Two-Layer-Magic-T-Based Bandpass, Quasi-Bandstop, and Dual-Passband Balanced Filters With Differential-/Common-Mode Reflectionless Behavior

An approach for the realization of radiofrequency (RF)/microwave balanced filters with simultaneous differential-mode (DM) and common-mode (CM) two-port-reflectionless behavior for bandpass, quasi-bandstop, and dual-passband applications is reported. These RF filtering components exploit as basic constituent circuit block a type of two-layer broadband magic-T, in which its two-side/coplanar arms are designed as the balanced feeding ports at the top layer. Specifically, two replicas of this wideband magic-T on a microstrip-to-slotline transition are back-to-back connected between their
$ {\Delta }$
terminals by means of a double-complementary-diplexer-based single-ended low-reflection filtering unit inserted at the bottom layer. In addition, their
$ {\Sigma }$
accesses are loaded with a resistor to perform the CM RF-power-absorption functionality. Thus, balanced symmetrical-absorptive RF filtering devices for both the DM and CM operations are developed. Such distinctive feature makes this family of microwave filters unique since most of related prior-art balanced filters mainly focused on obtaining the RF-power-absorption property only for the DM or the CM. The theoretical operational foundations and RF design principles of the engineered class of balanced filters with double DM and CM symmetrical-absorptive capabilities are described. Furthermore, their two-layer implementations exemplified in second- and fourth-order bandpass filters (BPFs), a second-order quasi-bandstop filter (BSF), and a second-order dual-band BPF are presented. For experimental-validation purposes, three different prototypes centered around 2 GHz corresponding to single-band BPF and quasi-BSF devices, as well as a dual-band BPF circuit with passbands located at 1.75 and 2.25 GHz, are manufactured and tested. For all these prototypes, 10-dB-referred DM and CM two-port-RF-power-absorption relative spectral ranges larger than 3:1 are measured. Their expected RF filtering functionalities and highly suppressed CM transmission at all frequencies are also verified.