  Using detailed Monte Carlo simulations we have characterized the features of
the radio emission of inclined air showers in the Ultra-High Frequency band
(300 MHz - 3 GHz). The Fourier-spectrum of the radiation is shown to have a
sizable intensity well into the GHz frequency range. The emission is mainly due
to transverse currents induced by the geomagnetic field and to the excess
charge produced by the Askaryan effect. At these frequencies only a
significantly reduced volume of the shower around the axis contributes
coherently to the signal observed on the ground. The size of the coherently
emitting volume depends on frequency, shower geometry and observer position,
and is interpreted in terms of the relative time delays. At ground level, the
maximum emission at high frequencies is concentrated in an elliptical ring-like
region around the intersection of a Cherenkov cone with its vertex at shower
maximum and the ground. The frequency spectrum of inclined showers when
observed at positions that view shower maximum in the Cherenkov direction, is
shown to be in broad agreement with the pulses detected by the Antarctic
Impulsive Transient Antenna (ANITA) experiment, making the interpretation that
they are due to Ultra-High Energy Cosmic Ray atmospheric showers consistent
with our simulations. These results are also of great importance for
experiments aiming to detect molecular bremsstrahlung radiation in the GHz
range as they present an important background for its detection.
