NATIONAL RADIO ASTRONOMY OBSERVATORY
                         Green Bank, WV


MEMORANDUM                                      November 14, 1997

TO:       R. Fisher, R. Lacasse, R. Norrod, M. McKinnon, C. Niday,
          T. Weadon
FROM:     R. Lacasse
SUBJ:     Minutes of Interference Protection Group 11/11/97 Meeting

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

Minutes
     
     Minutes from the last meeting were approved.

RFI Call-outs

     A signal at 4880 MHz (420 MHz in the IF) was observed by
Lacasse at the 140'.  Grammer did a variety of things to the
receiver (shutting down amplifiers, etc.) to show that the
interference was not internally generated.  Lacasse and Grammer
placed several aluminum plates on top of the large feed effectively
covering approximately 90% of the aperture.  This did not make the
spike disappear, but was the only test which affected the amplitude
of the signal.  Information was forwarded to Sizemore who is
pursuing it.

     Episodes such as this highlight the need for a good monitoring
station, and for a database of observed interference.


Budget Items

     Several people reported on the costs of items in the budget. 
The first of these was an A/D system on a PC to facilitate
continuum measurements.  Specification for this item were
discussed.  A sample rate of 1 to 10KHz was deemed adequate, as was
a resolution of 12 bits, although 16 bit resolution would be
preferable.  The intial estimates for 16-bit systems that will
sample at several hundred KHz (PC, A/D board, and software) is $6K
to $7K.  Weadon agreed to look into slower sampling devices that
have only the basic sampling functions to see whether something is
available at lower cost.  The monitoring station cost was estimated
at $7400.  A power meter and head was estimated at $4K, and a low
frequency antenna (30MHz to 1GHz) was estimated at $3500.

     Priorities of the items were discussed.  It was decided to
purchase the A/D system first, and spend any left over money on the
monitoring station.  


GB School LAN Status

     Although the current state of the LAN installation at the
school was unknown, the discussion quickly digressed into the best
ways of dealing with the interference potential of the school.  The
school has a technical committee, of which John Ford is a member;
this seems the most appropriate conduit for technical information
flow between NRAO and the school.  It was decided to invite John to
the IPG meetings every couple of months to keep him abreast of the
IPG's activities.  The possibility of providing presentations or
tours to school personnel to give them an appreciation for the
interference problem was also discussed.


Workstation/PC Shielded Enclosure

     The shop is working on it.  It should be ready in about 2
weeks.  Power filters for the box have been received.  Since the
decision to locate the GBT control building in the Jansky lab
addition has been made, the need for shielding workstations is not
as critical.  Preliminary shielding effectiveness measurements on
the second floor of the new addition show that the rooms have about
75db of shielding.


Equipment Testing

     Further tests of the 1500MHz oscillator for the laser ranging
system have been completed.  Some leakage still exists.  A separate
memo, documenting the tests, has been written.

Next Meeting

     TBD