2019_programme: PLANE/CYLINDRICAL WAVE SYNTHESIS IN A SMALL TANK APPLIED TO FAR-FIELD BEAM PATTERN MEASUREMENT OF A 300KHZ SONAR TRANSDUCER

• Session: 02. Underwater acoustic calibration, testing, facilities and standards
  Organiser(s): Robinson Stephen, Humphrey Victor, Linné Markus, Évora Victor
• Lecture: PLANE/CYLINDRICAL WAVE SYNTHESIS IN A SMALL TANK APPLIED TO FAR-FIELD BEAM PATTERN MEASUREMENT OF A 300KHZ SONAR TRANSDUCER
  Paper ID: 779
  Author(s): Troiano Luigi
  Presenter: Troiano Luigi
  Presentation type: oral
  Abstract: It is highly desirable to infer far-field behaviour of a radio antenna or SONAR transducer, from near-field measurements. This approach allows the measurements to be made in a small tank (or anechoic chamber), under controlled conditions at a fraction of the cost of the equivalent free-field procedure. One of the techniques available is to generate a plane wave in order to create far-field conditions at the transducer under test (TUT). The equipment required is often referred to as a Plane Wave Converter in the field of radio communications, or the Trott Array in underwater acoustics. In essence, the plane wave is synthesised by a 2-D array of point sources which, acting together, generate a plane wave according to the Huygens Wavelet principle. The 2-D array can be reduced to a line-array for a TUT with large aspect ratio, in which case a cylindrical wave is generated. A common criterion in the design of the array is that the Huygens element spacing should be less than four fifths of the wavelength. Applying this principle to the beam pattern measurement of the of a 300kHz SONAR transducer of length 108 wavelengths, would require an array at least 135 Huygens elements wide, in order to generate a uniform wave of the same dimensions. This would clearly require a significant investment, and is the reason that this method is usually applied at lower frequencies. An alternative implementation that is studied in this article, is to scan a single Huygens element mechanically. This allows the array to be composed of any number of elements. The spherical wavelets thus generated, when summed at the receiver, are equivalent to simultaneous transmission with multiple sources, and therefore indistinguishable from a plane (or cylindrical) wave. The transducer tested has a far-field distance of 58m, but the measurements presented were taken at only 67cm from the transducer (well in the near-field). The resulting horizontal beam pattern compares favourably with the calculated far-field pattern for a rectangular aperture
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• Corresponding author: Mr Troiano Luigi
  Affiliation: CMRE
  Country: Italy
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