High power transmitting antennas for VLF frequencies are very large wire antennas, up to several kilometers long. They consist of a series of steel radio masts, linked at the top with a network of cables, often shaped like an umbrella or clotheslines. Either the towers themselves or vertical wires serve as monopole radiators, and the horizontal cables form a "capacitive top-load" to increase the current in the vertical wires, increasing the radiated power and efficiency of the antenna. High power stations use variations on the umbrella antenna such as the "delta" and "trideco" antennas, or multiwire flattop (triatic) antennas. For low-power transmitters, inverted-L and T antennas are used.