VLF Transmitter Cutler



The VLF Transmitter Cutler is the United States Navy's very low frequency (VLF) shore radio station at Cutler, Maine. The station provides one-way communication to submarines in the Navy's Atlantic Fleet, both on the surface and submerged.

History
The station began operations in 1913 as a radio telegraphy station in Arlington, Virginia, at a facility next to Fort Myer. Although its broadcasts occasionally included band concerts and speeches, it was most famous for its nightly time signals. The current Cutler Naval Station was built in 1960 and became operational on January 4, 1961.

Transmitter
Cutler Naval Station has a transmission power of 1800 kW, and it was at one time, and perhaps still is, the most powerful radio transmitter in the world. As with all VLF stations, the transmitter has a very small bandwidth, and so cannot transmit audio (speech) but only coded text messages, at a very slow data rate. The transmission consists of a continuous encrypted FSK (F1B) signal at 200 baud. The transmitter operates on 24.0 kHz. In the past it operated on 17.8 kHz. The callsign of the station is NAA.

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.

Antenna maintenance
Antenna maintenance is performed during the summer months. During maintenance periods the station transmits on one array while people work on the other array, which is grounded. This allows continuous transmission, crucial since the Navy closed Annapolis (NSS), the only other East Coast VLF station.

The region where the two arrays come close together, near the transmitter house, is called the "bow-tie area". There are two panels and three towers from each array in this area. The fields on the grounded array are highest in the bow-tie area due to proximity to the active array. The present station operating procedure, based on a past RADHAZ survey, does not allow work on the bow-tie area towers or panels while transmitting on the other array. There is an ongoing tower painting project at Cutler scheduled for completion over the next few years. Under the present station policy, completion of this project would require several months of total downtime, which is unacceptable.

Test transmissions have been arranged, during which only four panels of one array shall be connected to the transmitter. The objective of the four-panel tests was to allow painting and normal maintenance on the bow-tie area towers of an inactive array. A secondary objective of the tests is to determine the antenna operating parameters which had not been measured since changing to 24.0 kHz.