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Jim Creek Naval Radio Station
Jim Creek Naval Radio Station is a United States Navy very low frequency (VLF) radio transmitter facility at Jim Creek near Oso, Washington. The primary mission of this site is to communicate orders one-way to submarines of the Pacific fleet while submerged. Established in 1953, the transmitter radiates on 24.8 kHz with a power of 1.2 megawatts and a callsign of NLK, and is one of the most powerful transmitters in the world.〔(【引用サイトリンク】title=Jim Creek Naval Radio Station )〕 Located near Arlington, Washington, in the foothills of the Cascades, north of Seattle, the site has 5,000 largely forested acres.
==Antenna==
Much of the site is devoted to the enormous overhead wire antenna array that is necessary to efficiently radiate the VLF waves. The antenna, shown above, consists of ten "catenary" cables, 5640–8700 ft (1719–2652 m, 1.1–1.6 mile) long, suspended on twelve 200 ft towers over the valley between Wheeler mountain and Blue mountain in a zigzag pattern. Each cable receives energy from a vertical cable attached at the center, which drops down to the valley floor where it is fed by one of two "bus" transmission lines which extend along the valley from the transmitter building in the center.
This type of antenna, called a "valley-span" antenna, functions as a capacitively top-loaded electrically short monopole antenna. The vertical cables are the main radiating elements, and the horizontal cables serve to add capacitance to the top of the antenna, to increase the power radiated. The antenna is divided into two sections of 5 elements, each fed with its own transmission line. These normally operate together as one antenna, but can operate separately so one section can be shut down for maintenance without interrupting transmission. The floor of the valley under the antenna is covered with a network of cables suspended a few feet above the ground which functions as a counterpoise ground system.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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