Research for mobile satellite tracking on a bike.

general thoughts on building a rotor ourselves:
http://www.eham.net/ehamforum/smf/index.php?topic=60746.0
Lets see, you will need:
1. A reversible motor.
2. A braking system.
3. The necessary gears.
4. A weatherproof housing.
5. Transformer.
6. Direction switch.
7. Starter capacitor.

The bottom line is that unless you have a machine shop and a bunch of materials laying around it will probably be less expensive to purchase a rotor.
they basically say: go for a rotor you can buy, it is worth the money.

For automatic selection of satellites and tracking of inclining satellites. Ready for yagi and radar antennas. Intelligent self-learning control system.
Designed for commercially available linear motors (actuators) with any discretional lift as well as for heavy geared motors (AC as well as DC motors, e. g. 3-phase models). 
http://www.egis.org/autot_us.html


diverse rotors incl prices
http://www.eham.net/reviews/products/24

https://gitorious.org/autotracker

An Antenna Rotator Controller
A very simple controller circuit, with fairly smart performance! 
http://www.qsl.net/zl1bpu/MICRO/ROTATOR/Index.htm