Image: Trideco

Description: Diagram of "Trideco" type VLF antenna. This is a very large wire antenna, often over a mile in diameter, used by powerful transmitters in the very low frequency band. A type of umbrella antenna, it consists of 13 steel radio towers arranged in two concentric circles with a network of cables linking the tops. The cables are arranged in six "panels" extending from the central tower in a hexagonal "snowflake" pattern. The central tower usually serves as the radiating element, acting as a monopole radiator, while the horizontal suspended cables act as a large capacitive top-load, forming a capacitor with the ground, which increases the current in the vertical element, increasing the radiated power. The transmitter power is applied between the bottom of the central tower and a ground connection. VLF antennas require an extremely low resistance ground, so the ground under the antenna is usually covered by a counterpoise, a radial screen of wires suspended a few feet off the ground. A major use of trideco antennas is in powerful Naval transmitters which communicate with submerged submarines.
Title: Trideco
Credit: NAVELEX 0101 113, Fig. 3-7
Author: US Navy, Naval Electronics Systems Command
Usage Terms: Public domain
License: Public domain
Attribution Required?: No