A nonlinear coupling theory was used to estimate the reflection coefficient for the 8 GHz lower hybrid and 433 MHz ion Bernstein wave (IBW) grills in the FTU tokamak. For the IBW grill, the decrease in plasma density due to ponderomotive effects is significant. At the full rated power of the grill, the reflection coefficient could be unacceptably high. (author). 4 figs., 9 refs.