What is quarter-wave transformer?

A quarter-wave transformer is a type of transmission line used to match the impedance of two circuits. It consists of a length of transmission line that is a quarter wavelength long at the frequency of interest. When properly designed, it can transform an input impedance to a desired output impedance. It is commonly used in radio frequency (RF) circuits and is a relatively simple and efficient method of impedance matching. The quarter-wave transformer is based on the principle that a quarter wavelength of transmission line acts as a short circuit if the load impedance is high and an open circuit if the load impedance is low, and thus the input and output impedances can be matched by using it.