Antenna tuning unit is usually used to match the transmitter output to the antenna over a wide range of fre3quency. To have an efficient transmission, the physical length of of antenna would have to be altered for each transmission frequency. The ATU is mounted as close to the main antenna as possible.
“ADDING INDUCTANCE IN SERIES, INCREASE THE LENGTH OF THE ANTENNA”.
“A CAPACITANCE IN PARALLEL DEREASES THE LENGTH”
ATU mainly used in MF/HF transmitter. It is a combination of variable capacitors and inductors (coil). It is used to match the length of the antenna to the radiating frequency which is also called Tuning of transmitter. Here length of antenna depicts electrical length of antenna.