TiltAz
Mounted Antenna
John Bunton CSIRO Telecommunications and Industrial
Physics, Australia john.bunton@tip.csiro.au
Abstract
It is almost
certain that SKA antennas will need mechanical motion of some type to achieve
full sky coverage for frequencies above 1GHz.
A wheel on track azimuth rotator is a low-cost and low-maintenance
option for providing this motion around one axis.
Use of an
azimuth rotator leads naturally to the TiltAz antenna mount proposed in this
paper. Any type of antenna that can provide
900 of scanning in the elevation plane will give full-sky coverage
with this mount. This includes
cylindrical reflectors and arrays of parabolic reflectors or Luneburg
lenses. For the latter, the TiltAz
mount allows close-to-filled aperture designs which increase the field of view
and eliminate grating lobes.
The Mechanical Imperative
It is
generally conceded that phased arrays are uneconomic much beyond a frequency of
1GHz and for most designs suitable for the SKA the number of mechanical drives
exceeds 7,000.
→
SKA antennas will have mechanical motion and we must minimise maintenance costs
This
can be done by –
¨
Reducing
the number to be maintained
¨
Reducing
the cost of maintaining each mechanical component
¨
Increasing
reliability
The first two
methods of reducing cost tend to work in opposing directions. Consider parabolic reflectors: replacing
components on a small reflector is much easier than on large dishes. On a larger reflector, elevation bearing
replacement may need weeks of work compared to only hours on a small
reflector. But in terms of time per
unit of collecting area the difference is small. Instead
→Choose designs that are inherently easy to maintain
→Wheel on Track
Azimuth rotator
This
provides
¨
ground
level access;
¨
mechanical
components are easy to replace.
The
need for the last point was impressed on the author after replacing the HA
drive pinion on a 14m diameter dish.
TiltAz Antenna Mount
The
antenna on an azimuth rotator need only scan in elevation by ±450
around 450. Maximum sky
coverage is achieved by tilting the antenna to the preferred look direction of
450. For such designs the
resulting antenna can be called a Tilted antenna on an Azimuth mount or TiltAz mounted antenna.
The
choice of antenna to mount on an azimuth rotator is very large and includes–
¨
Electronically
scanned phased arrays and cylindrical reflectors (eg
doublet as in Figure 1)
¨
Feed
scanned spherical reflectors, Luneburg lenses and R‑KR lenses
¨
Mechanically
scanned parabolic reflectors (AltAz mount)
¨
Arrays
of any of the above
Cylindrical and Spherical Reflector TiltAz Antennas
An existing cylindrical reflector TiltAz antenna is the
doublet[1] which provides a large field of view and effective area at
low cost. Alternatively, using a
spherical reflector trades aperture efficiency for a reduction in the number of
feeds, and allows operation to 22 GHz.
For both designs, the surface can be made very stable because of the
box-like nature of the backing structure and the lack of varying gravitational
distortion.
Figure 1 The Doublet or Dual Offset Cylindrical
Reflector TiltAz antenna
Arrayed Reflector or Lens TiltAz Antenna
Arrays
of reflectors or lenses suffer from a very low filling factor if they are to
observe sources at low elevations.
Mounting such arrays on a TiltAz mount can increase the filling (also
proposed by Weinreb[2]). As the whole antenna rotates for azimuth
steering, the individual antennas can be close packed horizontally. In the orthogonal direction along the tilted
plane the antennas can be packed with 80% efficiency. An example is shown in Figure 2.
Figure 2 Diagram showing general arrangement of
parabolic reflectors or Luneburg lenses on a TiltAz mount
The advantages of arrays on a TiltAz
mount are–