Here are some instructions to build a ground plane antenna suitable for FM transmitter:
http://www.geocities.com/tomzi.geo/antenna/antenna.htm
You can also use some other kind of antennas like dipoles. Here are some links with instructions:
http://www.kyes.com/antenna/build.html
http://www.kyes.com/antenna/antennadex.html
Also check the attachment!
Antenna_instructions.pdf