MEMORANDUM

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

Minutes of Previous Meeting

Approved.

Broad band, "shark-fin" interference was detected recently with the GBT.  It was traced to a heater in a shed near the 140 ft.

It was found that the GBT HVAC control unit emits 43 MHz strongly enough to interfere with the operation of the site radio.  Similarly, operation of the radios is compromised near the laser ranger control boxes.

1. RFI suppression of GBT equipment   (all)

Feed Arm Servo

Parts to shield and filter all feed arm servo components except the motors are on order or in the shop.  Work on the motors will probably begin this week.  There was some discussion about whether work on the non-motor cables should start in early January when all parts are received.  This may leave cables exposed in the receiver room for a considerable amount of time, depending on the pace of the motor work.  A decision was made to await preliminary results on the motors before starting.  Work on shielding and filtering the transducer end of the magnetostrictive transducers will be done as soon as parts are received from the shop and weather permits.  Ford will check on RFI-related items in the shop.

Main Drives

Recently, Maddalena and Heatherly noted that more RFI was seen at the 40-ft. with the GBT moving and pointed away from the 40-ft. than with the GBT moving and pointed towards the 40-ft.  Apparently the GBT surface was shielding the 40-ft. from the GBT Az/El drives.  Fisher will attempt to characterize the RFI from the drives with the Monitor Station.

Blue Warning Lights

McCullough tested both the blue and amber warning lights now in use around the GBT.  The blue lights employ both slip rings and motor brushes, and radiate quite a bit.  The amber lights use only an AC motor and no RFI was detected from them.  Results were written up.  Telescope operations will purchase new blue lights that use technology similar to the amber lights.

Lasers

A design to quench RFI from both the laser control panel and the laser ranger itself has been completed.  Many parts  to retrofit two units are on order or in the shop.  If these work as well as expected, the remainder of the units will be retrofitted in the same way.   A source for EMI absorber for the laser ranger is being sought.  A goal is to have the two units complete and tested by the end of January.

An enclosure is being designed for each laser station.  The IPG will try to stay in the loop on the detailed design to avoid having to retrofit these as well.

Lightning Detector Receiver

This unit will be tested today.  It will be located outside the GBT control room and will communicate with a computer inside the shielded control room via a fiber optic link.  Ford noted that the fiber transceivers may be noisy and that we may be able to design a better unit ourselves if necessary.

General Discussion

Recent experience has shown that observers and operators are unsure about whom to contact when interference is found.  Ford has let it be known that people should contact him with these types of problems.  Once Jeff Acree starts work, he will establish a contact roster.

Many people on site are purchasing and installing items without regard for the RFI they may cause.  A way needs to be found to raise the awareness of all those on site.  It is hoped that the site-wide interference policy will help in this regard.  Some type of RFI requirements on purchase orders may help.  New personnel, who will be starting soon, may also help by being available to test and fix items as they are received.

Jeff Acree is slated to start in late December,  2001.  Carla Beaudet will start in early January.  A coop student from the University of Cincinnati will be arriving in January to work with Fisher on his MRI.

The IPG has completed its review of the revised paper.  It will be presented to Jewell and Acree in the near future.  Shortly thereafter, a short summary of the relevant points of the policy will be distributed to the staff.

This item is in Jewell's court.

Ford is coordinating with Butch Wirt and will make some inspections when electrical work begins.  There was some concern expressed that the Education Center may interfere with the operation of the antenna range by causing unwanted reflections.  At this time there are no plans to do anything about this.

Efforts to interface the Advantest spectrum analyzer to the monitor station PC have been successful.  The next step is for Rosalind to develop a monitor program which will use the monitor station to observe the spectrum and catalog the results in a database.  Both the 200 to 1000 MHz and 1200 to 1700 MHz antennas will be used in this survey.

Anish wants to observe in this band, so the priority of doing this survey has risen.  The receiver will be installed on the telescope on Dec. 26, and remain on the telescope for two days.  Part of these days will be dedicated to the spectrum survey.

There have been reports that flat-panel display emit considerably less RFI than the CRT types.  Niday plans to test one soon.

Niday went to the NRAO offices in Charlottesville last week to do an RFI survey.  Measurements were made at the NRAO Ivy Road and Edgemont road facilities as well as a Caruthers Hall which houses the UVa computing center.  This was motivated by the suspicion that RFI is hampering the testing of low noise amplifiers in the lab.  Another motivation was the planned construction of a new electronics lab at the Edgemont Road facility.  The possibility of having a shielded lab is being considered, and Niday's measurements will be folded into this consideration.   Generally the RFI environment below 1 GHz was similar to that in Green Bank, but it was much worse than Green Bank's above that frequency.  Signals were seen up to 7 GHz.  Niday will generate a detailed report.