English: Diagram of the antenna array at the U.S. Navy Cutler VLF transmitter facility at Cutler, Maine. It transmits operational orders one-way to submerged U.S. submarines worldwide at a frequency of 24 kHz. With a transmitter power of 1.8 megawatts, it is known as the most powerful radio transmitter in the world. It consists of two identical antenna arrays called the 'North array' and 'South array', driven from a transmitter building between them. This type of antenna is called a trideco or umbrella antenna, a form of capacitively top-loaded electrically-short monopole antenna. Each array consists of a hexagonal pattern of 13 vertical steel masts attached at the top to a system of 6 diamond-shaped fans of cables suspended above the ground. The vertical masts actually radiate the VLF radio waves, while the top cables form a large capacitor with the ground, which serves to increase the efficiency of the antenna. Under each mast there is also a system of cables suspended a few feet above the ground radiating from the mast base, called a counterpoise (not shown) which functions as the other plate of the capacitor. Each central mast is 304 m (997.5 ft) high, and has a 'helix house' at its base, which contains massive coils of wire needed to match the antenna to the transmission line, which travels through an underground tunnel from the transmitter.