Microwave characterisation and detection of wood decay

Abstract

The transparency of wood to microwave radiation is subject to changes in the wood's properties caused by softening of the wood material during fungal attack or the addition of moisture. A prototype microwave system that measures microwave attenuation and phase delay between two antennae was developed to detect fungal decay in wood at equilibrium moisture content. A total of twelve wooden stakes of 200 x 65 x 65 mm Pinus radiata (softwood) and twelve stakes of Eucalyptus regnans (hardwood) of the same dimensions were exposed to soil containing decay fungi for 12, 16, 20 and 24 weeks in an Accelerated Field Simulator (AFS), which is a climate conditioned room that simulates humid tropical conditions. A control set of three stakes of each wood type was also included in the experiment. Samples were washed and dried in an oven before testing with the prototype system. Orienting the antennae such that the microwave field was perpendicular to the wood grain could not distinguish between sound and decayed wood; however orienting the antennae so that the microwave fields were parallel to the wood grain and having the antennae pressed against the wood sample easily distinguished between sound wood and decayed wood. The phase delay data could not distinguish between sound or decayed wood.