Factors Affecting the Operation of High-Power Microwave Heating Systems for Lumber Processing

It is shown that, in the microwave power spectrum, the field penetration depth in a wet material increases rapidly with temperature. The selectivity of coupling to a wet material is analysed for softwoods. It is shown that the selectivity is both moisture and temperature dependent, the action being self-regulating, except at low moisture contents. The average bulk dielectric constant for softwoods below fibre saturation point is determined experimentally at one frequency (9.21 Gc/s) and compared with theory. Approximate empirical relations are given for dielectric behavior in the microwave power spectrum, 1 to 10 Gc/s.