Green Bank, WV


October 17,  2000
TO: R. Fisher, J. Ford, F. Ghigo, R. Lacasse,  C. Niday, T. Weadon
FROM: R. Lacasse
SUBJ.: Minutes of Interference Protection Group 10-17-00 Meeting

In attendance: Fisher,  Ford, Ghigo, Lacasse, Niday, Weadon

Minutes of Previous Meeting

Interference Reports
The 85-3 telescope was used recently by a visiting group of teachers.  The telescope was used in total power mode with a center frequency of 610 MHz and a bandwidth of 40 MHz.  The observed fluctuations were in the 10 Jy range, whereas theoretically for a system temp of about 100 K an 85-foot telescope should have an rms noise of less than 10 mJy.
Review of Action Items:

  1. Site-wide interference suppression policy - position  paper (John, Rick)

John reported that comments received on the recently distributed draft caused him to re-think the approach to this paper.  Additional recommendations for improving the document were suggested during the meeting as well.  Two new drafts will be issued shortly.  These will overlap in content considerably.  One will be aimed at management and will include a section on resource allocation.  A second one will be for general consumption and will emphasize the reasons for requiring low interference levels on site.
  2. Microwave oven in shielded box (Chuck)
The box was tested on September 20 and a report was distributed.  At 2450 MHz, the box gives 34.8 db of attenuation, putting it close to our limits for levels emitted from the shielded room in the Jansky lab.  Ford will try to squeeze a few more db out of it.  It was noted that the oven could interfere with the IF systems in the shielded area of the lab.  This should be tested as soon as possible.
  3. RFI considerations for new Education Center (John, Frank)
Discussion deferred to the end of the meeting and to a meeting later in the morning.
  4. Hand-held HF/VHF radio for power switching RFI detection    (Chuck)
Chuck reported that he had identified a suitable receiver for a reasonable price.  The receiver is the ICOM R-10 which costs $339.95.  There are more expensive radios with additional features, but it is not clear that the features are needed.  A decision was made to purchase the receiver, and to revisit the issue when and if it becomes clear that additional features are needed.
  5. Measurements of PC/Workstation in a shielded box (Chuck)
No progress.
  6. Shielded room entrances (John, Rick)
Ford reported that a ray-tracing study of the baffled entrances for the new education center had been completed.  It appears that the baffles will provide the required 60 db of attenuation.  It was pointed out that diffraction around the corners of the baffles may diminish their effectiveness.  Since the walls are quite thick, this may not be much of an issue.  Fisher presented some data on emission limits from the Jansky lab for three PCs that have been measured in the anechoic chamber.  From the data, one could conclude that it could be possible to meet emission limits with a bit less attenuation.  This suggests a possible trade-off in the education center: shield a bigger area to a less stringent spec.

In preparation for the meeting on shielding the new Education Center, later in the day, some information was passed along to the group by Fisher and Ford.  In particular, there is concern over pieces of electronic equipment that will be in the center but outside the shielded rooms.  Some examples include a cash register, distributed HVAC controls, and interactive controls throughout the auditorium.  The question of whether we can really meet our own standard in existing buildings was also brought up.  It was pointed out that some effort is being expended in this direction (e.g., moving the network switch to the shielded room), but that it was not clear whether everything could eventually be clearned up.