If we look at Zin with a 1/4 wavelength transmission we line see the DUAL of Zl. (2-12-19 I posted a link to Mr Turners book on transistor theory) Mr Turner explains the theory of duals in his book. The simple statement is we see the opposite at the input and output. Some text say we see the conjagate.
In other words with the load end open we see a short. With the slider on the load end we see an open feed end. With a line shorter than 1/4 wavelength we see capacitance.
Here are some more variations. Notice the top right antenna has and inducter sticking off the side.
While Amie found an application in the broadcast band her system would be more common in microwave. When the wavelength is measured in centimeters lead lengths becomes more critical.
A parallel resonant circuit is high impedance. A shorted 1/4 wavelength transmission line produces a high impedance on its input. This equals a parallel resonant circuit at that wave length. At other wavelengths the impedance is lower. Using the slide makes it adjustable.