  The Tunka Radio Extension (Tunka-Rex) is an array of 44 radio antenna
stations, distributed over 3 km$^{2}$, constituting a radio detector for air
showers with an energy threshold around 10$^{17}$ eV. It is an extension to
Tunka-133, an air-Cherenkov detector in Siberia, which is used as an external
trigger for Tunka-Rex and provides a reliable reconstruction of energy and
shower maximum. Each antenna station consists of two perpendicularly aligned
active antennas, called SALLAs. An antenna calibration of the SALLA with a
commercial reference source enables us to reconstruct the detected radio signal
on an absolute scale. Since the same reference source was used for the
calibration of LOPES and, in a calibration campaign in 2014, also for LOFAR,
these three experiments now have a consistent calibration and, therefore,
absolute scale. This was a key ingredient to resolve a longer standing
contradiction between measurements of two calibrated experiments. We will
present how the calibration was performed and compare radio measurements of air
showers from Tunka-Rex to model calculations with the radio simulation code
CoREAS, confirming it within the scale uncertainty of the calibration of 18%.
