[en] The MFTF-B ECRH transmission system is designed to transmit the rf produced by each gyrotron quasi-optically from a launch point inside of the vacuum vessel wall in a linearly polarized guassian beam mode to an open cavity enclosing the plasma. The open cavity allows the microwave beam to be reflected numerous times through the plasma before it is lost. This increases the heating efficiency at low densities and temperatures when absorption on a single pass is small and also at high (relativistic) temperatures. In addition, the cavities allow unobstructed passage of neutral beams and simplify compensation for resonant zone shifts caused by beta depression, doppler shifting and relativistic effects. The microwave beam feeding the cavity has a guassian amplitude profile and a nominal diameter of 50 cm