Designs for broad-band microstrip vertical transitions using cavity couplers

Two designs for broad-band vertical transitions using microstrip-fed cavity couplers are presented in this paper. Both designs feature a cavity in an electrically thick ground plane between two parallel back-to-back microstrip lines terminated by open-cir-cuited stubs. The cavity in the first design can be treated as a section of a waveguide, whereas the second design has the cavity operate in nonresonant modes. The electrically thick ground plane can provide isolation between modules, thermal dissipation for on-board active devices, as well as structural support for thin substrates. The finite-element method is chosen to carry out the required simulations for the proposed designs because of its accuracy. Both rectangular and circular cavity couplers are investigated. More than 100% fractional bandwidth can be achieved. The validity of the simulation results is verified experimentally. For applications at millimeter-wave frequencies, sensitivity study is conducted to demonstrate that the proposed design is robust and not susceptible to potential errors in dimension and misalignment, which may occur during the fabrication process of the vertical transitions.