An Open-ended Coaxial Probe for Broad-band Permittivity Measurement of Agricultural Products

Sheen, N.I.; Woodhead, Ian

doi:10.1006/jaer.1999.0444

Cited by 65 publications

(54 citation statements)

References 14 publications

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“…This method allows broadband measurements while minimizing sample disturbance. The measured reflection coefficient is related to the sample permittivity (Sheen and Woodhead, 1999). The open-ended coaxial probe method is widely used to measure the dielectric properties of food and agricultural materials because it does not require a particular sample shape or special containers Feng et al, 2002;Nelson, 2003;Wang et al, 2003b).…”

mentioning

confidence: 99%

Temperature-Dependent Dielectric Properties of Selected Subtropical and Tropical Fruits and Associated Insect Pests

Wang¹,

Monzón²,

Gazit³

et al. 2005

Transactions of the ASAE

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mentioning

confidence: 99%

Temperature-Dependent Dielectric Properties of Selected Subtropical and Tropical Fruits and Associated Insect Pests

Wang¹,

Monzón²,

Gazit³

et al. 2005

Transactions of the ASAE

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“…Calibration of the VNA and probe is a two-step process. In previous work, we calibrated the VNA at the beginning of the probe using a coaxial open, short and 50 Ohm coaxial load, then calibrated the probe using the process described by [6]. Here we show that calibration of the VNA may be accomplished at the probe tip by using an open, a short in the form of Indium foil pressed firmly against the probe tip, and thirdly a bespoke calibration board.…”

Section: Methodsmentioning

confidence: 74%

A Broadband Spectroscopic Sensor Probe

Woodhead

Platt²,

Christie³

et al. 2008

International Journal on Smart Sensing and Intelligent Systems

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Abstract-The electrical properties of many materials are closely related to their composition and to their moisture content in particular. For sensor development, characterising the response of a particular volume of material over a broad frequency range is desirable, since separate measurements could generate errors due to spatial variability. A coaxial probe has been designed for measurement of the permittivity of smooth and flat, solid or liquid samples over the frequency range from 1 Hz to 6 GHz. Although the probe is capable of a very wide frequency range, separate instruments are generally required, and here we focus on measurements above 1MHz. We demonstrate measurements in the frequency domain using a vector network analyser, and in the time domain using a broadband oscilloscope. For switching, we employed a coaxial switch and demonstrate how that is included within the instrument calibration. Calibration of the probe used three references: an open circuit, short circuit (indium foil) and a reference liquid, ideally chosen for a permittivity similar to that of the sample. The sample complex permittivity was calculated by a numerical model which used as inputs, the measured reflection coefficient and physical measurements of the probe geometry.Index terms: coaxial probe, spectroscopy, calibration, broadband, network analyser. I INTRODUCTIONThe dairy, timber, wool, and many other primary processing industries demand rapid on-line measurement of product properties, particularly moisture. The chief advantage of the dielectric measuring technique is its relatively low cost compared with neutron backscatter, X-ray analysis, and magnetic resonance imaging. Dielectric techniques are also non-ionising, and offer clean, rapid, on-line measurements of bulk material, suited to many diverse products. For a particular product, there will be one or more frequency ranges where the dielectric constant has the highest sensitivity to changes in moisture content (or other The chosen measurement frequency for determining moisture content or composition may lie within a very broad range. Reasons for choosing a particular frequency will include sensitivity of the reading to changes in moisture content and the dependence of the reading on other parameters such as density and dielectric loss. For a particular polar molecular species, as the frequency is increased, the dielectric loss reaches a maximum and is accompanied by a reduction in the permittivity. The frequency of maximum loss is the relaxation frequency, and for free water begins to significantly affect the dielectric properties above approximately 3 GHz. In contrast, the relaxation frequency for bound water is typically 100 MHz [3]. Electrical conductivity is directly useful at low frequencies, particularly for materials with low moisture content where it provides a very useful measure of moisture content, and at high frequencies where it influences the dielectric loss. Apart from the Maxwell-Wagner effect which is caused by conducting inclusions in a dielectric, the p...

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“…In this case a coaxial line with a tip, which can sense the signal reflected from the investigated material, must be applied (Sheen & Woodhead, 1999). The tip is either in contact with the sample by touching the probe to a flat face of the solid material (Gardner & Anantaraman, 1995) or it is immersed into the liquid sample during the measurements (Guo, 2011).…”

Section: Open-ended Probe Techniquementioning

confidence: 99%