19 November 2002 GBIPG Meeting



1.         Discussion of 2002 RFI status briefing (All)

2.         Update on GBT receiver room filter integration (Lacasse)

3.         Discussion of 22 November, 2002 LIGO design review (Acree)

4.         ALMA/Chajnator, Chile RFI survey status (Beaudet)

5.         Astronomy Visitor Education Center (AVEC) update                                               

- Test results and mitigation efforts for HVAC components and light fixtures (Niday)

            - HVAC shielded enclosures and filters (Ford)

            - Computer room SE test results (Beaudet)

            - RF baffle issues (Acree)




1.         It was agreed that the RFI status briefing for Phil and Richard should be very high level and one hour or less. John noted that a key point that needs to be made is that the current IPG focus is not on the mitigation of existing RFI sources. The group’s current emphasis is to provide maximum resistance to new RFI proliferation. Some examples given of new RFI proliferation risks are as follows:

            - New Astronomy Visitor Education Center (AVEC)

            - New Dormitory

            - Changes in FCC philosophy and rules (geographic area licensing for example)

            - Local industrial development


It was noted that high-impact sources of existing RFI such as the GBT receiver room and the Jansky lab shielded room are being addressed, but there are insufficient resources to work low-impact issues, such as the atmospheric phase interferometer, at this time. It was also noted that all confirmed sources of RFI need to be addressed and will be addressed once some of the larger issues are under control, hopefully by the spring of 03.

            There was some discussion on the intended audience for the briefing.


2.         Rich reported that the GBT receiver room filter integration is on hold until a cross-talk problem can be resolved. Further testing is needed to characterize the problem and develop a solution. It was noted that Randy should have some useful input.


3.         A brief discussion of the review documents provided by LIGO was held. Several IPG members indicated that they had provided comments to Mike Zucker and planned to participate in the 22 November, 2002 teleconference. It was agreed that the LIGO group has made significant progress on their plans to deal with RFI. Jeff noted that he may not be able to participate in the 22 November review due to New Dormitory and AVEC design reviews.

4.         Carla reported that all the required equipment, about 600 lbs, was on the way to Chile.


5.         Astronomy Visitor Education Center update


- Chuck reported that the large collection of AVEC HVAC components he tested offered a few surprises, but in general, the items he thought would be RFI noisy, were RFI noisy. An exception was the Invensys damper motors. They were a significant and unexpected source of RFI. He was successful in mitigating them using a filter box on the power and control leads and some copper tape on the housing. He also noted that the two can-type fluorescent light fixtures he tested were major sources of broadband hash to 500 MHz. He also tested two under the counter fixtures. One of the units passed, the other failed by a few dB.

- John reported that all the modified 48x36" and 24x16" Hoffman shielded enclosures have been delivered to the HVAC contractor. These are NEMA 4, chromated steel, 70 dB COTS shielded enclosures. He also noted that they are much cheaper than the stainless steel verison, but would need to be painted for outdoor use. We may have a few spare 24x16" boxes if anyone needs them. John also noted that even though we have been successful in some areas of the AVEC, RFI is still creeping in. Light dimmers, audio / video equipment, and the can-type fluorescent light fixtures are examples.

- Carla reported that the computer room SE testing went well. The contractors test methodology is similar to ours , but is geared toward pass-fail indications rather than exact performance data. The final results indicate SE > 70 dB at all points on the enclosure. During the testing some weak spots were found around HVAC air vents. This was due to using to using RFI gasket material that was too thin. The problem was quickly corrected by using thicker material. Carla noted that, aside from the RF baffles, the exhibit hall shield is being fabricated using the same materials and construction techniques as the computer room and has a reasonable chance at comparable performance. She is concerned about the many penetrations and multiple ground points in the exhibit hall ceiling. The 4" steel pipes that support the light grid are an example.

- A discussion was held on the design of the AVEC RF baffles. Jeff noted that the RF absorber material to be used in the baffles does not match the requirement and is not expected to meet spec. Jeff also reported on some SE testing that was conducted by NWSC on the materials that could be used to protect the microwave absorber cones. The data indicates that the ½" Acrylite is more reflective than sheet rock. This is a concern, since reflectivity of Acrylite will degrade baffle SE. It was noted that a transparent material is highly desirable, since we want visitors to be able to see the absorber cones. Jeff indicated that he will be keeping close tabs on the baffles as the AVEC construction proceeds.