TO: R. Fisher, J. Ford, F. Ghigo, R. Lacasse, C. Niday, T.
FROM: R. Lacasse
SUBJ.: Minutes of Interference Protection Group 05-16-00 Meeting
In attendance: Fisher, Ford, Ghigo, Lacasse, Niday
Minutes of Previous Meeting
1. Microwave oven in shielded box (John)
Awaiting parts from shop. They should be ready at the end of May.2. RFI considerations for new Education Center (Frank)
An architect for the Education Center has been selected. Construction is slated to start early next year. Consequently, detailed plans will be drawn up in the second half of this year. The architect is aware of our needs for shielding. Ghigo will continue to serve as liaison for the IPG.3. Monitor station developments, summer student (Rick)
The company making displays for the tour center is also aware of our needs for RFI suppression. However, they anticipate that most of their designs will be in a shielded enclosure. NRAO needs to test prototypes and calculate interference at the GBT.
A short description and block diagram of the proposed design was distributed at the meeting. A summer student, who will work on antenna position controls, will be arriving shortly.4. Hand-held HF/VHF radio for power switching RFI detection (Niday)
A 3.6 meter dish is included in the monitor station mainly to measure out-of-band satellite interference. It can also be used for direction finding.
Niday reported on a few receivers manufactured by ICOM that could be used for detecting power switching. They have AM, FM and SSB capability, and are most probably computer controllable. The estimated cost is $300 to $400. Fisher stated that, to detect power switching, the AM mode, with as broad a band as possible, would be best.5. A turntable for the anechoic chamber (Rick)
Sizemore may have one of these radios. Possibly it can be borrowed for evaluation purposes.
Niday will look into less expensive short-wave radios for the same purpose.
Fisher distributed specifications for a turntable for the anechoic chamber to facilitate FCC Class B measurements. The need for another turntable in the anechoic chamber was also discussed. The second turntable would be for antenna measurements. The specifications for the two tables differ significantly, and it is practically impossible to make one table serve both purposes. The IPG will therefore pursue the building a a turntable to suit its needs only.6. Progress on suppressing RFI from equipment that has been measured in the anechoic chamber
The closed circuit TV camera has been modified since the last measurement and is ready for another round of measurements.
Anderson has requested that the control rack for the Prime Focus Receiver be tested.
The IF rack will be ready for testing soon. It consists mainly of filters and switches, and contains no mixers. It does include a noise source. Initially, it will be measured with a switching power supply. Then the supply will be replaced and the rack re-measured.
Some of the above testing would be facilitated by building an amplifier and filter assembly to feed the continuum detector. This will be moved up in priority.
Faint recombination lines at 610 MHz have been detected using the 85-3 antenna and the spectral processor. A significant amount of effort was required to reduce the interference level to the point where the observations could be made. Almost all equipment in the Interferometer Control Building (ICB) was turned off; the exceptions included Preston's workstation and the ethernet fiber interface box. Also the lasers and associated equipment in the laser lab were powered down. Still a fair amount of interference can be seen. The prime suspect is Preston's workstation.
There are plans afoot to turn the ICB into a tourist attraction with a complete simulated control room. The IPG re-stated its objections to "active" displays which will emit significant RFI very close to the GBT. Ford will continue to monitor progress on this project and speak for the interests of the IPG.
Ghigo noted that Backer's program on 85-3 can be interrupted for scientifically important reasons. This could include using the 610 and 327 MHz receivers to do interference surveys.
Next Meeting: June 20, 2000