  In radio astronomy, holography is a commonly used technique to create an
image of the electric field distribution in the aperture of a dish antenna. The
image is used to detect imperfections in the reflector surface. Similarly,
holography can be applied to phased array telescopes, in order to measure the
complex gains of the receive paths of individual antennas. In this paper, a
holographic technique is suggested to calibrate the digital beamformer of a
phased array telescope. The effectiveness of the technique was demonstrated by
applying it on data from the Engineering Development Array 2, one of the
prototype stations of the low frequency component of the Square Kilometre
Array. The calibration method is very quick and requires few resources. In
contrast to holography for dish antennas, it works without a reference antenna.
We demonstrate the utility of this technique for initial station commissioning
and verification as well as for routine station calibration.
