1. Possible new shielded
room for NRAO GB. (Ford)
2. Report on spectrum management class. (Wirt)
3. Discussion of possible policy for GBT control room shielded door. (Lacasse)
4. Update on LRF cover actuator RFI measures. (McCullough)
5. Update on power line RFI control efforts. (Sizemore)
6. Report on RFI scans using PF1 and PF2 (Ghigo)
Attendees: J. Acree, C. Beaudet, R. Fisher, J. Ford, F. Ghigo, C. Niday, W. Sizemore, D. Wirt
John reported that we will
be receiving a surplus shielded room to supplement our existing anechoic
chamber facility. Present thinking is to use this
room for RFI testing in order to increase the availability of the indoor antenna
range. The new room could be outfitted with the one
foot absorber we already have. This should greatly
improve anechoic performance below 1 GHz compared to our existing chamber. Carla noted that we should also consider using the new
chamber as a mode stir chamber. A crew of four from
the works area will be going to
Denise reported on her trip
to the NTIA spectrum management seminar she attended last week in
This item will be reported on next meeting.
Randy has conducted RFI
tests on the proposed DC motor which will be used for the laser range finder
covers. Preliminary findings indicate a fair, but
predictable, amount of RFI through 500 MHz. John Shelton
and Randy have partially dismantled one of the motor mechanisms and have
found that it will be possible to design and construct tubular shields with
flanges and integrated single line RFI filters which will adequately mitigate
all RFI generated by the motor’s commutator and
brushes. Randy is currently designing this shielding
arrangement and expects to have drawings complete for the Machine Shop by
At the request of Dave Parker, Randy has also recommended a number of small limit switches which might be used by contractor Triad in their supporting control circuitry. Upon completion of their design, Randy will come up with the appropriate R/C snubber circuits to be applied across all such limit switches. This design process should be completed the same day he receives final drawings from Triad via our Metrology Group. Final RFI testing will take approximately two hours and is tentatively slated for the first two weeks in August pending Triad’s design completion.
In response to an observer complaint logged by David Nice over the July 4th holiday weekend, Wes is searching out problem utility poles. He noted that any pole located within line of sight to the GBT poses an RFI threat. Presently, six poles need attention. About one week prior to low frequency observing, Wes will start routinely looking for power line noise using the monitoring station and portable equipment. If problems are found, they will be fixed prior to the observation. August 16th is David Nice’s next scheduled PF1 340 observation, and as best possible, all power line noise should be “quieted” by this time. It was noted that it is a good idea for members of the IPG to drop by the GBT Control Room during low frequency observations to offer support and learn of any issues from Observers.
Frank reported on RFI scans taken using PF1 and PF2 receivers. The GBT was rotated in azimuth 360° with spectra being collected every 2 seconds. Scans reflect day versus night comparisons at 4 frequency ranges. Plots represent the average of the each collective set. The frequency ranges used were requested by MIT for use in their radar program. Frank has a summer student who will work on a program to display features on an azimuth versus frequency basis. One feature readily identified on one of the plots by the IPG was 1090 MHz transponder used in the aircraft DME band 960 – 1260 MHz. Regarding another plot, the group questioned why data was collected for the range 894 – 896 MHz since it is allocated to air phones used in commercial aircraft.
An off-topic discussion of the 2 MHz comb that has been identified with the control modules for the active surface located in the actuator room was initiated. It was noted that a short term solution is to turn off the active surface during low frequency observations. The operators have been instructed to do this and it seems to be an effective interim solution. The long term strategy is to install the cages originally fabricated for this purpose. Filters already exist on most of the wires connected to these modules; however there are 2 – 4 wires per unit that appear to be unfiltered. It is not clear at this point if these will pose a problem. Software interlocks may be considered if the RFI mitigation using the cages is not successful.