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Researching fault location techniques for the oil industry requires data on the complex permittivity of the polyamide Rilsan at microwave frequencies. Rilsan is a vital material used in oil pipe construction. Currently there is no published data for the complex permittivity of Rilsan. The task for this project was to develop an accurate method to measure the complex permittivity of Rilsan to gain an understanding of the dielectric relaxations that occur within it. A fast, accurate method was developed using a vector network analyzer (VNA) and rectangular waveguide.

Waveguides used in the experiment.

LabVIEW graphical programming software from National Instruments (Austin, TX) was used to control the system, directly provide complex permittivity values, and save data. A novel error correction technique was applied using LabVIEW programs. This transformed the complicated task of permittivity measurement into a straightforward procedure that can be carried out even by nontechnical users. The measurement system produced very accurate results that were verified using a range of standards. Further investigation into the properties of Rilsan is planned around the methods developed in this project.

One previous attempt to solve this problem involved constructing parallel plate Rilsan capacitors to calculate permittivity, but this technique is only suitable for use at low frequencies and suffers from errors caused by the fringing effect. At microwave frequencies, the permittivity measurement requires a much more elegant solution than the standard parallel plate capacitor.

A Rilsan sample was placed in a holder at the joint between the waveguides, which causes the signals traveling between the ports to be attenuated and reflected. A VNA detected the attenuation and reflections, and interpreted these as S parameters. The S parameters were fed back into the PC and converted into permittivity values. This required a data acquisition application that could interface with the Agilent 8753 ES VNA. LabVIEW software offered compatibility with a wide scope of instruments and devices.

The experiment set-up.

This experiment was just a small part of a larger project at the University of Manchester (UK). The collected data had to be passed on to a number of other researchers in a common file format. LabVIEW was used to automate this process by mapping all of the data into a Microsoft Excel file; this allowed for a simple method of data transfer among researchers.

Error correction was another major hurdle to overcome while implementing this experiment. To tackle the errors the apparatus introduced into the measurements, a technique known as thru, reflect, line (TRL) calibration was used.

Measurements from these three stages give all the characteristics of the empty apparatus and are saved using a LabVIEW program. This means that when a sample is inserted between the waveguides, the resulting measurements can have the effects of the apparatus automatically removed by the VNA.

This article was written by James Gyves of the University of Manchester using National Instruments products. For more information, visit http://info.hotims.com/45608-542.