High-Pass Filtering Characteristics of Transmission-Line Combs

The filtering characteristics of a transmission line loaded at regular intervals with closely spaced shorted stubs are investigated theoretically, experimentally, and by means of rigorous electromagnetic simulations. It is shown that this periodic structure exhibits waveguide-like behavior with a first passband whose width is a function of the characteristic impedance and pitch of the stubs. It is also shown that when the stub pitch vanishes, the structure degenerates into a waveguide whose mode-cutoff frequencies correspond to a shorted-stub length that is an odd multiple of the wavelength. These features enable the design of compact high-pass filters, and make possible cutoff-frequency hopping and tuning. Implementations in microstrip are shown that demonstrate desirable characteristics such as wide bandwidth, low insertion loss, reduced size, easy integration with planar circuits, as well as common and differential-mode filtering, making such structures attractive for use in low-cost microwave and millimeter-wave applications.