Microwave probe for in situ observations of vegetation dielectric

Field-portable dielectric probes have been developed using a vector network analyzer incorporating a voltage-controlled oscillator (VCO) with two double-balanced mixers for single-frequency operation from 400 MHz to 10 GHz. A capacitor model is used to obtain the reflection coefficient from the measured admittance of an open-ended coaxial line terminated in the material under test. Although the technique was designed for media of uniform dielectric and with thickness greater than or equal to the probe radius, simple modifications permit measurements of thin dielectric sheets and also disaggregated particles (such as needles) immersed in a host material. The portability and relatively low cost of the instrument make feasible in situ observations at remote locations. For many applications where sharp resonances are not expected, continuous dielectric spectra are not required. The data acquisition rate of the device is commensurate with measuring a variety of natural media where the dielectric constant, ε_r, is found to be highly time variant. The device has been extensively field-tested