Broadband, nonuniform stripline directional couplers for use in VNA testsets

Scattering parameter testsets of vector network analyzer (VNA) base instruments require backward wave directional couplers with sufficiently high directivity. This paper describes a synthesis procedure of symmetric, nonuniform couplers based on common multilayer printed circuit board (PCB) technology. The utilized offset coupled striplines (STL) are characterized by 2D electromagnetic (EM) field simulations in the frequency domain. Two loose directional couplers with directivity values greater than 15 dB from 0.003 MHz to 40 GHz resp. 29 GHz and greater than 5 dB up to 50 GHz resp. 70 GHz are proposed. The necessity of well matched transitions from STL to microstrip line (MSL) is outlined and two possible realizations are discussed. Measurement data of the manufactured couplers is presented, the arising deviations are clearly mapped to specific manufacturing tolerances. The minimum requirements for the applied PCB technology that guarantee successful designs are highlighted.