Becker,, Robert O.; (1993) Wavefront quality in the anechoic chamber. Doctoral thesis (Ph.D.), University College London (United Kingdom).

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Abstract

The problem of non ideal conditions for antenna measurements in the anechoic chamber is addressed. The proposition of angular spectrum estimation via the electromagnetic field sampling at equally spaced positions smaller than the wavelength is examined. The Fast Fourier Transform (FFT) is examined as a tool for obtaining the angular spectrum from probing the field in the chamber. The angular superresolution technique provided by the Multiple Signal Classification (MUSIC) method is evaluated to be qualified in working out the angular spectrum too. Computer simulations with several signal combinations are analysed for both algorithms' performances. Restrictions existing with the interpretation of the FFT results in spatial spectrum analysis are reported and examined. Particularly its inability to place the spectrum peaks accurately, when not coincident with one of its discrete spectral harmonics, is addressed. FFT and MUSIC performances are compared by simulations and it is shown that the latter, presenting a higher resolution ability combined with better flexibility, is recommended in the anechoic chamber's spectrum investigation. FFT and MUSIC are analysed for two-dimensional angular spectrum estimation showing basically the same relative performance as for linear processing. Measurements collected in the E.E. Department's anechoic chamber are analysed with the aid of both FFT and MUSIC algorithms. Linear and planar array configurations are considered separately. Results of both linear and 2-D angular estimations show the basic angular spectrum pattern of a spherical wavefront along with side lobes located in the horizontal and vertical planes, with levels estimated about 35 dB below the main beam and with angles of arrival estimated around 60 degrees in the horizontal plane and 40 degrees in the vertical plane. Noise effects are investigated as a probable cause for the side lobes, however due to the stationary character of these side lobes this possibility is dismissed. The experimental set-up is described and some limitations are pointed out, which may affect the results in the vertical axis.

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