NATIONAL RADIO ASTRONOMY OBSERVATORY Green Bank, WV MEMORANDUM May 27, 1997 TO: R. Fisher, R. Lacasse, M. McKinnon, C. Niday, R. Norrod, T. Weadon FROM: R. Lacasse SUBJ: Minutes of Interference Protection Group 5/27/97 Meeting In attendance: Fisher, Lacasse, McKinnon, Niday, and Weadon RF: Changes to minutes of previous meeting? RL: They were corrected to reflect Niday's comment, and re- distributed Progress on Action Items: - Interference reports: MM: No call outs. Bania had some problems at 8.6GHz, but they turned out to be correlator problems. CN: Some interference was reported by Sizemore at 868MHz. It appeared to be coming from the lab, possibly from test equipment. It was transient in nature and could not be seen with a small ground plane antenna. - Tour of GBT receiver room. RF: Had a look at the GBT receiver room. Noted that the four input air ducts have honey-comb type filters. The implementation details of the RFI seal around the turret are presently undecided. The first try at this seal produced too much drag on the turret drive system. Most installed cables seem to be well filtered or shielded. It is not clear how NRAO installed cables will get shielded. There is no shielding goal in the spec for the room; expecting 60 to 80db is probably ambitious. The task of the IPG is to advise designers of equipment on how much shielding is required to meet the GBT performance goals. RL: People implementing the shielding would be interested in our guesstimates. TW: As the person responsible for much of the equipment in the room, I feel that some practical guidance would be helpful. RF: Really can't give any. I would be surprised to get 50db from the room. We should not rely on the room to provide all the shielding. - Indoor Range Instrumentation: CN: All filters in the new bulkhead are installed, except for the 37-pin D-connector, which is not in yet. A cable has been built and installed in the trough in the anechoic chamber for equipment requiring an MCB connection. Also the 208VAC outlet in the chamber has been connected to a source of 208V. Don't know if a transformer was ever installed, but it works! TW: Never received a bill for the transformer. RL: Are there comments on the block diagram of the proposed indoor range instrumentation design, distributed at the last meeting? RF: Looks like a reasonable starting point. TW: Plans on the computer system are a bit sketchy. RL: The computer system is a low priority item; not much effort was put into the details. However, one important feature of the system is the specification of interfaces to the test equipment. RL: With regards to priority of purchasing equipment, going by what we've had to borrow from other groups so far is a good starting point. The spectrum analyzer is a very high priority item. Since this device is expensive, the group will be "stuck" with whatever it purchases for a long time. For this reason, I am in favor of buying a unit with a bandwidth of at least 6GHz; 3GHz may not be adequate coverage in a few years. Clock rates in digital and communications equipment will continue to rise. CN: Agree. List prices of such units priced to date fall into the $20 to $30K range. Have brought in an Advantest unit for evaluation. Although this demo unit has a bandwidth of 26GHz, it has all the same features as a 6GHz unit which we could purchase; the vendor did not have a demo 6GHz unit available. I would still like to get demo units in from IFR. Units that can run off batteries are especially attractive for use with the mobile cart. RL: What about using a UPS system? CN: This would be another potential source of noise. RF: Is there an accuracy penalty on the high frequency units? CN: No, since the high frequency capability is implemented with a mixer and oscillator. RF: The choice of 3GHz as the highest frequency this group should worry about is a bit arbitrary. Due to the importance of L-band 2GHz is a definite requirement. MM: In recent measurements of the shielding effectiveness of a cage for a microwave oven, capability to 6GHz would have been useful. RL: What other items, besides those shown on the block diagram will need some money this year? RF: The monitor station will need a few K. RL: We need to proceed cautiously to not overspend our account. We will have a much better handle on this when the spectrum analyzer is purchased. One item that really needs to be bought ASAP is an amplifier to drive the fiber link. The link will likely be required for testing in the near future, and, without an appropriate amp, it would not be available. RF: Go ahead MM: OK with me too. - Discussion of GBT RFI limits document RF: I received several good comments from Rich on the preliminary document distributed at the last meeting. His main point was that the meeting minutes have a wide distribution and that we must be sensitive that anything we publish may be held against us. RL: In particular, it appears that we may be holding ourselves to a lesser standard than we do others. In addition, the document considers only interference to the GBT and not to other telescopes on-site. TW: Agree with Rich. Since this is an official group of the observatory, the meeting minutes and other statements have a lot of weight. CN: Service providers would complain if the observatory's standards for itself were less stringent than those for others. RF: The CCIR limits which we have been trying to meet are more stringent than those we impose on licensed service providers. RL: So it is correct to say that if we live by your proposed document, we are requiring more of ourselves than of licensed service providers? RF: Absolutely. For our purposes, the CCIR limits are a good goal. This document relaxes some of the limits imposed by the CCIR. The CCIR limits are general purpose; when the particular aspects of the GBT are factored in, the requirements may be relaxed in some areas. This intent here is cost saving; in some cases, getting the extra attenuation required by the CCIR may be quite expensive. The document is tailored to the needs of the GBT. Although we generally do a straight line interpolation between bands protected by the CCIR limits, these limits do not apply at all frequencies, but only in designated radio astronomy bands. The graphs in this report should probably be modified by placing notches in the radio astronomy bands, such that the GBT requirements match the CCIR limits in these bands. TW: Should be concerned about harmonics that fall into the radio astronomy bands? MM: There are FCC rules that cover such things. CN: Also the antennas tend to act as bandpass filters. - Next meeting: at 0800 on the same date as the next coordination meeting.