With the completion of the 1.4 GHz band upgrade in 1994, ground pickup at low elevations became a significant problem. The 1.4 GHz feed should be redesigned to eliminate spillover problems. There is also keen interest in observations at lower frequencies in the 1.4 GHz band. The extent to which the 1.4 GHz band may be extended and improved without eliminating or seriously compromising other systems on the feed ring (e.g., the 2.4 GHz band system that is also scientifically attractive) or at the prime focus must be considered carefully.
The best way to improve performance in the 1.4 GHz band alone would be to install a larger subreflector, similar to that used for the VLBA. This could provide good performance at a lower frequency for the same size feed mouth as at present. But this option is expensive and introduces compatibility problems for all feeds, especially at the prime focus (see Sec.5.2.3). In principle, the 1.4 GHz band can be extended down to 1.2 GHz with a new high-performance feed at the Cassegrain focus, illuminating the current subreflector. This approach would however require a long, geometrically awkward feed with an expensive and heavy lens, without which the feed would extend through the vertex room floor. A solid dielectric lens would weigh about a couple of tons; a compound lens such as that on the present feed would not be broad-band, though it is unclear how far such a design could be pushed. It will be important to assess how the performance of any proposed system degrades below 1.2 GHz, as reasonable performance down to about 1050 MHz might reduce the need for the most problematic prime-focus system (800-1200 MHz),