Green Bank, WV


October 22, 2001

TO: R. Fisher, J. Ford, F. Ghigo, R. Lacasse,  C. Niday, T. Weadon
FROM: R. Lacasse
SUBJ.: Minutes of Interference Protection Group 10-16-01 Meeting

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

Minutes of Previous Meeting

Interference Reports
The first observations with the 600 MHz receiver were performed in September.  Using just a spectrum analyzer, many unwanted signals that were way above (20 to 30 db) the noise in the bandpass were detected.  Most signals have been matched with Broadcast TV allocations.  A few have been matched with cable TV allocations.   It was noted that although the local cable company is now using fiber on their main trunk, there is still a lot of coax in the distribution system.  Furthermore, it was noted that there are federal regulations in place regarding leakage from cable distribution systems.

Wideband interference has also been noticed near the Dr. John Sharp's office.

Interference has been noted on the 40 ft. telescope when the GBT is moving at a high rate.

Review of Action Items:

  1. End-of-year purchases (all)

Fisher has purchased two antenna positioners, a lightning detector, and two RS232 fiber modems.   A communication service monitor was purchased; this is for maintenance of the site radio system.  A 1 - 18 GHz horn antenna and a set of hand-held EMI probes were also purchased.  There was no progress on buying a spectrum analyzer with frequency coverage to 10 GHz to allow monitoring of any part of the GBT IF band from the control room.
  2. Site-wide interference suppression policy - position  paper (John, Rick)
Comments from Jewell were reviewed by some members of the IPG.  Jewell wants to expand the scope of the IPG to include external interference as well.  Jewell's comments will be incorporated into a revised document by Ford, and will be sent back to Jewell for approval.
    3.  300 MHz spectrum survey (Rick)
Norrod has requested that the IPG survey the 300 MHz band so that appropriate filters may be purchased for the 300 MHz receiver now being designed.  This band is bound to be filled with locally generated interference from PCs and other digital equipment.  It was noted that the process of migrating computers that are required 24 hours a day (e.g. servers) to shielded rooms and enclosures is making pretty good progress
    4. RFI considerations for new Education Center (John, Frank)
No news.  Fords stated that, to the best of his knowledge, all of our interference related requests have been incorporated into the building's plans.

    5. Use of the GBT warehouse by Operations (John)

As reported last month, Bignell wrote a memo detailing Telescope Operation's plans to use the GBT warehouse as an operations base for the telescope mechanics.  During the past month, Ford drafted  a reply to this memo and circulated it to the IPG group members for comment.  The memo was then sent to the site director to assist him in making a decision with regards to the use of the GBT warehouse.
    6. RFI suppression of GBT equipment   (all)
Progress was made on the servo system length transducers.  Additional shielding and filtering was added in the wire termination box.  This reduced RFI to levels that were undetectable with the anechoic chamber equipment.  Many of the parts needed to filter and shield this subsystem are on order.

After the addition of a corona detector, Weather Station 1 was retested in the anechoic chamber.  No RFI was measured.

   7. Monitor station developments (Rick)
Efforts to interface the Advantest spectrum analyzer to the monitor station PC have been unsuccessful.  The Advantest spectrum analyzer uses an old GPIB interface specification which is incompatible with the new controller. Our efforts will be concentrated on GPIB control of newly purchased analyzers, one of which should arrive soon.
   8. Lab RFI suppression efforts (Rick)
Fibers have been laid in the lab to enable the computer network to to run on fiber instead of copper.  Recently, the connectorization of these fibers was completed.  Critical computers which must be up 24/7 are being moved to shielded rooms.
 Next Meeting:  November 20, 2001