# Radio wave **Radio waves** are electromagnetic radiation at the lowest-frequency, longest-wavelength end of the electromagnetic spectrum: the same kind of disturbance as light, only oscillating far more slowly, and for that reason able to be generated and detected with ordinary electric circuits rather than with atoms or lasers. Any accelerating electric charge radiates them, from the electrons sloshing back and forth in a transmitting antenna to the current surge in a lightning stroke to the thermal jostling of charged particles inside any warm object; the practical difference between a useful radio signal and background static is only how deliberately the charge's motion is controlled. The primary microsim on this page animates a travelling wave's electric and magnetic fields together and marks where the current frequency falls on the naming ladder that runs from the very lowest frequencies up through the boundary with microwaves, with a frequency slider that carries the reader across that whole range. Once above roughly 300 gigahertz, where the wavelength has shrunk to about a millimetre, the same radiation is conventionally called a [[Microwave|microwave]] rather than a radio wave; nothing about the wave itself changes at that line; only the name does. Below it, radio waves span more than eleven decades of frequency, and what a given frequency is good for, how far it travels, whether it bends around a hill, whether it reflects off the upper atmosphere, changes enormously across that range even though every one of these waves obeys the same equations. ## Discovery and exploitation [[James_Clerk_Maxwell|James Clerk Maxwell]]'s theory of electromagnetism predicted in the 1860s that a changing electric current should launch a wave that travels at the speed of light,[^maxwell] a bold claim at a time when no one had ever generated or detected such a wave deliberately. Heinrich Hertz settled the question in the laboratory some two decades later, building a spark-gap transmitter and a simple loop receiver and showing that the disturbance it threw off behaved exactly like light: it could be reflected, refracted and polarized, only at a wavelength enormously longer than anything the eye could see.[^hertz] Hertz's waves were, at first, a confirmation of theory rather than a technology; turning them into one, and building the transmitters, antennas and receivers that made [[Radio]] a business rather than a laboratory curiosity, is a separate story told there and in the history of the medium generally. ## Generation and reception A radio wave is generated whenever a charge accelerates, but a *useful* one is generated by driving an alternating current through a conductor shaped to radiate efficiently, an [[Antenna_(radio)|antenna]], at the frequency the application requires. The primary microsim shows the result of that process directly: an electric field `E(x,t) = E0·sin(2πft − 2πx/λ)` and a magnetic field of the same shape, in phase with it but at right angles to it, both travelling along the wave's direction of motion at a fixed speed `c = f·λ`; in the sketch that speed is fixed at 3×10⁸ metres per second, the same constant that sets the pace of light itself. Reception simply runs the process backward: the travelling field pushes charges in a receiving antenna, inducing a tiny alternating current that a [[Radio_receiver|receiver]] amplifies and decodes. Nature generates radio waves the same way without any circuit at all. Lightning is an enormous, brief current and radiates across a broad swath of the spectrum, which is why an ordinary AM radio crackles during a thunderstorm; a planet or a star radiates a weaker, steadier version of the same thing as part of the [[Black-body_radiation|thermal radiation]] every warm object emits, with radio wavelengths making up the very long tail of that glow. ## Properties Every radio wave is characterized by the same three linked quantities that describe any wave: a frequency, a wavelength and a speed, related by `c = fλ`. Because the speed is fixed, frequency and wavelength are two names for the same fact, and either one places the wave on the [[Radio_spectrum|radio spectrum]], the ladder of named bands that runs from the extremely low frequencies used for submarine communication up through the boundary with microwaves that this page's microsim marks directly. Amplitude, the third quantity the microsim lets the reader vary, sets how much power the wave carries rather than what band it occupies. ### Polarization The electric field's orientation as the wave travels is its polarization, and it matters because a receiving antenna coupled to a wave whose field points along the antenna's own length collects far more of it than one built at a right angle to that field. A straight [[Dipole_antenna|dipole]] naturally favours linear polarization, oscillating in a single fixed plane, of the kind the primary microsim draws; other antenna geometries produce circular or elliptical polarization, in which the field's direction rotates as the wave advances rather than staying fixed. The microsim's two field panels are an illustrative simplification in this respect: they hold the wave to one fixed linear polarization so that the E-field-and-B-field relationship stays easy to read, where a real transmission may use any polarization the antenna design allows. ## Propagation characteristics How far a radio wave travels, and by what route, depends heavily on its frequency. At the lowest frequencies a [[Ground_wave|ground wave]] can hug the Earth's curved surface for hundreds of kilometres, diffracting around hills the way water waves bend around a breakwater; at frequencies used for shortwave broadcasting, a sky wave instead reflects off the [[Ionosphere|ionosphere]] and can cover a continent in a single hop, a route called [[Skywave|skywave]] propagation. Higher still, from roughly the VHF band upward, a wave behaves increasingly like a beam of light: it travels essentially in a straight line, is blocked by hills and buildings, and reaches only as far as the horizon plus a small margin from atmospheric bending. Real paths are rarely a single clean route in any of these regimes; reflections off buildings, terrain or the sea surface deliver a receiver several delayed copies of the same signal at once, a condition called [[Multipath_propagation|multipath propagation]] that causes the rapid signal-strength dips known as [[Fading|fading]]. The general subject of [[Radio_propagation]] treats all of these mechanisms, and the paths they set up between transmitter and receiver, in more depth. ## Radio communication Communication rides on a radio wave by [[Signal_modulation|modulating]] it: varying the wave's amplitude, frequency or phase in step with a signal, so that a receiver tuned to the carrier can undo the same variation and recover what was sent. Every one of the propagation regimes above is a choice a communication system designer makes deliberately: a ground wave suits a service that needs steady, moderate-range coverage regardless of the ionosphere's mood, a skywave suits long-distance broadcasting when hopping over the horizon matters more than reliability, and a line-of-sight VHF or UHF link suits high-bandwidth, short-range use where multipath and fading can be engineered around instead of tolerated. Which regime is available is itself a function of frequency, which is why international [[Telecommunications|telecommunications]] policy allocates whole bands to particular services rather than leaving the choice to each transmitter individually. ## Biological and environmental effects A radio-frequency [[Photon|photon]] carries far too little energy to break a [[Chemical_bond|chemical bond]] or eject an electron from an atom, so radio waves are classed as non-ionizing radiation, unlike X-rays or gamma rays at the spectrum's opposite end. The physical mechanism behind their one well-established biological effect is heating: an oscillating field drives currents in tissue, and at sufficiently high power that dissipates as heat exactly as it does, far more intensely, inside a microwave oven. At the low power levels of ordinary broadcasting, mobile telephony and Wi-Fi, that heating is a small fraction of the body's own resting output, which is why exposure limits are set well below it, though the precise limit a given jurisdiction sets is a regulatory rather than a physical question.[^exposure] Environmentally, radio waves pass through the atmosphere with little absorption at most frequencies used for communication, which is exactly what makes them useful for that purpose in the first place. ## Measurement Measuring a radio wave almost always means measuring how much power a calibrated antenna and receiver deliver, expressed either directly in watts or, for a very faint natural signal, as an equivalent noise or brightness temperature. A receiving system aimed at the cosmic microwave background reads about 2.7 kelvin; the same system aimed at the quiet Sun reads anywhere from a thousand to a million kelvin depending on frequency, and aimed at the Moon reads roughly 200 kelvin.[^brightness] None of these figures is a physical temperature in the everyday sense; each is simply the temperature a resistor would need to be at to radiate the same amount of power the antenna actually collects. The practical consequence is that a receiver's own internal noise, typically tens to hundreds of kelvin for a well-built front end, has to sit well below the brightness temperature of whatever is being measured, or the measurement is swamped before it starts. ## Microsims The primary microsim animates a travelling radio wave's electric and magnetic fields side by side with a chart of the named frequency bands, so that the equation above and the naming ladder in Properties stay in view together. A frequency slider sweeps the wave from the low end of the spectrum toward the microwave boundary; an amplitude slider changes how tall the field traces are drawn without changing the frequency; an animation-speed slider changes only how quickly the wave appears to travel across the screen, not the physics. A three.js companion, a variant of the sketch built for [[Radio_spectrum]], renders the same wavelength-against-frequency relationship in more depth elsewhere on the site. *Try:* in the sketch, raise the frequency slider slowly and watch the E-field and B-field traces oscillate faster while staying locked in phase and at right angles to each other, and watch the band marker slide from one named region toward the next. *Try:* in the sketch, hold the frequency fixed and raise only the amplitude slider; the field traces grow taller but the spacing between their peaks, which is what the band marker actually reads, does not move. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Radio_wave) : [Wikitube](https://en.wikitube.io/wiki/Radio_wave) Skeleton mirrored at revision 1374667691. Prose, emphasis and the microsims are Wikitube's own. ## See also - [[Radio]] - [[Radio_spectrum]] - [[Radio_propagation]] - [[Antenna_(radio)]] - [[Ionosphere]] - [[Skywave]] - [[Microwave]] ## References [^maxwell]: Maxwell, J. C. "A Dynamical Theory of the Electromagnetic Field." *Philosophical Transactions of the Royal Society of London*, vol. 155, 1865, pp. 459-512. [^hertz]: Citation needed: the specific paper and date of Heinrich Hertz's first published demonstration of electromagnetic waves (1887-1888), to confirm the exact experiments described. [^exposure]: Citation needed: the specific regulatory exposure-limit figures (for example SAR limits in W/kg) for a named jurisdiction and year; these vary by regulator and are not sourced here. [^brightness]: Ellingson, S. *Radio Systems Engineering - Revised First Edition*. 2023, pp. 104-105 (PDF pages). Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/radio-systems-engineering . CC BY-NC. ## Further reading - Jeff Sanny; Samuel Ling. *University Physics Volume 2*. 2016. CC BY. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/university-physics-volume-2 - Steven Ellingson. *Radio Systems Engineering - Revised First Edition*. 2023. CC BY-NC. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/radio-systems-engineering ## External links - Live sketch: https://editor.p5js.org/sciencenibber/full/z9m02RxYS - Editor (fork to remix): https://editor.p5js.org/sciencenibber/sketches/z9m02RxYS <!-- Hubs: Signal_processing. Portals: PORTAL_Radio. Radio portal wave 1 · 2026-09-17 · drafted. -->