The Ionospheric Research Instrument (IRI) at HAARP is an HF antenna array that is currently the most powerful, and the most versatile, in the world. It transmits electromagnetic radiation towards the ionosphere in the radio frequency range.
The array consists of 180 towers, each with 2 pairs of crossed dipole antennas (see Jackson, Chapter 9). They are arranged in a 12 by 15 grid that takes up 33 acres. The array produces a beam in the HF range (2.8-10 MHz) which is controlled by the operations center located at HAARP. The phase of each tower's radiation can be changed to produce a beam (or a split beam) that can be directed to a range of locations in the ionsphere above HAARP. The beam can be modulated and moved, depending on the needs of the particular research group.
The antennas are powered by an on-site power plant consisting of five 2500 kW generators, each powered by a 3600 hp diesel engine (a diesel truck is on the order of a few hundred hp). The engines are referred to as Angels 1-5. Apparently even engineers have a sense of humor.