 Some early mobile radios were the size of a suitcase or had separate boxes for the transmitter and receiver.As time went on, power supply technology evolved to use first electromechanical vibrators, then solid-state power supplies to provide high voltage for the vacuum tubes.These circuits, called "inverters", changed the 6 or 12 V direct current (DC) to alternating current (AC) which could be passed through a transformer to make high voltage.