Attendees: J. Acree, C. Beaudet, R. Fisher, R. McCullough, C. Niday, D. Wirt
Rick reported on the 28.75 Hz RFI that has been problematic for pulsar observations. This problem is usually reported when automated pulsar searches are conducted using the BCPM . The RFI seems to come and go and sometimes is very strong. It is believed the problem is a result of the 28.75 Hz sidebands on the maser reference signals from the timing center. Rick noted that the timing center provides a frequency reference to all the LOs, so all the LOs are affected - per Mike Stennes. The receiver group has agreed to work this problem as the situation warrants and direct IPG involvement is not required at this time.
The LO receiver module is a rich
source of RFI per IPG testing. Chuck has
taken on the job of working it over so he provided some background on his analysis
of it and the planned RFI retrofit. He
noted that the module requires so many sources to operate that it would be
difficult to test in the anechoic chamber as the various sources would likely
generate as much RFI as the LO receiver and would be difficult to
differentiate. Thus, he conducted
pinpoint RFI testing in his lab using a test loop and spectrum analyzer.
RFI from the module extended to 4 GHz. The major source of the RFI is the
synch/divide circuit, which phase locks the 5 and 10 MHz reference outputs and
the 1 pps output with the 500 MHz “round trip” timing
signal. As a part of the retrofit, a PC
board will be fabricated to replace an existing
Jeff provided some background on the Nextel Partners issue. He explained that in an effort to gain control of a license application backlog in 2001, the FCC hired a contractor to process applications. Since the contractor was unaware of or decided to ignore NRQZ coordination requirements, Nextel Partners ended up with approximately 34 licensed sites inside the NRQZ that were not coordinated with the NRAO. We are now working with Nextel Partners to clean up the mess. Jeff noted that another issue complicating the process is inaccuracy of coordinates for tower locations. Many of the erroneous coordinates are based on 2C surveys. Chuck noted that this might be due to the type of radio service that the surveyor typically deals with because there are different FCC accuracy requirements for different radio services.
Denise indicated that Wes would be inspecting 19 of the affected sites in late June. The intent is for him to inspect all the sites where the main beam power exceeds the NRQZ limit. He will verify coordinates and site configuration based on engineering data submitted to the NRAO. Upon successful confirmation of site configuration, the bulk of the coordination effort should be complete with the applications becoming a formality at that point. Denise noted that she had received and processed two more revised applications from the Nextel Partners backlog in June.
Chris Clark requested to have his plasma ball tested for RFI emissions. This is a typical example of how looks can be deceiving: By the arcing inherent to the plasma ball operation, all assumed it would be a strong source of RFI; but in fact, no emissions above 30 MHz were seen in the anechoic chamber. In the office, Carla and Jeff noticed some interference on an AM radio, but nothing that should impact observers was found. It was noted that requests like this bring to light the fact that some staff members are sensitive to RFI concerns and do not want to cause interference that would be harmful to the observatory. Thanks Chris!
Due to a belief that the 2 MHz comb that has been showing up in PF1-340 data is GBT self RFI, some SE testing was conducted on the receiver room. The intent was to see if the PF1 feeds are being properly protected from RFI sources inside the receiver room. Of particular interest was the integrity of the turret gasket. Maintenance has not been performed in approximately five years on this gasket. On the roof of the receiver room, four points directly above the gasket and four points away from it were tested. No significant leakage was found to be associated with the turret gasket, and the receiver room SE was found to be about 60 dB with a few localized exceptions. The Q-band receiver tested poorly and needs work. Carla will follow up on this with Galen when the Q-band receiver is removed in late June. The conduit to the blower was also suspect, but it appears that the poor SE near it is most likely from the conduit bulkhead, not the actual conduit assembly. The plate covering the opening for the future receiver also tested poorly. The receiver room door is not making sufficient contact to the RFI gasket; light can be seen at the bottom of the door opening when the door is closed. A plate to plate interface on the PF1 bulkhead is also questionable. These will be further investigated and mitigated as the situation warrants.
This item will be reported on next meeting.
In the absence of Rich Lacasse, information regarding the latest problem with feed arm servo filters was solicited. Carla reported that water appears to be entering the filter enclosure via one or more of the cables and corrosion of a few filters is already taking place. It is uncertain how the water is breaching the cable, but it a damaged jacket on one or more cables may be to blame.
Jeff was approached by Jerry Turner about re-establishing the radio controlled airplane club on site. Jeff requested that members provide feedback as to how the IPG should approach this. The following are examples of some of the questions that were raised: Should we allow radio controlled airplanes to be operated on site? If so, where? Should they be asked to sign an agreement similar to the Hams? Should a minimum acceptable distance from the GBT be established? The group was asked to consider these questions and provide feedback to Jeff.
Randy reported on the temperature sensor boxes being implemented as part of PTCS. The full measure of RFI control will be used. Sensors are placed in 2x2 boxes with expensive Spectrum Control filters being used for inputs/outputs. A prototype is on the GBT and is working extremely well. Jeff noted that the temperature sensor boxes, including the integrated filters, are a very elegant design. This illustrates one advantages of incorporating RFI control in the early stages of design.