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Nonlinear Permeability of Composites for Nonlinear Transmission Lines at Microwave Frequencies
Nonlinear transmission lines (NLTLs), which exhibit permittivity as a function of electric field and/or permeability as a function of magnetic field strength, are of increasing importance for sharpening pulses to less than 100 ps and serving as radiofrequency (RF) sources; however, NLTLs often are not easily modified to achieve different output parameters. One method under investigation involves combining inclusions of nonlinear dielectric (barium strontium titanate (BST)) and/or magnetic (nickel zinc ferrite (NZF)) inclusions to tune NLTL properties by adjusting inclusion loading fractions. This thesis focuses on measuring the nonlinear permeability and magnetic loss tangent of composites comprised of various volume loadings of NZF or a combination of NZF and BST inclusions encapsulated in a silicon matrix. We measured the relative permeability from 1 - 4 GHz using a coaxial airline while biasing the samples in an external DC magnetic field from 0 – 171 kA/m. The permeability decreased from 1 to 4 GHz for each volume fraction but increased with increasing magnetic field strength at low magnetic field strengths with sufficient NZF volume loading. The magnetic loss tangents of the composites increased with increasing frequency and/or NZF volume fraction but were suppressed by increasing the external magnetic field strength. Adding BST to an NZF composites did not cause a significant change in permeability compared to NZF alone, based on an analysis of variance (ANOVA) and multiple comparison test. These results elucidate the frequency, magnetic field, and volume loading dependence of NZF at microwave frequency and provide initial information for simulating NLTLs and examining more comprehensive RF system behavior.
We then investigated the feasibility of tapering the NLTL by modifying the inclusion loading down the line to control the device’s electric and magnetic properties to achieve greater system flexibility and rigidity. Unlike conventional tapering by gradually changing the line size, this method bypasses the complex, difficult to manufacture geometries by leveraging the experimental results obtained for our NZF and BST composites. Potential drawbacks of extending this approach to nonlinear materials and NLTLs is discussed.