Plans for new prime focus receiver systems are less well defined. The VLA's current low frequency capability consists of a fixed 300-340 MHz band and a dismountable, trial 73.4 - 74.2 MHz band. Both use crossed, on-axis dipole feeds with the (curved) subreflector as a backplane - a very low efficiency arrangement. The scientific potential of these lower frequency systems is high, so it is desirable to replace both systems with an arrangement which would permit continuous frequency coverage below 1 GHz from the prime focus with higher efficiency if this can be done without degrading antenna performance at high frequencies.
Figure 2.3: A sketch of the modified quadrupod and rotating feed assembly at the prime focus. The horizontal quadrupod legs would be replaced with narrow tubes, allowing the subreflector to be rotated away from the prime focus about a vertical axis and exchanged for any of four potential UHF feeds (shown in outline).
The current plan is to replace each of the horizontal quadrupod legs with a pair of thinner tubes which would permit rotation of the subreflector between them. Three or more low frequency feeds could then be mounted as part of the entire rotating package to permit excellent low frequency coverage. One of these could, for example, be a broad-band UHF (150 to 1000 MHz) system. A suggested receiver suite is given in Table 2.2. A sketch of the proposed new feed-rotation system is shown in Figure 2.3
|U1||700 - 1100||1.57|