NATIONAL RADIO ASTRONOMY OBSERVATORY
Green Bank, WV
June 22, 1999
TO: R. Fisher, J. Ford, F. Ghigo, R. Lacasse, C. Niday, T. Weadon
FROM: R. Lacasse
SUBJ.: Minutes of Interference Protection Group 06/22/99 Meeting
In attendance: Fisher, Ford, Lacasse
Minutes of Previous Meeting
Call Outs for Interference
Action Item Review
of OVLBI transmission field strengths at the GBT.
A schedule was received from Minter. Need to gather equipment
and try a test measurement.
2. Site radio system (Chuck)
In Chuck's absence, Rick reported that Chuck was having problems with
the resistive power combiner for the two transmitters. Intermod products
are produced when both transmitters are on. Several options are being
considered to remedy the situation.
3. Monitor station and
anechoic chamber developments (Rick, Rich)
Pole setting for the monitor station has been postponed twice.
4. Microwave oven in shielded
test box (John)
The continuum detector system for the anechoic chamber was tested.
First a modulated carrier was run through the system to show that the detector
system could properly detect and filter it. Second, two copies of
a wide white noise bandpass were generated. One was attenuated by
10 db and the other by 9. These two signals were run through an RF
single-pole double-throw switch whose switching state was controlled by
a TTL signal generated by the continuum detector PC. The output of
the switch was detected and filtered by the detector/filter unit and sampled
by the acquisition board in the PC. The PC was then able to display
the acquired waveform and calculate the SIG/REF quotient. The quotient
was in error by about 12%. This may be due to bad calibration of
the pads (they were not checked) or to large switching transients.
A blanking option will be added to the software and the experiment repeated
The factory claims to have never received the enclosure that was returned
to them due to shoddy workmanship. Ford will trace the package.
5. Shielded room entrances
A second enclosure is required for the cafeteria. Another vendor,
AAMCO will be used as supplier for this one; AAMCO has supplied shielded
racks for several systems at NRAO and their quality is expected to be better
for the same price.
Some 7 GHz absorber was ordered. This will allow for the construction
of a 10:1 scale model. There was some discussion about how to handle
the floor of the serpentine entrance. As a first cut, a metal floor
will be tested. Alternatively, plywood over absorber can be tested.
6. Progress on suppressing
RFI from equipment that has been measured in the anechoic chamber
Ford will work with the laser group to shield their control unit enclosure.
Most signals in and out of the control unit are low frequency so should
be easy to shield. The 1500 MHz reference also must come out of the
unit. It was recommended that this signal be well shielded, both
inside and outside the enclosure, and bandpass filtered as it leaves the
enclosure to keep internally generated noise from following this conductor
out of the enclosure.
7. Telescope operator
participation in IPG activities (Rick)
Eric Knapp is a 140-ft. operator who has recently transferred to the
lab to prepare for his new position as GBT operator. He has been
designated the "RFI expert" operator, and part of his training will entail
working with the IPG group. One possible task for him is to work
on a triangulation system to locate RFI with the monitor stations.
Other possibilities include setting up and making RFI measurements.
8. RFI considerations
for new Education Center (Frank)
No progress. The project is still looking for money and and a
9. RFI survey plan for
GBT commissioning (Rick)
Fisher explained that this plan is to spell out the details of the
installation of the monitor station on the GBT. This will be used
by McKinnon for coordinating the commissioning effort.
10. GBT RFI 'harmful levels'
11. Lab RFI suppression
12. Interference at
13. RFI monitor for
All of these are on hold.
14. Cal and sig/ref switching
system system for anechoic chamber (Rich, Chuck, Rick)
For continuum measurements, it is desirable to switch the DUT on and
off to look for turn-on transients. After some discussion, it was
decided to design a switch when we have a device in hand that needs to
be switched on and off. This switch will probably take the form of
an optically-isolated, zero-crossing solid-state relay, with I/O well filtered.
Lacasse will check on the feasibility of using a power supply test load
built by Schiebel as a test load for an AC power switch box.
15. FCC Class B requirements
and test procedures (Rick, Chuck)
Requirement for a turret for anechoic chamber
Next Meeting: July 6, 0800.