MEMORANDUM                                                                                                                                  June 1, 1999
 

TO: R. Fisher, J. Ford, F. Ghigo, R. Lacasse,  C. Niday, T. Weadon
FROM: R. Lacasse
SUBJ.: Minutes of Interference Protection Group 06/01/99 Meeting
 

In attendance: Fisher, Ford, Lacasse, Niday
 

Minutes of Previous Meeting

    Approved.
 

Call Outs for Interference

    Both Niday and Lacasse reported interference from Iridium satellites.  In a VLBI run, the first LO had to be moved to take advantage of a 40 MHz band pass filter which centered the 32 MHz observing band in the filter and the Iridium signal on the skirt of the filter.

     Fisher reported interference at 9200 MHz, while observing at 9400 MHz.  Since the interference did not hamper the observing, it was not pursued.
 

Action Item Review

1 - Measurement of OVLBI transmission field strengths at the GBT

     The OVLBI system transmits in two bands, 7 GHz and 15 GHz.  To test the 7 GHz field strength, the IPG needs an amplifier (ours goes to 3 GHz).  The IPG does not own any equipment to test the 15 GHz signal.  However, some equipment could be borrowed for this test.

     Niday will try to assemble a 7 GHz test system in the coming month.  Some preliminary tests can then be made.
 

2 - Progress on RFI Suppression from equipment that has been measured in the anechoic chamber

     The laser ranging equipment was tested after the last IPG meeting.  The tests indicated that additional shielding is required.  Ford will make recommendations to Parker on ways to shield and filter the equipment.

     The fiber optic E-stop system was also tested.  The receiver tested clean.  The transmitter emitted radiation at 170 MHz.  As tested,  the system consisted only of bare boards.  Packaging the system to meet spec should not be difficult.

     RFI measurements of most of the equipment destined for the GBT receiver room have been made.  These results will be tabulated and reviewed in light of the measured attenuation of the receiver building.  Thus, systems needing additional work will be identified.

     Norrod is doing some integrated testing of the GBT system.  Presently, he is searching for self-generated RFI using the Digital Continuum Receiver.  Some types of interference would be more easily found using a spectrometer.  This will no doubt be done when the spectrometer or spectral processor is available.
 

3 - Site Radio System

     On the paperwork side, both frequency coordination and licensing need to be completed.  On the hardware side, transmitter equipment is being rack-mounted.  The location of an antenna on the water tower is still in question.  Most portables have been tested.  Completion of a demonstration system is imminent.
 

4 - Monitor station and anechoic chamber development

     New poles for the antennas for the monitor station will be installed shortly.

     Weadon and Lacasse need to test the continuum detector system ASAP.
 

5 - Microwave Oven Enclosure

     A commercial enclosure was purchased ($1500) from EMCOR. It was received recently and found to be poorly constructed. It has been returned to the vendor for repair. Ford will follow-up with the vendor.
 

6 - Shielded Room Entrances

     No progress.  Fisher needs dimensions on the absorber.
 

7 - RFI considerations for the new Education Center

     No progress.  Project is stalled for lack of funds.

8 - Use of Interference Reporting System

     Balser has used the system to document interference at the 140 ft.  He has also been informed of the new interference call-out order (Fisher, Lacasse, Niday).
 

Agenda items  9 to 14 were not discussed.
 

Next Meeting: June 22, 0800.