This resonant circuit allows you to tune end-fed antennas within a certain range.
The idea originally comes from my SotaOne transceiver concept using the bushAntenna (c), which in turn was inspired by the Elecraft KH1:
To get on the air quickly and discreetly, you lay a wire approximately half the wavelength long on the ground, on bushes, or, if possible, in trees. The tuner’s capacitor compensates for the detuning. This way, you can even work two or three bands with a single wire length.
73 Chris