The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
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Shaped Reflector Antenna Design and Antenna Measurements at Sub-mm Wavelengths
Janne Häkli
Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Electrical and Communications Engineering for public examination and debate in Auditorium S1 at Helsinki University of Technology (Espoo, Finland) on the 29th of September, 2006, at 12 o'clock noon.
Dissertation in PDF format (ISBN 951-22-8340-9) [11929 KB]
Dissertation is also available in print (ISBN 951-22-8339-5)
Abstract
In this thesis, a ray-tracing based synthesis procedure for shaped dual reflector antennas at the sub-mm wavelengths is developed. A dual reflector feed system (DRFS) consisting of two shaped hyperboloid reflectors for hologram based compact antenna test range (CATR) is designed. The DRFS provides a shaped hologram illumination improving the hologram performance and manufacturability. The DRFS operation is verified with simulations and planar near-field measurements at 310 GHz. The measured and simulated beams are within ±0.3 dB from each other.
The applicability of the DRFS for providing a shaped illumination for the holograms in CATRs is verified experimentally with quiet-zone field amplitude and phase measurements of test holograms at 310 GHz. The shaped hologram illumination eliminates narrow slots in the hologram pattern, which simplifies the etching of the pattern. In addition, the shaped illumination allows the holograms to operate also at the linear horizontal polarisation as previously the hologram operation was limited to the vertical polarisation.
Cross-polarisation properties of the holograms are investigated using a polarization grid to suppress the cross-polarisation in the hologram illumination by 21 dB. It is concluded that the hologram contributes mostly to the cross-polarisation level of a hologram-based CATR. By the placing the cross-polarisation grid behind the hologram, the cross-polarisation level in the quiet-zone can be reduced to below −33 dB.
For improving the measurement accuracy of planar near-field measurements at sub-mm wavelengths, the measurement errors and their contribution to the measurement uncertainty are determined. The measurement system is improved for more accurate near-field measurements. Error correction techniques based on error measurements are used to reduce the phase measurement uncertainty from ±14° to ±8° at 310 GHz in one-dimensional scans of a plane wave corresponding to the quiet-zone of a hologram.
A hologram-based CATR is constructed for testing a 1.5 m reflector antenna (ADMIRALS RTO) at 322 GHz. The measured radiation pattern of the RTO corresponds reasonably well to the simulated pattern, but the quiet-zone field non-ideality affects the measurement results. As estimated effect of the quiet-zone does not explain all the discrepancies between the measurements and simulations, it is concluded that the reflector surface differs from the simulated surface.
Keywords: reflector antennas, compact antenna test range, near-field measurements, sub-millimetre wave
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© 2006 Helsinki University of Technology
Last update 2011-05-26