KFPA Beam Spacing

• K-band hexagonal feed spacing in the focal plane and on the sky:
  

• The above figure shows the hexgonal close-packed spacing used for the KFPA.
  • a) feed geometry
  • b) resulting beam pattern on the sky (I assume the indicated circles correspond to the fwhm of a K-band beam)
• The spacing between elements there is dictated by the outside diameter of the feed horns, which is 8.8cm, and results in a beam spacing of 2.3 to 3.4 half-power beamwidths for 18 to 26.5 GHz.
• The current KFPA is thus undersampled by a factor of ~ 36.
• A maximum feed-horn efficiency (illumination and spillover) of 68% is predicted for the on-axis feed and the efficiency is nearly the same for the next two rings consisting of 19 elements. The large receiver cabin turret holes are 36" in diameter. For the feeds at the edge of the mounting ring the aperture efficiency would drop to about 57%.
• Numerical values as a function of wavelength are given in this GBT K-band Beam Spacing document