A new method for holographic measurement of microwave antenna radiation patterns

Leach, Mark (2005) A new method for holographic measurement of microwave antenna radiation patterns. Doctoral thesis, Northumbria University.

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Abstract

The rate at which new communications technologies are being developed has been immense and will continue to intensify for the foreseeable future. This evolution is fuelled by the desire to meet the wants and needs of the global community, by developing devices able to offer ever-increasing functionality, with greater complexity. To achieve this designers are forced to move to higher and higher frequencies.
The antenna, as one of the fundamental building blocks of any radiated wave system, and as such, must develop along with the evolution of the communication system be it
for, mobile, satellite or point to point systems. Antenna designers need to be able to test antenna, to ensure they exhibit the characteristics to which they were designed. Antenna test becomes progressively more difficult and costly as both, the operating frequency and the size of the antenna increase, especially for the measurement of the
antenna Far-Field radiation pattern. Either the distance over which the measurement must be made becomes unfeasibly large or expensive measurement equipment is required to attain the phase component of the antenna field, where traditional methods for measuring close to the antenna are applied.
Techniques have been developed to eliminate the need for the expensive phase measurement at reduced distances. Specifically of interest in this thesis, is the optical
process of Off-Axis holography. The process allows phase information to be retained in a scalar measurement by use of a phase coherent known reference source. The reference desired reference source is a plane wave, which although possible at optical frequencies with the use of lasers is problematic at microwave frequencies. To date the plane wave characteristic required has been approximated using conventional radiating elements, which degrades the quality of the recorded holographic image.
This thesis proposes a novel implementation of the Off-Axis hologram system, for application in the microwave frequency region. The novel system developed here addresses the problem of generating the desired plane wave characteristic. The conventional radiating element used to provide the near plane wave reference is replaced by a synthetic equivalent, which allows the magnitude and phase of the reference beam to be directly controlled at every measurement location required.
Practical verification of the novel system proposed has been performed, with comparisons made between the results obtained from the novel technique and standard techniques used in industry. The comparisons show that the novel implementation is valid and able to provide good repeatable results.

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