Green Bank, WV


February 22, 2001

TO: R. Fisher, J. Ford, F. Ghigo, R. Lacasse,  C. Niday, T. Weadon
FROM: R. Lacasse
SUBJ.: Minutes of Interference Protection Group 02-20-01 Meeting

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

Minutes of Previous Meeting

Interference Reports
Tests at L-band using the GBT system showed quite a bit of interference across the band.  Some of this may be internally generated.  Some follow-up work is required before more definitive statements can be made.  See additional discussion in item 2 below.
Review of Action Items:

  1. 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.
  2. Recent RFI experience with the GBT   (all)
The interference at S-band is similar to that at L-band.  Fisher stated that this level of RFI makes the GBT incapable of doing research quality observations.  It is still believed that the Feed Arm Servo System is the first layer of the onion.  It was resolved that Niday, Weadon, and Lacasse start spending at least a half day per week locating and fixing RFI problems.  The use of the spectrometer for this purpose is a bit clumsy, so a decision was made to attempt to work with a spectrum analyzer and an antenna, and the new hand-held receiver.  Characterization of the Az/El servo in a similar way was deemed to be a high priority.

The group charged with commissioning the GBT found interference with the 800 MHz receiver in total power mode.  The interference had the appearance of shark fins, building up slowly over the course of a few seconds and then abruptly disappearing and were therefore dubbed "sharks."  The source of this interference was identified to be the thermostats in heaters located in some of the outbuildings on the construction site.  These were turned off, and the observations were much more fruitful.

   3. RFI considerations for new Education Center (John, Frank)
No news...
  4. Progress on suppressing RFI from equipment that has been measured in the anechoic chamber
Interference from the vibration monitors for the receivers was fixed and the monitors are now installed.  Several pieces of equipment were measured.  The operator's radio console was "pretty bad" and a sample fire alarm for the new education center was "terrible".  Quantitative reports are on file.  Ford noted that the architects for the new building are having a hard time hearing the message that we want a "low tech" building to avoid interference from numerous microprocessor controlled devices (e.g. toilet flushers and the like).
  5. Monitor station developments (Rick)
The 70 to 1000 MHz antenna has been received.  It will be mounted at the monitor station near the 45-ft and the 200-1000 MHz log-periodic antenna will be moved to atop the GBT along with L and S band horns on a rotator.  A gimballed mount and rotator still need to be designed for these.  A fiber transmitter and receiver for the "deer stand" monitor station are ready and will be installed when the weather breaks.
  6. Measurements of PC/Workstation in a shielded box (Chuck)
A high-clock-rate PC has been tested in this box and it works well.  No emissions whatsoever can be detected.  Plans call for parts of the PC (keyboard, mouse, etc.) to be moved outside the shielded box, and made to operate without generating interference.
Next Meeting:  March 20, 2001.