NATIONAL RADIO ASTRONOMY OBSERVATORY
                         Green Bank, WV


MEMORANDUM                                      February 25, 1997

TO:       G. Behrens, R. Fisher, R. Lacasse, M. MkKinnon, R.
          Norrod, T. Weadon
FROM:     R. Lacasse
SUBJ:     Minutes of Interference Protection Group 2/25/97 Meeting

     In attendance: Behrens, Fisher, Lacasse, McKinnon, and Weadon 

RF:  Changes to minutes of previous meeting?
All: NO.

RF:  Highest priority of group is to test equipment going on GBT.

Discussion of connector panel for indoor test range:

GB:  Sample layout for panel shown in distributed sketch.  The
     panel includes 4 Type N coax connectors, 2 filtered DB 25's,
     two multi-pin cylindrical (32 pin and 6 pin) connectors
     capable of 7 amps per pin, a telephone filter, and two
     feedthru pipes for fiber cables.  The multi-pin cylindrical
     connectors provide 1 db of attenuation at 1 MHz and 42 db at
     30MHz.  
RF:  The room itself is spec'd for 60 to 70 db.  It would be
     preferable to maintain that level of isolation if it's not too
     expensive.  
GB:  The DB 25's are even worse, providing about 27db at 30MHz. 
     How was the frequency span for testing decided.
RF:  Low end is determined by room.  To get quantitative
     measurements, need to have antenna at least a wavelength away
     from the device under test, to get in the far field of the
     antenna.  Given that the range is 11 meters long, the low
     frequency limit is about 30MHz.  The high end (3GHz)is a bit
     arbitrary.  Most interference is below this frequency, and the
     area of the feeds is getting quite small at this frequency. 
     Also the expense of going higher in frequency is a
     consideration.
GB:  Which DB-25 should we get?
RF:  The one with most attenuation, since we have no immediate
     plans to run high frequency digital signals through these
     filters; they are mostly for control signals.
GB:  Feedthrus for fibers are 0.5" diameter.

     Some discussion of whether two is enough and whether 0.5" is
     big enough.  
TW:  Prefer 4 feed-thrus.
***: Action item for Behrens to check with Turner and Shank on
     sizes of connectors prior to committing to diameter.
GB:  Suggest communications into the room via speaker phone.
All: Good idea.  Separate extension desirable.
GB:  Showed gasketing plan for panel.
MM:  Plans for screws around panels?  Question is prompted by the
     poor performance of a microwave oven enclosure with a door
     make of aluminum.
GB:  Hex head screws every few inches usually work.
RL:  Good success (60 - 70db?) with a box being used by Schiebel
     using Zeuss fasteners every few inches.
RL:  Is there room for expansion on the panel?
GB:  The panel is the maximum size permitted by the room
     construction.  Additional capability would be difficult to
     come by.
TW:  Would people want a tour of the room led by GB?
All: Yes.
***: Action item.  All to attend tour at 1:00 on 2/26/97.

     Some discussion of other connector requirements for the
     shielded room.  

RL:  The lack of any power except 120VAC has already proven to be
     a limitation.  It would be desirable to have 208 3-phase and
     possibly 240V single phase.
***: Action item for Weadon to follow this up with Fleming.

Discussion of other items on agenda:

RF:  Review of call outs for RFI?
MM:  None.
RF:  Report on Sizemore's inventory?
MM:  It is started and is due to be complete by 3/4/97.  Each item
     on the inventory will be designated as to whether it can be
     shared with, or not shared with, or given to, the IPG.
RF:  Reaction to requirements document produced by RF?
All: Not studied yet
***: Action item for RL, MM, TW to study this document and have
     comments prepared by the next meeting.
RF:  List of equipment to be tested.  Norrod has started on a list.
TW:  The list, as presented, does not include anything in the
     receiver room.  However, it has to be assumed that everything
     in the room will be tested.
RF:  Agreed.  Shielding from the room probably won't exceed 20db. 
     Until recently, perhaps none of us appreciated how quiet the
     equipment being installed in or near the feeds has to be.  The
     noise floor in the recent Schiebel/Norrod tests is 20db higher
     than it really needs to be.
RL:  Need to be able to get a signal to the spectral processor from
     the test facility to achieve this level.
***: Action item for Weadon to get this fiber installed.
TW:  Inventory of receivers and transmitters.  Found the one that
     White was supposed to have.  It includes a receiver and
     transmitter with 3GHz BW.  Norrod also has a narrow band
     (30MHz) receiver/transmitter pair.  White and Norrod have
     recommended amplifiers and optical isolators in other single
     mode links; this will most probably be required in the link
     from the test facility to the spectral processor.  Isolators
     cost about 1K and have 8 week delivery.

     General discussion of interference detection systems to be
     designed and built by this group.  There is general agreement
     that there are various designs possible.  These designs need
     to be properly engineered and the various trade-offs studied. 
     RL expressed interest in being on a subcommittee involved in
     the design and evaluation, assuming time can be found to do
     this.

TW:  A technician who will replace Jim Oliver will start work soon. 
     Part of his duties will include taking care of routine tasks
     generated by the IPG.  The most immediate task is to calibrate
     equipment presently in use in the test range for interference
     measurements.  Norrod will do some of this himself before the
     new technician starts.

Next meeting: Friday, March 7, at 0800 in new conference room.