Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.atnf.csiro.au/projects/askap/techdocs/JB_Doublet_Revisited_pdf.pdf Дата изменения: Wed Aug 30 04:26:38 2000 Дата индексирования: Sat Sep 6 22:17:29 2008 Кодировка: Поисковые слова: arp 220

The Cylindrical Reflector Doublet Antenna Revisited

John Bunton

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CSIRO Telecommunications and Industrial Physics, Australia

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Description
The cylindrical reflector doublet antenna achieves full sky coverage by tilting the reflector at a fixed angle and using a line feed to scan from the horizon to zenith. In azimuth the beam is positioned mechanically by rotating the whole antenna. In the Doublet proposed by James and Parfitt1 the two offset reflectors are mounted on a single backing structure. The use of offset reflectors eliminates blockage and enables the use of several offset feeds to provide both instantaneous azimuth coverage and full frequency coverage from 0.1 to above 10GHz. Dual Line feeds Elevation scanning. Dual Offset Cylindrical Reflectors

Wheel on Track Azimuth Rotator

A Doublet antenna Operational Advantages
· MULTIBEAMING - Elevation multi beam forming with array line feeds - Limited multiple beams in azimuth using several offset line feeds · ADAPTIVE NULL STEERING - Quality of cancellation proportion to degree of freedom - First line of defence against interference - Does not degrade Tsys · POSSIBLE SINGLE UNIT ARRAY STATION - Collecting area of 2000m2 practical
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James, G.M. & Parfitt, A.J., `A Low-Cost Cylindrical Reflector For The Square Kilometre Array', in Perspectives on Radio Astronomy: Technologies for Large Antenna Arrays, (eds.) Smolders &. van Haarlem, ASTRON,1999

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Astronomy Advantages
· WIDE BANDWIDTH CONSTANT COLLECTING AREA -High frequency to 10GHz and above -Low frequency 100MHz with 15m reflector · FULL SKY COVERAGE - Mechanical azimuth steering - Electronic scanning zenith to horizon · HIGH SURFACE BRIGHTNESS SENSITIVITY - Due to reduced shadowing, Antenna has low and constant profile, does not tilt Antennas can be closer together more than doubling the surface brightness sensitivity -Inter line feed correlation High isolation between line feeds allows cross correlations with baselines ~5 to ~70m · DUAL POLARISATION with good purity

Mechanical Advantages
· LOW · HIGH · HIGH COST No towers or counterweights Minimal backing structure with multipoint support Ease of mechanical maintenance, main mechanical components at ground level FREQUENCY SURFACE Easily achievable high surface accuracy, curvature in one direction Easy to align surface Reasonable feed costs at 10 GHz No gravity deflections WIND RESISTANCE

Limitations ­ not really
· LIMITED INSTANTANEOUS SKY VISIBILITY - But necessary for high frequency operation to limit feed numbers - Still have large Field-of-View but not whole sky - Instantaneous multiple beams possible over part sky. Up to 1000 deg2 · LARGE, MOVING PHYSICAL STRUCTURE - Must have movement for full sky visibility If high frequency operation desired - This design reduces movement to single Azimuth drive No other option simpler except phased arrays - Azimuth drive is the lowest cost option
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