UHF Antenna

To achieve the acceleration there are two 30 hp motors both for azimuth and elevation.

Even though an UHF antenna is strong and sturdy, it is also a precision instrument, which, contrary to most other EISCAT hardware, is standing naked under a rather hostile sky. Occasionally situation can arise that place limitations to its use. These occasions are rare but must be recognized because failure to do so may well stop all measuring actions with that antenna for time intervals ranging from a few days to infinity. Thus, attention should be given to exceptional values of the following factors of weather:

wind speed
temperature
snowfall
large and rapid temperature changes during wintertime.

Wind

The main mirror has

diameter 32 m
cross section 804 m²
total surface area about 1000 m².

Wind blowing perpendicularly to the mirror at 25 m/s causes a load of about 60 tons at the level of the azimuth drive wheels. This kind of wind still allows operations to continue. But according to the statement by the manufacturer, stronger winds will distort the mirror shape and thus modify its receiving properties (the beam shape).

So

UHF measurements are (literally) pointless in winds stronger than about 25 m/s.

And moreover, in such windy situations, one should minimize the antenna area open to wind by either turning the antenna sideways with respect to the wind in azimuth, or lifting the elevation to 90 °. According to the manufacturer, with elevation at 90 ° the antenna will stand up in winds as strong as 45 m/s.

Cold

In temperatures under about -35 ° the metallic structures themselves start to get brittle. Therefore

Rapid temperature variations in winter

When temperature rises rapidly from well below zero to above the freezing point, water from melting snow will flow down to the pintle bearing which is the central azimuthal bearing of the antenna. The pintle bearing is mounted on a massive concrete pillar that will preserve freezing temperature for a long time. Hence the down running water will freeze at the surface of the bearing, forming a very hard and tightly sticking cover. Under these circumstances the Teflon coating of the sliding surface of the bearing (the one that moves with the antenna) is in danger of tearing off when it touches the rough sheet of ice on the fixed surface.

Before the antenna is moved for the first time in the beginning of a sudden thaw, the ice formation situation on the pintle bearing ought to be checked.

Snow

The volume of the bowl formed by the main mirror is of the order of 2000 m³ ( 804 m² × 2.5 m ). Wet snow may have density of the order of 200 kg/m³. If the antenna is kept at elevations 80 .. 90 ° and gets filled with such kind of snow either from direct showfall or (more likely) because of snow lifted by wind during a snowstorm, the weight of the antenna will be increased by about 400 tons. The manufacturer does not recommend this kind of powerlifting exercise, but nevertheless promises that the antenna will not break under it. However,

Failure to stick to the above rule will topple the antenna without exception.

Having above listed the cases when the antenna should not be used, we next turn to the more normal situation.

Antenna operating modes

In the receiver hall the status of the antenna is readable from the lights on the Antenna Control Unit (ACU) front panel. For instance, the parking (sleeping) mode of the antenna is called STAND-BY mode, and correspondingly, this state is marked by having the light with the label STAND-BY in the ACU illuminated. During normal experiments the antenna is kept in COMPUTER mode. In some situations the antenna may also be used in MANUAL mode. In addition there is a special maintenance mode that can be recognized by the REMOTE and/or TRANSFER ENABLE light(s) being illuminated. (If the latter lights are illuminated when one is needing the antenna, the staff person responsible for the antenna maintenance should be contacted).

Moving the antenna

Each of the four antenna motors has a mechanical brake that is released using electromagnetic means. These brakes are used to keep the antenna locked when the antenna is in the STAND-BY mode. However, when the antenna is in COMPUTER mode or in MANUAL mode, the deceleration and locking of the antenna is achieved not by mechanical brakes but by actively controlling the drive motors. As was briefly mentioned above, there are two motors for azimuth, and two motors for elevation. When the antenna pointing is kept fixed, the two motors of a pair are working against each other using about 10% of their maximum power.

This energy consuming way of doing nothing implies both good news and bad news.

The good news is that the arrangement allows a servo system that is monitoring the motors and the antenna to keep the relative pointing direction well within 0.01 ° from the commanded direction, even during strong winds. (The absolute calibrated pointing direction accuracy is at the 0.05 ° level).

The bad news is that the user should remember, especially during summertime, to put the antenna to STAND-BY when the experiment is over. This is because the ventilation of the motors is working well only when driving at full speed. Near zero speed most of the 10% × 30 hp of power must be get rid off by radiation. This usually heats the motors considerably, significantly shortening their expected life time.

To decrease the risk of damage to the drive gear boxes and to slow down the wearing out of the pintle bearing a special feature has been added to Tromsø and Sodankylä antenna control hardware. The circuitry makes it possible for the antenna control software to detect the so called MINOR-FAULT situations. These are instances when one of the four motors is not working. (The MINOR-FAULT situation has its reserved light in the ACU panel.) The control software can then inhibit pointing commands generated in experiment control files from actually going to the antenna. More precisely the effect of this extra safety measure is the following:

In Tromsø

In Sodankylä:

Even while the antenna in MINOR FAULT situations is not respecting file commands, it can be moved by putting it into MANUAL mode. (Actually it can be moved even in COMPUTER mode, using the "POINT-DIR" -command).

In general, the following button pushing exercises in front of the ACU may be tried by the experimenter if the antenna refuses to start moving when the experiment file is giving commands for it:

If no red lights on the ACU are illuminated, check your file/try to give commands from terminal/curse/contact staff.

If one or more red lights are illuminated, push DISABLE both in azimuth and in elevation so that the respective red lights come on and go off. Now also the extra red light(s) should have vanished. But if after about seven seconds the red light comes on again, push POWER, killing the respective green light. Then wait about five seconds, then push POWER again. The green light should now be back. If also the red DISABLED lights came on, push them off, then push COMPUTER. There should be no red lights now. If there are, contact staff.