Computer Science – Performance
Scientific paper
May 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011phdt........34h&link_type=abstract
PhD Thesis, Universiti Tunku Abdul Rahman, Malaysia, May 2011, 193 pages
Computer Science
Performance
Scientific paper
In THz radio astronomy, waveguide heterodyne receivers are often used in signal mixing. To ensure that the energy of the waves from the incoming waveguide couples efficiently to the microstrip probe, an accurate and versatile mathematical model that computes losses in waveguides is desirable in the development of a mixer circuit. In this thesis, a new and novel method to compute the propagation constants in guiding structures is presented. This method is based on matching the fields at the boundary with the constitutive properties of the wall material. Compared to existing methods which assume lossless fields, the field expressions in the new method can accommodate both lossless and lossy cases. Unlike the existing methods which are geometry specific, the new method is applicable to various structures including the circular and rectangular waveguides, superconducting waveguides, and microstrip lines. For circular and rectangular waveguides, simulation and experimental measurements were carried out to validate the new method. It is found that this method is able to account the additional loss induced by mode coupling effects in degenerate modes. This is in contrast to existing methods which fail to account for multimode propagation. In the study of superconducting waveguides, the real conductivity is replaced with a complex conductivity derived from the Bardeen-Cooper-Schrieffer theory. It is found that at frequencies below the gap frequency, the waveguide exhibited lossless transmission behaviour while above the gap frequency, Cooper pairs breaking dominates and the loss increases considerably. Considering that THz signals from astronomical sources are extremely weak, the result suggests that superconducting waveguides that operate at frequencies below the gap frequency can be applied in SIS receivers to minimize the loss of such signals. A full-wave analysis has also been performed on microstrip lines. Since the new method accounts for the propagation of hybrid modes and fringing loss, it is found to be more accurate compared to the conventional quasi-static methods which only assume TEM mode propagation. Superconducting microstrips are found to be dispersionless and exhibit a much lower loss. A comparison is also made between the performance of a microstrip line and coplanar waveguide (CPW). Preliminary studies suggest that at dimensions comparable with the wavelength, CPW exhibits lower loss than a microstrip. The lower loss found in CPWs strongly suggests that CPWs can be considered as a better alternative to microstrip line for THz waves coupling in hete!
rodyne receivers.
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