Studies on the Reentrant Four-Layer Quasi-Elliptic Bandstop Filter

This study propose for the first time a novel approach to design a reentrant structure with additional
symmetrical reflection zeros at the lower and upper pass bands of the stop band, leading to the quasielliptic
frequency response that improves the manufacturing and selectivity of the filter. The novel
microwave quasi-elliptic bandstop filter, which uses the initially uncoupled strip transmission lines, is
considered in this work. The microwave bandstop filters (BSFs) play an important role in the support
of distortion reductions through the radio frequency (RF) and microwave communities. The proposed
filter is based on the reentrant structure, where the metallic body with a floating potential is
asymmetric. The internal and external dielectric fillings must have their unequal relative permittivities.
As a result, additional symmetrical reflection zeros are reached at the lower and upper pass bands of
the stop band leading to the quasi-elliptic function response that improves the filter selectivity. The
general transverse electromagnetic (TEM) circuit model for the proposed filter in terms of a series
connection of the multi-ports is presented and then used to predict the initial electrical and geometrical
parameters. An experimental printed circuit prototype has been manufactured and evaluated to
validate the design concept. The measured filter parameters correlate well with the simulation-derived
ones and that increases the degree of freedom in the fabrication of microwave frequency selective
components. Due to the asymmetric metallic body with the floating potential, the proposed reentrant
filter, as a basic building block, is expected to be used in various types of frequency selective devices
that pave the way for a variety of applications, including LTCC and SIW technologies.