Coaxial Line to Waveguide Transition

Almost all waveguides are usually fed using a coaxial line. The purpose of a coaxial line to waveguide transition is to convert the TEM mode of the coaxial line into the TE mode of the waveguide, and vice versa. This can be achieved in two ways: E-plane coupling (probe coupling) and in-line coupling. These are illustrated in Figures 1, 2, and 3.

In the E-plane transition, also known as orthogonal transition, the center conductor of the probe (coaxial line) is inserted into the waveguide. This excites the TE01 mode in the waveguide. The probe position is carefully chosen so that reflected electromagnetic fields combine in phase with the incident electromagnetic fields. The distance between the back wall and the probe center is optimized to achieve good impedance matching, typically kept at λc/4.

In the H-plane transition, a short circuit concept is employed to create a time-varying magnetic field. These fields then couple with the waveguide. The short circuit is created using a right-angled metallic piece connected to the probe. This type of coaxial line to waveguide transition can handle high power levels but tends to have higher losses due to a higher Voltage Standing Wave Ratio (VSWR).

Transmission Line Transitions

• Coaxial to microstrip transition
• Coaxial to waveguide transition
• Waveguide to microstrip transition
• Microstrip to slotline transition