OVLBI EARTH STATION PROJECT REQUIREMENTS FOR FEEDS AND OPTICS ON THE 45FT ANTENNA IN GREEN BANK Larry R. D'Addario 90/09/07 1.0 Frequency Coverage It is necessary to feed the antenna so as to put coincident beams on the sky at any combination of frequencies in the Space Research allocations between 7 GHz and 18 GHz. These allocations are: Frequencies Direction Space Research Allocation 7.190 - 7.235 GHz uplink only primary 8.450 - 8.500 downlink only primary 13.400 -15.350 both directions secondary The situation in the 12 to 15.5 GHz region is a bit complicated, as is illustrated in Figure 1. Excerpts from the Radio Regulations are shown in Figure 2. It will be necessary to use circular polarization in each band. For convenience, we would like to have both senses of polarization available at all frequencies, but in fact we will only use one in each subband; preliminary selection of the polarization sense is LCP for all downlinks and RCP for all uplinks. Although we want coverage of all the allocated frequencies to allow for future work, the specific missions for which support is funded will use the following, which therefore have the highest priority for good performance: 7.200 GHz: Radioastron CW uplink, LCP. 8.472 GHz: Radioastron CW downlink, RCP. 13.400 GHz: VSOP CW uplink (preliminary), polarization TBD. 15.050 GHz: VSOP data downlink, pol TBD, 150 MHz bandwidth. 15.063 GHz: Radioastron data downlink, RCP, 150 MHz. 2.0 Optical Constraints The antenna can support either prime focus or cassegrain optics. The basic optical dimensions are: Primary diameter D 540 in (13.7 m) Focal length f 200 in (5.08 m) [f/D=0.37] Clear access at vertex 16 in (0.81 m) radius Inner panels stop at 24 in (1.22 m) radius Blockage from focal package 54 in (1.37 m) radius In principle, a subreflector up to 1.37 m radius (108 inch diameter) could be installed, but it would be heavy and expensive. A diameter of 1.0 m is 25 wavelengths at the lowest supported frequency, so we will assume this subreflector size for any cassegrain design. A prime focus design is slightly preferred because the antenna already has good access to the prime focus, along with most of the required cabling. But a cassegrain design will be selected if there is a significant performance advantage. Both types should be studied. 3.0 Performance Required The highest priority of the design is to maximize the receiving sensitivity (gain-to-system temperature ratio) at 15 GHz. Cryogenically cooled receivers will be used, and as much as possible of the input circuit will be cooled (including at least the polarizer). A receiver temperature of about 20 K is expected at this frequency. Operation at elevations from 5 deg to 90 deg is planned, and it can be assumed that the sky temperature is 5 K at 90 deg and 50 K at 5 deg, for a system temperature less spillover of 25 to 70 K. For receiving at 8.5 GHz, a separate cryogenically cooled receiver will be used, but the spacecraft signals are expected to be strong (see OVLBI-ES No. 2), so some compromise of efficiency is allowable. For transmitting at 7.2 GHz and 13.4 GHz, gain should be maximized and spillover is unimportant. But performance can be compromised in favor of 15 GHz sensitivity. At each frequency, polarization loss must be held to 0.5 dB mzximum, with a goal of 0.2 dB. The feed system must provide a minimum of four I/O ports: Port Polarization Optimized at 7-8 GHz uplink input LCP 7.20 GHz 7-8 GHz downlink output RCP 8.47 13-15 GHz uplink input LCP 13.4 13-15 GHz downlink output RCP 15.05 Isolation between any two ports must be 15 dB minimum, perferably more than 20 dB, at least at the optimized frequencies, and preferably across the bands.