Design of the broadband admittance tunnel for high fidelity material characterization

This dissertation is mainly focused on designing a broadband admittance tunnel for high fidelity material characterization. It is realized by creating a local plane wave illumination region as the quiet zone free of diffraction and other factors falsifying measurement results.; The admittance tunnel was chosen on the basis of the evaluation of existing measurement techniques and design specifications. The plane wave spectrum analysis of the focused beam measurement setup shows its sufferance from the existence of the longitudinal mode, faster phase velocity, lens spillover and movable feeding antenna phase center, which are all negative factors degrading high fidelity measurement.; The low diffraction iris design was proposed to enable the admittance tunnel suitable for high fidelity material characterization. A well modeled broadband double-ridged horn provides a steady gain over 700.0 MHz to 18.0 GHz as the transceiver. The iris diffraction illuminated by the horn was characterized with Geometrical Theory of Diffraction (GTD) and Finite-difference Time-domain (FDTD), based on which serration and resistive taper were used to reduce the diffraction. The prototype of a serrated iris was generated with GTD and verified with FDTD in both near and far zones. The serrated iris shows the effect of sending diffraction out of the quiet zone. And the resistive taper attenuates the incident wave interacting directly with the conducting edges. Both the time history analysis and the simulation of the transmission coefficient in the spectral domain show that the resistively tapered serrated iris has the best performance in diffraction reduction and redirection. It proves to be superior over the traditional admittance tunnel with square iris.; As an alternative to the iris admittance tunnel, solid dielectric polyrod and hollow dielectric pipe can work as broadband antennas in near field applications. A dielectric polyrod with specifically chosen tapering profile, dielectric constant and geometrical dimensions was proposed in a compact tunnel design. Also the design of a hollow dielectric pipe combined with the broadband horn and proper tapering and wedging was presented. Both designs demonstrate a good guiding effect of the traveling plane wave and forms a Gaussian spot over a distance range outside of the guide structure with planar phase front and finite spot size. Their performance in wave guidance and spot illumination was examined and the system proves to generate transmission coefficients similar to the resistively tapered iris in a more compact, affordable and efficient tunnel for broadband material characterization.; In support of the modeling of the measurement setups, FDTD was used incorporated with the state-of-the-art teleportation, re-Radiating Boundary Condition (rRBC), wrapping around technique and Perfect Electric Conducting (PEC) blocks. Besides, a novel algorithm was implemented to model curved structures in Cartesian FDTD grids. New FDTD grids aiming to create better propagation isotropy were proposed and the implementation of teleportation and rRBC was proved to be feasible in these new grid stencils.