Dual-band filter with fully independent passbands based on unequal-length coupled lines and stubs

In this work, a dual-band filter that has completely independent passbands is demonstrated. The complete independence of the two passbands is achieved by simultaneously abandoning the classical restriction on electrical lengths for dual-band operation and allowing all the components to have different lengths. As a result, the proposed filter has different sets of J-inverter values and susceptance slope parameters independently at two centre frequencies. Therefore, the two passbands can be designed to have not only different bandwidths, but also different passband responses simultaneously; i.e. one passband can be designed to be a Chebyshev type while the other is a Butterworth type, or a Chebyshev type with different passband ripple levels. Rigorous design equations are derived and various design curves are provided to discuss the practicality of the proposed method. Finally, two filters with different bandwidths and passband types are demonstrated experimentally. The excellent agreement between the simulated and measured results verifies the practicality of the proposed method.