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Microwave Holography of Radioastronomical Antennas

Microwave Holography of Radioastronomical Antennas

Principles

The problem of measuring the surface accuracy of a large reflector antenna is not simple. Usually it takes several days of painful measurements, and requires that the receiver block be removed and a particular instrument set (theodolite, metric gauges) placed in a difficult-to-access location.

Using the Fourier relationship connecting the shape (and phase) of the radiation pattern and the antenna illumination function, a measurement of the former can be used to infer the latter. The phase of the antenna illumination is proportionale to deformations of the reflector, and so these can be measured.

The main problem is that, although measuring the amplitude of the radiation pattern is simple, and can be done with the usual radioastronomical receivers, phase measurement is difficult. We have developed, in cooperation with IRAM and the University of Napoli, a technique to infer the phase from two (or more) out-of-focus measurements of the radiation pattern.

Results

We have performed three set of measures on the CNR-IRA 32-m radio telescope (image)
  • 1988: Measurements using Misell phase retrieval algorithm, and the 22 GHz Orion water vapor maser as a reference source.
  • 1990: Measurements using Misell phase retrieval algorithm, and a geostationary TV broadcast satellite iat 12 GHz as a reference source.
  • 1995: Measurements using a phase fitting algorithm and the 22 GHz W49N water vapor maser as a reference source.

    Phase image Amplitude image

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