Defects and cracks in dried natural timber (relative permittivity 2–5) may cause structural weakness and enhanced warping in structural beams. For a pine wood beam (1200 mm × 70 mm × 70 mm), microwave reflection (S11) and transmission (S21) measurements using a cavity-backed slot antenna on the wood surface showed the variations caused by imperfections and defects in the wood. Reflection measurements at 4.4 GHz increased (>5 dB) above a major knot evident on the wood surface when the E-field was parallel to the wood grain. Similar results were observed for air cavities, independent of depth from the wood surface. The presence of a metal bolt in an air hole increased S11 by 2 dB. In comparison, transmission measurements (S21) were increased by 6 dB for a metal screw centered in the cavity. A kiln-dried pine wood sample was saturated with water to increase its moisture content from 17% to 138%. Both parallel and perpendicular E-field measurements showed a difference of more than 15 dB above an open saw-cut slot in the water-saturated beam. The insertion of a brass plate in the open slot created a 7 dB rise in the S11 measurement (p < 0.0003), while there was no significant variation for perpendicular orientation. By measuring the reflection coefficient, it was possible to detect the location of a crack through a change in its magnitude without a noticeable change (<0.01 GHz) in resonant frequency. These microwave measurements offer a simple, single-frequency non-destructive testing method of structural timber in situ, when one or more plane faces are accessible for direct antenna contact.

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While the earth provides a relative secure environment in the event of flood, fires and storm events, the design of antennas which operate satisfactorily to a receiver on or above the ground has been a design challenge. A cavitybacked slot antenna previously used for biomedical applications was modified for subsurface propagation at 433 MHz at shallow depths in soil. A 433 MHz +10 dBm beacon located in an aluminium box (163 × 114 × 42 mm) with a thin (1 mm width) slot buried to a depth of 100 mm. Vertical electric field measurements were made on the surface of the earth from the antenna buried in moist lawn-covered soil. The antenna was placed below the grassed surface with the slot directing radiation horizontally into the soil. Propagation across the surface suffered a loss of 0.53 dB/m. This technology shows significant promise in the case of sensors buried in soil.

Cavity-backed slot antennas can be used to detect anomalies and measure the dielectric properties of lossy media such as timber at microwave frequencies. An enhanced antenna design with the feed plate directly behind the rectangular slot allows maximum E-field coupling to the wood and provides more space for electronics inside the aluminum box. The E-field in a rectangular wood piece (100 mm × 70 mm × 70 mm) was calculated using CST simulations at the resonant frequency of 7.4 GHz. The wood material was modeled as isotropic with a relative permittivity of 2 and low conductivity (10 -6 S/m). A comparison between simulations and a spherical wave calculation showed significant change in E-field in the very nearfield region of the antenna at the distances smaller than 4 mm. The electrical field penetrated the 70 mm width, and reflection from the opposite wood-air interface was observed.