Joachim Köppen Kiel Dec 2022

• Parameters of Aperture Antennas computes the gain, half-power beam width, effective area and other parameters of circular paraboloidal antennas, as well as the antenna temperature and signal to noise ratio for a source of given radio flux and with specified system temperature.
• Antenna Patterns are computed for square and circular aperture antennas, illuminated either uniformly or according to some laws. This is done for a point source, which reveals the theoretical pattern, as well as for extended sources of specified angular diameter and with a few simple surface brightness distributions.
• Performance of Parabolic Dish Antennas is computed for illumination by various feeds. As in FeedPatt by W1GHZ, the feed pattern, the antenna efficiency, spillover losses and illumination losses, and the resulting antenna radiation pattern are calculated for dishes with specified f/D ratio of focal length and diameter.
• The Beam shape factor: The Half-Power Beam Width (HPBW) of the antenna's main lobe is influenced by the illumination of the reflector dish by the feed antenna. When the illumination is tapered to the outer edge of the reflector - to e.g. -10 dB of its intensity towards the dish centre - the main lobe becomes slightly wider, as the strength of sidelobes and spill-over is reduced. The true HPBW can be computed from HPBW = WF(taper) λ/Diameter The plot below shows the dependence of WF(taper) on the edge taper for several different illumination functions:
  
  Quite interestingly, for tapers between 0 and about -15 dB, the actual shape of the illumination function is unimportant. Thus, one gets the same results for various illuminations, as long as the intensity ratio at edge and centre is as specified.
  The ratio b = WF(taper)/WF₀ with WF₀ = 58.95701° for the uniformly illuminated circular dish, is called the beam shape factor. For the quadratic illumination profile Baars (2007; The Paraboloidal Reflector Antenna in Radio Astronomy and Communication, Springer) gives this analytic fit: b = 1.269 + t*(-0.566 + t*(0.534 + t*(-0.208))) with t = 10**(taper/20) and taper in dB. The results of Hansen (1976; A One-Parameter Circular Aperture Distribution with Narrow Beamwidth and Low Sidelobes, IEEE Transactions, Ant.and Prop., p.477) can be fit with (and taper in dB): b = 1.029 - 0.01195*taper
• Antenna Spill-Over computes from measurements of the sky noise at various elevations the system temperature of the radio telescope and the zenith temperature for the thermal emission of the Earth atmosphere. It then allows the user to add spill-over noise to account for the enhanced sky noise measured at elevations close to the zeinth. In this way, one can measure the spill-over contribution to the system noise.
• Measures of the noise at Sun Moon & Ground along with the empty sky gives three estimates for the system temperature. Significant differences between the values are a sign for errors in some of the measured values, which indicates that the measuring methods or/and equipment need revision or improvement. If the three values agree with each other, they very likely indicate the true noise figure of the receiving system of antenna and receiver. This 'overall' noise figure includes also how much noise from the surroundings is picked up by the antenna's side and rear lobes. It thus gives an indication for the true efficiency of the antenna. Knowing this better helps in optimizing the antenna system.

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last update: Dec. 2022 J.Köppen