Free-wave transmittance determination of the complex permittivity of textiles and composites in the W-band

Summary form only given. By measuring the oblique transmittance of planar samples of textile and composite materials in the W-band (75 - 110 GHz) the frequency dependent complex permittivity is determined using a non-linear fitting algorithm. Samples of clothing fabrics, fabrics used in composites and different coloured leathers are not suitable for the "open resonator" technique for determination of complex permittivity as these samples are, in general, too lossy for the loss tangent to be well determined using this method. A free wave method that only measures the transmittance of the sample is used here to determine the complex permittivity of cotton clothing materials, various leathers and fabrics used in composite materials. The common problem of standing wave interference between emitter horn, sample and detector horn is here addressed by a polynomial curve fitting method that removes the rapid oscillatory response in transmittance, typified by such interference, while revealing the much slower change in transmittance of the sample. Although the results are expected to be less accurate than would be obtained by other commonly employed methods, e.g. free-wave reflectance and transmittance, the ease and simplicity of both the apparatus and data processing means this method has application where a large number of samples need to be processed quickly and where very precise permittivity values are not required. Common clothing fabrics are found to be moderately absorbing and highly transparent across the W-band with loss tangents in the range ~ 0.02 - 0.15 and reals parts of permittivity ~ 1.4 - 2.3.