# Dipole antenna A **dipole antenna** is a radio antenna built from two identical, usually straight, conductive elements fed at the point where they meet, so that the transmitter's [[Alternating_current|current]] flows in opposite directions along each half. It is one of the two simplest antenna types, the other being the monopole, which uses a single element worked against a ground plane instead of a second element; the "rabbit ears" antenna once common on television sets is a well-known dipole variant, tunable by hand to different broadcast channels. The dipole's behaviour is set almost entirely by its length measured in wavelengths. An idealised, infinitesimally short dipole with uniform current, the Hertzian dipole, is the building block from which every other length is derived by integration; a half-wave dipole, one of the most common practical lengths, radiates efficiently from a feedpoint impedance close to a standard 50 or 75 ohm line, while a full-wave or longer dipole splits its once-simple donut-shaped pattern into multiple lobes. The primary microsim on this page drags a dipole's length from a small fraction of a wavelength to two wavelengths and recomputes its far-field radiation pattern and directivity live, in both a two-dimensional slice and a rotating three-dimensional view. A companion three.js sketch, built from the same family of models, renders a pair of half-wave elements side by side to show how two dipoles combine into a broadside array. ## History The dipole is among the oldest deliberately engineered antennas. Heinrich Hertz used a pair of collinear rods, driven at a spark gap between them, to generate and detect the electromagnetic waves that confirmed [[James_Clerk_Maxwell|Maxwell]]'s prediction of radio waves in the late 1880s, and the two-rod, centre-fed geometry he used is recognisably a dipole in the modern sense.[^cite-hertz] Early wireless telegraphy favoured long-wire and umbrella antennas driven against ground more often than the symmetric dipole, and it was the growth of shortwave and then broadcast radio, where a resonant half-wave element sized conveniently to the wavelength in use, that made the half-wave dipole the default reference antenna it remains today in broadcast and [[Telecommunications|telecommunications]] engineering.[^cite-dipole-broadcast] ## Dipole variations Every dipole is a variation on the same idea, a centre-fed conductor radiating a standing-wave current, and the variations mostly trade length, shape or feed arrangement for bandwidth, impedance or physical convenience. ### Short dipole A short dipole is much shorter than a wavelength, so its current distribution is close to a simple triangular taper to zero at each open end. Its radiation pattern is nearly identical to the idealised Hertzian case, but its very low radiation resistance means ordinary conductor and ground losses can absorb a large share of the power fed to it. ### Dipole antennas of various lengths Lengthening a dipole toward a half wavelength raises its radiation resistance from a few ohms to a value convenient to feed directly, and lengthening it further changes the pattern's shape: the single broadside lobe of a short or half-wave dipole progressively narrows, then splits into several lobes once the length passes about 1.25 wavelengths, so a longer dipole is not simply a higher-gain one. ### Half-wave dipole At exactly half a wavelength the standing current completes one half-cycle along the element, the input reactance is small, and the input resistance is close to the well-known 73 ohm figure in free space, near enough to common 50 or 75 ohm feed lines that a half-wave dipole is usually fed directly, with only a modest mismatch to absorb. ### Folded dipole A folded dipole runs a second conductor parallel to the first and joins the two at both ends, so the same total current splits between the halves while the antenna keeps the same overall length. The folding raises the feedpoint impedance by a factor near four and broadens the antenna's bandwidth, both useful when the antenna must match an unbalanced feed. ### Other variants Bent, sleeved or loaded dipoles trade a straight rod for a shape that shortens the physical length needed for a given resonant frequency, trims the feedpoint reactance, or widens the bandwidth, at some cost to the clean two-lobe pattern of a straight half-wave element. ### Vertical (monopole) antennas A vertical monopole over a conducting ground plane is electrically equivalent to half of a dipole, because the ground plane's image currents supply the missing half; a quarter-wave monopole therefore behaves like a half-wave dipole cut in two, with half the feedpoint resistance and a pattern that is the dipole's upper half reflected back down to the ground. ## Dipole characteristics ### Impedance of dipoles of various lengths A dipole's feedpoint impedance varies continuously with its length in wavelengths, not just at the handful of lengths given closed-form names above: resistance rises from a few ohms at a small fraction of a wavelength toward a broad resistive peak near a full wavelength, while the reactance swings from capacitive at short lengths through zero near resonance to strongly inductive just past it, a full [[Complex_analysis|complex]] trajectory that a [[Smith_chart|Smith chart]] displays more compactly than a pair of separate curves. ### Radiation pattern and gain In the plane containing the wire, a dipole's far-field pattern narrows from the broad donut of a short dipole to a somewhat flatter, still single-lobed donut at half a wavelength; in the plane perpendicular to the wire the pattern is a circle, since nothing distinguishes one direction around the axis from another. Directivity, expressed in [[Decibel|decibels]] as dBi, rises only modestly over this range before multiple lobes appear. ### Feeding a dipole antenna A dipole is a balanced structure, with equal and opposite current in each half, while a coaxial feed line, a [[Transmission_line|transmission line]] in its own right, is unbalanced, its shield grounded on the outside; connecting one directly to the other lets stray current flow on the outer shield and [[Distortion|distort]] the pattern. #### Current balun A current balun forces the two halves of the feed to carry equal and opposite current by presenting a high impedance to any current that tries to flow in the common, or shield, mode, typically using a choke formed by coiling the coaxial cable itself into an [[Inductor|inductor]] or threading it through ferrite beads. #### Coax balun A coax balun, sometimes called a bazooka balun, uses a quarter-wave sleeve of cable shield around the feed line to present an open circuit to common-mode current at the operating frequency, an entirely passive, narrowband solution built from the feed line's own outer conductor; without it, the same shield current that a receiving antenna is meant to reject can reappear as picked-up [[Noise_(electronics)|noise]]. #### Sleeve balun A sleeve balun is the same quarter-wave-sleeve idea packaged as a separate metal tube around the coaxial line near the feedpoint, moving the choke action off the cable itself and onto a dedicated fitting. ## Common applications The half-wave dipole and its close relatives appear throughout consumer and [[Broadcast_engineering|broadcast engineering]] equipment, wherever a simple, resonant, moderately directional element is enough. ### "Rabbit ears" TV antenna The two telescoping rods once standard on top of a television set are a half-wave dipole whose length is adjusted by hand, extending or collapsing each rod to bring its resonance closer to the channel being watched; because broadcast television spans a wide band, the compromise length rarely matches every channel equally well. ### FM-broadcast-receiving antennas An [[Frequency_modulation|FM]] broadcast receiver commonly ships with a folded dipole taped to a wall or hung behind a receiver, its length set for the middle of the FM band and its folded form chosen for the wider bandwidth that a whole broadcast band demands. ### Shortwave antenna A half-wave wire dipole strung between two supports, fed at its centre with open-wire or coaxial line, remains one of the simplest effective shortwave antennas, cut to resonate near the middle of whichever shortwave band it is meant to work. ### Dipole towers Some broadcast installations mount a dipole element, or a stack of them, on a supporting tower rather than working the tower itself as a grounded monopole radiator, decoupling the antenna's electrical length from the physical height of the structure that holds it up. ### Dipole arrays Feeding several dipole elements together, with a chosen amplitude and phase at each, combines their individual patterns into a single, more directional pattern; the resulting [[Antenna_array|antenna array]] can be steered or shaped well beyond what any single dipole element could produce alone, from simple two-element television antennas up to large phased [[Radar|radar]] arrays. ### Yagi antennas A Yagi-Uda antenna uses one dipole as the driven element, fed directly from the transmitter or receiver, alongside additional unfed rods, a reflector behind it and one or more directors ahead of it, that reradiate the dipole's field with a phase that reinforces gain in one direction.[^cite-yagi] ### Dipole as a reference standard Antenna gain is often quoted in dBd, decibels relative to a half-wave dipole, rather than in dBi relative to a hypothetical isotropic radiator, a convention formalised in [[Institute_of_Electrical_and_Electronics_Engineers|IEEE]] usage because a real dipole, unlike an isotropic source, can actually be built and measured against.[^cite-dbd] ## Hertzian dipole The Hertzian dipole is the mathematical idealisation behind every result above: an element so short compared with the wavelength that its current can be taken as uniform along its whole length and its location as a single point, mathematically a [[Dirac_delta_function|Dirac delta]] source, rather than an extended structure. ### Radiation resistance Because an idealised Hertzian dipole carries the same current, a flow of [[Electron|electrons]] back and forth, at every point along its length, integrating the power it radiates over a surrounding sphere gives a radiation resistance that grows with the square of its length in wavelengths; a physical short dipole's real current tapers toward each open end, so its actual radiation resistance is a further fraction of the idealised figure. ### Directive gain A Hertzian dipole's far-field pattern is proportional to sin θ, measured from the wire's own axis, the same donut shape a short physical dipole shows; integrating the radiated power density over the full sphere against this pattern gives a directivity of 1.5, or about 1.76 dBi, meaning the peak of the donut is that much stronger than an isotropic radiator carrying the same total power. ### Comparison with the short dipole A short physical dipole shares the Hertzian dipole's sin θ pattern and its 1.5 directivity almost exactly, because both are dominated by the same uniform-current approximation. In the sketch, the same donut narrows only slightly and the directivity readout rises only from about 1.5 toward the half-wave dipole's 1.64, or 2.15 dBi, as the length is dragged from a small fraction of a wavelength up to half a wavelength. ## Detailed calculation of dipole feedpoint impedance Finding a dipole's exact feedpoint impedance from first principles, rather than reading it off a length chart, means integrating the fields of its assumed sinusoidal current distribution by one of two related [[Electrical_engineering|methods]]. ### Induced EMF method The induced EMF method treats the dipole's own radiated field as though it were induced by a second, identical current distribution, and computes the impedance from the work that field would do driving that second current; because the assumed current shape is only approximately correct, the method is exact for the assumed distribution but approximate for a real antenna, close enough to be the classical route to the dipole's 73 ohm figure. ### Integral methods Directly integrating the Poynting vector of the radiated field over a large sphere gives the same radiation resistance without the intermediate induced-voltage picture, at the cost of an integral that has no elementary closed form and is usually left in terms of the sine and cosine integral functions Si and Ci evaluated at the antenna's electrical length. ## Microsims The primary sketch draws a dipole of adjustable length and computes its far-field radiation pattern from the closed-form current-distribution formula, live. A length slider sweeps from a twentieth of a wavelength to two wavelengths, defaulting to the half-wave case; a second slider drags a measurement cursor around the pattern to read off its value at any angle, and a button switches between a two-dimensional polar slice through the wire's axis and a rotating three-dimensional view formed by revolving that slice into a donut. The directivity readout, as both a linear ratio and in dBi, updates as the length changes. A companion three.js sketch, part of the [[Antenna_array]] family of models, renders two half-wave elements side by side to show a broadside array built from exactly this element. *Try:* Drag the length past about 1.25 wavelengths and watch the single donut-shaped lobe split into several smaller lobes, then check the directivity readout at exactly half a wavelength against the 1.64, or 2.15 dBi, this article quotes. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Dipole_antenna) : [Wikitube](https://en.wikitube.io/wiki/Dipole_antenna) Skeleton mirrored at revision 1375131763. Prose, emphasis and the microsims are Wikitube's own. ## See also - [[Antenna_array]] - [[Transmission_line]] - [[Smith_chart]] - [[Broadcast_engineering]] ## Notes The radiation-pattern and directivity figures in this article (1.5 and 1.76 dBi for the Hertzian and short dipole, 1.64 and 2.15 dBi for the half-wave dipole) are the values the primary sketch itself computes by numerically integrating the pattern formula in its header comment; they are standard, textbook-level results and are not separately footnoted here (style guide §6.1). ## References [^cite-hertz]: Citation needed: a primary citation (paper or apparatus description, author, venue, year) for Hertz's original dipole transmitter and receiver has not been pinned to a Portal Book page in this pass. [^cite-dipole-broadcast]: Citation needed: a primary or secondary source dating the half-wave dipole's adoption as the standard shortwave and broadcast reference antenna has not been pinned to a Portal Book page in this pass. [^cite-yagi]: Citation needed: a primary citation (authors, paper title, venue, year) for the Yagi-Uda antenna has not been pinned to a Portal Book page in this pass. [^cite-dbd]: Citation needed: the specific standard or document that codifies the dBd (gain relative to a half-wave dipole) convention has not been pinned to a Portal Book page in this pass. [^cite-dipole-textbook]: Citation needed: a specific textbook edition and page range for the classical elementary, short and half-wave dipole derivations has not been pinned to a Portal Book page in this pass. ## Sources for elementary, short, and half-wave dipoles Classical closed-form results for the elementary (Hertzian), short and half-wave dipole, the radiation-resistance and directivity figures used throughout this article, are standard results reproduced in essentially every antenna-engineering textbook; a specific pinned edition and page range for this article's own figures is queued for a later pass.[^cite-dipole-textbook] ## External links - Dipole antenna, live sketch: https://editor.p5js.org/sciencenibber/full/fHUmn6tkh - Dipole antenna, editor source: https://editor.p5js.org/sciencenibber/sketches/fHUmn6tkh <!-- Hubs: Signal_processing. Portals: PORTAL_Signal_Processing. Signal Processing portal wave 1 · 2026-09-17 · drafted. -->