The folded unipole design near 1/4 wavelength tall works like a 1/4 wave transformer, with a short circuit at the top and open at the bottom. It creates a high impedance at the base and electrically disconnects the tower making it non-resonant without a matching network. when the L-network is disconnected, the antenna does not re-radiate noise to the receive antenna as when it is resonant.
Tuning the unipole design with the L-network results in a very broad banded transmit antenna.
