Green Bank, WV


April 25, 2001
TO: R. Fisher, J. Ford, F. Ghigo, R. Lacasse,  C. Niday, T. Weadon
FROM: R. Lacasse
SUBJ.: Minutes of Interference Protection Group 04-24-01 Meeting

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

Minutes of Previous Meeting

Interference Reports
Review of Action Items:

   1. Interference Manager position status (John)

Two applications have been received.  John will conduct a preliminary phone interview soon.
  2. RFI considerations for new Education Center (John, Frank)
A meeting with the RF consultants and architect was held.  They were receptive to our input, and it should start to appear on drawings soon.  Additional help may be required from the IPG to review the baffle design.  The RF consultant is the same one who re-designed the shielded rooms in the new wing of the lab.  He did a good job on that project so we are optimistic about a good result on this project as well.

NRAO has committed to build shielded and filtered enclosures for several control systems to be used in the new building.

   3. Site-wide interference suppression policy - position  paper (John, Rick)
John and Rick have not received any feedback from Jewell or McKinnon yet.  This item will be brought to their attention again.  Feedback is required to give guidelines for  the paper to be distributed to the rest of the staff.  In the absence of formal guidelines, the spirit of the policy is presently being enforced to the extent possible.
   4. Recent RFI experience with the GBT   (all)
As reported previously, the GBT feed arm servo cabinet has been established as a strong source of interference.  Work is underway to limit these emissions to acceptable levels.  Tests were performed on a magnetostrictive transducer sub-system associated with this cabinet.  It was found that the sub-system as a whole radiated quite a bit.  Filters compatible with the signals in the transducer were specificed and purchased.  Modifications to the PC enclosure were designed.  The filters and modifications should allow the contribution of the two parts of the transducer (head and counter card) to be measured separately.  The ball is now in the machine shop's court.  Also associated with this cabinet are Kollmorgen motors and motor drivers.  Efforts to purchase a motor so that it can be tested are continuing; the factory has not been very forthcoming with information needed to purchase the motor.

The thermostat that has been added in the receiver room to protect the feed arm servo breaches the shielding of the system.  This is fairly straightforward to fix and will be done shortly.

An effort to identify the "top 10" RFI offenders in the receiver room has not been completed due to lack of time when the telescope has been at access position.

  5. Progress on suppressing RFI from equipment that has been measured in the anechoic chamber
A shielded/filtered rack was recently built to house one of the site's network switches.  Tests in the anechoic chamber showed no emissions from the rack when the doors were properly closed.  The same design will be used on a second switch rack.

The optical guide telescope for the GBT was successfully tested recently and will be re-tested when it returns from the paint shop.

Several fluorescent fixtures and associated ballasts have been tested.   A fixture with an RFI filter will be tested soon.

   6. Monitor station developments (Rick)
White and Shank continue to make progress in outfitting the monitor station.  The station will include monitoring antennas from 200 to 1000 MHz and from 1.2 to 1.7 GHz.  The Advantest spectrum analyzer that was formerly used at the 140 ft.  has been refurbished moved to the GBT control room to monitor the signals from the monitor station.  The analyzer will be interfaced to a PC to enable storage of the analyzer outputs and to make the data available on the web.
   7. Measurements of PC/Workstation in a shielded box (Chuck)
The shop is modifying the enclosure to facilitate the mounting of filtered connectors.  The enclosure will be used soon in conjunction with the testing of the magnetostrictive transducers mentioned in (4) above.

 Next Meeting:  May 15, 2001.