What impedance does a 1/4-wavelength transmission line present to an RF generator when the line is shorted at the far end?
Why A transmission line an odd multiple of 1/4 wavelength long acts as an impedance inverter: Zin = Z0^2 / ZL. With the far end shorted, ZL is essentially zero, so Zin goes toward infinity and the generator sees a very high impedance. This is why shorted quarter-wave stubs behave like parallel resonant circuits at their design frequency and are used as notch filters and metal insulators.
Watch out Very low impedance is what a shorted line presents when it is a half wavelength (or any multiple of a half wavelength) long, since a half-wave line repeats the load impedance rather than inverting it.
Quarter wave flips it: short in, open out; open in, short out. Half wave repeats what it sees.