The Paraboloidal Reflector Antenna in Radio Astronomy and Communication: Theory and Practice (Astrophysics and Space Science Library)
Language: English
Pages: 253
ISBN: 0387697330
Format: PDF / Kindle (mobi) / ePub
Radio astronomers have developed techniques of calibration of large reflector antennas with radio astronomical methods, but these have not been comprehensively described. This text aims to fill this gap, taking a practical approach to the characterisation of antennas. All calculations and results in the form of tables and figures have been made with Mathematica by Wolfram Research. The reader can use the procedures for the implementation of his own input data.
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ephemerides readily available to overcome this handicap. Considerable effort has been devoted over the last 30 years to establish absolute brightness temperatures of the planets (Ulich et al, 1980). With care, the mm-wavelength brightness temperature of most of the planets can now be predicted with an absolute error of 5 to 10 percent over the entire mm- and submm- wavelength region. This is less accurate than the strong sources in the cm-range and improvements in the accuracy would be necessary
to achieve, for instance, the calibration goal for ALMA of five percent in flux density. A considerable part of this book is concerned with the methods for antenna characterisation with the aid of cosmic sources. In addition to the determination of the gain, we shall want to measure the beam shape and near sidelobe level. In general, the interaction between the beam with its angular structure and the radio source, which normally will have a finite angular extent and perhaps an irregular
ÅÅÅÅÅÅÅÅ ÅÅÅÅÅ = ÅÅÅÅxr = ÅÅÅÅÅÅÅÅ ÅrÅÅÅÅÅÅÅÅÅÅÅ . WL ê2 AC sin f We apply the sine-rule to the triangle ABC to obtain AB AC AC ÅÅÅÅÅÅÅÅ ÅÅÅÅÅÅÅÅÅÅÅÅ = ÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅ ÅÅÅÅÅÅÅÅ = ÅÅÅÅÅÅÅÅ sinHy-aL sinHpê2+aL cos ÅaÅÅÅÅ . Thus we get sinHy-aL W W r L y = ÅÅÅÅ2ÅFÅÅÅÅ r ÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅ ÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅ = ÅÅÅÅÅÅÅÅ ÅÅÅÅ H1 - tan a cot yL . AB cos a sin y 2 AB r Using some trigonometric relations and the fact that tan ÅÅÅÅy2Å = ÅÅÅÅ ÅÅÅÅ (Eq. 2.10a), we 2f obtain 2 WF y = ÅÅÅÅÅÅÅÅ ÅÅÅÅ 9r - f
parabola and hyperbola 2. Geometry of the Cassegrain reflector antenna 3. Geometry of the lateral defocus in the plane of feed translation. 4. Pathlength error for lateral defocus as function of aperture radius 5. Difference in the pathlength error: exact minus approximate 6. Geometry of the axial defocus 7. Pathlength difference for several approximations 15 16 22 24 24 25 28 ü Chapter 3 1. Geometry and coordinate systems for the paraboloidal reflector 2. Geometry of the aperture integration
it will be necessary to regularly check for the optimum focus position, both in the axial and lateral direction. This can be done with relative ease using a celestial radio source of angular size smaller than the HPBW of the antenna at the frequency of operation. For the determination of the axial focus we exploit the rather fast variation of the antenna gain with axial feed position as discussed in Chapter 4.3.1. We saw there (Eqs. (4.22-23) that for relatively small defocus the gain varies as
