Plasma processing apparatus frequently incorporates an antenna fed from a power supply and in this invention a power supply feeds a conventional matching circuit (10), which in turn is connected to the primary (11) of a transformer (12). The antenna (15) is coupled across the secondary winding (13) of the transformer (12) and that winding is tapped to ground at (16). This creates a virtual earth (17) near the mid point of the antenna (15) significantly reducing the variation, along the length of the antenna, in the power supplied to the plasma.