A microwave applicator includes an elongated chamber, preferably having a cylindrical shape, for processing materials therein. A waveguide, connected to the elongated chamber, couples microwave power into the elongated chamber. The cross-sectional area of the elongated chamber can be mechanically adjusted to control and maintain the microwave field uniformity and resonant mode, preferably the length independent mode TM.sub.010, during the processing of the material. The applicator can thus provide microwave energy having a substantially uniform field distribution over a large area for processing materials, for example, a web or sheet like product, plywood and pre-impregnated cloth, in a continuous manner.