Green Bank, WV


March 23, 2001
TO: R. Fisher, J. Ford, F. Ghigo, R. Lacasse,  C. Niday, T. Weadon
FROM: R. Lacasse
SUBJ.: Minutes of Interference Protection Group 03-23-01 Meeting

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

Minutes of Previous Meeting

Interference Reports
Discussed with item 3 below.
Review of Action Items:

   1. Interference Manager position status (John)

The position has been approved.  Management is currently defining the detailed responsibilities of the position and where it fits in the chain of command.  The internal posting should appear soon.
  2. 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 version of the policy that will be distributed to the site.
   3. Recent RFI experience with the GBT   (all)
Measurements were made in and around the receiver room using both the spectrum analyzer and the ICOM receiver to try to isolate causes of interference within the Feed Arm Servo.  Unfortunately, it seems as if most components of this system are contributors, since the interference level worsens as more parts of the system are powered up.    It appears that filtering all lines leading out of the room is the most viable alternative.  Measurements were made to get an idea of how large a box could be built to accommodate the filters.  Data sheets from filter vendors are being accumulated.  Subsystems will be configured and tested individually in the anechoic chamber.  It was also found that the shields of cables leaving the servo system are not as well terminated as they could be.  This will be worked on in the near future.

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

The azimuth servo is still considered a potential offender, but no work has been done on it to date.

The interference that was dubbed "shark fins" was traced to some heaters in the antenna contractor's sheds.  These are now routinely turned off for observations.  However, similar, but much lower level, interference is still seen.  The monitor station, which is almost in an operational state, may be a useful tool in finding these.

Fisher requested that the dozen or so strongest signals in the receiver room, as observed with a local antenna and spectrum analyzer, be documented.  He will compare these with signals observed with the GBT receivers.

   4. RFI considerations for new Education Center (John, Frank)
A final design review is scheduled two weeks from now.  The IPG is discouraged that there has been no contact with the architect or RFI consultant to review preliminary plans.  Ford and Fisher will attempt to meet with Jewell today to express the group's concern.  Over the past 15 years, the site seems to have lost much of the culture which drove it minimize RFI.
  5. Progress on suppressing RFI from equipment that has been measured in the anechoic chamber
Most of the testing recently has concentrated on testing new items as opposed to items which have been previously measured and improved.  New items tested include a variety of fluorescent fixtures which are candidates for the new building.  A sonic wind speed monitor was re-tested and  found improved but still wanting.
   6. Monitor station developments (Rick)
White and Shank continue to make progress in outfitting the monitor station.  The control PC is being shielded. Cables at the station will be terminated when weather permits.  the station will included monitoring antennas from 200 to 1000 MHz and from 1.1 to 1.7 GHz.  The Advantest spectrum analyzer that was formerly used at the 140 ft.  will be refurbished (front panel keys don't work well) and used in 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.
   7. Measurements of PC/Workstation in a shielded box (Chuck)
No change.
    8. Microwave oven
The microwave oven for which the IPG designed an RFI enclosure is in regular use.  Some minor improvements to the enclosure are still possible, but are fairly low priority.
    Other Discussion
Niday is working with Anderson to assure that modifications to the anechoic chamber for the purpose of installing an feed measurement system will not interfere with the ability to use the chamber for RFI measurements.

The GBT spectrometer sampler has been identified as a strong source of 1.6 GHz interference.  Tests will be conducted to see if improving the cables which carry the signal decrease the level of RFI.  If so, all samplers will be retro-fitted in conjunction with the installation of improvements to the module's phase locked loop.

Next Meeting:  April 17, 2001.