 |
2. Antenna Structure |
|
|
2. Antenna Structure |
2.2 Quadrupod and Secondary Reflector
The primary reflector backup structure substains the secondary mirror, placed at a distance of 9 m, through 4x45° inclined beams (quadrupod).
The secondary mirror is a hyperbolic reflector, 3.2 m diameter, made of a single aluminium panel (rms = 0.35 mm). On the backup structure 3 mechanical actuators are installed and allow the mirror to tilt around the 3 axes. In addition the whole system can translate along the x and y axes.
Fig. 2.2 : Hyperbolic mirror
The mirror must completely be retracted along the y axis when the primary focus is used.
Fig. 2.3 : Configuration for Cassegrain focus usage (plain line) and primary focus usage (dotted line)
The mirror and the quadrupod induce an obstruction on the primary reflector of nearly 4%.
|
Cause |
Obstruction |
| Sub-reflector | 2 % |
| Quadrupod | 2 % |
|
Total |
4 % |
Tab. 2.2 : Primary reflector obstruction
The system that rotates the secondary mirror has been optimized in order to enhance the number of receivers that can be installed at the Cassegrain focus, but actually it is used only to realize the "Wobbling" technique, using one receiver at once; typical shifting times, shorter then those used in Position Switching, are listed in the following table.
|
Frequency (GHz) |
Beam HPBW (“) |
Mirror rotation 2.5 beam (°) |
Required time (sec) |
Mirror rotation 5 beam (°) |
Required time (sec) |
| 5 | 450 | 2.56 | 1.16 | 5.12 | 2.12 |
| 6 | 390 | 2.22 | 1.03 | 4.44 | 1.86 |
| 22 | 120 | 0.68 | 0.45 | 1.37 | 0.71 |
Tab. 2.3 : Wobbling time for 2.5-beam and 5-beam throws
