Early non-destructive detection of biological degradation in wood is important if remedial treatments are to be effective. When microwaves are transmitted through wood, the wave will be partially reflected, attenuated and delayed compared to a wave travelling through air. The amount of reflection, attenuation and phase delay depends on the dielectric properties of the material and the dielectric properties depend on the physical condition of the material.
The dielectric properties of timber, over a wide range of frequencies, can be determined using a dielectric probe [1]. The open-ended coaxial probe is a cut off section of transmission line. The dielectric properties, or permittivity, are measured by touching the flat face of a solid material to the probe. The EM fields at the probe’s end penetrate into the material under test and the reflected signal
(S
11
) can be measured. The complex permittivity is computed from the reflected signal. The permittivity determines how easily microwaves pass through the material.
Sound wood, at equilibrium moisture content
(EMC), is relatively transparent to microwave frequencies [2]; however its permittivity changes with moisture and density [2]. Wood density decreases as wood material is removed due to fungal decay. Therefore it is expected that fungal decay should be detected by measuring the transmission behaviour of microwaves passing through the wood samples.
This paper reports on the non-destructive detection of moisture and fungal decay in wood using microwave technologies.
Initially, the dielectric properties of wood samples at 4 different moisture levels were measured using a dielectric probe attached to a network analyser.
Finally a microwave system that measures microwave attenuation and phase delay between two antennae was developed to detect fungal decay in wood at equilibrium moisture content (EMC).
The microwave system performs a frequency sweep between 2.2 and 2.7 GHz. The transmitting and receiving antennae were identical micro-strip patch antennae, designed to operate at a central frequency of 2.4 GHz with a 14 % band width.
A total of twelve wood stakes of 200 x 65 x 65 mm
Pinus radiata (softwood) and twelve stakes of
Eucalyptus regnans (hardwood) of the same dimensions were exposed, with three replicates each, to soil containing decay fungi for 12, 16, 20 and 24 weeks in Accelerated Field Simulator (AFS).
The stakes were oven dried and weighed before being exposed to decay fungi in the AFS to speed up the decay processes for testing wood stakes. A control set of three stakes of each wood type was also included in the experiment.
After removal from the soil, the samples were washed to remove all soil and oven dried at 60 °C for three days. The stakes were allowed to equilibrate with the laboratory’s ambient temperature and relative humidity prior to testing with the microwave system.
Attenuation and phase delay measurements were made with the microwave fields oriented parallel and perpendicular to the wood grain. Data was analysed using a multifactor analysis of variance, with the factors being wood type (softwood or hardwood), fungal exposure period and microwave field orientation. The prototype microwave system also analysed three other replicates of each wood type of the same size. These were not decayed, but were preconditioned to two different moisture contents of 17 % and 12 % to evaluate moisture effect on microwave transmission without decay.

As anticipated the presence of moisture increased the dielectric constant of the material (Figure 1).
This was encouraging for the development of the microwave decay detection system. The prototype system could easily distinguish between the two moisture levels in the wood samples when the antennae were oriented parallel to the wood grain.
12
Moist_Level2
Moist_Level3
Moist_Level1
Solid_Dry
10
8
6
4
2
0 2 4 6
Frequency (GHz)
8 10
Figure 1: Dielectric properties of wood as a function of moisture and frequency (error in this measurement is less than 5%)
750
700
650
600
550
500
450
400
LSD (P = 0.05)
= 45.7 kg/m
3
Softwood
Hardwood
350
0 12 16 20
Exposure Time (Weeks)
24
Figure 2: Mean wood density as a function of exposure time to fungal decay
Exposure to decay fungi significantly reduced wood density compared to the controls (Figure 2). The softwood was less dense and more susceptible to fungal decay than the hardwood (Figure 2).
Microwave attenuation measurements easily distinguished between sound and decayed wood when the microwave fields were oriented parallel to the wood grain (Figure 3). Orienting the microwave fields perpendicular to the wood grain resulted in less microwave attenuation, as expected from literature [2]; however, this orientation could not distinguish between sound and decayed wood
(Figure 3) or the different moisture levels in the samples. The phase delay data could not distinguish between the various treatments and the controls.
36.0
34.0
32.0
LSD (P = 0.05)
= 1.6 dB
Softwood - Parallel to the grain
Hardwood - Parallel to the grain
Softwood - Perpendicular to the grain
Hardwood - Perpendicular to the grain
30.0
28.0
26.0
24.0
0 12 16 20
Exposure to Decay Fungi (Weeks)
24
Figure 3: Mean microwave attenuation (dB) as a function of exposure time to fungal decay
A system for non-destructively detecting wood decay, based on transmission of microwave energy through the wood sample, can distinguish between decayed samples and sound wood when the microwave fields are oriented parallel to the wood grain.
This research has been generously supported by the
Australian Research Council, PowerCor, CitiPower,
Archicentre and RedStart (ARC Project No. LP
0776778).
1.
2.
Agilent. 85070E Dielectric Probe Kit 200 MHz to 50 GHz Technical Overview.
Torgovnikov, G. I. " Dielectric Properties of
Wood and Wood-Based Materials", Springer-
Verlag, Berlin (1993).