Picture Album of the
DEEP SPACE NETWORK

Microwave Holographic Images of the Beam-Waveguide Antenna at the Venus Research and Development Site

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These images of the first DSN 34-meter diameter beam-waveguide antenna were derived using the microwave holographic technique. The raw data used in this technique are the far-field amplitude and phase response patterns of the antenna. These were obtained by raster-scanning the antenna beam across a commercial geostationary satellite operating at Ku-band (12.1795 gigahertz). The very high lateral resolution is the result of a complex data array of 38,809 (197 x 197) samples of the far field of the antenna; the data array includes sampling to the 150th sidelobe, approximately. In this example, the nearly 40,000 samples were taken at a rate of just under one point per second, requiring nearly 12 hours of antenna scanning.

Surface current (intensity) and surface error (phase) holographic images confirm features of the antenna's mechanical and electromagnetic designs; these images also provide information that can be used to physically correct a broad range of possible design deficiencies. Irregularities in the shape of the reflecting surface and in the intensity (power) distribution are revealed in the "light" of the microwave illumination.

The holographic technique processes the raw data according to physical principles involving the Fourier transform. This transform is effectively the complex mathematical connection between the antenna's far-field spatial response (the data from the raster scan) and its complex aperture illumination (the desired output image information). A satisfying way to think of the physical process is that the microwave fields in the aperture plane (in an input/stimulus- output/response sense) uniquely define the far-field response of the antenna. After the far-field response from the measurement was obtained, and with the mathematical "connection" known, the images seen here were derived by using the inverse transform process.

For these images, the available signal-to-noise ratio allowed an accuracy in the axial measurement of the surface contour of better than one part in 200 of the observing wavelength. The lateral resolution on the surface of the antenna is determined by the size of the data array and is about 200 millimeters in this case. Thus, each of the 348 individual reflecting panels, as outlined in the surface-error map, is characterized by tens of accurate data cells, from which information to mechanically adjust each panel is derived and applied.

Earlier beam-waveguide antenna measurements showed that the initial surface adjustment was adequate for X-band, but the antenna would be largely unusable at the shorter Ka-band wavelength. However, based on three repeated applications of the holographic technique, a very smooth (0.38-millimeter root-mean-square) surface was achieved after careful adjustment of the 1716 trimming screws on the 348 reflecting panels. The surface achieved is capable of operating at Ka-band. Estimates based on holographic measurements before and after adjustment of the surface indicate that over 4 decibels of performance was gained at 32 gigahertz. Radiometric measurements of antenna efficiency (effective collecting area) at 32 gigahertz confirm the excellent results obtained through the application of microwave holography. Microwave holography has proven to be an invaluable tool in the development and maintenance of large, high-performance ground antennas.

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