TO: J. Acree, C. Beaudet, R. Fisher, J. Ford, F. Ghigo, R. Lacasse, R. McCullough, C. Niday, W. Sizemore
FROM: R. Fisher
SUBJ.: Minutes of Interference Protection Group 5-21-02 Meeting
In attendance: Acree, Beaudet, Fisher, McCullough, Niday, Sizemore, Ford, Lacasse, Emerson
Minutes of Previous Meeting
In a few IPG emails over the past month the question was asked about what should be the lower frequency limit to which RFI/EMI from equipment on site should be measured and suppressed. Rick opened the discussion with the suggestion that it should be 50 MHz on the basis of expected observations on the GBT over the foreseeable future. Carla raised the point that quantitative anechoic chamber measurements are quite difficult below 100 MHz because of wall reflections due to the ineffectiveness of the absorber at low frequencies. She showed a plot of broadband noise measurements where the amplitude of the signal below 100 MHZ varied by +/- 10 dB, possibly due to constructive and destructive multi-path interference. A lively discussion followed.
The preliminary conclusion was that even qualitative measures of radiation below 100 MHz are still useful with the understanding that we cannot make an accurate statement about how far above the IPG radiation limits a given measurement falls. The two antennas that can be used below 100 MHz are the 100-1000 MHz log periodic and the bi-conical antenna that is calibrated to as low as 20 MHz. Carla and Chuck were of the opinion that the bi-conical antenna was no more sensitive than the log periodic below 100 MHz, even though the latter is not designed for that frequency range. Pending further information from the people involved with anechoic chamber measurements, the proposal is to simply extend the spectrum analyzer measurements with the log periodic down to 30 MHz. Any signals found below 100 MHz will be noted in the reports, but comparisons with R.A. 769 limits will be of marginal value. The point was made that shielding and filtering steps taken to suppress radiation above 100 MHz will generally be equally effective down to 30 MHz. We just want to be sure that there are no hidden surprises in the 30-100 MHz range.Status of current action items (Jeff):
The action item document is self-documenting so the item reports will no longer be included in these minutes. The next action item list will contains the results of reports on each item.Feed arm servo filtering status (Rich, Randy):
The first batch of filters has been delivered, and the expected degree of suppression has been achieved in tests. The filters have no adverse effects on motor operation. An issue regarding final design of the shield box for the filters remains to be resolved, and a meeting with Jeff Cromer is planned. No extensive summer shutdown of the telescope is anticipated so installation of the shielding box and filters may have to be worked into the painting and maintenance schedule.Laser shielding and filtering status (Randy):
Technician work is under way on implementing the completed shielding and filtering design for the laser units. This task will be worked into the lab's regular technician priority process. Installation of the first modified unit is expected by the end of July.Shielded room repair status:
Jeff reported on some research into bi-metallic corrosion and confirmed that corrosion at the zinc-copper interface on the shield room window flanges is no surprise. The difference of the anodic indices of these two metals is 0.9, which is quite high. The plan is to replace the zinc plating with nickel, which will reduce the anodic index difference to about 0.05. The current plan is to do baseline testing of the room's shielding effectiveness, fix the windows, and retest. An assessment of any necessary wall repairs will be made after the widow repair tests. A detailed written shielding update report is available from Jeff.RFI considerations for new Education Center (John):
The immediate issue regarding construction of the new Education Center is radiation from fluorescence light fixtures proposed by the contractor. The best available lights tested so far are 3 dB above the IPG's radiation limits times the number of fixtures to be installed. A final decision is required within the next few weeks. John and Carla are on top of this issue.