Borchalooei, Abolfazl Khosrowbeygi; (1994) A new free-wave technique for measuring materials dielectric and magnetic properties at millimetre wavelengths. Doctoral thesis (Ph.D.), University College London (United Kingdom).

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Abstract

The complex permittivity and permeability of materials can be measured by different techniques, but none of the conventional methods is appropriate for measuring lossy materials at millimetre wavelengths. In this thesis a new free-wave technique for measuring the properties of lossy materials at these frequencies is introduced. Free-wave and conventional measurement techniques are reviewed. The sources of error in free-wave measurements are explained, and ways to reduce their impact from the measurement results are discussed. Various methods for calculating the sample properties from the measured parameters are introduced, and the dependence of the sample properties on these parameters are studied. The total error in the sample properties is investigated, and the optimum way of using the measured data is presented. The use of time gating for removing unwanted reflections from the measurement results is explained, and the gating error due to discarding higher order terms of multiple reflections within the slab for an ideal time domain gate is calculated. The gating error for a real gate is simulated and compared with the calculated values for an ideal gate. Removing multiple reflections within the sample by the time gating technique is introduced, and the minimum electrical length of the sample is estimated. It is also shown that ignoring multiple reflections within the sample improves the total accuracy. An angular spectrum model considering the measurement in the near-field region is introduced. The accuracy improvement delivered by this model is demonstrated by simulation techniques, and the minimum antenna distance to ignore the wavefront curvature error is estimated. The key parameters in constructing a compact single-pass system for measuring material properties in the range of 26.5-40 GHz are presented, and examples of the results obtained from measurements of ferrite and dielectric samples are presented and discussed. Finally the achievements obtained in this project are summarised, and further work to improve free-wave measurement techniques is suggested.

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