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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.

The Frequency of Moderate-Speed Oscillations

The decihertz (dHz) is a unit of frequency equal to 0.1 hertz (10⁻¹ Hz), meaning one cycle occurs every 10 seconds. This frequency range is useful for studying moderate-speed oscillations in various fields such as geophysics, astrophysics, and engineering.

In geophysics, decihertz frequencies are relevant in the analysis of certain seismic waves and ground vibrations caused by natural events like earthquakes or volcanic activity. These waves help scientists understand the Earth’s interior and predict the effects of seismic events on structures and populations.

In astrophysics, decihertz frequencies are significant for observing gravitational waves generated by binary star systems and other massive celestial objects. These waves fall within a frequency range targeted by upcoming space-based gravitational wave detectors, such as the Laser Interferometer Space Antenna (LISA), which aims to study sources inaccessible to ground-based observatories.

In engineering, vibrations in the decihertz range can influence the behavior of mechanical systems and large infrastructures, such as tall buildings or bridges, under environmental loads like wind or traffic.

Decihertz frequencies fill the gap between slower oscillations and those in the audio range, making them crucial for understanding a wide variety of natural and man-made phenomena occurring on timescales of seconds to minutes.