VLF COMMUNICATIONS


INTRODUCTION

The VLF (Very Low Frequency) part of the electromagnetic spectrum is defined as covering the range from 3 to 30 kHz. There are natural signals that exist in this range in space, but they have not been used for man made communication away from the Earth. It has been used for terrestrial communication, particularly for communication with submarines.

VLF, with wavelengths from 10 to 100 km and the existence of an ionosphere around the Earth offers reliable worldwide propagation although it does require high power to achieve this. It has been used for navigation, standard signalling and where surface penetration is required.

VLF also has scientific uses to explore the near space environment and as means of radiation belt remediation when upper atmospheric nuclear detonations have contaminated this environment with artificially enhanced radiation belts.


VLF TRANSMITTERS

VLF propagation requires large and powerful transmitters and large antennae. The latter are usually one electrical quarter wave electrical antenna. Because it is not possible to make a physical quarter wave vertical antenna, a shorter physical antenna is bottom loaded with a large inductance and top loaded with 'top hat' capacitance to produce the electrical quarter wave.

Because of the high powers involved VLF transmitters continued to use vacuum tube technology long after higher frequency equipment was fully solid state. US VLF transmitter manufacturers included Continental and Collins (Rockwell Collins).

The diagram below shows a full VLF transmitting station with not only the transmitter but also the loading inductors.

Powers range from a few tens of kilowatts to over a megawatt. Most final power amplifiers are still vacuum tube although exciters and intermediate power amps have mostly been converted to solid state electronics. The image below right shows the ceramic vacuum tubes of the final power amplifier sitting on metallic stands containing fluid to remove the large amount of heat produced.

Transmitting efficiencies range from 10 to 50%, mostly dependent upon the antenna and the conductivity of the ground on which it stands. It is sometimes arranged to flood the terrain underneath the antenna to increase its conductivity, particularly during times of drought.

A large one megawatt VLF station may require its own power station (of 2MW capacity in the case of 50% transmitter efficiency) to supply the power involved. Backup generators must be able to assume the load within seconds if continuous transmission is required for security purposes.

Because of the limited bandwidth available at these frequencies, modulation is generally one of the following:

  1. On/Off Keying (OOK)

  2. Frequency/Phase shift keying (FSK/PSK)

  3. Minimum Shift Keying (MSK) - a PSK variant

Minimum shift keying is the preferred variant which changes the signal phase smoothly at the signal max or min values (instead of at the zero crossing points). This places less strain on the power system. OOK is particularly bad in this regard as the power station fluctuations are intense.

Bandwidth is typically 50 to 200 Hz. The wider bandwidth is often multiplexed between 2 to 4 modulation sources or streams (channels).