A **quarter-wave transformer** is a segment of transmission line used to match two different impedances. It works by transforming one impedance into another through a section exactly ¼ wavelength long (electrically).

The required characteristic impedance is calculated by: $$Z_0=\sqrt{Z_s\cdot Z_l}$$ Where:

• Zₛ = source (transmitter or feedline) impedance
• Zₗ = load (antenna or device) impedance

If you specify a frequency and velocity factor, the calculator will also estimate the physical length of the ¼-wave transformer section: $$\text{Length}=\frac{\lambda }{4}\cdot VF=\frac{300}{4f}\cdot VF$$ This is useful for building matching stubs using coax or open wire line.