Title page for ETD etd-11042011-103540


Type of Document Dissertation
Author Harun, Mahmud
Author's Email Address mharun@vt.edu
URN etd-11042011-103540
Title Modification of Large Reflector Antennas for Low Frequency Operation
Degree PhD
Department Electrical and Computer Engineering
Advisory Committee
Advisor Name Title
Ellingson, Steven W. Committee Chair
Manteghi, Majid Committee Member
Patterson, Cameron D. Committee Member
Safaai-Jazi, Ahmad Committee Member
Simonetti, John H. Committee Member
Keywords
  • Low Frequency Radio Astronomy
  • Reflector Antennas
  • Feeds
  • Sensitivity
Date of Defense 2011-11-28
Availability unrestricted
Abstract
Modifications of large reflector antennas, such that their observing capabilities are enhanced in the range of about 10-500~MHz without affecting operation of the pre-existing higher-frequency systems, are addressed in this dissertation. The major contributions of this dissertation can be divided into two parts: 1) designing new low frequency feeds, and 2) developing new analysis methodologies which, as opposed to traditional techniques, are suitable for analyzing low frequency systems.

First, we consider the performance of existing schemes that provide low frequency capability. Then, a new class of dipole-based low frequency feeds - namely, the ``distributed feed array'' - is designed to cover the frequency range of interest without affecting operation at higher frequencies. As an example, distributed feed arrays are designed for the Expanded Very Large Array (EVLA) to cover the range of 50-250~MHz. A method of moments (MoM) model of an EVLA antenna is developed for this purpose. The new design shows performance comparable to the existing 4 m system on the EVLA in the range of 50-88~MHz, and introduces observing capabilities in the range of 110-250~MHz (currently not covered by the EVLA). Moreover, the blockage presented to the existing EVLA L-band system is reduced significantly when the existing 4 m system is replaced by the proposed system.

At low frequencies, external noise can be a significant or dominant contribution to the total noise of the system. This, combined with mutual coupling between the array elements of the distributed feed array, makes it difficult to predict the sensitivity of these systems. This dissertation describes a system model and procedure for estimating the system equivalent flux density (SEFD) - a useful and meaningful metric of the sensitivity of a radio telescope - that accounts for these issues.

We consider the efficiency of methods other than MoM - in particular, Physical Optics (PO), Uniform Geometrical Theory of Diffraction (UTD), and hybrid methods - for accelerated computation at low frequencies. A method for estimating the blockage presented by low frequency systems to the pre-existing higher-frequency systems is also described.

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