Green Bank, WV


December 22,  2000

TO: R. Fisher, J. Ford, F. Ghigo, R. Lacasse,  C. Niday, T. Weadon
FROM: R. Lacasse
SUBJ.: Minutes of Interference Protection Group 12-19-00 Meeting

In attendance: Fisher,  Ford,  Ghigo, Lacasse, Niday

Minutes of Previous Meeting

Interference Reports
None, except as discussed in item 6 below.
Review of Action Items:

  1. Site-wide interference suppression policy - position  paper (John, Rick)

The management version of the policy was distributed on 12/18.  Jewell and McKinnon will provide input with regards to changes required for the second version whose intended audience is the technical and scientific staff and visiting observers.
  2. Hand-held HF/VHF radio for power switching RFI detection    (Chuck)
A receiver has been ordered.  We have requested that the analog cell phone bands be not filtered out of the receiver.  Apparently we can do this since we are a government agency.
  3. RFI considerations for new Education Center (John, Frank)
We have received replies from two RFI consultants.  One consultant would charge about $16K to modify the architectural plans to meet our shielding requirements.  Another would charge $20K for this service and estimates that additional building costs due to shielding and filtering would be about $800K.  This is all very preliminary.  Holstine is presently handling negotiations so Ford could not give an up-to-date report.

Additional money has been found for the building, thanks to the efforts of Senator Byrd.  This should increase the likelihood that the building will be constructed using good RFI practices.

  4. Progress on suppressing RFI from equipment that has been measured in the anechoic chamber
RFI from the GBT monitor boards was decreased significantly using several techniques.  First, additional decoupling capacitors were installed around the voltage regulator on the microprocessor board.  Second, surface-mount inductors were installed in series with some particularly noisy lines.  A 20 to 30 db improvement in performance was measured.
  5. Measurements of PC/Workstation in a shielded box (Chuck)
No progress.
  6. Initial RFI measurements on the GBT L-Band receiver ( Rick)
The output of the L-band system was analyzed with the Spectral Processor.  Many unwanted emissions were observed, and several of them are probably self-inflicted.  In particular, there are some spikes in the protected band and there are 4 MHz and 1 MHz combs in the data.  A strong 1500 MHz signal, most probably from the laser-ranging system, was also observed.

Work is planned to isolate the causes of as many unwanted signals as possible.  In particular, tests will be conducted with the receiver room door opened and closed as a first step in determining which, if any, of the signal originate in this room.  Similarly, tests will be conducted with parts of the servo system on and off, with the active surface system off and on, etc.  Rick will request that time be scheduled for these tests.

Rick also noted that strong signals, like the 1500 MHz signal mentioned above, may create intermodulation products with other interfering signals, resulting in additional lost spectrum.

  7. Monitor Station Developments
White has some equipment ready to install and is waiting for reasonable weather.  It was noted that the laser group has a tent which can be used to provide shelter from the wind.
 Next Meeting: January 16, 2001