NATIONAL RADIO ASTRONOMY OBSERVATORY Green Bank, WV MEMORANDUM September 25, 1997 TO: R. Fisher, R. Lacasse, R. Norrod, M. McKinnon, C. Niday, T. Weadon FROM: R. Lacasse SUBJ: Minutes of Interference Protection Group 9/23/97 Meeting In attendance: Fisher, Lacasse, McKinnon, and Niday, Weadon Minutes There was general agreement that the minutes did not have to be as detailed as they have been recently. Minutes from the last meeting were approved. Meeting Time The meeting time was discussed. It was agreed to move the regular meeting time to 0800 on the Wednesday following the GBT coordination meetings. RFI Call-outs No call-outs for interference were experienced by the group since the last meeting. However, several reports have come in after the fact, indicating that the observer did not want to give up observing time to track down the interference. Balser is usually the first staff member to be made aware of interference problems. McKinnon agreed to ask Balser to keep up aware of what he is hearing. Butler Burton saw broadband interference, hundreds of KHz wide sweeping through his passband during a recent 21cm run on the 140'. There was some interference reported, after the fact, during a VLBI run at OH (around 1600 MHz). The most probable cause was Glonass. It was also reported that Lockman did an all-sky survey of OH IR stars, mainly for the purpose of assessing the RFI environment at OH. He found that 3% of his data was corrupted. McKinnon will get more information on this test run; the percentage lost seems surprisingly low. The possibility of training the operators to run a program which provides Glonass satellite positions in real time was suggested. Also suggested was a the use of a database for cataloging instances of interference. GBT Equipment RFI Testing Equipment tested recently includes the Switching Signal Mux, the Sig/Cal Buffer and a power supply for OVLBI. Status of Equipment Purchase All information from vendors of our top three selections for spectrum analyzers have been received. There has been no time to collate this information and generate a purchase requisition. There was some discussion as to whether the lowest priced unit that "met spec" had to be purchased. The conclusion was that usually, this is not the case. Richard Fleming is the person who decides whether a justification for a more expensive, similar piece of equipment would satisfy an auditor. Budget There is $19K in RE equipment and $8K in an RFI account remaining for this year. Most, if not all, of this will be consumed by the new spectrum analyzer. A discussion of other items that might be purchased if money were found ensued. These items included: - An antenna to monitor the 30 - 100MHz frequency range. - A power meter. - A switch and noise source for switched power measurements with the spectral processor. - An RFI monitoring station. - A mobile cart for chasing interference within and around the lab. Interferometer Control Building Fisher reported on a conversation with Heatherly concerning plans to build a display for tourists at the Interferometer Control Building. She was made aware of the potential for interference to the GBT. The possibility of using the display to showcase NRAO's concern with interference was suggested. New Control Building It is anticipated that the proposed GBT control building will use windows with a gold film which will provide 20db of shielding in our frequency range of interest, and that some type of conductive cloth would also be placed over the windows to provide an additional 20 to 30 db of attenuation. Additional shielding will have to be provided for each piece of equipment. In this vein, a fine mesh screen suitable for use over a 17" monitor has been procured. Niday will finish plans for a metal box which will incorporate this and some power filtering connectors. An entire workstation will be placed within the box, and shielding effectiveness will be measured. Subsequently, the keyboard and mouse will be filtered and moved outside the box, and further measurements made. RFI Monitoring Station Fisher discussed a block diagram for a proposed RFI monitoring station. It would be housed in the 140 foot service tower, use multiple antennas for direction finding, and use the spectral processor as a back end. Report on Visit to ATNF Fisher reported that the Australia Telescope National Facility is experiencing serious problems with self generated RFI and is investigating shielding and filtering for some components. He has been put on their distribution list for RFI memos. URSI Link A link to a site, maintained by Don Backer for URSI Commission J, concerning RFI will be added to the IPG's web page.