A design technique for symmetric dualband filters

This work describes a method for designing a class of microwave filters presenting a rejection bandwidth inside the passband; these filters are then characterized by two passbands separated by a stopband, which is created by suitably inserting transmission zeros inside the original passband. In case of passbands of equal extension and with the stopband symmetrically displaced between the passbands, the proposed method (based on a suitable frequency transformation) allows the analytical synthesis of a low pass prototype, presenting an equiripple response both in the passbands and in the stopband. The method proposed has been employed for designing a multiple-coupled coaxial cavity filter presenting two pass-bands (835-849 MHz and 880-894 MHz) separated by a stopband (851-877). The filter has been fabricated and the measured results have validated the proposed design technique.