GREEN BANK INTERFERENCE PROTECTION GROUP
Minutes from the October 24, 2002 Meeting
1. Rick reported that the BYU students have completed the 3.6m RFI antenna installation by integrating the required satellite tracking hardware and software and the low noise amplifiers and 1.4 -1.6 GHz feed. They were able to track a Glonass satellite and had some success at cancelling Glonass RFI from a GBT observation. He noted that the narrow frequency coverage and the 20° minimum elevation in the eastern quadrant will limit the ability of the IPG to do useful terrestrial RFI work. In the future, we hope to develop additional feeds to provide coverage to 300 MHz. We also have some plans to address the azimuth limitation.
2. The portable RFI measurement system to be used for characterization of the spectrum at the ALMA site in Chajnator, Chile will be ready for shipment by the end of October, 2002. The system includes a HP8563E spectrum analyzer, Yaesu antenna positioner, and miscellaneous antennas and low-noise amplifiers. The spectrum analyzer will be controlled over it’s GPIB port by a notebook computer running an instrument control program Carla developed using Vee. Vee is a high-level, COTS instrument control language marketed by Agilent Technologies. Carla’s program allows spectral data to be stored, retrieved, analyzed and plotted. Additionally, it automatically records the center frequency and power level of signals above a user defined threshold, in a similar fashion to the Anritsu we use at the chamber. Carla also noted that Vee has GPIB panel drivers for most HP instruments and that computer control of the Yaesu antenna positioner and other serial and USB devices is also possible. It was noted that Vee should be useful in many future IPG efforts. Carla mentioned that for frequencies below 2 GHz, the amplifiers in the new setup are powered through “bias Ts” which eliminates the need for separate wires for DC power. The antenna positioner is a source of significant RFI, so it is likely that it will need to be powered down during data collection. If time permits, some mitigation may be attempted prior to the trip.
3. Wes noted that we currently do not have the time and resources to properly administer the quiet zone. We are keeping up with the paper chase side pretty well, but do not do enough follow-up compliance verification and routine monitoring. He sited some real life examples of this. John noted that the management of the NRQZ and the in-house RFI mitigation are different sides of the same coin and that one should not be ignored for the sake of the other. It was noted that we should try to get additional resources or redirect some of our existing resources to NRQZ administration. It was agreed that we cannot monitor and police the entire radio spectrum real time, but that if we are smart about it, we can do a respectable job.
4. Rich reported that Nathan and Tracy have 8 or 9 more power cables to the filter in the GBT receiver room. The power cable filters are being installed on a filter panel which has been temporarily mounted between a shielded rack and the exit hole for the wires. It will take about 32 hours to complete the power cable filter installation. The number of days this will equate to depends on the amount of access time they can get. Once the power cables have been completed, all the wiring will need to be disconnected from the filter panel, about 56 wires total, then the panel will be installed in an RFI tight enclosure and rewired. This may involve 3-4 days of telescope downtime. We hope to get the SE of the receiver room back to 50 + dB.
5. Chuck tested a rebuilt Varion vac ion pump with new and old controller cards and found the new setup to be RFI quiet with the exception of a weak emission at 43 MHz. The 43 MHz emission was a few dB lower on the old controller, but a 102 MHz emission was also seen on it.
6. John reported that RFI enclosures and filter panels are in the works for the new tour center HVAC system. Filters are on order. He also reported that other RFI sources, such as incandescent light dimmers, had worked their way into the building, in spite of recommendations against using them. It was agreed that the contractor team has not managed RFI concerns as well as they should have. It was agreed that it is likely that we will have to do our own shielding work in the future if we want it done correctly. The lessons learned from our experiences with the Jansky Lab shielded room overhaul and the NTC shielding are being captured in a new shielded enclosure specification. It will be distributed to other observatories and used for future NRAO GB requirements.
7. The possibility of screening in-house purchase orders for RFI risks was discussed. The group agreed that a single approval point should be identified, but at the same time, we don’t want to be a bottleneck. There was some preliminary discussion on what would and would not be screened and what the pass-fail criteria would be. Rick recommended that we collect and review a months worth of purchase orders to see what we are getting into and to assist in the development of evaluation criteria. This will be done.
8. It was also noted that off-site developers are not always aware of the need to coordinate with NRAO. It was agreed that participating in some county commission meetings might facilitate this. It was also noted that an occasional newspaper article may be helpful.