science

Very low frequency

Very low frequency is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Very low frequency rather than just read about it. In short: Very low frequency or VLF is the ITU designation for radio frequencies (RF) in the range of 3–30 kHz, corresponding to wavelengths from 100 to 10 km, respectively. The band is also known as the myriameter band or myriameter wave as the wavelengths range from one to ten myriameters (an obsolete metric unit equal to 10 kilometers).

Key takeaways

Very low frequency belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Very low frequency to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Very low frequency from memory before moving on to harder problems.

Reference excerpt

Very low frequency or VLF is the ITU designation for radio frequencies (RF) in the range of 3–30 kHz, corresponding to wavelengths from 100 to 10 km, respectively. The band is also known as the myriameter band or myriameter wave as the wavelengths range from one to ten myriameters (an obsolete metric unit equal to 10 kilometers). Due to its limited bandwidth, audio (voice) transmission is highly impractical in this band, and therefore only low-data-rate coded signals are used. The VLF band is used for a few radio navigation services, government time radio stations (broadcasting time signals to set radio clocks) and secure military communication. Since VLF waves can penetrate at least 40 meters (130 ft) into saltwater, they are used for military communication with submarines.

Propagation characteristics Because of their long wavelengths, VLF radio waves can diffract around large obstacles and so are not blocked by mountain ranges, and they can propagate as ground waves following the curvature of the Earth and so are not limited by the horizon. Ground waves are absorbed by the resistance of the Earth and are less important beyond several hundred to a thousand kilometres/miles, and the main mode of long-distance propagation is an Earth–ionosphere waveguide mechanism. The Earth is surrounded by a conductive layer of electrons and ions in the upper atmosphere at the bottom of the ionosphere called the D layer at 60–90 km (37–56 mi) altitude, which reflects VLF radio waves. The conductive ionosphere and the conductive Earth form a horizontal "duct" a few VLF wavelengths high, which acts as a waveguide confining the waves so they don't escape into space. The waves travel in a zig-zag path around the Earth, reflected alternately by the Earth and the ionosphere, in transverse magnetic (TM) mode. VLF waves have very low path attenuation, 2–3 dB per 1,000 km, with little of the "fading" experienced at higher frequencies. This is because VLF waves are reflected from the bottom of the ionosphere, while higher frequency shortwave signals are returned to Earth from higher layers in the ionosphere, the F1 and F2 layers, by a refraction process, and spend most of their journey in the ionosphere, so they are much more affected by ionization gradients and turbulence. Therefore, VLF transmissions are very stable and reliable, and are used for long-distance communication. Propagation distances of 5,000–20,000 km have been realized. However, atmospheric noise ("sferics") is high in the band, including such phenomena as "whistlers", caused by lightning.

VLF waves can penetrate seawater to a depth of at least 10–40 meters (33–131 feet), depending on the frequency employed and the salinity of the water, so they are used to communicate with submarines. VLF waves at certain frequencies have been found to cause electron precipitation. VLF waves used to communicate with submarines have created an artificial bubble around the Earth that can protect it from solar flares and coronal mass ejections; this occurred through interaction with high-energy radiation particles.

Antennas

A major practical drawback of the VLF band is that because of the length of the waves, full size resonant antennas (half wave dipole or quarter wave monopole antennas) cannot be built because of their physical height. Vertical antennas must be used because VLF waves propagate in vertical polarization, but a quarter-wave vertical antenna at 30 kHz (10 km wavelength) would be 2.5 kilometres (8,200 feet) high. So practical transmitting antennas are electrically short, a small fraction of the length at which they would be self-resonant. Due to their low radiation resistance (often less than one ohm) they are inefficient, radiating only 10% to 50% of the transmitter power at most, with the rest of the power dissipated in the antenna/ground system resistances. Very high power transmitters (~1 megawatt) are required for long-distance communication, so the efficiency of the antenna is an important factor.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Very low frequency

Start with the simplest possible case. Write down what Very low frequency claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Very low frequency before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Very low frequency ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Very low frequency

In research: Very low frequency appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Very low frequency in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Very low frequency is common in secondary-school and first-year university syllabi. It links to neighbouring topics Radio electronics, Radio spectrum, so understanding it makes those chapters shorter.
In everyday life: Look for Very low frequency outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Very low frequency in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Very low frequency means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Very low frequency out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Very low frequency in simple terms?

Why does Very low frequency matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Very low frequency?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Very low frequency.