regular grid of dual-polarisation dipole elements arranged on a 4m x 4m steel mesh ground plane. Each antenna (with its 16 dipoles) is known as a "tile". Signals from each dipole pass through a low noise amplifier (LNA) and are combined in an analogue beamformer to produce tile beams on the sky. Beamformers sit next to the tiles in the field. The radio frequency (RF) signals from the tile-beams are transmitted to a receiver, each receiver being able to process the signals from a group of eight tiles. Receivers therefore sit in the field, close to groups of eight tiles; cables between receivers and beamformers carry data, power and control signals. Power for the receivers is provided from a central generator. The receiver contains analogue elements to condition the signals in preparation for sampling and digitisation. The frequency range 80–300 MHz is Nyquist-sampled at high precision. Digital elements in the receiver (after the digitiser) are used to transform the time-series data to the frequency domain with a 1.28 MHz resolution – 5 bits real and 5 bits imaginary for each resolution element. Sets of 1.28 MHz coarse frequency channels are transmitted via an optical fibre connection to the correlator subsystem, located in the CSIRO Data Processing Facility near the MWA site. MWA shares the CSIRO facility with the ASKAP program. In Phase I the majority of the tiles (112) were scattered across a roughly 1.5 km core region, forming an array with very high imaging quality, and a field of view of several hundred square degrees at a resolution of several arcminutes. The remaining 16 tiles are placed at locations outside the core, yielding baseline distances of about 3 km to allow higher angular resolution observations. In Phase II the MWA operated in two configurations, a compact configuration and an extended configuration of 128 tiles each. The compact configuration consists of seven Phase I receivers and 56 tiles, plus 72 new tiles arranged in two dense hexagonal configurations each of 36 close-packed tiles. The new hexagonal super tiles in the compact configuration make use of the concept of "redundant spacings" to help calibrate the array to high precision for detection of the EoR. The extended configuration consists of nine Phase I receivers and 72 original tiles, plus an additional 56 new long baseline tiles which provide baselines distances of about 5 km. The original correlator subsystem comprises Poly-phase Filter Bank