  We present predictions for the radio pulses emitted by extensive air showers
using ZHAireS, an AIRES-based Monte Carlo code that takes into account the full
complexity of ultra-high energy cosmic-ray induced shower development in the
atmosphere, and allows the calculation of the electric field in both the time
and frequency domains. We do not presuppose any emission mechanism, but our
results are compatible with a superposition of geomagnetic and charge excess
radio emission effects. We investigate the polarization of the electric field
as well as the effects of the refractive index n and shower geometry on the
radio pulses. We show that geometry, coupled to the relativistic effects that
appear when using a realistic n>1, play a prominent role on the radio emission
of air showers.
