A Low-Pass Filter with Miniaturized Size and Broad-Stopband Using Helical Transmission Line and Dielectric Rings

A miniaturized broad-stopband helical cavity low-pass filter is proposed in this article. The filter is composed primarily of a helical transmission line and a SIR-like structure with two dielectric rings. In order to miniaturize the cavity low-pass filter, the helical transmission line structure is implemented to increase the length of the transmission line in order to reduce the cutoff frequency of the filter within a restricted dimension. Furthermore, by increasing the capacitance to ground through the placement of two dielectric rings over low-impedance transmission lines, the cutoff frequency of the low-pass filter can be decreased. A low-pass filter is subsequently designed, featuring a cutoff frequency of 125 MHz. With high miniaturization, this filter structure can be utilized in mobile communication systems operating in the very-high-frequency (VHF) and ultra-high-frequency (UHF) ranges.