Also known as: electromagnetic spectrum, EM spectrum
The electromagnetic spectrum is the full range of electromagnetic radiation ordered by frequency (or, equivalently, wavelength). It spans from low-frequency radio waves through microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays — all the same physical phenomenon, an oscillating electric and magnetic field propagating through space, differing only in how fast it vibrates.1 Every region is one continuous whole; the named bands are human labels of convenience, not physical boundaries.
How it works
Every part of the spectrum is electromagnetic energy travelling at the speed of light c (≈299,792,458 m/s in vacuum); only the frequency — and therefore the wavelength, since λ = c / f — changes. A single physical law, Maxwell’s equations, governs the whole range: a changing electric field creates a magnetic field and vice versa, and the pair sustains itself as a wave. What differs from one region to the next is the energy each cycle carries. Photon energy rises with frequency (E = hf), so gamma rays are ionising and dangerous while radio photons are far too weak to break chemical bonds.
Radio occupies the low-frequency end, conventionally about 3 kHz to 300 GHz. This range is special for engineering reasons: the oscillations are slow enough that ordinary electronic circuits — oscillators, amplifiers, and antennas — can generate, radiate, and detect them directly. Above the radio range, into infrared and visible light, we mostly resort to optics and photonics because no circuit switches fast enough. The radio portion is itself subdivided by the ITU into named frequency bands — VLF, LF, MF, HF, VHF, UHF, SHF, EHF — each a decade of frequency (a factor of ten), with characteristic propagation and antenna sizes.
In practice
Where a signal sits in the spectrum decides almost everything practical about it:
- Propagation. HF (3–30 MHz) waves refract off the ionosphere and can travel worldwide; VHF/UHF are largely line-of-sight; microwaves are blocked by terrain and attenuated by rain. See radio propagation.
- Antenna size. A resonant antenna is a fraction of a wavelength, so lower frequencies demand physically larger antennas — a practical limit at the low end.
- Available bandwidth. Higher bands have more absolute spectrum to spare, which is why 5G and Wi-Fi keep climbing toward millimetre waves for capacity.
- Regulation. The ITU and national regulators (FCC, Ofcom, and others) allocate slices of the radio spectrum to services, so a given frequency legally belongs to broadcasting, aviation, land-mobile radio, and so on.
Radio itself is a small, crowded strip of an enormous continuum: the visible-light octave alone spans more frequency than the entire radio range below it.
Relevance to SDR
Software-defined radios operate strictly within the radio portion of the spectrum, limited by their tuner and analog-to-digital converter — an RTL-SDR reaches roughly 24 MHz–1.7 GHz, wideband devices like the HackRF up to ~6 GHz. The trunking systems GopherTrunk decodes (P25, DMR, NXDN, TETRA) live in the VHF and UHF land-mobile bands, typically 136–174 MHz, 380–520 MHz, and 700–900 MHz. Where a target sits in the spectrum dictates the antenna, how far it will be heard, and which SDR hardware can receive it at all.
Sources
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Electromagnetic spectrum — Wikipedia, overview of the full range of electromagnetic radiation and its named regions. ↩