VLF Transmitter Cutler - Antenna

Antenna

The extensive antenna system consists of two separate identical arrays, designated the “north array” and the “south array”.

Each array consists of a ring of 13 tall metal masts connected at the top by a network of horizontal cables. The cables form six diamond-shaped "panels" radiating from the central tower in a hexagonal pattern shaped like a snowflake. The two arrays normally operate together as one antenna, but each is designed to function independently to allow maintenance on the other array. The central tower of each antenna system is 304 m (997.5 ft) tall. It is surrounded by six 266.7 m (875 ft) tall masts, placed on a ring with a radius of 556 m around the central tower. The remaining six towers of the array are 243.5 m (799 ft) tall and placed on a circle of 935.7 m (3070 ft) around the central tower.

Each element (“panel”) of the antenna is suspended between the central tower, two towers of the inner ring and one tower of the outer ring. The entire array is 6140 ft (1.871 km or 1.16 miles) in diameter.

This type of antenna array is called a trideco or umbrella antenna. It functions as a capacitively top-loaded electrically short monopole antenna. The vertical steel masts of the antenna radiate the VLF radio waves, while the suspended cable array functions as a large capacitor, increasing the efficiency of the vertical radiators, At the base of each mast is a star-shaped network of cables suspended a few feet above the ground, extending out hundreds of feet, called a counterpoise, which serves as the bottom plate of the "capacitor".

The climate in Maine results in severe icing of the antenna wires during the winter, resulting in unacceptably large loads on the supporting structures. Therefore the antennas have a deicing system which runs large 60 Hz electric currents through the wires, heating them, to melt the ice. The power required for deicing is 3 MW, higher than the transmitter output power. An antenna array cannot transmit while it is being deiced, and one reason for having two arrays was to allow one array to be deiced while the other provides crucial uninterrupted transmission capability. Repairs and adjustments to an array can also be made without interrupting transmission.