Low reflection and TEM mode preserving coaxial/non-coaxial adapting fixture

Coupling Decoupling Network CDN and Common Mode Absorption Device CMAD are examples of one-port and two-ports DUTs with non-coaxial terminals commonly used in EMC testing labs. They must be verified with Vector Network Analyzers (VNAs), which unfortunately have coaxial ports. These devices can be connected to the VNA only if they are embedded in adapters. In the verification process, the DUT must be de-embedded, i.e. the contribution of adapters must be extracted from the measured S-parameters. The Thru-Reflect-Line TRL calibration of the VNA is a suitable tool for that task, provided the necessary hardware and software facilities of the VNAs are available. If not, alternatives are required. The Authors presented such an alternative calibration method in their previous publications. Frequency limitations of the de-embedding process were reported in them. The reason is an abrupt transition from 50 ohms characteristic impedance of the coaxial structure by the cable connector to the 207 ohms characteristic impedance of the cylindrical rod wire above the reference ground plane. In this paper, the Authors present a newly developed adapter, with smoothly increased characteristic impedance, for which noticeable improvements can be observed.