Very low-frequency (VLF) radio waves can carry signals through land and water with little attenuation. Unlike higher-frequency electromagnetic waves used for most communications, VLF waves are reflected by the ionosphere, and the space between it and Earth’s surface acts as a waveguide through which the waves travel beyond the horizon. So, whereas higher-frequency waves travel in straight lines, VLF signals follow Earth’s curvature and can transmit information to locations hundreds of kilometers away. The military uses VLF waves for navigation and communication with aircraft and submarines.
Although VLF signals are routinely generated, their use is limited by an antenna’s size. To be reasonably efficient, an antenna’s length should be at least a tenth of the signal’s wavelength. For VLF waves, which are 3–30 kHz, the length would be more than a kilometer. Antennas whose length is much less than the signal’s wavelength are considered “electrically small.” They can still transmit...
