Compact Dual-Band Bandpass Filters With Stub Resonating Lines

Several new compact microstrip dual-band bandpass filters using admittance inverters and open-ended resonating stubs are introduced. Operating at the center-band frequencies of 1.8 and 2.4 GHz, with moderate bandwidth passbands, the proposed devices are suitable for LTE mobile cellular networks and Wi-Fi technologies. The design procedure includes the use of simple equations to extract the lumped element values of the circuit model. Next, the equivalent distributed layout which consists of 3 admittance inverters and 4 shunt-connected stubs (two g/2 and two g/2) is presented. The design is adjusted to attain further enhancement in the electrical and physical characteristics. These include spurious passband suppression, layout miniaturization, and reduction in the capacitive coupling between stubs. Measured responses are compared with simulated results, and very good agreement is observed. These features make the proposed filters suitable for integration into various microwave planar technologies.