Research And Design Of Dual-band Microstrip Bandpass Filter

With the rapid development of the communication technology, the dual andmulti-communication systems become more important. Because the filter is animportant component of the communication systems, so the dual-band bandpassfilter can have an effect on the dual-communication systems. This paper will studythe dual-band bandpass filters based on design theory of the single-band bandpassfilters.First this paper discusses the basic theory of the filter. The characteristics ofthe electromagnetic coupling between resonators and between resonator and thefeed line will be stated, by analyzing the electromagnetic of the resonators, whichprovides the design theory of the dual-band bandpas filters.Second a reasonable resonance structure is designed by analyzing the resonantcharacteristics of the step impedance resonator. Then, according to the requirementcoupling coefficient of the resonators and the external quality factor of the first/lastresonator, a dual-band bandpass filter is accomplished by the resonant structure. Auniform impedance resonator (UIR) is derivative from the step impedance resonator(SIR), which the ratio of the dual resonant frequency less than2.The uniformimpedance resonator (UIR) is used to design a new dual-band bandpass filter,which is similar to the SIR for the filter. Because the uniform impedance resonator(SIR) has the same characteristic impedance on the whole structure, so it is easyoptimized by adjusted its structure. According to the distribution ofelectromagnetic field on the resonator, adding some open short stubs on theresonators, the coupling coefficient of the two resonant frequency for the uniformimpedance resonators will be turn small. Two dual-band bandpass filters simulatedand optimized with the electromagnetic simulation software (Sonnet) are designed.One is proposed for WLAN applications working on dual-frequencies, and the otheris proposed for mobile phone and WLAN applications.