Materials Measurement Phil Bartley Shelley Begley

Transmission LineMaterial assumptions:Reflection(S )11lTransmission(S )21 sample fills fixture cross section no air gaps at fixture walls smooth, flat faces, perpendicular tolong axis Known thickness > 20/360 λThe material sample is assumed to completely fill the cross section of thefixture with no air gaps, have smooth flat faces and to be uniform throughout.Coaxial airline fixtures are broadband, but the samples are more difficult tomachine. Waveguide fixtures extend to the mm-wave frequencies and thesamples are simpler to machine, but their frequency coverage is banded.Because the coax and waveguide transmission line size scales withfrequency, the practical sample size determines the frequency limits.Waveguides at frequencies much lower than 5GHz start to get very large,and a large sample is needed. Below 1GHz for many materials, samplelength also becomes an issue. The sample must be long enough to containenough of the wavelength of interest to be measurable, 20 degrees at least,but ideally 180 degrees, or ½ λ. At frequencies above 75GHz, both coaxialand waveguide dimensions get very small, and it becomes too difficult tomachine the tiny samples.34