  Air showers induced by cosmic rays create nanosecond pulses detectable at
radio frequencies. These pulses have been measured successfully in the past few
years at the LOw Frequency ARray (LOFAR) and are used to study the properties
of cosmic rays. For a complete understanding of this phenomenon and the
underlying physical processes, an absolute calibration of the detecting antenna
system is needed. We present three approaches that were used to check and
improve the antenna model of LOFAR and to provide an absolute calibration of
the whole system for air shower measurements. Two methods are based on
calibrated reference sources and one on a calibration approach using the
diffuse radio emission of the Galaxy, optimized for short data-sets. An
accuracy of 19% in amplitude is reached. The absolute calibration is also
compared to predictions from air shower simulations. These results are used to
set an absolute energy scale for air shower measurements and can be used as a
basis for an absolute scale for the measurement of astronomical transients with
LOFAR.
