Haarp

The Developmental Prototype (DP) had 18 antenna elements, organized in three columns by six rows. It was fed with a total of 360 kilowatts (kW) combined transmitter output power.

The DP transmitted just enough power for the most basic of ionospheric testing.
The Filled Developmental Prototype (FDP) had 48 antenna units arrayed in six columns by eight rows, with 960 kW of transmitter power. This was used for a number of successful scientific experiments and ionospheric exploration campaigns over the years.
The Final IRI (FIRI) will be the final build of the IRI.

It has 180 antenna units, organized in 15 columns by 12 rows, yielding a theoretical maximum gain of 31 dB. As of the summer of 2005, all the antennas were in place, but the final quota of transmitters had not yet been installed.

As of March 2007, the final phase was completed and the antenna array was undergoing testing aimed at fine-tuning its performance to comply with safety requirements required by regulatory agencies.
Each antenna element consists of a crossed dipole that can be polarized for linear, ordinary mode (O-mode), or extraordinary mode (X-mode) transmission and reception. Each part of the two section crossed dipoles are individually fed from a custom built transmitter, that has been specially designed with very low distortion.

The ERP of the IRI is limited by more than a factor of 10 at its lower operating frequencies. This frequency range lies above the AM radio broadcast band and well below Citizens' Band frequency allocations.