GBIPG Meeting

18 March, 2003



1.        Management's take on the IPG Progress Report for 2002 – Ford

2.        Update on Astronomer efforts to characterize GBT RFI and make the data available to Observers – Ghigo

3.        Overview of the GBT RFI Evaluation Spreadsheet – McCullough

4.        Review of the derivation of our in-house limits as discussed in Rick's 1997 paper titled the "RFI Radiation Limits in the Vicinity of the GBT" – Fisher

5.        Overview of the graphical representation of our in-house limits – Niday

6.        Discussion of short and long term recommendations to minimize the impact of modeing in our Anechoic Chamber when testing at < 2 GHz – Beaudet

7.        Progress report on IPG web site, office move, prop studies and settling in - D. Wirt

8.        Progress on Feed Arm Servo Filters – Lacasse

9.        J. Lab shielded room update – Sizemore

10.     Spectrum Control Resin Sealed PI filter installation tips and traps – Acree


*Due to time constraints, agenda for next meeting will include items 7 through 10


Attendees:  J. Acree, R. Fisher, F. Ghigo, R. McCullough, C. Beaudet, C. Niday, R. Lacasse, D. Wirt




1.       Management's take on the IPG Progress Report for 2002 (Ford, Acree)


Per John’s input (read by Jeff), the IPG Progress Report to Management has received excellent reviews.  Jeff will present the briefing at Green Bank in April and later in Charlottesville.  Jeff noted that even though he presented the slides; the development of the briefing was a cooperative effort involving the whole group and that it would not have been a success without the contributions of all the members. 


The following are a few more things that were noted about the briefing:


(a)     Jeff noted that preparing for the briefing gave him a better appreciation of the five (5) discrete “tools” we have at our disposal to control RFI.   He now realizes that a key part of his role will be to understand the capabilities and limitations of each tool so that he can allocate resources to them in a way that will best support Observers.

(b)     Jeff also noted that a suggestion from Management that we should compare last year’s spectrum data to current data to see how much things have “improved”, indicates a failure on our part to communicate effectively our current mission.   Though some 02 mitigation efforts, such as the feed arm servo filter installation and the shielded room overhaul were intended to reduce existing RFI; the emphasis in 02 was to prevent new RFI sources from being born.  He cited IPG coordination in the AEVC construction and our efforts to suppress RFI before new systems are deployed as examples.  He also noted that NRQZ administration is key in this.  He suggested that another way to measure 02 progress would be to compare current IPG infrastructure to that which existed prior to 02, since infrastructure improvement was an area of focus.  He noted that the comment was early in the briefing, so hopefully by the end, the mission was made clearer.  Jeff also noted that he feels good about our 02 efforts, but we need to make it clear that it will take a deliberate and relentless effort on our part, just to keep the RFI situation from getting any worse.  He noted that even as we throw everything we have at the problem, we are doing well to hold our own.

(c)     Rick noted improving the RFI situation should be the goal.  He also noted that even though the NRQZ is not totally “quiet”, there are chunks of spectrum that are relatively free of RFI.  Thus, we can do science at Green Bank that would be difficult or impossible elsewhere.  The group agreed that we need to build on that.  


2.        Update on Astronomer efforts to characterize GBT RFI and make the data available to Observers (Ghigo)


Per John’s input (read by Jeff), the Planning Committee has agreed that the astronomers should take this on as a commissioning task.  Glen Langston is trying to set up a web-based tool that will provide high level descriptions of various bands in the spectrum.  Links to more detailed data would also be provided.  Jeff noted that he found the format used in the 02 GBT RFI reports prepared by Anish, Glen and Frank very useful.


As the discussion branched out a bit, Jeff noted that during the progress report to management, Phil had requested that the IPG develop a network-based tool Observers could use to get real-time spectrum data for observation planning purposes.  The intent would be for them to be able to identify strong, persistent RFI prior to conducting observations.  Jeff noted that such information could be provided using a spectrum analyzer and networked PC running HP VEE.  It was noted that our existing RFI monitoring stations could play a role in this, but the comparatively low elevation of our antennas would be a serious limitation.  Carla mentioned that Arecibo has a fairly elaborate database to provide legacy RFI data.  Rick said he found the Arecibo RFI data base to be interesting, but not particularly useful.   Jeff got the same impression from Glen.  Rick suggested that legacy RFI data would only be useful it was routinely updated; at least every 6 months to a year.  It was also agreed that real-time spectral data would be much more useful if the receive antenna could be located on the GBT feed arm.    Carla stated that Observers should not base RFI on a “quick look” at the spectrum.  Long-term data needs to be used and be statistically significant.  Jeff noted that HP VEE would certainly make that possible.


The final consensus of the IPG was to mount an OMNI antenna on the feed arm and to use a spectrum analyzer/PC arrangement as described above to get started.  The tool would be accessible to select network users on the IPG web site.   Rick suggested working with one or two observers over the course of a year to develop a prototype and get practical feedback.  He noted that we will need to get input from our visiting Observers.  Not just our in-house Observers.  Jeff would like to implement a basic set up this year, if we can get to it.  Frank asked about the current availability of archived RFI data and it was noted that Glen is pursuing this presently.


3.        Overview of the GBT RFI Evaluation Spreadsheet (McCullough)

Calendar year 02 enhancements to our RFI evaluation spreadsheet included fully adopting the limits Rick suggested in his May 9, 1997 paper titled “RFI Radiation Limits in the Vicinity of the GBT.”  The spreadsheet attempts to spread measured RFI over the proper bandwidth in order to compute a GBT protection limit that is consistent with the ITU RA limits.  One note of caution:  The resolution bandwidth must be chosen carefully (i.e., small enough) to be able to resolve the details of a feature.  It is often necessary to determine the 3dB points of the feature being evaluated to properly characterize it.  It was also noted that smaller resolution bandwidths provide better sensitivity, but may result in painfully slow sweeps.


This topic spawned discussion of adding the capabilities of the spectral processor to Anechoic Chamber.  This had been done previously, but does require considerable effort.  The sensitivity improvement is about 20 dB.  Another possibility is to use the continuum receiver designed by Rich Lacasse.  This unit does on/off comparisons to identify RFI. 


A workable test system needs to be in place in 4 to 6 weeks to support PTCS.


*Due to the depth of this topic a separate meeting was scheduled for the afternoon.


4.        Review of the derivation of our in-house limits as discussed in Rick's 1997 paper titled the "RFI Radiation Limits in the Vicinity of the GBT"  (Fisher)

Rick provided an overview of the derivation of our in-house RFI limits.  He pointed out, our in-house limits are based on ITU-R RA.769 and are more stringent than the NRQZ limits.  The NRQZ limits were based on what already existed when the NRQZ was born and had practical limitations.  In general the NRQZ limits are intended to address a different type of signal and a different type of concern.  He also noted that the NRQZ limits pertain to bands outside the radio astronomy bands.  Given that much of the spectrum is open, the assumption in adopting these in-house limits is to not make the situation worse with self-generated RFI.


The consensus of the group is these are the right limits to apply in the design and installation of equipment here at NRAO - GB, which begs the question, “Have we installed equipment outside these limits?”  The answer is an emphatic, yes!  However, we are trying to resolve these instances on a case by case basis.


5.        Overview of the graphical representation of our in-house limits (Niday)

Chuck presented a graphical representation of Rick’s limits which he previously prepared for University of Pennsylvania.  He also presented a graph that showed the acceptable ERP for an isotropic source, 10.5 meters from the GBT feed at the specified bandwidths.  The graphs were generated for University of Pennsylvania to show the RFI constraints for a receiver they were building.  The graphs were generated using MS Excel.  The University of Pennsylvania folks were startled by how stringent our limits are.


A recommendation was made to publish on the IPG web site Chuck’s plot of Rick’s limits along side the NRQZ limits and the FCC limits, graphically showing the magnitude difference of each.


6.        Discussion of short and long term recommendations to minimize the impact of modeing in our Anechoic Chamber when testing at < 2 GHz (Beaudet)


Carla noted that our anechoic chamber was originally intended to be effective for an EUT in a 1m sphere at each end of the chamber at 2 GHz and higher.  Since we are using it well below its intended frequency range, she has been concerned that modeing may cause significant errors in our measurements.  She presented the preliminary data from our initial investigation of this concern.


The data was collected using our standard anechoic chamber setup.  The EUT was a sweep oscillator connected to a 200–1000 MHz LP antenna.  The oscillator was swept from 200-1000 MHz during testing.  The following is a summary of her tests:


·         The first measurement was taken in the anechoic chamber with the EUT right behind the turntable (without floor absorber in place).  The affects of modeing were evident.

·         Then the entire setup was moved to the cafeteria parking lot and the measurements were repeated.   This data was used as baseline data to determine the departure from the “ideal” response of the anechoic chamber setup.  Up to 30 dB differences were seen.  Modeing appeared to be the cause.

·         The first measurement was then repeated in the anechoic chamber with floor absorber in place (modeing worsened)

·         In the chamber, on-axis rotation of the EUT was then used to simulate a turntable (no appreciable difference in modeing was seen)

·         She noted that moving the EUT around in the chamber did reduce the negative impact of modeing significantly.  The “sweet spot” seemed to be to the right and slightly in front of the turntable.


Randy was concerned that the “sweet spot” for every EUT may be vastly different.  It was agreed that we should take this test further by testing an actual RFI heavy device.   The EUT would be moved to various spots in the room to see if Carla’s results can be corroborated.  It was noted that above 2GHz, this should not be a big issue.  Carla’s current belief is that we can greatly reduce the impact of modeing below 2 GHz by rotating the EUT, off-axis, on a turntable.  She noted that the extent that this will be possible is limited by the size and weight of the EUT.  It was agreed that for the time being, we need to place the EUT to several places, not just the center to minimize the impact of modeing on our measurement accuracy.  It was agreed that our test methodology should be consistent, thus we need some written guidelines to stay on the same page.  Chuck said he had a document that attempted to do that and would provide it to Jeff.


* The remaining agenda items will be addressed at the next meeting