|Submitter:||APERTIF team (Wim van Cappellen)|
|Description:|| In dense Focal Plane Arrays the signals received by the individual elements are combined to create compound beams. Recently a new series of measurements has been performed to characterize the radiation patterns of the individual elements of the DIGESTIF Focal Plane Array (FPA) and the compound beams in more detail. Above the first results are presented. The angular size and color scale of every panel in the left image is the same as the right image.|
The left image shows the measured patterns on the sky of all 56 horizontally polarized elements. The 5 MHz bandwidth measurement is centred around 1421.5 MHz and used the Cas A as source. The black dot in the centre indicates the position of the geometrical focus. As expected, the beam pattern of elements close to the focus has a circular main lobe and relatively low sidelobes. The pattern of elements further away from the focus is scanned and suffers from a distorted main lobe and higher sidelobes.
In the right image, the individual element beams are combined into a compound beam. The weights of the elements have been determined to achieve a maximum sensitivity in the on-axis direction (in the near future the shape of the offset compound beams will be revealed). The result is a nice smooth beam with much lower sidelobes than the individual element patterns. Comparison with a MFFE beam at this frequency learns that the half power beamwidth is slightly smaller. Although additional measurements need to be performed to confirm this, it suggests that the dish illumination is more uniform. This would also imply a better aperture efficiency of the system, which is exactly what Focal Plane Arrays are aiming at.