The Current Drive performances of the JT‐60U multijunction launcher, fed by oversized waveguides, have been investigated under poor matching conditions. In the experiments the main waveguides reflection coefficient reached values of 35%, moving the grill in the shadow of the port. Good agreement is found between measured and expected reflection coefficient computed without taking into account the generation of higher modes in the oversized guides and using a model for the density in front of the grill deduced from Langmuir probe measurements. Good current drive performances are observed up to reflection coefficients as high as 25%, well above the theoretical expectation.