GBIPG Meeting

22 July 2003



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




  1. Possible new shielded room for NRAO GB - Ford


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 Charleston, WV to dismantle the “panelized” room and transport it back to Green Bank.  The panels will be stored in the basement of the interferometer building, but a final location has not yet been decided. Possible candidates include the interferometer building, the building at 85-1, or perhaps the basement of the 140’ telescope.


  1. Report on spectrum management class - Wirt


Denise reported on her trip to the NTIA spectrum management seminar she attended last week in Washington, DC.  She noted that the seminar provided an excellent overview of domestic and international spectrum management processes as well as presentations on current issues.  Denise made several contacts within the NTIA, FCC, and other seminar participants.  She noted that a wide variety of organizations were represented.  Several speakers mentioned spectrum use and availability for scientific research, including radio astronomy, which she found particularly encouraging.  At least from a planning and engineering level, there seems to be consideration for carefully managing the spectrum such that assignments will not interfere with other services, including radio astronomy.  Joe Camacho, who facilitated the seminar, offered the opportunity for NRAO to present a one hour session during the upcoming seminar in January 2004.  The group agreed this is a good opportunity, and that radio astronomy as a whole should be represented, either by NSF or NRAO.  This opportunity will be discussed with Phil.


  1. Discussion of possible policy for GBT control room shielded door – Lacasse


This item will be reported on next meeting.


  1. Update on LRF cover actuator RFI measures - McCullough


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 Friday, August 1, 2003.


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.


  1. Update on power line RFI control efforts -  Sizemore


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.



  1. RFI scan results using PF1 and PF2 - Ghigo


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.