NATIONAL RADIO ASTRONOMY OBSERVATORY Green Bank, WV MEMORANDUM Aug. 25, 1998 TO: R. Fisher, R. Lacasse, R. Norrod, M. McKinnon, C. Niday, T. Weadon FROM: R. Lacasse SUBJ.: Minutes of Interference Protection Group 8/25/98 Meeting In attendance: Fisher, Ford, Lacasse, McKinnon, Niday Minutes Approved. Call Outs McKinnon reported on several cases of interference. The first of these was from Ed Murphy, observing at L-band on the 140'. There were two spikes in his plots, one at 1419.86 MHz and another at 1419.99 MHz. The latter appeared in almost all the plots. They were quite low level, requiring many minutes of integration to detect. These were not identified. A second report was also originated by Murphy. Again spikes were noticed in his plots at 1382.4 MHz. Although the identity of the signals was not confirmed, it was hypothesized that they were probably third harmonics of the radios used by the GBT contractor. Finally, Minter complained of interference at 1624 MHz which was probably caused by the Iridium satellites. The signal strength was time variable, making it consistent with orbiting satellites. Lacasse reported that Minter had requested that the Glonass band be filtered out of observing passband. As presently configured, the L-band receiver cannot do this. The question of how to deal with the Iridium satellite constellation operationally has not yet been considered. Fisher noted that Arecibo is planning on using ITC filters. These are very high Q filters that are cooled. Possibly NRAO could piggy-back on this development. The Iridium passband is centered at 1624 MHz and appears as a 5 MHz wide pedestal. ****Action Item: Fisher to speak with Norrod about plans to deal with Iridium and Glonass. GBT Receiver Room Shielding Effectiveness Test McKinnon reported that considerable progress has been made towards testing the shielding effectiveness of the GBT receiver room. All cables in and out of the room have been removed and bulkhead openings have been shielded. The turret weather seal has been removed, and will be replaced by the RFI shield in the next few days. Niday is gathering equipment to perform the tests. The goal is to finish the tests by Sep. 15. A few suggestions were made concerning the test configuration. First, it might be wise to grind the paint off the door frame and treat it with conductive primer, just the same as the bulkheads. Second, using copper tape to insure a good seal on the door might be advisable. PC based Continuum Detector Lacasse reported that an active filter chip has been located to do the post-detection filtering. Also detectors have been located in several old chasses. ****Action item: Lacasse to locate and evaluate specs on the detectors. Ahead of the detector, there is a need for bandwidth defining filters, a local oscillator, etc. There is a need for an overall block diagram of the continuum detector system. ****Action item: Fisher to produce an overall block diagram of the continuum detector system. Green Bank Elementary/Middle School Ford brought in the architect's plans for an addition to the Green Bank Elementary/Middle school. Methods for mitigating the radiation towards the site from the new computer room were discussed. Shielding the room completely was considered impractical. At most 10 db could be gained from "half-way" measures, e.g, copper wall paper and a wooden door. Four measures were considered worth bringing to the attention of the school: 1 - requesting that twisted-shielded pairs be used for networking, 2 - installing snap-on toroids on the power cords of the computers, 3 - avoiding the use of noisy electronic ballasts in fluorescent light fixtures, 4 - requesting that computers be turned off at night. ****Action item: McKinnon to check to see how fluorescent fixtures were handled in the Jansky lab addition. Device Testing One of the new Gateway computers will be tested today. Lab Addition Shielded Doors Telescope operations has requested that one of the shielded doors in the control room area of the lab addition be removed. This would have the effect of turning two shielded enclosures into one larger one. No one voiced any objections. There has also been a request to install windows so people could see into the control rooms and easier-opening shielded doors. There was general agreement that the effect of leaving the doors open could be measured with the monitoring station when it becomes available. However, this approach to operations goes against the wishes of those at NRAO who originally specified the shielded rooms, and those who are trying to minimize interference to the telescopes. Interference Reporting System Modifications to the interference reporting system suggested by the IPG have been incorporated into the software. The time is ripe for training the 140' operators. ****Action item: Fisher to speak with Balser about initiating the training of 140' operators. Site Radios Niday reported that he is re-tuning the duplexer, and that crystals have been ordered for the demonstration system. A loaner repeater will be delivered shortly. Antennas for the system need to be ordered. PC Enclosure The shop is working on a power entry box for the enclosure. Next Meeting Sep. 8, 1998, at 0800