Arthur Haigh

Complex Permittivity of Solid, Liquid and Granular Materials using a four layer boundary cell

The four boundary cell is an important arrangement for the measurement of complex permittivity of bulk solids and liquids. Transcendental equations for reflection and transmission are derived and previous errors in the literature are noted. This measurement method is extended to granular samples where a compression technique is introduced to minimize the dispersion in S-parameter measurements associated with interface problems.

Terahertz time-domain spectroscopy of crushed wheat grain

The terahertz time-domain spectroscopy (THz-TDS) is used to measure absorption spectra in crushed wheat grain over the frequency range of 0.1-2.0 THz. A number of well resolved absorption peaks are observed for dry, 12%, 14% and 18% moisture content levels. Subtracting the transmission signal spectra of the dry flour from that of the wetted flour provides a calibrated result which relates attenuation to moisture content level.

Bimodal Method of Determining Fat and Salt Content in Beef Products by Microwave Techniques

A bimodal microwave technique is presented as a rapid analytical method of estimating the fat and salt content of beef products. The dielectric properties of lean meat, fat, salt, and other nonmeat ingredients (dry and aqueous solutions) and meat blends of typical ingredients used in meat product manufacture are all separately investigated. A broadband coaxial probe technique is used to select frequencies of maximum sensitivity. The optimum frequency for fat determination lies between 8 and 20 GHz, and the optimum frequency for salt determination lies below 4 GHz. For example, by using a WR-90 cell, either the real or imaginary part of the complex permittivity can effectively resolve fat content, whereas the imaginary part of the permittivity correlates well with salt content when using a WR-284 cell. The technique is sufficiently robust to ensure that the dielectric effects of other nonmeat ingredients, temperature, and density do not adversely affect this measurement approach.

Application of an electromagnetic sensor for detection of impact damage in aircraft composites

This paper presents a comprehensive experimental study of barely visible impact damage detection for carbon-fibre reinforced polymer (CFRP) composites using an electromagnetic sensor: coupled spiral inductors. A multi-frequency inspection is performed, where the resultant images indicate its great potential for the evaluation of damage propagation. The accuracy and efficiency are compared with the open-ended waveguide imaging, near-field microwave microscopy and microwave time-domain reflectometry. Advantages, disadvantages and applications of these methods used for impact damage detection are described. It is found that the phase can also be used for evaluation other than the magnitude. The experimental results demonstrate that the sensor could offer an alternative method for the structural monitoring and maintenance of aircraft composite structures.

A Twin Toroid Ferrite Phase Shifter

Ferrite phase shift and control components designed with low reluctance magnetic bias circuits help to reduce space, weight and energy consumption in microwave systems. Avoiding air-gaps is essential and one suitable geometrical arrangement is the toroidal shaped ferrite positioned inside a rectangular waveguide. A finite element method is presented to model the nonuniform magnetization and partially magnetized state of this type of phase shifter. Using a viscous plastic processing (VPP) method, homogeneous ferrite toroids are fabricated and a twin toroidal phase shifter is designed, assembled and tested. The insertion loss was less than 1 dB over the 9.5 GHz to 10.3 GHz band and return loss of 20 dB was achievable. The phase shift calculation agrees to within 10% of the measured values.

An Instrument to Measure the Characteristics of Single Wheat Grain Kernels

This paper describes an instrument, based on microwave techniques and a gas displacement method, to measure the wet mass, moisture content, volume and hence the density of single wheat grain kernels. A low-cost microwave source and detector is designed to drive a microwave cavity to determine the wet mass and moisture content of the grain. The microwave cavity is compactly integrated with the volume meter to combine the measurements into a single assembly with one loading cycle. The loading mechanism is separated from two novel gas compression bellows to improve the accuracy and stability of this measurement. All of the results are processed using a microcontroller. The measured wet mass, moisture content, volume and density of the grain are shown on an LCD display

An Automated Microwave Waveguide Measurement Technique

The tedious and often difficult measurement of dielectric permittivity and permeability as a function of microwave frequency has been simplified using an automated microwave waveguide cell measurement system. The measurement system comprises a microwave VNA under PC control and a data processing section. The overall experimental setup and computer program for the automation of data acquisition are fully discussed. This includes the automatic solution of the transcendental equation to determine the dielectric properties for a frequency band. Experimental data taken using this system is presented to show the feasibility of this automated system for material characterisation. The simplicity, applicability and robustness of this proposed system has applications throughout manufacturing and food processing industries.

Coplanar waveguide scanning microwave profiler

A robust near-field microwave profiler is constructed from a micromachined coplanar waveguide probe coupled to a half wavelength coaxial cavity. This microwave profiler can perform both high resolution scanning microwave measurements and more robust, contactable, in-situ handheld measurements. High resolution measurements are undertaken with the sensor on an X-Y-Z scanning table and results for the investigation of metallic strips on dielectric are compared with measurements using a previously reported tapered conical cavity open-ended coaxial probe. The new micromachined probe is considerably more robust than the fine tipped coaxial probe and yet has been shown to yield higher sensitivity for \S11\ and Q measurements when scanning metallic tracks on a dielectric substrate.

Detection of Impact Damage in Carbon Fiber Composites Using an Electromagnetic Sensor

ABSTRACT This article presents a comprehensive experimental study of impact damage detection for carbon fiber-reinforced polymer (CFRP) composites using an electromagnetic (EM) sensor with coupled spiral inductors (CSI). Two representative types of damage are detected and evaluated, i.e., barely visible impact damage (BVID) and delamination. A multifrequency inspection is performed, where the resultant images indicate the potential of the CSI sensor in the characterization of damage extent. The accuracy and efficiency of the CSI sensor are compared with the open-ended waveguide imaging, near-field microwave microscopy, microwave time-domain reflectometry, the complementary split-ring resonator, and ultrasonic scanning. Applications and limitations of these nondestructive testing (NDT) methods for identifying impact damage are discussed. There is a free edge effect on the electromagnetic signal, which is illustrated for the first time with the proposed EM technique. Detection of the air gap produced by inserting a thin piece into a machined subsurface groove is carried out. It is found that the developed CSI sensor is able to accurately resolve the location and extent of the air gap. The experimental results demonstrate that the sensor could offer an alternative relatively low cost method that can be fully automated for structural monitoring of aircraft and other composite structures.

An Overview of Microwave Techniques for the Efficient Measurement of Food Materials

There is critical demand for rapid, low-cost methods to determine food composition to provide a responsive control mechanism and an efficient quality control system in food processes. Four non-destructive microwave techniques used to measure the relative complex permittivity (e*) of food components are reviewed. The open-ended coaxial probe is found to be quick and effective for broadband measurements of liquids and pastes. Narrowband waveguide cells require careful sample preparation but they provide accurate permittivity measurements for solids, particulates and liquids. The resonant cavity method provides a high Q measurement at a single frequency and is suitable for low loss samples that can be inserted into the resonator. For scanning and conveyer belts we describe a horn antenna, free-space transmission line method. Each method is described and real examples are demonstrated for the characterisation of a range of foodstuffs.