Green Bank, WV


April 18, 2000

TO: J. Ford, F. Ghigo, R. Lacasse, C. Niday, T. Weadon
FROM: R. Fisher
SUBJ.: Minutes of Interference Protection Group 4/18/00 Meeting

In attendance: Fisher, Ford, Ghigo, Niday

Minutes of Previous Meeting

Review of action items:

1. Microwave oven in shielded box (John)

Modifications to the commercial shielded box to get better shielding at the seams are nearly complete in our shop, and the box should be ready for tests in the anechoic chamber in next couple of weeks.

2. RFI considerations for new Education Center (Frank)

A contractor has been selected for the new tour center building, but the details are not public yet. The contractor is fully aware of the shielded room requirements. Displays, which are being built by a separate contractor, are not expected to be completed until at least next year, but we will suggest that one or two prototypes be assembled for preliminary test in the anechoic chamber later this year.

3. Monitor station developments (Rick)

The three single-mode fiber connections between the RFI monitor station and the GBT control room are now complete. Roger Norrod, Bill Shank, and Ann Wester have helped with installing modems and making cable splices. The passband was found to be usable up to about 2.7 Ghz even though the modem spec is only for 1 GHz. A problem with intermittent gain dropouts on all channels remains to be tracked down. One of the 30-120 MHz Cas A log periodic dipole array antennas has been mounted above the platform for first tests.

4. Switching transient noise on GBT

There was some discussion of the best type of receiver for detecting switching transient noise in the GBT power system. Fred Crews has been using an AM broadcast band receiver. We generally agreed that a shortwave or 121.5 MHz emergency band receiver with an AM detector would be more appropriate. Chuck will look into what is available for this purpose.

5. Budget and manpower for the year (Rick, John)

In addition to the $30K of RE money for all signal processing projects mentioned at the last meeting, the IPG has $10K in the M&S budget this year. A little bit of this has already been spent. A bit of junior technician time is available for modest assembly projects on an occasional basis, and some engineering help may be freed for IPG projects later in the year as GBT commissioning gets underway. Galen Watts is spending a bit of time on DSP-related projects, and Steve White and Gary Anderson may have some free time later.

6. Progress on suppressing RFI from equipment that has been measured in the anechoic chamber

A few items, such as the GBT IF rack, are expected to be tested in the anechoic chamber in the near future. The spectral processor software needs to be updated to the new M&C revision. The fiber link to the GBT control room and, hence, to the spectral processor needs to be checked out. A 9 Mhz spectral feature in the empty anechoic chamber (possibly the smoke detector) and broadband noise around 150 MHz need to be tracked down before making further sensitive measurements with the spectral processor. The power supply for the test antenna preamp remains to be RFI suppressed.

7. Shielded room entrances (John, Rick)

No progress.

8. Disabled persons transport on site for tours

A note from Sue Ann indicates that no plans are currently being made for wheelchair access to the tour bus. This would be an expensive retrofit. The current fallback is to take the person around the site in their own (gasoline powered) vehicle. Rick will contact Mike Holstine to begin finding out how frequently a handicapped person is expected to want to take the tour and then discuss how to minimize any impact on the GBT's RFI environment.

9. Measurements of PC/Workstation in a shielded box (Chuck)

This is something to be done in the next few months.

10. Emissions from the new diesels on site

No discussion.

11. GBT RFI 'harmful levels' document and measurement procedures (Rick)

No discussion.

12. Measurement of OVLBI transmission field strengths at the GBT.

No discussion.

13. RFI survey plan for GBT commissioning (Rick)

No discussion.

14. Lab RFI suppression efforts (Rick)

It was generally agreed that shielded telephone cables would not be an effective use of funds. Filters on newly purchased network switches will be suggested to Ed and Gareth by John. Modular telephone jack RF filters are available at modest cost, and these will be considered in connection with shielding of the PBX and similar equipment. These filters are specified for suppression up to 30 MHz but may be effective at higher frequencies. This needs to be tested.

15. Interference at 1612.17 MHz

No discussion.

16. FCC Class B requirements and test procedures (Rick, Chuck)

Chuck uncovered some FCC information on Part 15 RFI measurement procedures. Only a few aspects of our current measurement procedure need to be changed to comply with these standards: conversion of field strengths to microvolts per meter, use of a turntable to find the peak emission from a computing device setup, and use of quasi-peak detection on our spectrum analyzer. Rick will set up a meeting with all persons interested in turntables for various purposes in the anechoic chamber to work toward a plan for designing and building an RFI measurement turntable. Chuck will look into quasi-peak detection on our spectrum analyzer.

Other items

Frank suggested that we use the 327 and 610 MHz receivers on 85-3 to look for interference from the GBT lasers while they are running laser tests over the next couple of weeks. Rick will pursue this with Dana Balser.

Next Meeting:  May 16, 2000