Instrumentation of a submillimetre wave hologram compact antenna test range

摘要

This thesis presents the developed instrumentation and measurement techniques suitable for use in a submillimetre wave compact antenna test range (CATR) for testing high-gain antennas and the quiet-zone quality of the CATR, but also for use in antenna testing with planar near-field scanning at submillimetre wavelengths. The thesis work is focused on improving the phase measurement accuracy and the dynamic range of a commercial submillimetre wave vector network analyser. The full angular scattering properties of radiation absorbing materials (RAM) suitable for the CATR are also analysed in the thesis.A CATR can be used for testing of electrically large antennas at millimetre and submillimetre wavelengths. These high-gain dish antennas are required for spaceborne astronomy and limb sounding of the Earth atmosphere. The most common CATR configuration at millimetre waves uses a reflector as the collimating element. However, the surface accuracy requirement of the reflector becomes very stringent at frequencies over 200 GHz, and the manufacturing of the reflector thus very expensive. An alternative collimator to the reflector is the binary amplitude hologram which is studied in this thesis. The hologram is a planar transmission type device, which is realised as a slot pattern on a metallised dielectric film. The surface (pattern) accuracy requirement of the hologram is less stringent than that of a reflector and it is potentially of lower cost. The amplitude and phase ripples of the CATR quiet-zone field need to be below ± 0.5 dB and ± 5°. The hologram CATR operating at 310 GHz discussed in this thesis is shown to be able to achieve these limits even at submillimetre wavelengths.The amplitude and phase measurement accuracies of a vector network analyser largely depend on the strength of the detected signal. The quiet-zone tests of planned large hologram CATRs require larger dynamic range than is possible with the standard solid-state source configuration, so a phase-lock system for submillimetre wave backward-wave oscillators (BWO) had to be developed. The powerful phase-locked BWO source described in this thesis can improve the dynamic range and the accuracy of the measurement system considerably. The improvement in dynamic range over the standard source based on a frequency-multiplied Gunn oscillator is 16-40 dB over the frequency range of 300-700 GHz.Problems in the phase measurement accuracy arise when the receiver is moved across the quiet-zone area with microwave cables connected to it. The flexing of the cables causes phase errors reaching tens of degrees due to changes in their electrical lengths. The novel phase error measurement and correction system described in this thesis is based on the use of a pilot signal to track changes in the electrical length of a microwave cable. The error analysis shows that phase correction of the detected submillimetre wave signal is possible down to a level of 2° with the constructed system. Accurate operation has also been verified by measurements.The CATR facility needs large quantities of high-performance absorbers. In order to select suitable absorbers, the specular and non-specular reflectivities of several commercially available, state-of-the art absorber materials have been measured between 200-600 GHz. Selected wool and synthetic floor carpet materials were also included in the tests. The results show that specular reflectivities between −40…−50 dB are possible over a considerable angular range when the materials are oriented properly. The best floor carpet materials have reflectivities below −15 dB over a wide angular range and are useful in the less critical areas by reducing backscatter. The published report is the first in the open literature showing the full angular performance of these materials across a wide frequency range.