Dielectric properties of tobacco and tomato plant leaves over a broad frequency range

Complex permittivity of tomato and tobacco leaves have been measured over the 20 Hz − 2 MHz frequency range using a precision LCR-Meter, and over frequency range from 500 MHz to 15 GHz using a Vector Network Analyzer. Complex permittivity of leaves is found to increase as the moisture content in the leaves increases, at a fixed frequency. At lower frequencies, the dielectric constant ε’ of leaves decreases with increase in frequency up to 2 MHz. At 2 MHz, the complex permittivity ε* = ε′ – jε″ for Tobacco leaves varies from 3855 – j 2863 to 6.12 – j 0.29 with decrease in moisture content, whereas for Tomato leaves ε* varies from 5900 – j 3116 to 3.06 – j 0.29 with decrease in moisture contents in the leaves. Over the microwave frequency range, the dielectric constant and dielectric loss of low moist leaves decreases with increase in frequency. For the leaves having higher moisture content, the dielectric loss decreases rapidly with increase in frequency from 500 MHz to about 5 GHz, after which it starts to increase with increase in the frequency, approaching a relaxation peak. At 14 GHz, the complex permittivity ε* for Tobacco leaves varies from 26.86 – j 26.45 to 2.31 – j 0.034 with decrease in moisture content, whereas for Tomato leaves ε* varies from 23.37 – j 17.50 to 1.67 – j 0.004 with decrease in moisture contents in the leaves. A good agreement is observed between measured values of complex permittivity, with the values calculated using Debye-Cole dual-dispersion dielectric model. Brewster’s angle corresponding to vertical polarization in emissivity calculation is found to increase with increase in moisture content in the leaves.