This is one of the aspects in PCB design that might be familiar in concept and purpose, yet surprisingly, you may not have known it had a specific name. During my work with RF boards, I frequently encountered the need for a trapezoidal shape to connect two traces with different widths. Initially, having a background in digital design, I perceived it as an ornamental detail with little significance. However, my perspective changed drastically when I witnessed the impact it made, especially with the rise of signal frequency, as demonstrated in simulation software.
In essence, a taper, as defined by Altium, is a transmission line structure that efficiently facilitates the transfer of a broadband signal between two transmission line structures or from a transmission line to a load, minimizing signal reflection.
To illustrate, consider a scenario where you have a trace with a width of 20 mils that needs to be connected to an 8 mil trace. The most effective approach is to employ a trapezoidal connection, with one side measuring 20 mils and the other side measuring 8 mils, while making the transition as elongated as possible. This configuration substantially reduces signal reflections.
In contrast, envision the worst case scenario: a direct connection between the traces with a 90-degree angle, leading to increased reflections. The best-case scenario involves minimizing this angle, ideally approaching zero, or on another words an infinite transition.
In practice, the linear taper (trapezoidal shape) is often the preferred choice, accounting for about 90% of cases. However, it's worth noting that there are other taper types available. For more detailed information, you can refer to the paper here: [Link to the paper: https://resources.altium.com/p/rf-trace-taper]