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Dielectric characterization of vegetation at L band using an open-ended coaxial probe

Abstract. Decoupling the integrated microwave signal originating from soil
and vegetation remains a challenge for all microwave remote sensing
applications. To improve satellite and airborne microwave data products in
forest environments, a precise and reliable estimation of the relative
permittivity (ε=ε′-iε′′) of trees is required. We
developed an open-ended coaxial probe suitable for in situ
permittivity measurements of tree trunks at L-band frequencies (1–2 GHz).
The probe is characterized by uncertainty ratios under 3.3 % for a broad
range of relative permittivities (unitless), [2–40] for
ε′ and [0.1–20] for ε′′. We quantified
the complex number describing the permittivity of seven different tree
species in both frozen and thawed states: black spruce, larch, red spruce,
balsam fir, red pine, aspen and black cherry. Permittivity variability is
substantial and can range up to 300 % for certain species. Our results
show that the permittivity of wood is linked to the freeze–thaw state of
vegetation and that even short winter thaw events can lead to an increase in
vegetation permittivity. The open-ended coaxial probe proved to be precise
enough to capture the diurnal cycle of water storage inside the trunk for the
length of the growing season.
