Tubular microwave applicator

A method of heating a load using a microwave applicator to automatically compensate for changes in the complex permittivity of the load comprising the steps of:

• a) positioning a generally cylindrical load along a central axis of a microwave applicator wherein the load diameter is constrained to a predetermined value selected according to a sum of modal patterns based on a set of cylindrical Bessel functions J.sub.m of the first kind and mth order such that the load will operate substantially at the largest slope of frequency increase with increasing complex permittivity;
• and
• b) sizing the internal radial dimension of the applicator to form a resonant system to maintain the resonant frequency of the combination of the load and free space within the applicator relatively constant with changes in the complex permittivity of the load.

A microwave applicator assembly for automatically compensating for different complex permittivities of loads heated by the applicator comprising:

• a) a source of microwaves operating at a predetermined frequency;
• b) load support means for positioning a generally circular cross section load along a central axis;
• c) a microwave cavity sized to support only a single mode therein and causing internal resonance in the load and an E field maximum parallel to an outer surface of the load such that the microwave field in the combined load and cavity remain resonant at or near the predetermined frequency for loads having complex permittivity in a range above about 20 and dielectric loss tangent in a range above about 0.3.

A microwave applicator assembly for automatically compensating for different complex permittivities of loads heated by the applicator comprising:

• a) a source of microwaves operating at a predetermined frequency;
• b) load support means for positioning a generally circular cross section load along a central axis;
• c) a microwave cavity sized to support only a single mode therein and is further sized to maximize the derivative df.sub.R /d.epsilon.', where f.sub.R is an increasing resonant frequency of the load with respect to increasing .epsilon.', the dielectric constant for the load such that a microwave field in the combined load and cavity remain resonant at or near the predetermined frequency for loads having complex permittivity in a range above about 20 and dielectric loss tangent in a range above about 0.3.