Coaxial Impedance Dielectric Reflectometry

The Coaxial Impedance Dielectric Reflectometry (also referred to as “Ratiometric Coaxial Impedance Dielectric Reflectometry”) method of soil moisture measurement employs an oscillator to generate an electromagnetic signal that is propagated through the sensor by metal tines and into the soil. It characterizes the ratio of the amplitudes of reflected radio waves at 50 MHz with a coaxial wave guide. A numerical solution to Maxwell’s equations first calculates the complex impedance of the soil and then delineates the real and imaginary dielectric permittivity (Seyfried 2004, Campbell 1990). The mathematical model that delineates the real and imaginary component from the impedance of the reflected signal resides in the microprocessor inside the sensor. These computations are based on the work of J. E. Campbell at Dartmouth College (Campbell 1988, Campbell 1990, Kraft 1988).

The term “ratiometric” refers to the process by which the ratio of the reflected signal over incident signal is first commutated which eliminates any variability in the circuit boards from one probe to the next. This step is performed on several reflections. The term “coaxial” refers to the metal wave guide that is inserted into the soil. It has three outer tines with a single tine in the center that both receives and emits a radio frequency at 50 MHz. “Impedance” refers to the intensity of the reflected signal, and “dielectric reflectometer” refers to the reflected signal that is used to measure the dielectric.

The HydraProbe is the only commercially available sensor to use the Coaxial Impedance Dielectric Reflectometry method along with complex computations in soil measurement, resulting in very high measurement accuracy through the lifetime of the sensor. One advantage is that sensors using this technology do not require calibration for most soils.

Unlike most soil moisture methods, Coaxial Impedance Dielectric Reflectometry measures both the real and the imaginary components of the dielectric permittivity as separate parameters. The sensor bases the soil moisture calibration on the real dielectric permittivity while most other soil moisture methods base their soil moisture estimation on the apparent permittivity which is a combination of the real and imaginary components. (Logsdon 2010). Basing the soil moisture calibration on the real dielectric permittivity instead of the apparent permittivity has many advantages. Because the Coaxial Impedance Dielectric Reflectometry separates the real and imaginary components, soil moisture calibrations are less affected by soil salinity, temperature, soil variability and inter-sensor variability than other methods.

Complex mathematical computations performed by an onboard microprocessor process the reflected signal measurements to accurately determine the soil’s dielectric permittivities, the key parameters behind the soil moisture and bulk EC measurement. Because the soil measurement computations are performed by a microcontroller inside the sensor, the probe can output results in standard engineering units.

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