Memo to: Interference Protection Group, File 12/20/99 From : Fred Crews Subject: Known Sources of Possible RFI on GBT This memo will deal with known possible sources of RFI on the GBT (mainly thermostat) as of this date. NRAO supplied equipment and Contractor supplied equipment is noted. Other areas are addressed where there may be possibilities of interference, and some ideas for an improvement in some areas is given. I. Existing thermostats: A. Substation mounted on the alidade. Has several strip heaters mounted length wise in the cabinet with one thermostat. Do not currently know current carried. This was Comsat/RSI supplied with NRAO approval. B. Cryogenic Compressor room on the alidade. There are 3 separate cooling (air flow only) thermostats in this room. Thermostats control fans, and louvers are opened by the airflow. These were Comsat/RSI supplied with NRAO approval. However, it is to be noted that original drawings show manually operated switches, but all agreed to change to thermostat. Probably better identified as a cryostat. C. Manlift #1 (alidade to el bearing) control circuit cabinet containing (in part) programmed IC's mounted inside cabin has 2 thermostats. One is set to 80 degree F for a heater. The other is set to open the control circuit and shut down the manlift when temp drops below 32 degrees F. There are no doubt overtemp cutouts in the 2 motors on this unit, but these are not obvious from the drawings. Note, this is the manlift that has the control circuit for which no parts will be available after 12/31/99. There is a new control circuit that is available and has been approved for use by the manufacturer, and rumor has it that Comsat/RSI will have this manlift retrofitted to a new controller, but nothing has happened. Comsat/RSI has been told that NRAO will not approve this equipment unless spare parts are available. Supplied by Comsat/RSI and approved by NRAO. D. Manlift #2 (on the feedarm). This manlift has the new control circuit (we have been told). There is an 80 degree set thermostat for a heater in the control box, a 32 degree thermostat to shut down control (consequently the manlift), an overtemp thermostat in each of 2 motors, and an overtemp thermostat in each of 2 motor brakes. Supplied by Comsat/RSI and approved by NRAO. E. In the lower pintle room, Comsat/RSI has agreed to 1 kw of single phase electric heat with fan that is thermostatically controlled. This is on drawings. We might have a little leverage on this- not likely much. F. 600kw generator- There is a thermostatically controlled room heater, and a thermostatically controlled engine block heater. This unit was purchased by NRAO. G. 50kw generator- Has a thermostatically controlled block heater. Unit purchased by NRAO. H. Generator Load Bank- NRAO has installed heaters in this NRAO supplied equipment which are switched on and off by a humidistat. I doubt that it has any special low RFI features. I. Elevation encoder housing- We still don't know what Comsat/RSI is going to do to control moisture in this housing. Probably John Ford has the most recent word on this. Summary: As far as I know, there are no other thermostats at this time. There is a Comsat/RSI requirement to run a 120 volt line completely off the telescope to the 4160 connection to site power nearby to run a heater to keep down humidity. My guess is that this will be thermostat controlled. II. Other possible sources of RFI A. Aircraft Warning Lights- Required to be functional continuously by the FAA. Most concern is for the ones that go on and off located at the top of the feed arm. These are incandescent and were tested and approved by NRAO regarding interference. However, there was never a report written and no information was given as to the testing conditions, in spite of mine and Fleming's insistence. B. Telescope drive motors, clutches and brakes- total of 24 of these assemblies, plus a number of motor drives around the feedroom, although these are brushless. Numerous possibilities of interference. In my experience, these are not a problem, since the designer has to worry about interference levels that could induce false triggering and instability. C. HVAC All the circuits I have seen use solid state switches to turn things on and off. Proportional temperature control is used. There may be contactors in each of the 2 refrigerant pump circuits, but they only cycle if the on-line one fails. The chiller goes on/off at roughly 35-36 degrees F. D. Telescope Access Lighting- All circuits are in conduit. There are 3 zones of telescope access lighting using 4 control contactors (2 on alidade, and 2 on tilting structure). A several line bus at 120 volts ac is used to control this zone lighting, each zone having contactors as indicated above. E. Automatic and Manual Power Switches- There are several automatic power switches, and 1 manual switch. These are not likely to be a problem, but in any event the power transfer contactors have snubbers across the coils. I only bring this up to suggest that the use of these (or even better thermistors) will lessen the chance of interference, and lengthen the life of the contactor considerably. F. Generator Load Bank- Automatically and manually adjusted to keep adequate load on 600 kw generator. This is done with a number of fixed resistors which are switched in and out by relays. This load operates at 480 volts.