The radio spectrum, from DC to daylight.

The electromagnetic spectrum is every possible frequency of electromagnetic wave, from 0 Hz (direct current, “DC”) up through radio, microwaves, infrared and visible light (“daylight”), and on to ultraviolet, X-rays and gamma rays. The ITU counts everything below 3,000 GHz as radio, though nearly all of it in everyday use sits between a few kilohertz and 300 GHz, divided by the ITU into named bands, each ten times higher than the last: VLF, LF, MF, HF, VHF, UHF, SHF and EHF. Lower frequencies have longer waves, travel further and bend around obstacles but carry little data; higher frequencies carry far more data over shorter, straighter paths.

One rule for the whole spectrum

Everything on this page travels at the speed of light, about 300,000 km a second. So the frequency and the wavelength are locked together:

Wavelength in metres = 300 ÷ frequency in MHz

The same rule, in units that suit each end of the spectrum:

That one table is the whole story of this page. As the waves get shorter, everything about how they behave changes, and the uses change with them. Our introduction to SDR shows how the same rule sets the length of an aerial.

How the bands are named

The International Telecommunication Union (ITU) splits radio into bands that each cover a factor of ten, so each band’s waves are a tenth the length of the one below. The names describe frequency, the metre names describe wavelength, and radio people use both.

Source: ITU-R Recommendation V.431-8. *The ITU gives bands 1, 2 and 12 numbers but no letter names; ELF, SLF and THF are common usage. The ITU’s own “ELF” label actually sits lower still, on 0.3 to 3 Hz, so you’ll see different edges for ELF in different places.

The ITU’s Radio Regulations stop calling it radio at 3,000 GHz (3 THz), but the boundary is blurry: infrared is usually counted from about 300 GHz (1 mm) up to 430 THz, visible light runs from about 430 to 750 THz, and beyond it come ultraviolet, X-rays and gamma rays. Australian law draws the line higher still: the Australian Radiofrequency Spectrum Plan defines radio waves as anything below 420 THz, deep into the infrared.

DC: 0 Hz

Direct current doesn’t wave at all: a battery pushes current one way, steadily, forever. It is frequency zero, the left-hand end of every spectrum chart, and the “DC” in “DC to daylight”. The phrase is old radio-engineer shorthand for an instrument or idea that covers everything.

ELF and SLF, 3 to 300 Hz: the hum in the walls

• Mains power. Australia’s electricity grid runs at 50 Hz, a wavelength of 6,000 km. The hum from a transformer or a badly earthed amplifier is 50 Hz and its multiples.
• The Earth itself. Lightning strikes all over the planet ring the gap between the ground and the ionosphere like a bell, at about 8 Hz and a series of higher tones, called the Schumann resonances.
• Submarines. Waves this long penetrate seawater. The US Navy built an “ELF” system on 76 Hz, with aerials 22 to 45 km long in Wisconsin and Michigan, to reach submerged submarines; it shut the system down in September 2004.

Almost no information fits down a channel this low: the US system could send only a few characters every 15 minutes.

ULF and VLF, 300 Hz to 30 kHz: sound-speed radio

These frequencies overlap human hearing, which runs from about 20 Hz to 20 kHz. The numbers are the same, but sound is air pressure, not an electromagnetic wave; you can’t hear a radio wave without a receiver.

• Exmouth, WA. The Naval Communication Station Harold E. Holt, near Exmouth, transmits on 19.8 kHz (callsign NWC) to Australian, US and allied submarines. Its aerial is a web of wires over more than 400 hectares, hung from 13 towers, the tallest nearly 400 m high. At about 1 megawatt it is one of the most powerful VLF transmitters in the world, and radio hobbyists pick it up with a long wire and a VLF receiver, or an SDR fitted with a VLF upconverter.
• Lightning. Every strike is a burst of VLF noise. Listened to through a VLF receiver, distant storms crackle (“sferics”) and occasionally whistle (“whistlers”, lightning energy that travelled out along the Earth’s magnetic field and came back down in the other hemisphere).
• Navigation. VLF signals hug the curve of the Earth and are stable enough to steer by; the Omega navigation system used them (10 to 14 kHz) worldwide until 1997.

LF, 30 to 300 kHz: long wave

• Ground wave. At these frequencies the signal follows the ground over the horizon, reliably, for hundreds or thousands of kilometres, day and night.
• Time signals. Several countries broadcast atomic-clock time on LF that radio-controlled clocks set themselves from, such as Japan’s JJY on 40 and 60 kHz. Australia doesn’t run one: the National Measurement Institute distributes official time over GPS and the internet instead.
• Aviation beacons. Non-directional beacons (NDBs) for aircraft sit between about 190 and 526.5 kHz, straddling LF and MF. Australia switched off 181 of its ground-based navigation aids, NDBs among them, from May 2016 as aircraft moved to satellite navigation, keeping the rest as a backup network.
• Amateur radio. A narrow band at 135.7 to 137.8 kHz.

Australia has no long-wave broadcasting; Europe and North Africa still have a few stations.

MF, 300 kHz to 3 MHz: medium wave, AM radio

• AM broadcasting. Australian AM stations sit between 531 and 1,602 kHz, on a 9 kHz raster, inside the AM broadcasting band of 526.5 to 1,606.5 kHz. By day they travel by ground wave, tens to a couple of hundred kilometres. At night the ionosphere reflects them, and AM stations from New Zealand and beyond come in.
• Marine. The old international distress frequency, 2,182 kHz, sits here, alongside marine HF radio further up.
• Amateur radio. The 160 m band near 1.8 MHz, and a band at 472 to 479 kHz.

The aerial problem starts to bite: a quarter wave at 1 MHz is 75 m, which is why AM radios hide a coil on a ferrite rod and AM transmitters are a tall mast.

HF, 3 to 30 MHz: shortwave, the band that goes around the world

HF is where radio becomes global. The ionosphere, layers of electrically charged gas from about 50 km to more than 500 km up, bends HF signals back to Earth, and a signal can bounce between ionosphere and ground several times to reach the other side of the planet. Which frequencies work depends on the time of day, the season and the roughly 11-year sunspot cycle: higher frequencies work by day, lower ones by night.

• Shortwave broadcasting, in bands around 6, 7, 9, 11, 13 and 15 MHz, among others.
• Marine and aviation HF: ships and aircraft over the ocean, beyond the reach of VHF. AMSA’s distress watch and the Bureau of Meteorology’s marine weather broadcasts are here.
• Amateur radio: eight bands between 3.5 and 29.7 MHz, from which hobbyists talk to every continent.
• 27 MHz CB radio, and toy radio control.

An RTL-SDR reaches HF only by “direct sampling” (the NESDR Smart v5 does this below about 25 MHz) and needs a long wire aerial to hear much.

VHF, 30 to 300 MHz: line of sight begins

Above about 30 MHz the ionosphere mostly stops bending signals back, and radio becomes line of sight, plus a bit: a VHF signal reaches roughly as far as a tall tower can “see”, usually 50 to 100 km. That is a feature: the same frequency can be reused in the next city.

• FM radio, 87.5 to 108 MHz.
• Aviation, 108 to 137 MHz: navigation beacons up to 118 MHz, then air-traffic-control voice. 121.5 MHz is the aviation emergency frequency.
• Marine VHF, 156 to 162 MHz, with channel 16 on 156.800 MHz for distress and calling, and AIS ship tracking on 161.975 and 162.025 MHz.
• DAB+ digital radio, blocks 9A to 9C between 202 and 207 MHz.
• VHF digital TV, 174 to 230 MHz.
• Amateur radio: the 6 m band (50 to 54 MHz) and the 2 m band (144 to 148 MHz).

This is the RTL-SDR’s home ground. Our Australian frequency tables list the exact channels.

UHF, 300 MHz to 3 GHz: where your phone lives

UHF is the most crowded stretch of spectrum, because it is the best compromise there is: short enough for a small aerial in a pocket, long enough to get through walls and around corners reasonably well.

• UHF TV, from 520 to 694 MHz.
• UHF CB, the 4WD and worksite radio, 476.425 to 477.4125 MHz.
• 433 MHz devices: garage remotes, weather stations, tyre-pressure sensors.
• Mobile phones: Australian networks use bands around 700, 850, 900, 1,800, 2,100, 2,300 and 2,600 MHz, and 5G on about 3.4 to 3.8 GHz in SHF (ACMA spectrum auctions).
• Aircraft tracking (ADS-B), 1,090 MHz: planes broadcasting their position, a favourite SDR project.
• GPS, 1,575.42 MHz: a signal so weak that it is below the background noise, recovered by maths.
• Microwave ovens and Wi-Fi, 2.4 GHz. Not a coincidence: 2.4 GHz is an “industrial, scientific and medical” band anyone can use, so the oven and the router share it, and an old oven with a leaky door can drop your Wi-Fi.

An RTL-SDR stops at about 1,750 MHz, so it hears ADS-B and GPS but not Wi-Fi.

SHF, 3 to 30 GHz: microwaves

At centimetre wavelengths, signals behave almost like light: they travel in straight lines, reflect off buildings, and are blocked by walls, hills and even heavy rain. In exchange there is a vast amount of room, so this is where high-capacity links live.

• Wi-Fi on 5 GHz and 6 GHz.
• 5G, mid-band, about 3.4 to 3.8 GHz.
• Weather radar: the Bureau of Meteorology’s radars work in the S band, around 2.7 to 2.9 GHz (just below SHF), and the C band, around 5.6 GHz. The colours on the rain radar map are microwaves bouncing off raindrops.
• Satellite TV: VAST (free-to-air TV for the bush) comes down from Optus satellites parked at 156° East, in the Ku band at about 12.25 to 12.75 GHz, to a small dish. The dish is a mirror that focuses the waves, which only works because they are a couple of centimetres long.
• Point-to-point links: the dishes on mobile towers and rooftops.

EHF, 30 to 300 GHz: millimetre waves

• 5G millimetre wave, Australia’s 26 GHz band (the mobile networks’ licences cover 25.1 to 27.5 GHz, which sits just below EHF), in some city centres and venues: huge speed, a range measured in hundreds of metres.
• Car radar at 76 to 81 GHz: adaptive cruise control and collision warning.
• Oxygen absorption. Around 60 GHz, oxygen in the air absorbs radio strongly. Engineers use that for short-range links that can’t carry far enough to interfere with anyone.
• Radio astronomy: millimetre-wave telescopes see cold gas and dust where stars form.

Terahertz, 300 GHz to 3 THz: the gap

For a long time this was the “terahertz gap”: too high for radio electronics, too low for light-based optics. It is now used in research, security imaging and astronomy, and in some industrial inspection, but little in daily life yet.

Infrared, about 300 GHz to 430 THz: heat and remotes

Somewhere between 1 mm and 0.1 mm, radio shades into infrared. Infrared is the next stretch, invisible to us but familiar:

• Heat. Everything warm gives off infrared. A thermal camera sees wavelengths of about 8 to 14 micrometres (roughly 20 to 40 THz), which is why it shows people and leaky windows in the dark.
• Your TV remote flashes an infrared LED at about 940 nanometres, around 320 THz, the wavelength of standard remote-control LEDs. Point it at a phone camera and press a button: many cameras show the flash. Your TV remote is a light-based transmitter, more than a hundred thousand times higher in frequency than the Wi-Fi in the same room.
• Fibre optics. The NBN’s fibre (a system called GPON) carries data as infrared light: around 1,490 nanometres towards your home and around 1,310 nanometres back, as set by ITU-T G.984.2. Glass fibre loses far less light at these infrared wavelengths than at visible ones, which is why fibre uses infrared.

Daylight, about 430 to 750 THz

Visible light is a tiny sliver of the spectrum: wavelengths from about 700 nanometres (red, around 430 THz) to 400 nanometres (violet, around 750 THz). Everything else on this page, from 50 Hz up to the TV remote, is invisible to us.

It is the same physics all the way along. Sunlight through a window and the signal from an FM tower are both electromagnetic waves travelling at the same speed; the only difference is that light’s waves are about six million times shorter.

Beyond daylight, and the safety question

Above violet comes ultraviolet, then X-rays and gamma rays. Ultraviolet is still classed as non-ionising, but it carries enough energy to cause sunburn and skin cancer, which is why sun protection matters. X-rays and gamma rays go further: they have enough energy to knock electrons off atoms. That is ionising radiation, and it can damage DNA, which is why X-rays and nuclear radiation are carefully controlled.

Everything below light on this page is non-ionising, radio and microwaves included. Radio can heat things at high power, which is how a microwave oven works, and the safety limits are set well below the point where that could cause harm. In Australia those limits are ARPANSA’s radiofrequency standard, RPS S-1, and ACMA requires mobile phones, mobile towers, Wi-Fi equipment and broadcast transmitters to comply with them. ARPANSA’s 5G fact sheet confirms 5G’s frequencies are covered by the same limits, and when ARPANSA measured 50 public places across Melbourne in 2022, the average exposure was more than 700 times below the limit.

Who decides who gets which frequency?

Internationally, the ITU’s World Radiocommunication Conferences agree what each band is for. In Australia, the ACMA turns that into the Australian Radiofrequency Spectrum Plan, whose table runs from 8.3 kHz all the way to 420 THz: it allocates services up to 275 GHz and lists everything above that as “not allocated” for now. The ACMA then licenses who may transmit. Broadcasters and mobile networks buy or are granted licences. Some bands, such as CB radio and the 433 MHz and 2.4 GHz device bands, are covered by class licences: anyone may use them with suitable equipment, without applying.

Where an SDR fits

A cheap RTL-SDR covers about 0.1 to 1,750 MHz: the upper part of LF, all of MF, HF (with a wire aerial), VHF and most of UHF. On the chart above, that is a surprisingly wide slice, and every everyday signal in it, from AM radio to aircraft and GPS, can be seen on a laptop screen. Our introduction to SDR is the place to start.

Common questions

Sources

All checked 12/10/2026 unless an edition date is given.

Standards and international bodies

• Recommendation ITU-R V.431-8, Nomenclature of the frequency and wavelength bands used in telecommunications (08/2015), ITU — itu.int
• Recommendation ITU-R P.676-13, Attenuation by atmospheric gases and related effects (08/2022), ITU — itu.int
• Recommendation ITU-T G.984.2, G-PON: Physical Media Dependent layer specification (08/2019), ITU — itu.int
• Broadcasting Assignment Plan GE75 (LF/MF plan for Regions 1 and 3), ITU Radiocommunication Bureau, 2014 — itu.int

Australian law and regulators

• Australian Radiofrequency Spectrum Plan (2025 Update) 2021, including general information, ACMA (compilation 09/10/2025) — acma.gov.au
• Summary and response to submissions, draft ARSP 2021 (ITU 3,000 GHz vs Australian 420 THz limit), ACMA, May 2021 — acma.gov.au
• Radiocommunications (Low Interference Potential Devices) Class Licence 2025, ACMA, F2025L01047 — legislation.gov.au
• Radiocommunications (Citizen Band Radio Stations) Class Licence 2025, ACMA, F2025L01088 — legislation.gov.au
• Radiocommunications Licence Conditions (Amateur Licence) Determination 2025, ACMA, F2025L01087 — legislation.gov.au
• Broadcasting apparatus licences (broadcasting services bands), ACMA — acma.gov.au
• Mapping of allotment numbers to frequency blocks (DAB+ blocks), ACMA — acma.gov.au
• Spectrum auctions, ACMA — acma.gov.au
• Frequency operating standard (effective 09/10/2023), Australian Energy Market Commission — aemc.gov.au
• Time and frequency services, National Measurement Institute — industry.gov.au

Radiation safety

• Standard for Limiting Exposure to Radiofrequency Fields – 100 kHz to 300 GHz, RPS S-1 (Rev. 1) (2021), ARPANSA — arpansa.gov.au
• Radiofrequency electromagnetic energy and the ARPANSA safety standard (Q&A), ARPANSA — arpansa.gov.au
• 5G: the new generation of the mobile phone network and health (19/03/2019), ARPANSA — arpansa.gov.au
• Survey of radio wave exposure 2022, ARPANSA — arpansa.gov.au
• What is ionising radiation?, ARPANSA — arpansa.gov.au
• Ultraviolet radiation, ARPANSA — arpansa.gov.au

Government, defence and services

• Australia and the United States celebrate 50 years for Naval Communication Station Harold E Holt (16/09/2017), Minister for Defence — minister.defence.gov.au
• Introduction to HF Radio Propagation, Bureau of Meteorology Space Weather Services — sws.bom.gov.au
• Communication services (marine weather by HF radio), Bureau of Meteorology — bom.gov.au
• HF radiotelephone monitoring in Australia, AMSA — amsa.gov.au
• Switch on about the navaid switch-off (May 2016), Flight Safety Australia (CASA) — flightsafetyaustralia.com
• nbn achieves 20 Gbps speeds in live network technical trials (25/10/2023), nbn co — nbnco.com.au
• Transmission information (VAST on Optus satellites at 156°E), SBS — sbs.com.au
• Standard frequency and time signal: JJY transmitting stations, NICT (Japan) — jjy.nict.go.jp
• Schumann resonance animation, NASA Scientific Visualization Studio — svs.gsfc.nasa.gov
• Omega Navigational System Collection, Smithsonian National Air and Space Museum — airandspace.si.edu

Manufacturers

• Optus C1 payload information (Ku-band frequency plan, 156°E), Optus — optus.com.au
• TSAL6200 high power infrared emitting diode, 940 nm (datasheet rev. 2.4), Vishay — vishay.com
• NESDR SMArt v5, Nooelec — nooelec.com
• AN1610: Using MRFIC1502 in GPS receivers (GPS L1 power level), NXP/Freescale — nxp.com
• Microbolometers (8–14 µm long-wave infrared), Teledyne DALSA — teledynevisionsolutions.com

Further reading

• Feng, Liu & Zhou, “Observations of ionospheric disturbances produced by a powerful VLF radio signal…”, Remote Sensing 15, 3307 (2023): NWC at 19.8 kHz, about 1 MW — doi.org
• Altgelt, The world’s largest “radio” station (US Navy ELF, 76 Hz), University of Wisconsin physics archive — pages.hep.wisc.edu
• “Navy closes Project ELF” (October 2004), Nuclear Watch New Mexico — nuclearactive.org
• VLF station list (SID space-weather monitors), Stanford SOLAR Center — solar-center.stanford.edu
• Receiving VLF with an upconverter and direct-sampling RTL-SDR, RTL-SDR.com — rtl-sdr.com