Closed-Form Equations of Conventional Microstrip Couplers Applied to Design Couplers and Filters Constructed With Floating-Plate Overlay

This paper presents a new method of designing tightly coupled directional couplers on printed circuit board substrates. The coupler is realized by placing a piece of floating metal plate on parallel-coupled microstrip lines to enhance the coupling between the lines. This structure yields a tightly coupled directional coupler without wire bonding and using high-resolution fabrication processes. A design procedure based on closed-form design equations of the parallel-coupled microstrips is proposed to design the new coupler. An S-band 3-dB coupler constructed from the new structure is fabricated and measured to verify the proposed design procedure. Another wideband coupler, comprising two sections of conventional microstrip couplers and one section of the new type coupler, is also fabricated and measured to demonstrate the flexibility of connecting the new structure and conventional microstrip structures or devices. The measurements of the two couplers demonstrate that the coupler operates properly with approximately 54% and 94% fractional bandwidth, respectively, showing good agreement with the electromagnetic simulations. Additionally, a broadband filter consisting of four sections of the proposed coupling structure is fabricated and tested. The measured insertion losses are around 0.8 dB associated with return losses of greater than 15 dB over a frequency range of 4.5-7.1 GHz, corresponding to a fractional bandwidth of 45%.