Abstract
Dielectric properties (i.e., permittivity) are essential
in designing, simulating, and modeling microwave applications.
The permittivity of stacked leaves of alfalfa (Medicago sativa) were
measured with a network analyzer and a coaxial probe, and the
effect of moisture content (MC: 12%–73% wet basis), frequency
(300 MHz to 18 GHz), bound water (Cole–Cole dispersion equation),
temperature (−15 ◦C and 30 ◦C), leaf-orientation, and
pressure (0–11 kPa) were investigated. The measured permittivity
increased with MC. A critical moisture level (CML) of 23% was
reported, below which the permittivity decreased with increasing
frequency at 22 ◦C. Above CML and up to 5 GHz, the dielectric
constants followed the Cole–Cole dispersion, and the dielectric loss
factors consisted of ionic and bound water losses. Above 5 GHz,
the behavior of the dielectric constant was similar to that of free
water, and the polar losses became dominant. Above 0 ◦C, the
measured permittivity followed a trend similar to that of free
saline water and was characterized by the Debye equation. Below
0 ◦C, it was dominated by nonfreezing bound and unfrozen supercooled
moistures. The relaxation parameters and the optimum
pressure (9 kPa) for the leaf measurements were determined. The
effects of variations among the samples, and their orientations had
negligible effects on the measured permittivity.