NATIONAL RADIO ASTRONOMY OBSERVATORY Green Bank, WV MEMORANDUM December 14, 1998 TO: R. Fisher, J. Ford, F. Ghigo, R. Lacasse, R. Norrod, M. McKinnon, C. Niday, T. Weadon FROM: R. Lacasse SUBJ.: Minutes of Interference Protection Group 12/08/98 Meeting In attendance: Fisher, Ford, Ghigo, Lacasse, Niday, Weadon Minutes Approved. Call Outs Ghigo and Lacasse reported seeing interference while setting up for a VLBI run. The interference was between 1624 and 1626 MHz. Several members of the group identified this part of the spectrum as being polluted by Iridium. A birdie at 4880 was again seen with the C-band receiver on the 140 ft. The signal was phase locked to our maser, so must be internally generated! Lab RFI Survey Niday reported on his recent survey of the lab for RFI. A formal report will be generated at later. Some highlights follow: The worst offenders in the upstairs of the old wing included the network hub room and the student office. The shielding effect of a solid wood core door was estimated at 2 db (500 MHz) and 5 db (850 MHz). In the new wing of the lab, the worst offenders on the first floor were the LAN room (room 121) and the projection room in the auditorium. The characteristics of the RFI in the first floor of the old wing of the lab differed from most other places: in this area, many CW signals, along with their harmonics were seen. Analog phones were only seen while they were ringing; digital phones were detected below 400 MHz. The PBX room was quite bad, with the worst offender being the Meridian mail sub-system. Sometime the survey will be repeated to look for RFI at above 1 GHz. The group then discussed what could be done to improve the situation. No agreement was reached on whether anything could be done with the PBX, although several possibilites, such as grounding unused lines, filtering lines, and shielding parts of the system There was general agreement that network hubs are servers could live in shielded areas. The possibility of installing a sinuous entrance to the shielded control rooms on the second floor was discussed. Fisher stated that he would "warn Mike" (Holstine) that the IPG would experiment with shielding worst offenders as time allowed. Microwave Oven for Telescope Operators Telescope operators would like to have a microwave oven for cooking purposes. The IPG has been asked for an opinion on this matter. Measurements of a sample oven indicated that 92 DB of attenuation at S-band would be required to meet our limits, if the oven were installed in the lab. Various members expressed their opinions. Although no votes were taken, the majority opinion seemed to lean toward enforcing our limits by some means or another. There are several open questions. For instance - would a microwave oven in the shielded control rooms interfere with the equipment in the rooms? could a practical RFI enclosure be made and how would it hold up? - would sort of testing schedule would be required to guarantee that the oven stayed within limits? Next Meeting: Dec 22, 1998, at 0800