The L-type tuner is very simple in concept and for use with SHORT random wire antennas.  It contains several coils in series with the antenna, which are selected by a switch to input to allow inductance to be added to the antenna making up for it being too short for the wavelength required. 

The joint of the inductor and the antenna has a variable capacitor to ground and the output to the receiver is taken from the other end of the inductor chain.  Suitable selection of inductance and tuning of the capacitor achieve 1/4 wave resonance and where the impedance is about 36 Ohms.

It is really quite effective and a TUNER/FILTER switch is included so you can switch the ATU out of circuit to compare with and without.  Often it is possible to achieve up to 2 S-points (about 12dB) improvement in signal using this tuner. 

It is designed for use with antennas that are too short for the required band and so it has more inductance than usually available in other types of tuner.  This unit has been tested successfully down to 150kHz so would even be of benefit to VLF enthusiasts.

Most radio enthusiasts will be able to have an antenna longer than 3.5 metres so that sets the practical limit to about 21MHz for which this tuner can be of benefit.  The one in the picture is fitted with a 4mm binding post for the antenna wire and a BNC connector to the receiver BUT it will now normally be supplied with an SO239 connector.  If you want BNC just send 'a note to the seller'.