Very Low Frequency - Antennas

Antennas

A major practical drawback to this band is that because of the length of the waves, full size resonant antennas (half wave dipole or quarter wave monopole antennas) cannot be built because of their physical height. Vertical antennas must be used because VLF waves propagate in vertical polarization, but a quarter-wave vertical antenna at 30 kHz would be 2.5 kilometers high. So practical transmitting antennas are electrically short, a small fraction of a wavelength long. Due to their low radiation resistance they are inefficient, radiating only 10% to 20% of the transmitter power at most. with the rest of the power dissipated in the antenna/ground system resistances. So very high power transmitters (~1 megawatt) are required to radiate enough power for long distance communication.

Transmitting antennas for VLF frequencies are very large wire antennas, up to a mile across. 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 efficiency of the antenna. High power stations use variations on the umbrella antenna such as the "delta" and "trideco" antennas, or multiply-tuned flattop (triatic) antennas. For low power transmitters, inverted-L and T antennas are used.

To minimize power dissipated in the ground, these antennas require extremely low resistance ground systems. Because of soil resistance, counterpoise systems are often used, consisting of radial networks of copper cables supported several feet above the ground under the antenna, extending out radially from the mast or vertical element.

The requirements for receiving antennas are less stringent, because the transmitter power is generally high to overcome atmospheric noise. Atmospheric noise and not receiver noise determines the signal to noise ratio, so small inefficient receiving antennas can be used. Loop antennas are often used for reception.