The Antenna Tuning Unit
Modern day transcievers are designed to work into a low-impedance, unbalanced co-axial feeder of 50-80 Ohms resistive impedance. Solid state transmitters must achieve these conditions otherwise damage may result. This would suggest that only antennas with driven elements based on half wave centre fed dipoles carefully trimmed for resonance around the operating frequency would be suitabe for radio communication..That is not necessarily the case since, in many instances, these restrictive conditions may be overcome by imposing a variable impedance rf transformer between the antenna and the transmitter. Such a transformer may be adjusted so that the transmitter "sees" a resistive 50-70 Ohm impedance. At the same time the output from the matching transformer is made to accurately match the impedance of the antenna or its feeder.
Such an rf transformer is known as an antenna tuning unit, antenna matching unit or transmatch. In its simplest form a transmatch consists of two components,an inductor and a capacitor. In reality some additional components are used to make the unit more flexible in the range of reactances and impedences with which it can cope.
Tuning units come in a wide range of shapes and sizes. They may be designed for manual operation or automatic use. ATU's may be stand alone units or they may form an integral part of an amateur transceiver. Their specifications must be carefully checked to ensure they are suitable for the bands on which you plan to operate and capable of handling the power level you anticipate using.
Manual ATU's require greater effort to use than their automatic counterparts but they offer a greater degree of control than their automatic counterpats. They are particularly suited to matching long wires and G5RV's, something which the automatic units (with a few exceptions) are not so good at.
The RSGB and ARRL have published many articles and reviews on antenna tuning units. They are worthy of study and commended to you for further reading.
The photograph shows an LDG AT-100Pro antenna tuning unit sitting on top of a Yaesu FT897D in the GI0DVU shack.
