GREEN BANK INTERFERENCE PROTECTION GROUP

Minutes from the August 20, 2002 Meeting

1.         Jeff provided a high level status report on agenda items 1-3, 5, 6.

2.         Metrology System RFI Status - The metrology system is currently presumed to be noisy at all frequencies and is therefore powered down during all observations. The Metrology Group would like to have more flexibility on when the system can be operated, so Kim asked the RFI group to help develop an emissions profile for the system and some individual subsystems to help evaluate the feasability of operating “as needed”, for maintenance and development efforts. At a minimum, Kim would like to be able to leave the rubidium standard powered 24–7.

During the week of August 12, 2002, Carla, Randy, Zach, Steve and Kim did some emissions and shielding effectiveness testing pursuant to these goals. Randy provided the following summary on what was learned:

•          The metrology trailer has the potential to act as a shielded enclosure.

                        •          It’s current shielding effectiveness is 20-40 dB per IEEE-STD-299 testing.

                        •          It has the potential of providing 60-70 dB of shielding effectiveness by: changing the RFI gaskets on the doors, cleaning the doors, filtering the cable entry panel, and fixing the AC.

                        •          The cable entry panel is the area of lowest shielding effectiveness and is pointed in the direction of the GBT.

            •          If the rubidium standard is to be left on 24-7, it will be necessary to properly terminate the 100 MHz output and feedline when the system is not in use. The possibility of using a low-pass filter should also be explored. Rick noted that since the 100 MHz fundamental falls in the FM broadcast band, the IPG would be unlikely to object to it. The final configuration should be tested in the anechoic chamber to assure compliance with in-house limits.

            •          Other than 1.5 GHz, no emissions above 500 MHz were seen.

            •          Contrary to expectations, no emissions were seen at 3.0 GHz, 4.5 GHz, etc.

            •          It seems likely that all the equipment in the metrology trailer could be used as needed if the trailer’s shielding effectiveness is restored to 60 dB and the cable entry panel is properly filtered.

            •          Efforts to quiet the laser monuments and associated cabling are ongoing and appear to be on the right track.

            •          The coax network segment that connects the monuments may be particularly difficult to mitigate.

It was also noted that since the Metrology System is so close to the GBT, the Metrology Group should continue to coordinate system installs and modifications with the IPG.

3.         Device Measurement Worksheet - The “Receiver RF Bandwidth” test setup parameter in the “Device Measurement Worksheet” should be set to the spectrum analyzer resolution bandwidth, but it should not be used in the limit calculations. The bandwidth used in the limit calculations should be based on the bandwidth used to generate the CCIR limit and should also be a function of whether the RFI is line or continuum. Rick and Randy will modify the spreadsheet to accommodate these concerns, and will send out a report explaining how to use the new format.

4.         The IPG is involved in too many projects to give a detailed report out on each project at each meeting. Rick suggested that a very high level (2-5 minute) report could be provided on each agenda item and one or two items could be chosen for detailed review. The item of focus could be rotated monthly, so that all the major IPG efforts could be discussed every three months or so. Let’s give it a try.                                

            The next IPG meeting will be held on 17 September, 2002.