NATIONAL RADIO ASTRONOMY OBSERVATORY
                          Green Bank, WV


MEMORANDUM                                       October 29, 1998

TO:      R. Fisher, J. Ford, F. Ghigo, R. Lacasse, R. Norrod, M. McKinnon,
	 C. Niday, T. Weadon
FROM:    R. Lacasse
SUBJ.:   Minutes of Interference Protection Group 10/27/98 Meeting

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

Minutes

     Approved.

Call Outs

     Ghigo and Lacasse reported seeing interference while setting up for
a VLBI run.  The interference was between 1050 and 1100 MHz.  Niday
stated that there is a lot of activity in this band from radar, DME
(distance measuring equipment) and VOR (VHF omni range).

Square Kilometer Array

     Fisher reported that, due to the proposed Square Kilometer Array,
many are now interested in interference monitoring and excission.  There
is widespread recognition that it is important to fix an observatory's
own RFI problems, and to design for RFI suppression.  This does not
detract from or replace the function of the IPG group.  Fisher will keep
us posted.

Advice for Computing Division

     The computing division has requested some advice from the IPG on
what measures it should take for mitigating RFI from all the computers
on site.  Several suggestions were proposed:
 
    1 - Turn computers off when they will not be used for a good while,
	e.g. at night and on weekends.
    2 -  Put "round the clock" computers in shielded rooms.
    3 -  Get computers with auto-power-down feature.
    4 - To make suggestions 1 and 2 above work, re-configure the network
	so there is only one main server, rather than a network of
	interdependent computers.

     It was noted that most of the interference from the newer PC's is
in the 100 to 300 MHz range.  This is despite the fact that clock
frequencies on these computers are 400 MHz or better.  The speculation
is that the RFI is coming from the switching supply.

     There is some interest in knowing how much shielding standard
construction provides.

     Fisher proposed that we try to find the worst offenders in the lab
by walking down the hallways with the interference cart and making
measurements every 10 to 20 feet.

****Action item: Niday will make the lab sweep.

In conjunction with the lab sweep, Niday will attempt to measure
shielding fromstandard construction.

Review of Action Items

     Due to lack of time just a few action items were reviewed.

     1. Measurement of Receiver Room Shielding
          Initial measurements have been completed.  The room appears to
provide 55 to 60 db of shielding, both in the horizontal and vertical
directions, from 100 to 1000 MHz. A calibration run needs to be
completed.
          Ford suggested that magnetic field measurements be attempted
in the room to see if there was much interference from the motor
drivers.  The servo cabinet shielding is not effective for magnetic
fields.  Weadon mentioned that Stennes had some type of instrument to do
this.
          In conjunction with measurements of the feed room shielding,
measurements of RFI from the feed arm servo electronics were made from
inside the room.  These indicate that the system generates interference
at a level about 10 db over our limit.  Lacasse pointed out that the
system is not properly terminated.  Ford rebutted that PCD ought to be
informed of the situation nevertheless.  It was also decided that a
baseline measurement of the servo system ought to be made from outside
the room.

****Action item: Niday and Weadon to measure servo RFI from outside the room.

     2. Education Center
          Ghigo stated that the project is in an early development
phase at present.  No exhibits have been designed yet.  He does not know
how much the architect knows about RFI shielding.  Fisher suggested that
we have a meeting with the architect soon to communicate our concerns,
before any significant design work is done.  Lacasse suggested that the
concept adopted in constructing the VLBA control rooms might be useful.
The concept there was to tell the contractor how to screen the room, but
not hold him to any RFI performance specs.  RFI tests of one of these
buildings were made and documented in a VLBA memo. ****Action item:
Ghigo to find out at what stage the planning is, and find out how much
the architect knows about RFI.

     3. Site Radio Tests
          Progress continues to be made.

     4. RFI Web Page
          There is an initiative to start an observatory wide web page on RFI.

     5. Filters on Fluorescent Fixtures
          To close an action item, McKinnon distributed information on
filters used on fluorescent fixtures in the lab addition to attenuate
RFI.  (The filter is Catalog No. RIF-1 from Advance Transformer Co.)

     6. Continuum Detctor System
          Detector amps have been built and tested.  Detectors must be
purchased and post- detection filters designed and built.

Next Meeting: Nov. 10, 1998, at 0800