1 VSRT M EMO #041 MASSACHUSETTS INSTITUTE OF TECHNOLOGY HAYSTACK OBSERVATORY WESTFORD, MASSACHUSETTS 01886 June 19, 2008 Telephone : 781-981-5407 Fax : 781-981-0590 To: VSRT Group From: Alan E.E. Rogers Subject: Satellite TV dish offset geometry The Ku band TV dishes are offset parabolas with the following parameters: Direct TV dish with 3 LNBFs Focal distance 13 d Offset angle 18 Ú Height 18 d Width 21 d f /D 0.65 Effective aperture 18 d Winegard DS-4047 Focal distance 10.6 d Offset angle 24 Ú Height 20.9 d Width 19.2 d f /D 0.59 Effective aperture 18.1 d For both these dishes the bottom of the dish is close to the vertex of the parabola as illustrated in figure 1. Typical feeds have a diameter of 1.5 wavelengths which illuminates the reflector with about 10 dB taper at the edge as illustrated in figure 2.
With a focal length to effective diameter ( f /D ratios) of about 0.60 to 0.65 the aperture efficiency is about 60-70%. The offset illumination eliminates the feed blockage present in a symmetric illumination of a parabolic dish. For the ozone observations the performance could be improved by reducing the cspillover d to improve the beam ... more. less.
efficiency at the cost of reduced aperture efficiency.<br><br> This Winegard dish is cdeeper d with a lower f /D ratio than the Direct TV dish and is expected to lower the noise temperature by reducing the spillover. Overall performance on the ozone line is expected to be about 1 dB by using the Winegard dish instead of the Direct TV dish. 2 Figure 1.<br><br> Figure 2.