Very low frequency

Lore & Background

VLF radio waves can diffract around large obstacles and propagate as ground waves following the curvature of the Earth, not limited by the horizon. Ground waves are absorbed by the Earth's resistance and become less important beyond several hundred to a thousand kilometres. The main mode of long-distance propagation is an Earth–ionosphere waveguide mechanism: the conductive D layer of the ionosphere at 60–90 km altitude and the conductive Earth form a horizontal duct a few VLF wavelengths high, confining the waves in transverse magnetic mode. VLF waves have very low path attenuation, 2–3 dB per 1,000 km, with little fading, making transmissions stable and reliable.

VLF waves can penetrate seawater to a depth of at least 10–40 m, depending on frequency and salinity, and are used to communicate with submarines. At certain frequencies, VLF waves have been found to cause electron precipitation. VLF waves used for submarine communication have created an artificial bubble around the Earth that can protect it from solar flares and coronal mass ejections through interaction with high-energy radiation particles. Atmospheric noise, including 'whistlers' caused by lightning, is high in the band.

Reader's Guide

The VLF band's significance lies in its unique propagation characteristics and practical applications. Because VLF waves are reflected from the bottom of the ionosphere rather than refracted through higher layers, they experience very low path attenuation and little fading, enabling reliable long-distance communication over 5,000–20,000 km. This stability makes VLF essential for government time signal stations and secure military communication, particularly with submarines, as the waves can penetrate saltwater to depths of at least 40 m.

A major practical drawback is the difficulty of building efficient transmitting antennas. Full-size resonant antennas would be impractically tall—a quarter-wave vertical antenna at 30 kHz would be 2.5 km high. Practical transmitting antennas are electrically short, with low radiation resistance (often less than one ohm) and efficiencies of only 10% to 50%. High-power stations use capacitively-toploaded monopole antennas, such as umbrella, delta, trideco, or flattop (triatic) designs, requiring extremely low-resistance ground systems and large loading coils. The narrow bandwidth of these antennas (50–100 Hz) necessitates dynamic tuning for FSK and MSK modulation, sometimes using saturable reactors. Receiving antennas are less demanding due to high atmospheric noise; ferrite loop antennas are commonly used. The band's legacy includes its role in submarine communication and its inadvertent creation of a protective artificial bubble around Earth.

Did You Know?

• VLF waves propagate via an Earth–ionosphere waveguide, with path attenuation as low as 2–3 dB per 1,000 km.
• High-power VLF transmitting antennas are electrically short and require large loading coils to cancel capacitive reactance.