A frequency-variable single-mode microwave (MW) reactor system was designed and applied for selective heating of a palladium dispersed layer deposited on a porous alumina tube. This MW reactor system automatically detects the suitable resonance frequency and provides the optimum MW irradiation conditions in the cylindrical cavity via a power feedback loop. The temperature program ensured simultaneous MW power response and fast heating of the palladium dispersed layer of the reactor tube. The high-power MW amplifier was oscillated using a semiconductor instead of a conventional magnetron-type oscillator. The semiconductor device provides a narrower distribution of frequencies, resulting in an intense and sharp heating zone along the tubular reactor. High reaction conversion with efficient energy use was demonstrated via continuous oxidation of ethylene by focusing the electric field along the palladium dispersed reactor tube.