wide Stopband Filter With Antenna Front-end

Beidou Satellite Navigation System is now in the stage of rapid development, and the end product based on navigation application has come out in large numbers. The potentially interfered between different products is getting more seriously. Considering the filter plays an irreplaceable role in the prevention of interference, the filter’s requirements get more stringent, one of the most important requirements is to suppress the antenna’s 2-10 harmonic. The classic filter design method is based on source and load impedances equal to 50Ω, when the filter is terminating antenna, antenna’s input impedance is a frequency response function as the filter’s load impedance, then the original filter response curve has changed greatly and couldn’t suppress the harmonics of antenna any more. This thesis proposes a filter design thought that each resonator’s lowest resonance frequency is the same, and high-order spurious resonance frequency interlaced with each other. Finally several wide stopband filters based on the thought are designed. The antenna’s input impedance is influenced small after terminated antenna, and the filter can be well suppressed antenna’s 2-10 harmonic. The main contribution of this paper has the following several aspects:1, Analyze the conditions to form passband of microwave filter and how to influence the filter response curve when the load impendence changes, then propose the filter design thought innovatively that each resonator’s lowest resonance frequency is the same, meanwhile high-order spurious resonance frequency interlaced with each other;2, Design a L-band single feed circular polarization microstrip antenna which is used in Bei Dou Navigation Satellite System, and bring in the perturbation to incent degenerate mode thought cutting angle to achieve circular polarization;3, Analyze the way to get small insertion loss, and select different stepped impedance resonators to achieve the same lowest resonant frequency meanwhile intertwined high-order spurious resonance frequency, and a bandpass filter with a small insertion loss and wide upper stopband is proposed. It could suppress antenna’s 2-10 harmonic well after the antenna termination, and the advantage of this method is that a higher suppression degree or higher frequency rejection filter according to the design requirements will achieve;4, Add open stub line in the symmetry plane of half-wavelength stepped impedance resonator to control the dual-mode independently, then design a lowpass filter and a bandpass filter respectively with wide stop band which comprises L-band of Bei Dou Navigation Satellite System, and it could suppress antenna’s 2-10 harmonic well after the antenna termination.