|
The Ka-band frequency spectrum has recently gained attention thanks to the successful launch of several new satellite services, all of which utilize the advantages of a high frequency range. The beam of an 18.3 to 20.2-GHz signal transmitted from a satellite to an earth station antenna has the advantage of a smaller beam width, which enables smaller footprints and creates the opportunity for considerable frequency reuse and much higher power. However, the significant advantages gained when using Ka-band come with unique challenges for earth station antenna (ESA) engineers.
Ka-band gateway antennas generally range in size from 3.5 to 8.1 meters (Figure 1). While all parabolic antennas share similar characteristics, it generally is true that the larger an ESA antenna is, the smaller the signal's beam width will be. Similarly, the higher the frequency, the narrower the beam width will be. It is obvious then, that the beam width of a Ka-band 8.1 m antenna will have a much narrower beam width than a 4.5 m Ku-band antenna. This narrow beam width requires precise antenna surface regularity, high pointing accuracy, and calculations for rain attenuation, wind deviation, and other design characteristics that need to be accounted for. This article focuses on sub-reflector tracking, which is a method for overcoming the challenge of achieving increased pointing accuracy.
1. Andrew's 8.1 meter Ka-band SRT antenna.
Sub-Reflector Tracking (SRT)
Conventional ESA antennas use either step track or monopulse tracking techniques. At Ka-band frequencies, however, the antenna beam widths approach the size of pointing errors associated with the limited motion mounts typically used on ESAs. Conventional antenna mount structures have limitations when pressed into service for Ka-band, even though they may work perfectly well for Ku- and K-band applications.
Problems areas like movement screw backlash, axis bearing wobble, structure windup, and mount wind deflection can severely compromise the integrity of step track algorithms and become significant contributors to step tracking error. When step tracking a conventional 8 m Ka-band on a limited motion mount, the tracking losses from operational winds are significant enough to render the effective achievable gain toward the satellite as comparable to a 6 m antenna or smaller. The traditional solution for improving tracking error losses on an antenna with this much gain (>65 dBi) is to implement a very costly closed loop servo drive monopulse tracking controller.
Sub-reflector tracking (SRT) steering of the antenna beam, in place of beam steering via movement of the main reflector, has significant cost advantages and yields negligible operational constraints. The cost savings are attributed to (1) a significant reduction in the complexity of the mount, (2) elimination of the massive tracking servo system, (3) elimination of the precision position indicator subsystem, (4) allowing for a significantly stiffer mount and reflector structure design because neither has to move, and (5) allowing for the use of a low-cost step track controller.
A SRT mount design works in concert with the recommended foundation design to significantly enhance total antenna system stiffness over a motion mount design. Further analysis has shown that the removal of the ground struts has little to no impact on the performance of the antenna. The recommended foundation includes a reinforced concrete column that supports the mount turning head steel superframe. Incremental foundation costs that include the column are small compared to the costs associated with an equivalently strong steel structure, including costs to ship and erect it. The accuracy or verticality of the column is of no importance because tracking is performed on orthogonal axes on the sub-reflector servo carriage assembly. The turning head assembly is equipped with motors that will allow motion in both azimuth and elevation axes.
SRT products can eliminate or significantly reduce the tracking error components comprising the tracking error budget. In the Andrew Corp. SRT antenna, for example, step tracking losses are minimized as a result of improved mount rigidity and elimination of movement actuators, jackscrews, or movement bearings in the load paths during operation. Taking this type of approach enables superior tracking in step track mode and makes a more costly monopulse tracking subsystem unnecessary.
The SRT shown in Figure 1 keeps an 8.1 m Ka-band antenna beam within the orbital box, while allowing the large basic antenna structure to be fixed and locked down to the concrete foundation. As compared to conventional AZ/EL motion mount antennas, this type of design offers the following advantages: lower cost, longer life, higher reliability, improved stiffness and structural rigidity, and higher wind load capability.
Some key features of an SRT design include:
- A fixed antenna structure with a moveable scanning sub-reflector for stable, long-life tracking operation in high winds
- Dual reflectors using an axis-symmetric Gregorian feed system with shaped optics to assure low cross polarization performance (stable beam position) and very high efficiency
- A small corrugated feed horn with circular symmetry, which achieves closely matched patterns for all polarizations and assures low cross polarization performance with the integrated, high velocity air system, keeping all moisture and ice off the feed window
- A mechanical structure with a rigid turning head assembly fastened to a concrete column with full hemispheric adjustment allowing installation at any location; a 2.1-m diameter central hub housing with adequate room to mount phase-combined or redundant high-power amplifiers (HPAs), microwave converters, and other electronics close to the feed for low loss
|
