Beam Widths vs lambda. All rcvrs, 2004

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.
 

• Fig 1: The hpbw/lambda. This should be a constant. Colors differentiate receivers. The scatter at the higher frequencies is partly coming from the source sizes. Many of the sources are 15 to 20 asecs which can add 15 to 20% to the hpbw at xband. The alfa hpbw's include all of the pixels (central pixel and outer 6) so there is more scatter in the hpbw than lbw.
• Fig 2:  The hpbw is converted to diameter using diam=1.02*lambda/hpbw (1.02 is the constant for uniform illumination). The median value from each frequency in fig 1 was used for these plots. The dashed line is a diameter of 225 meters. The 50cm (green 610 rcvr) and 90 cm (black 327 rcvr) sit above the other receivers. These two receivers use dipole feeds. The other receivers have horns.
• Fig 3: Blowup showing xband thru lband. The curvature in a receiver is caused by the horns illumination pattern of the tertiary vs frequency. The light blue sbn and red cbh both have smaller diameters. This is because these two receivers are under illuminating the tertiary. This gives them the lowest system temperatures while sacrificing some gain from the under illumination. 
• Fig 4: Blowup showing xband thru cband. The cband hi receiver has a horn illumination that is falling off with frequency. This causes the diameter to decrease at lower wavelengths. The xband horn has a flat response over the 8500-9200 Mhz of the measurements.

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