  The generic properties of the emission of coherent radiation from a moving
charge distribution are discussed. The general structure of the charge and
current distributions in an extensive air shower are derived. These are
subsequently used to develop a very intuitive picture for the properties of the
emitted radio pulse. Using this picture can be seen that the structure of the
pulse is a direct reflection of the shower profile. At higher frequencies the
emission is suppressed because the wavelength is shorter than the important
length scale in the shower. It is shown that radio emission can be used to
distinguish proton and iron induced air showers.
