Beam Widths vs lambda. All rcvrs, 2004

feb 2005

     The Half Power Beam Width (hpbw) is determined by the size of the dish and the  wavelength:

hpbw=K * lambda/Diameter

    K is a constant that varies with the edge taper. For uniform illumination on a spherical dish K=1.02. The AO dish optics is 213m by 237 m (700 by 776 feet). It has a geometric taper of about 9 db.

    When measuring the gain of the telescope, there can be errors in the cal values used and the flux of the sources. The beam width is not sensitive to errors in the cal or the source flux. It is affected by collimation errors, surface errors, and the size of the source (it should be small compared to the beam).

    Calibration data taken mar04 thru feb05 was used to plot the hpbw versus wavelength (.ps)  (.pdf) for all receivers. The value plotted is the average hpbw (the average of the major and minor axis). The zenith angle has been limited to 5 < za < 14 degrees.

The illumination pattern is elliptical. The gregorian optics is designed for a  700 x 776 feet (213 x 237 meters) illumination. The geometrical mean of this is  225 meters. The hpbw of the measured receivers fall close to this value. Those receivers that are larger are over illuminating while those that are below this may be under illuminating the optics. The tertiary skirt will be placed around the tertiary so that the tertiary spill over will see the sky rather than the 250+K of the radiation inside the dome. When this is installed we could open up the illumination of the sbn/cbh receivers without drastically affecting the system temperature. It should also help some of the receivers that over illuminate (although no so much with the lower frequencies since the sky temperature is picking up).
processing: x101/sysperf/misc/hpbw_04.pros