Numerical studies on a radio frequency hologram and its use in antenna measurements

Abstract

This thesis deals with the compact antenna test range (CATR) based on a radio frequency hologram. The emphasis is on the development of the numerical analysis of the hologram. Several holograms have been fabricated and tested assessing the quality of the numerical analysis methods described in this thesis.

A CATR is a potential method for testing the electrically large antennas operating at millimetre and submillimetre wavelengths. These high-gain antennas are needed for remote sensing and radio-astronomical applications. A parabolic reflector is most often used as the collimating element in the CATR. However, at frequencies above 100 GHz, the reflector surface accuracy requirement becomes very stringent. To overcome this problem, it is proposed to use a binary amplitude hologram in the CATR. The hologram is a planar, transmission type of structure and the hologram pattern is realised on the metal plated dielectric film.

Typical requirements for the quiet-zone field quality are maximum amplitude and phase ripples of ±0.5 dB and ±5°, respectively. To achieve these requirements in the iterative hologram design procedure, an accurate method for evaluating the electromagnetic wave transmission through the hologram is needed. In this thesis, the finite difference time domain (FDTD) method is used for calculating the transmission of the hologram. However, the hologram structure is so large (at least several hundreds of wavelengths) that the whole hologram structure cannot be simulated at one time.

Two approaches are used for the simplification of the hologram structure: either 1) only one cut of the hologram is analysed at a time or 2) transmissions of electromagnetic wave through slots of different widths are studied and applied to the whole hologram analysis. The first method is used for optimising the holograms as it gives more accurate results. In addition, it is used when the bandwidth, tolerances, polarisation dependence, and planarity requirements of the hologram CATR are studied. The second method provides a tool for predicting cross-polarisation performance of the hologram CATR.

Experiments have been made at several frequencies: 39, 119, and 310 GHz. The results of these experiments show that the method of analysis developed in this thesis is adequate for designing the holograms. In addition, the feasibility of the hologram CATR operation for antenna measurements is demonstrated.