What impedance does a 1/4-wavelength transmission line present to an RF generator when the line is open at the far end?

A quarter-wavelength transmission line has the special property of acting as an impedance inverter. It mathematically transforms a given impedance at the load end to its exact opposite at the source end. If the far end is left open, it has an effectively infinite impedance. The quarter-wave line transforms this infinite impedance into a very low impedance (acting like a short circuit) at the generator. This is why open quarter-wave stubs are sometimes used as notch filters for specific frequencies; at the resonant frequency, they present a dead short to the unwanted signal.