I like to understand things... 
Why does an antenna radiate? What is a electro-magnetic field? What makes a antenna resonant?
On the Internet there are a lot of good sites which explain the basic principles and theory of antennas. No need to repeat it here.
Therefore a list of sites I discovered:
- antenna-theory.com
- wikipedia.org/wiki/Antenna_(radio)
- wikipedia.org/wiki/Transmission_line
- AntennaTheory (www.cv.nrao.edu)
