  Cosmic ray air showers emit radio pulses at MHz frequencies, which can be
measured with radio antenna arrays - like LOPES at the Karlsruhe Institute of
Technology in Germany. To improve the understanding of the radio emission, we
test theoretical descriptions with measured data. The observables used for
these tests are the absolute amplitude of the radio signal, and the shape of
the radio lateral distribution. We compare lateral distributions of more than
500 LOPES events with two recent and public Monte Carlo simulation codes, REAS
3.11 and CoREAS (v 1.0). The absolute radio amplitudes predicted by REAS 3.11
are in good agreement with the LOPES measurements. The amplitudes predicted by
CoREAS are lower by a factor of two, and marginally compatible with the LOPES
measurements within the systematic scale uncertainties. In contrast to any
previous versions of REAS, REAS 3.11 and CoREAS now reproduce the shape of the
measured lateral distributions correctly. This reflects a remarkable progress
compared to the situation a few years ago, and it seems that the main processes
for the radio emission of air showers are now understood: The emission is
mainly due to the geomagnetic deflection of the electrons and positrons in the
shower. Less important but not negligible is the Askaryan effect (net charge
variation). Moreover, we confirm that the refractive index of the air plays an
important role, since it changes the coherence conditions for the emission:
Only the new simulations including the refractive index can reproduce rising
lateral distributions which we observe in a few LOPES events. Finally, we show
that the lateral distribution is sensitive to the energy and the mass of the
primary cosmic ray particles.
