Transmission Line Transducer from Dominant-Mode Circular Waveguide to Microstrip

This investigation deals with a compact transducer from a polystyrene-filled, dominant-mode circular waveguide to an axially parallel 100 Ω microstrip transmission line for use at X-Band frequencies. Three different circuit concepts are compared: One- and two-section quarterwave impedance transformers as well as alternative components applying evanescent-mode waveguide techniques. The transformers are connected to the microstrip transmission line by means of a probe transition in the E-plane of a reduced-height wave-guide. The transducer showed a VSWR of 1.25 within a relative bandwidth of 33%. The corresponding insertion loss was 0.3 dB.