English: Drawing of large wire antenna used by US Navy to communicate with submarines at VLF frequencies, on US naval base at Yosami, Japan. It consists of a string of parallel horizontal wires 4575 ft long, suspended on 800 ft towers, fed at one end by feeders slanting down diagonally to the transmitter. This is called a "flatttop" antenna It is used at frequencies in the VLF band, around 20 kHz, because these frequencies can penetrate water to reach submarines. At these frequencies, with wavelengths of around 15 km, even this large antenna is a small fraction of a wavelength long, so it has a very low radiation resistance of less than an ohm. Therefore the resistances of the antenna/ground system have to be extremely low to avoid wasting energy. The vertical wires function as radiators, while the huge horizontal "flattop" serves as a capacitive "top-load" to increase the current in the vertical portion. Multiple parallel wires are used to increase capacitance and decrease the resistance. The ground under the antenna is covered by a "counterpoise" (not shown), a network of wires a few feet above the ground, which functions as the bottom plate of a huge capacitor.