TABLE OF CONTENTS Transmission line transitions are required in nearly every microwave subsystem and component. A transition is an interconnection between two different transmission lines that possesses low insertion loss and high return loss. These characteristics can be achieved only through careful matching of the impedances and electromagnetic fields of the two transmission lines. The designs of the signal and ground current paths through a transition are also critical. For a transition to function properly, these paths must often be continuous, in close proximity to suppress radiation, and as short and closely matched in length as possible. As we study transitions between planar, coaxial, and waveguide transmission lines in this chapter, we focus on the relationship between the design of signal and ground current paths and transition performance.
We already know that a coaxial transmission line possesses wide bandwidth and provides high isolation from external signals, so it is well suited for interconnecting microwave modules and systems that cannot tolerate interference. From Section 3.2 we know that to avoid the propagation of higher order modes as frequency increases, we must reduce the circumference of coaxial line. However, as the inner and outer conductor diameters decrease, the current density increases and so does the resistive loss. Consequently, at millimeter-wave frequencies (above 28 GHz), waveguide, which has the lowest insertion loss of conductor-based transmission lines, often replaces coaxial line as the energy transporter of choice. In general, though, the weight, size, and cost of coaxial line and waveguide preclude their use...
