An experimental study of artificial isotropic chiral media at microwave frequencies

[摘要] ENGLISH ABSTRACT: An artificial chiral medium can be made by embedding metal helices (chiral or handed structures)with random orientation inside au achiral hnst medium. The electromagnetic behaviourof such artificial chiral media can be explained by an additional (to p. and t') constitutive parameter,the chirality parameter e. Chiral media have certain special properties of whichoptical activity is the most prominent. Optical activity is the term used to describe therotation of the polarization plane of a linearly polarized wave as it travels through a chiralmedium. The chirality parameter is directly linked with this rotation.The reflection and the transmission coefficients from a chiral slab are well known in terms ofthe constitutive parameters and the thickness of the sample. In the thesis a set of .inversionequations are derived that can be used to determine the constitutive parameters (p., £, e) ofa chiral medium from the measured S-parameters 811, S21x and S2l'jJ (Le. the reflection andco- and cross-polarized transmission coefficients respectively).An accuracy analysis of the measurement method is made. This is done by using the firstorder partial derivatives of the inversion equations. A sensitivity analysis is done on theinversion equations and the results are in an analytical form. This makes it possible todetermine the contribution of each of the measurement errors to the total expected error. Astatistical root mean square method is used to predict the expected error in the measuredconstitutive parameters.Several artificial chiral samples are measured and an accuracy analysis done on the measurements.The inaccuracy of the method of inversion for low loss samples that are multiples ofhalf a wavelength thick is illustrated by example and explained by the sensitivity analysis.The free-space system (11-17 GHz) used to measure the S-parameters is described in detail.The radiation patterns of the focused lens antennas are measured and compared to thoseobtained from two theoretical models. A free-space calibration procedure is developed and thedispersion in the focal region of the antennas incorporated into it. The results from differentcombinations of calibration standards are compared and related to the idea of minimumsensitivity calibration.The possible use of an artificial chiral medium as a microwave absorber is also discussed.

[发布机构] Stellenbosch University