NATIONAL RADIO ASTRONOMY OBSERVATORY Green Bank, WV MEMORANDUM April 22, 1997 TO: R. Fisher, R. Lacasse, M. MkKinnon, C. Niday, R. Norrod, T. Weadon FROM: R. Lacasse SUBJ: Minutes of Interference Protection Group 4/22/97 Meeting In attendance: Fisher, Lacasse, McKinnon, Niday, Norrod, and Weadon RF: Changes to minutes of previous meeting? All: No. Progress on Action Items: - Interference reports: MM: Had a complaint from Ed Murphy about interference at 1421.58 MHz. Weadon and Niday tracked it down to the computer in the audio-visual system in the new auditorium. We may need to shield this rack. TW: This signal seemed different from a similar signal that Weadon and Norrod tried to find in January: it was less intermittent and about 2MHz higher. RL: Is Ed happy with environment right now? MM: Yes. RF: Are there lessons to be learned? MM: Needed the 140' to get an azimuth on this interference. It would be better if this were not the case. This puts some pressure on getting the monitoring station built. The fact that Ed was willing to take some of his observing time to track this signal was critical. Also, as reported last meeting, we really need a monitoring station to check for transmitters which violate their license restrictions. CN: What about the present monitoring station? MM: It is not as good as it could be. CN: For monitoring it is quite good, given the available equipment. RF: It needs some additions. In particular, it should be accessible from the 140' control room, and it should be interfaced to the spectral processor. It may be worth moving it to the 140'. CN: Most of the operation of the station is coordinated by a PC. It may be possible to remote the station through the PC. MM: Another interference problem is broadband interference at 600MHz. It comes on at 6:08AM every day and shuts off at 6:36AM every day. Have asked for ideas, and things like automatic coffee pots have been suggested. The interference started at 6:08AM both when we were on standard time and daylight savings time. MM: During the last meeting, we asked why we received so few call-outs for interference. I went to the 140' during observations at OH of the comet. They were getting clobbered by GLONASS. RN: Was it hurting their data? MM: Don't know. RN: Since they were looking for narrow band emission, they were probably OK so long as nothing was saturating. - Discussion of Policy for Use of Radios on Site TW: RSi uses radios often. RN: This is not a permanent thing. Concerned more about routine operation. MM: Radios would be really useful for communication on the GBT. In cases where personnel safety is an issue, there is no question that radios should be used. Have to be concerned about interfering with other telescopes. CN: Common bands where communication equipment is available: 30 - 50 MHz (LO-band VHF) 150-174 MHz (HI-band VHF) 220-222 MHz (hard to get equipment) 450-470 MHz (used by RSi) 806-866 MHz MM: Pulsar observing is usually done in the 800 to 820 MHz band. CN: FCC specs require that the second harmonic be at least 60dBc down. LO-band radios have problems with building penetration. High band and UHF have better penetration in structures like the GBT. The safety issue is certainly significant. Communication on the 20M was inadequate recently. MM: The benefits of radios are obvious. Limitations on their use should be strictly enforced. TW: If the operator were to hand them out they could be well controlled. MM: In addition, they would be another way of knowing who is on the structure. RN: We have had surprisingly few problems attributed to RSi's use of radios on site. We should probably test these for harmonics. TW: Some NRAO personnel presently use 460 MHz radios for communications on-site. This does not seem right. Also the ban against microwave ovens on-site on the one hand, and allowing the use of ham radios on site on the other does not seem right. CN: A radio coordinator would be required if many people were allowed to use radios. RL: We could get into an endless testing debacle if everyone bought their own radios, and we saw a need to test them all. We should qualify a certain model and use this exclusively. RN: Another application of radios is the safety of individuals, like telescope operators, who work alone. Some observatories provide these people with a small transmitter that they clip to their belt. When keyed, the transmitter causes a phone to dial a sequence of numbers and play a pre-recorded message. Several phone numbers can be programmed and the phone goes through the list until it gets an answer. CN: There are several manufacturers of these types of devices. RF: Should we discuss our ideas with divisions heads soon? RN: Not yet. Just want to get the thinking started. RL: Radios are useful, but they should not be considered substitutes for existing systems. For example, the 140' has an excellent intercom to the service tower. Something similar should be provided at the GBT. RN: Phones and intercoms are planned. When Gerald Lawrence arrives, this is one job he will be asked to do. RF: Others should be aware of our thinking, and we need to be aware of their needs. **Action item for Fisher: To summarize the group's thinking on constraints that should be placed on the use of radios on site. - Storage for equipment No progress - Cable entry into anechoic chamber control room. CN: The bulk head is complete. Some cables still need to be built and tested. Not sure about the status of the 37 pin D connector. - 208VAC to chamber. CN: A receptacle and conduit have been installed in the chamber. A filter has been installed above the chamber. Plant maintenance is looking into how to feed the filter. - Testing of Fiber Link and Feed Arm Servo RL: Construction of link continues. Bandwidth is limited to 2GHZ. We will live with this for now. MM: The spectral processor is free this week to test this link. During the following 2 weeks, pulsar observations are scheduled on the spectral processor. TW: The SIG/REF signals are available in the lab. Two direct runs from the 140' were made. One goes to the mock-up room, and the other to the anechoic chamber control room. Receivers and transmitters are available everywhere. RL: There may still be a splice required to get to from the "splice room" to the control room. - RFI Test of Other Equipment RL: Tests of the workstation Rigel will have to be repeated since we found that amplifiers were saturated during the previous tests. Tests of the PC "Spica" were also recently completed. RN: The Sun monitor emitted less RFI than the Gateway. We were told that Sun builds for the Swedish market, which has tougher specs for terminals. We should try to shield the CPU to get a feel for what other components are doing. - Tektronix EMI Course RF: Presented a brochure from a Tektronix EMI course, and asked if it might be useful. RL: Have looked at the syllabus of some of these courses and they are not very advanced. CN: Have some experience with Tek. They generally seem to do a good job. - Concept for Monitor Cart and Anechoic Chamber Test System RL: Would like to have three backends, the spectral processor, a spectrum analyzer, and a power meter on line simultaneously, fed by a broadband resistive power splitter with buffered outputs for isolation. Need several front ends which include an antenna and matched low noise amplifier to cover our band of interest. At the 140' plan to use the VLBI patch panel to get signals within the frequency range of the spectral processor. **Action Item for Lacasse: to produce a block diagram of the above by the next meeting. TW: Recent experience has shown that the monitor cart should be equipped with good "air" wheels, and some type of cover for the equipment. - Next meeting: at 0800 on the same date as the next coordination meeting.