Think of Alaska and the Aurora Borealis comes to mind. Solar particles are captured by Earth’s magnetic field and flung to the North and South magnetic poles. There, they collide with atmospheric gases to emit photons of colored light, resulting in breathtaking displays of shimmering green, red, blue or violet.
There’s more to the ionosphere at auroral latitudes than celestial beauty, and that’s just what the U.S. government’s High Frequency Active Auroral Research Program (HAARP) facility aims to find out. One of a number of global sites devoted to upper atmospheric and solar-terrestrial research via a phased array transmitter, HAARP is intended to help researchers improve the general understanding of ionospheric behavior and properties.
Located near Gakona, AK, the HAARP phased array radio transmitter (or ionospheric research instrument) comprises a planar array of 180 multiple transmitter and antenna elements that transmit RF signals in an upward direction as a focused beam. By introducing this small, known amount of energy into a specific ionospheric layer, the complex physical processes that occur in plasma regions created each day by the sun can be studied.
Designed, installed and operated by the engineering group of Advanced Power Technologies (now owned by BAE Systems), the HAARP array commenced operations in 1993 with 18 antenna elements. By 1998, this had expanded to 48. Now, the array is undergoing expansion to a total of 180 antenna elements to cover an area of 36 acres.
The final expansion will take the array to full size and will increase effective radiated power (ERP) from 84dBW to around 96dBW.
Radio Frequency Systems (RFS) has supplied all outdoor coaxial transmission line to the HAARP facility since its inception. As a result, many feet of foam-dielectric CELLFLEX cable provide the critical link between each specially designed radio transmitter located on the ground and its corresponding antenna dipoles on each 72ft-tall mast.
Each antenna element is composed of two pairs of crossed dipoles at the top of the mast. The upper crossed dipoles are used for low-band operating frequencies between 2.8MHz and 8.1MHz. And a pair of wire-crossed dipoles mounted lower on the tower is used for high-band operating frequencies between 7MHz and 10MHz. For both the low- and high-band crossed dipoles, the north-south (NS) and east-west (EW) dipoles are treated independently. In other words, two main 1-5/8in-transmission line lengths are required for each antenna mast — one to feed the NS dipoles (low- and high-bands), and the other to feed the EW dipoles (low- and high-bands). Switching between the high- and low-band dipoles is achieved via a mast-mounted RF switch box, into which the main input NS and EW transmission line lengths are connected.
At the other end of the transmission line is the dual 10kW transmitter: one half of the transmitter drives whichever NS dipole is in operation, and the other the corresponding EW dipole.
Instead of transmitting a single modulation, they can transmit a variety, including FM, AM, pulse and continuous wave. Other features of the transmitters, jointly designed by BAE Systems and Continental Electronics, include an ultra-clean signal to minimize interference with other communications systems; sophisticated phase manipulation to achieve precision beam steering control of the array; tunability to any frequency between 2MHz and 10MHz and rugged ceramic metal transmitting tubes.
The extreme freeze-thaw cycles unique to Alaska — ranging from -60 degrees Fahrenheit in the depths of winter to peak summer temperatures of 90 degrees Fahrenheit — also presented a challenge in feed system design. With the operating conditions so far beyond normal, BAE Systems needed a solution to further protect the cable connectors on the mast from water ingress.
The first part of the solution was the use of specialized PLAST2000 EIA-flanged connectors. PLAST2000 is a specially designed silicone material that is injected into the connector as a sealant. It helps provide a barrier for any condensation.
Additionally, RFS pioneered the development of a silicone shrink boot, which is applied to the back of the connectors to provide extra stability and protection from water penetration.
The 180-mast HAARP array is scheduled for completion in 2007. Twelve of new masts are already in service, having been fast-tracked to prove the design of the high-band antenna system. If HAARP lives up to its potential, it will reveal new insights into the ways the ionosphere responds to a wide variety of natural conditions, and through this make great leaps forward in the understanding of satellite communication systems and the unique physics of the Earth’s ionosphere.