Maximum power is transferred from the transmitter Power Amplifier to the Antenna for radiation into space when the Antenna is perfectly tuned circuit i.e., it is in resonance with the frequency being transmitted.
This usually achieved when the length of the Antenna is half the wavelength of the frequency being transmitted.
The Antenna is of fixed physical length (6-8m). It has to cover a large band of frequencies (from 300 kHz – 40 Mhz). Therefore the electrical length of the Antenna is adjusted by means of capacitors and inductors inside the Antenna Tuning Unit (ATU) which makes the Antenna a perfect tuned circuit so that maximum power is transferred from the power amplifier to the antenna and out into space.
By pressing Tune buttons a motorised control brings in different values of capacitors and inductors to electrically match the antenna length to the frequency being transmitted.