There are many antenna types. A lot of bots use a small inductor loaded whip antenna.
 

Antenna Type	Pros	Cons
1/4 wave	Good directional control Not as dependent on ground plane	Size (several feet)
inductor loaded Whip	Smaller	Must extend outside of bot several inches
Top Hat	Very small (1 inch)	Dependent on battlebot construction

The whip antenna must have 1/2 inch clearance from any metal. This includes the mounting base of the antenna.

Top Hat antennas

This antenna may not look like much, however it gets 80 feet of range with the transmitter antenna down.

The inductor came from a old TV. I removed the wire, punched two holes and then spiraled the antenna wire around the spool. In the above picture I cut off the snap end and glued the shaft part in a hole drilled in the aluminum. The direction that the wire is wound is not important, however it is very important that the wire does not cross itself since this can sometimes cause strange effects. The wire then connects to the 2 inch x 1.5 inch piece of aluminum. I used aluminum soldering flux to attach the wire to the aluminum foil. By using a inductor with a large (>.5 inch) ferrite core I did not have to be as concerned with the exact number of winding and size of the aluminum piece since this gave me a large tuning range.
 

Tuning
To tune the unit I simply turned the transmitter on and left the antenna down. I started close and then saw how far away the unit functioned. I then turned the core up or down and then saw if it got better or worse. Once it worked at 80 feet I kept turning it until it stopped working at 80 feet and then turned it between the two points. (count turns)