NATIONAL RADIO ASTRONOMY OBSERVATORY Green Bank, WV MEMORANDUM May 13, 1997 TO: R. Fisher, R. Lacasse, M. McKinnon, C. Niday, R. Norrod, T. Weadon FROM: R. Lacasse SUBJ: Minutes of Interference Protection Group 5/13/97 Meeting In attendance: Fisher, Lacasse, McKinnon, Niday, and Weadon RF: Changes to minutes of previous meeting? RL: They were corrected to reflect Niday's and Norrod's comments, but distributed with an incorrect meeting date. They will be reissued with the correct meeting date. Progress on Action Items: - Interference reports: MM: No official reports. However, I visited the 140' during Zaven's observing run and noted broadband interference coming from the North. Zaven noted that it was worse than in the past, but did not want to stop observing to track it down. CN: What frequency? MM: I noted only the total power as displayed on the strip chart recorder, which showed huge swings in total power. RF: Did it appear man-made? MM: Yes. CN: Not much to add to the memo on the transmitter in the new auditorium. The transmitter has been left disconnected. Fleming said he'd get with the contractor to try to get it fixed. The radiation exhibited broadband spurs to 1700MHz; the fundamental is at 72.9MHz. RF: We are usually concerned about radiation above 100MHz; however, there is a radio astronomy band at 74MHz and Bill Erikson has observed in that vicinity. CN: Several things could be done to reduce the EMI from the transmitter system. The synthesizer is on an unshielded PC board - lots of room for improvement there. Filters could be added to restrict radiation to the fundamental. The Americans With Disabilities Act may mandate the transmitter capability in the auditorium, so we may have to fix it rather than leave it permanently disconnected. RL: In light of other things we need to do, this little project is probably best deferred. RL: This brings up the issue of monitoring below 100MHz. Do we need continuous coverage from 30MHz to 100MHz? Do we need a carefully calibrated system for our measurements in this range? RF: Monitoring a few selected frequencies from 30MHz to 100MHz will probably suffice. With some knowledge of the equipment under test, we can probably select frequencies to measure where we expect trouble. A dipole is about the only reasonable antenna to use in this frequency range. Calibration is not necessary; just go by the theoretical value. A worthwhile experiment to do would be to tune a dipole to a CW tone, and then see how fast the signal drops off as you de-tune the antenna. CN: Have borrowed a telescoping dipole antenna from Sizemore that sounds suitable. A new stand for it, which would allow the measurement of vertical polarization would be nice. RF: Agreed. One band we should surely check is the radio astronomy band at 74MHz. For the type of measurements now being made, it is not necessary to test two polarizations; it is extremely unlikely that the antenna in the range would be in a polarization null. - EMI measurements of workstations RL: Many devices have been measured. These are documented in a three ring binder which is kept in my office. Chuck and I have worked together on commercial equipment and Chuck and the cognizant engineer have worked on NRAO- built equipment. Getting the cognizant engineer in the loop at the measurement stage has several advantages. Almost all equipment measured is significantly over limits we would like to observe. RL: I have spoken with Brandt about shielding workstations, since he has had some experience in this area. He recommended we get in touch with Sun Federal division. CN: Dreher, of the SETI group, has told me that there are businesses that specialize in repackaging computers to meet EMI requirements. Contact him to get specifics. RL: We can spend a lot of time shielding our equipment, however, there are lots of other PC's in the community which can be a source of trouble. The elementary/middle school is of particular concern due to its proximity to the GBT. RF: It may well be possible to work with the school, and get them to turn off their computers when they are not in use at night and on the weekends. CN: Aren't there provisions in the Quiet Zone that can be helpful here? MM: The quiet zone rules apply to fixed transmitters only. There is a state law (House Bill No. 2) enacts zoning restrictions to govern the use of electrical equipment near the observatory. Also, we may be able to enlist the help of the school children by asking them to help the observatory by turning off the computers at the school at night. TW: Reducing the EMI from racks destined for the GBT receiver room will have a significant time and dollar impact on the project. Manpower will be available in July to work on shielding; we should have all devices measured before then. RF: After a representative number of racks have been measured, a meeting should be held between people who know the interference numbers and people who control the project dollars, to discuss approaches to solving this problem. ****Action item for Weadon: Tell the IPG group when a representative sample of racks have been measured, so that a meeting can be scheduled. RL: One big unknown in the equation is the shielding effectiveness of the receiver room, where much of this equipment is housed. It will be difficult to measure due to the fact that it is not in its final configuration, e.g, the shielded door is not installed, and there are several holes in the wall covered only by plywood. RF: McKinnon and I will go look over the room and then discuss what we find with Lacasse. - EMI testing of the GBT Feed Arm Servo RL: This is on hold, since the system is presently inoperable. - Indoor range cable entry TW: Holstine has decided to buy a transformer to get 208V/3- phase to the chamber. CN: Conduit for 208 has been run into a closet off the main hall on the first floor of the new building. The bulkhead panel is in, without the 37-pin Bendix connector which is still on back-order. RF: Is all cabling into the chamber going through the bulkhead so that the doors can be closed during testing? CN: Yes. - GBT RFI limits RF: Circulated a draft of a document, "RFI Radiation Limits in the Vicinity of the GBT" for review by members of the IPG. - Indoor Test Range configuration RL: Circulated a block diagram of a strawman design for the indoor test range. Would appreciate comments by next meeting. Also attached are specs on several items being considered for purchase, for review by the group. Niday has received an Advantest spectrum analyzer for evaluation. Interested members of the group should look the instrument over and be prepared to comment. - Storage for equipment RL: Cleaned off some space in the anechoic chamber control room. This should provide adequate storage for our immediate needs. CN: Fleming said that he has asked the contractor to move out of that room completely. TW: Plan to make more space available in the lab basement. - Next meeting: at 0800 on the same date as the next coordination meeting.