Microwave permittivity measurements of two conifers

Complex permittivity spatial distributions of two conifers, a 5.0-m high Caucasian fir (Abies normanniana) and a 1.9-m high spruce (Picea omorika), have been measured in the frequency range 1-10 GHz. Unlike earlier studies, particular attention was paid to the anisotropy of the dielectric properties. The measurements were performed in the frame of polarimetric scattering, and imaging experiments conducted on both trees in the European Microwave Signature Laboratory (EMSL), Space Applications Institute, Joint Research Centre, Ispra, Italy. They will be used, together with carefully established architectural models of the trees, to validate forest remote-sensing algorithms. The measurement method was based on an open-ended coaxial probe reflection technique with a rational function approximation model for the probe tip aperture admittance. With this model, no calibration on reference liquids is required and sufficiently accurate results for the dielectric constant and loss factor can be obtained. Results are presented for branches, parts of the trunks, and needles from different tree heights. Values obtained with the probe oriented along different stem directions of the trunk confirm the anisotropic nature of wood. The longitudinal complex permittivity is roughly 1.5-three times higher than the transverse component. Inside the trunk, early and late wood layers and small heterogeneities give rise to fluctuations; the phloem layer and the new needles have the highest complex permittivity. Representative average values for trunks, branches, and needles for modeling purposes are also given.