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Describing Very Low Frequency Radio Waves

A hectometre (hm) is a unit of length equal to 100 metres, and it is used to describe very long wavelengths in the Very Low Frequency (VLF) and Low Frequency (LF) bands of the electromagnetic spectrum. Wavelengths in the hectometre range correspond to frequencies between approximately 3 kHz and 3 MHz. These long wavelengths are typically used for maritime navigation, military submarine communication, AM radio broadcasting, and time signal transmissions.

For example, a signal at 300 kHz has a wavelength of 1 kilometre, or 10 hectometres, and a signal at 1 MHz corresponds to 3 hectometres. These long wavelengths have the unique ability to travel long distances and penetrate water and the ground, which is why they are used in submarine communications and emergency broadcast systems.

Using hectometres to express wavelength offers a practical scale for understanding wave propagation over great distances. It also aids in antenna design, where very large antennas—often hundreds of metres long—are needed to efficiently transmit or receive these frequencies. Understanding wavelength in hectometres is important in geophysics, radio astronomy, and large-scale communications infrastructure.

Measuring Ultra-Low Frequencies

The attohertz (aHz) is an extremely small unit of frequency equal to 10⁻¹⁸ hertz, or one cycle per 10¹⁸ seconds (about 31.7 billion years). This unit is used to describe ultra-low frequency phenomena that occur on cosmic or geological timescales, far beyond everyday human experience.

Attohertz frequencies are relevant in cosmology, astrophysics, and geophysics, where they help scientists study processes that evolve over billions of years. For example, gravitational waves generated by massive cosmic events or the oscillations of the Earth’s magnetic field can be characterized by frequencies in the attohertz range. These waves have enormous wavelengths, often spanning millions or billions of kilometres.

Because the attohertz corresponds to such a long period between cycles, it is mostly used in theoretical research rather than practical applications. Understanding phenomena at this scale gives insight into the fundamental workings of the universe, including the slow evolution of cosmic structures, the expansion of space-time, and the early conditions following the Big Bang.