Waveguide to Microstrip Transitions: Design and Types

Waveguide to microstrip transitions are essential for connecting waveguide and planar microstrip circuits, facilitating efficient signal transfer. This guide delves into their design, operational principles, and applications in contemporary RF systems. At higher frequencies, coaxial to microstrip transitions tend to exhibit higher losses, making them less suitable. Waveguides, on the other hand, offer lower losses at such high frequencies and are therefore preferred for interfacing with microstrip circuits.

The waveguide to microstrip transition functions using the dominant mode bandwidth. The $TE$ mode is supported within the waveguide.

The impedance of each mode in both the waveguide and microstrip varies with frequency. Consequently, achieving full dominant mode operation is challenging due to dispersion in the waveguide to microstrip transition.

Types of Waveguide to Microstrip Transitions

There are primarily two types of waveguide to microstrip transitions:

1. Orthogonal Transition: In this configuration, the microstrip is positioned inside the waveguide through a narrow slot. This slot is specifically designed as part of the waveguide to excite only the microstrip mode within the operational frequency band, effectively cutting off all other waveguide modes. The ground plane of the microstrip is connected to the waveguide walls. This arrangement allows the microstrip probe to function as an antenna.

2. End Launch Transition: This type of transition is utilized in parallel plate waveguide to microstrip transitions.

   Here, both the waveguide and the microstrip support the $TEM$ mode, enabling a broader bandwidth. The parallel plate waveguide is of a balanced type, while the microstrip line is unbalanced. A balun is therefore necessary to match these two transmission lines.

   In this configuration, the ground plane of the microstrip is connected to one plate of the waveguide, and the strip line is connected to the other plate. The balun used in this waveguide to microstrip transition typically features a flared structure, which aids in the divergence of ground and signal currents.

Conclusion

Waveguide to microstrip transitions are indispensable components in hybrid RF systems. Proper design and seamless integration are crucial for ensuring smooth signal transfer and overall system efficiency.