Microwave Dielectric Properties of Alfalfa Leaves From 0.3 to 18 GHz

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.