Microstrip to Slotline Transition for Broadband Applications

For broadband applications, a microstrip to slotline transition is generally used. It provides a return loss of about -10dB with a bandwidth on the order of 10:1.

In a microstrip to slotline transition, the slotline acts as a ground plane for the microstrip line. As shown in the figure above, the microstrip is printed on one side of the substrate, and the slotline is on the other side. Both are orthogonal to each other.

To achieve multi-octave bandwidth operation, radial stubs are often used at the ends of the slotline and microstrip line, typically with a radius of approximately $\lambda/4$.

The characteristic impedance of the microstrip line is typically designed to be 50 Ohms, while that of the slotline is around 70 Ohms. To match these impedances, a multi-section impedance transformer is often employed.

This type of microstrip to slotline transition is widely used in planar microwave integrated circuit designs. It’s also frequently found in tapered-slot antenna designs.

As mentioned, the dotted microstrip radial stub acts as a short circuit, and the slotline radial stub acts as an open circuit at higher microwave frequencies. These radial stubs effectively cancel each other’s reactance, which helps in achieving a broader bandwidth.

Transmission Line Transitions

Here’s a quick list of other common transmission line transitions:

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