June 1, 1999
TO: R. Fisher, J. Ford, F. Ghigo, R. Lacasse, C. Niday, T. Weadon
FROM: R. Lacasse
SUBJ.: Minutes of Interference Protection Group 06/01/99 Meeting
In attendance: Fisher, Ford, Lacasse, Niday
Minutes of Previous Meeting
Call Outs for Interference
Both Niday and Lacasse reported interference from Iridium satellites. In a VLBI run, the first LO had to be moved to take advantage of a 40 MHz band pass filter which centered the 32 MHz observing band in the filter and the Iridium signal on the skirt of the filter.
Fisher reported interference at 9200 MHz, while
observing at 9400 MHz. Since the interference did not hamper the
observing, it was not pursued.
Action Item Review
1 - Measurement of OVLBI transmission field strengths at the GBT
The OVLBI system transmits in two bands, 7 GHz and 15 GHz. To test the 7 GHz field strength, the IPG needs an amplifier (ours goes to 3 GHz). The IPG does not own any equipment to test the 15 GHz signal. However, some equipment could be borrowed for this test.
Niday will try to assemble a 7 GHz test system
in the coming month. Some preliminary tests can then be made.
2 - Progress on RFI Suppression from equipment that has been measured in the anechoic chamber
The laser ranging equipment was tested after the last IPG meeting. The tests indicated that additional shielding is required. Ford will make recommendations to Parker on ways to shield and filter the equipment.
The fiber optic E-stop system was also tested. The receiver tested clean. The transmitter emitted radiation at 170 MHz. As tested, the system consisted only of bare boards. Packaging the system to meet spec should not be difficult.
RFI measurements of most of the equipment destined for the GBT receiver room have been made. These results will be tabulated and reviewed in light of the measured attenuation of the receiver building. Thus, systems needing additional work will be identified.
Norrod is doing some integrated testing of
the GBT system. Presently, he is searching for self-generated RFI
using the Digital Continuum Receiver. Some types of interference
would be more easily found using a spectrometer. This will no doubt
be done when the spectrometer or spectral processor is available.
3 - Site Radio System
On the paperwork side, both frequency coordination
and licensing need to be completed. On the hardware side, transmitter
equipment is being rack-mounted. The location of an antenna on the
water tower is still in question. Most portables have been tested.
Completion of a demonstration system is imminent.
4 - Monitor station and anechoic chamber development
New poles for the antennas for the monitor station will be installed shortly.
Weadon and Lacasse need to test the continuum
detector system ASAP.
5 - Microwave Oven Enclosure
A commercial enclosure was purchased ($1500)
from EMCOR. It was received recently and found to be poorly constructed.
It has been returned to the vendor for repair. Ford will follow-up with
6 - Shielded Room Entrances
No progress. Fisher needs dimensions
on the absorber.
7 - RFI considerations for the new Education Center
No progress. Project is stalled for lack of funds.
8 - Use of Interference Reporting System
Balser has used the system to document interference
at the 140 ft. He has also been informed of the new interference
call-out order (Fisher, Lacasse, Niday).
Agenda items 9 to 14 were not discussed.
Next Meeting: June 22, 0800.