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 near-field 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.