Monopole_antenna - Wikiparency

# Monopole antenna A **monopole antenna** is a class of [[Antenna_(radio)|radio antenna]] built from a single straight, rod-shaped conductor, usually mounted perpendicular to a conductive ground plane, with the transmitter or receiver connected between the base of the rod and the ground plane itself. It is mathematically related to the [[Dipole_antenna|dipole antenna]]: image theory turns a monopole and its ground plane into the equivalent of a dipole twice as long, and most of a monopole's properties follow directly from that equivalence, as the sections below show. A three.js companion sketch renders the resulting radiation pattern as half of the dipole's, the half a ground plane leaves standing once it removes the other. A resonant quarter-wave monopole is omnidirectional in azimuth and radiates most of its power toward the horizon, with a modest gain of a few decibels over an isotropic radiator; real ground planes fall short of the ideal, so a practical monopole typically manages 2 to 5 dBi rather than the somewhat higher figure theory predicts for a perfect one. The whip, the mast radiator and the loaded antennas built for handheld and vehicle-mounted radios are all monopoles in this sense, however different they look. ## Types and uses The whip antenna, a flexible or semi-rigid rod fed at its base, is the everyday form seen on vehicles and portable radios; when it must fit inside a compact handheld [[Transistor|transistor]]-based device, the same electrical length is often coiled into a short, flexible-cased "rubber duck" instead of left as a straight rod.[^cn-rubberduck] Where a full quarter-wave rod would be inconveniently tall, capacitive top-loading shortens the visible structure: an umbrella antenna slopes several wires outward from the top of a shorter mast to add the same capacitance a taller plain rod would supply on its own, and an inverted-L or a T-antenna achieves much the same effect with one or two horizontal top wires instead of a full umbrella. A folded unipole wraps a second conductor around a broadcast tower to adjust its feed-point impedance without changing its physical height, and, at the largest scale, the mast radiator dispenses with a separate antenna altogether by making the entire supporting tower the radiating element; some mast radiators built for [[AM_broadcasting|AM broadcasting]] rank among the tallest human-made structures ever erected, and a longwave mast at Konstantynów in Poland reportedly stood 646 metres tall from 1974 until its collapse in 1991.[^cn-warsawmast] Where no natural ground is convenient, a ground-plane antenna supplies its own artificial ground from several radial rods instead of relying on the earth beneath it. Tall broadcast monopoles favour [[Ground_wave|ground-wave]] propagation, which follows the Earth's curved surface out to a useful daytime range, while the same tower's [[Skywave|sky-wave]] radiation, reflected back down by the [[Ionosphere|ionosphere]] after dark, can carry the same signal far beyond that range at night, a difference station engineers must plan around rather than ignore. ## History Early wireless telegraphy commonly worked a single vertical wire against an earth or counterpoise ground, a configuration close in spirit to the monopole described here and later paired, at the receiving end, with the simplest of all radio receivers, the [[Crystal_radio|crystal set]]; [[Guglielmo_Marconi|Guglielmo Marconi]]'s pioneering transmitters are usually counted among the first practical examples, though the precise wire arrangement in his most famous long-distance experiments was more elaborate than a single rod, and remains debated by historians in its details.[^cn-marconi] Through the twentieth century, the mast radiator and the top-loaded umbrella became the standard antenna configuration for medium-wave and longwave broadcasting, a role tall monopole towers still fill today.[^cn-broadcaststandard] ## Elementary description of operation A monopole is fed as a two-terminal device, exactly like a dipole, except that one terminal is the base of the rod and the other is the ground plane rather than a second, matching rod; the current a transmitter drives into the base returns through the ground plane instead of through a symmetric element on the far side. ### Ground plane The ground plane's role is explained by image theory, itself a consequence of the boundary conditions [[Maxwell's_equations|Maxwell's equations]] impose at a conductor's surface: a conductor fed against an infinite, perfectly conducting ground plane produces exactly the fields, above the plane, that the same conductor together with a mirror-image conductor would produce in free space with no ground plane at all, the image carrying an equal and similarly phased current. A quarter-wave monopole over such a plane is therefore equivalent, above the plane, to a half-wave dipole in free space: the same current distribution and the same radiation into the space it actually occupies, but with all of the equivalent dipole's power now confined to the upper half-space instead of shared with a lower half that the ground plane removes, giving twice the power density in every direction that remains. A real ground plane is finite and imperfectly conducting rather than infinite and perfect, and the antenna's performance departs from the ideal case by however much the real ground falls short. ## Current distribution on antenna As on a dipole, current and voltage form a standing wave along a monopole's length rather than travelling smoothly from base to tip: for a monopole no longer than about a half wavelength, current is greatest at the base and falls to zero at the free top end, approximately following one arch of a [[Sine_wave|sine wave]], while voltage does the opposite, reaching its maximum at the tip and its minimum at the base. Monopoles built longer than a half wavelength develop additional current maxima and minima along their length, in the same way a longer dipole does, and the simple base-fed picture above no longer captures the whole pattern. ## Input impedance The impedance a monopole presents at its base is purely resistive only at specific lengths where the standing wave described above happens to leave no reactive component at the feed point; at every other length it carries a reactance as well, capacitive for a monopole shorter than the nearest such length and inductive for one longer, and any mismatch between that impedance and the [[Transmission_line|feedline]]'s own characteristic impedance shows up as a raised [[Standing_wave_ratio|standing wave ratio]] the whole way back to the transmitter. Because the ground plane's image doubles the equivalent structure without doubling the feed current, a monopole's input impedance at any given electrical length is exactly half of the corresponding dipole's: at resonance, close to a quarter wavelength, a monopole over an ideal ground plane presents about 36.5 ohms of resistance with a small inductive remainder, half of the roughly 73 ohms a half-wave dipole shows at its own resonance. ## Resonant frequencies and lengths A monopole is resonant, in the sense of presenting a purely resistive impedance, at a series of lengths related to the wavelength, and whether a given resonance is a low-impedance or a high-impedance one depends on which multiple of a quarter wavelength the length falls on. ### Series resonances At an electrical length of one quarter wavelength, and again at three quarters, five quarters and every further odd multiple, a monopole's input resistance reaches a local minimum, typically tens of ohms, an easy match for common feedlines; these are its series resonances, named for the low-impedance series-resonant condition a tuned circuit shows under the same description. ### Parallel resonances At an electrical length of one half wavelength, and again at every further multiple of a half wavelength, the input resistance instead reaches a local maximum, commonly thousands of ohms, mirroring the high-impedance parallel-resonant condition of a tuned circuit; a monopole fed at one of these lengths needs a step-up matching network rather than the simple, nearly direct match a quarter-wave feed allows. ### End effects A physical monopole resonates at a length several percent shorter than the idealised quarter wavelength computed from frequency alone, because a conductor of finite diameter is not the infinitesimally thin wire the simple theory assumes: charge collects at the free tip and adds a small shunt capacitance there, an end effect the antenna compensates for automatically by resonating slightly early. For a monopole cut for 1 MHz, in the middle of the medium-wave broadcast band, the idealised quarter-wave length is 75 metres; for one cut for 146 MHz, a common amateur-radio frequency in the [[Very_high_frequency|VHF]] range, it is close to half a metre, and a real antenna at either frequency is built a little shorter than either figure to allow for the end effect. ## Radiation pattern Because a quarter-wave monopole over a ground plane is electrically half of a half-wave dipole, its [[Radiation_pattern|radiation pattern]] is likewise half of the dipole's: omnidirectional in azimuth around the rod's own axis, exactly as the dipole's doughnut-shaped pattern is, but present only in the upper half-space the ground plane leaves standing, strongest toward the horizon and absent entirely straight up along the rod itself. ### Gain and input impedance An ideal quarter-wave monopole over a perfectly conducting, infinite ground plane has twice the dipole's [[Directivity|directivity]], since the same total power is confined to half the solid angle, giving a directivity of about 3.28, or roughly 5.15 dBi. Real monopoles fall short of this figure because real ground planes are neither infinite nor lossless, which is why practical vertical monopoles are usually quoted at 2 to 5 dBi rather than at the higher ideal value; the shortfall is ground loss, power the ground system absorbs and converts to heat rather than returning as radiation, and the same input-impedance departures discussed above track right along with it as the antenna moves away from an ideal quarter-wave length. ### Directivity equation The directivity of a monopole of arbitrary length above an ideal ground plane can be written as an integral over its current distribution, in the same way a dipole's can, and for most lengths that integral has no simple closed form: it is evaluated numerically or by way of tabulated cosine-integral functions rather than by a short formula. Only a few special lengths reduce to plain numbers, among them the quarter-wave and half-wave cases above; between roughly a quarter and five-eighths of a wavelength, directivity toward the horizon continues to rise with length, which is why base-station and vehicle antennas built a little longer than a plain quarter wave, often close to five-eighths of a wavelength, are common where a little extra gain toward the horizon is worth the added height. ## Types of feed Most modern monopoles are fed with [[Coaxial_cable|coaxial cable]], the centre conductor connected to the base of the rod and the shield connected to the ground plane, a natural match to the unbalanced, single-ended nature of the antenna itself. Where the antenna's impedance at the chosen operating length departs from the feedline's characteristic impedance, a matching network of the kinds used for antennas generally is inserted at the base; a shunt or gamma-type feed, tapping the coaxial connection onto the radiator some distance above its grounded base rather than exactly at it, is a common way to raise a naturally low input resistance to match a standard cable without a separate matching network at all. ## Electrically short monopoles When a monopole's physical height is well under a quarter wavelength, common for mobile and handheld antennas working at lower frequencies than their length comfortably supports, its radiation resistance drops to a few ohms or less while its capacitive reactance grows correspondingly large, following the same square-law relationship that governs any electrically short antenna. The small radiation resistance is then easily comparable to, or smaller than, the ordinary [[Electrical_resistivity_and_conductivity|resistance]] of the conductor and loading components in series with it, so a growing share of the power delivered to the antenna is lost as heat rather than radiated, and radiation efficiency falls well short of what a full-size antenna at the same frequency would achieve. ### Capacitively top-loaded monopoles Adding a capacitive top load, a disk, a wire umbrella or a simple ball at the tip of a short monopole raises its effective height for a given physical height by making the current distribution along the rod more nearly uniform rather than tapering linearly to zero at the top; a more uniform current radiates more strongly for the same peak value, so a top-loaded short monopole reaches a usefully higher radiation resistance, and hence a better efficiency, than the same physical height achieves with base loading alone. ## Definition of variables The formulas above use a small, consistent set of symbols, gathered here for reference. | Symbol | Meaning | |---|---| | h | Physical height of the monopole | | lambda | Wavelength of the operating frequency | | f | Operating frequency | | Z_in | Input impedance at the feed point (resistance plus reactance) | | D | Directivity | | G | Gain | | eta | Radiation efficiency | ## Microsims A three.js companion sketch renders the quarter-wave monopole's radiation pattern as half of the dipole's pattern, split at the ground plane exactly as the Elementary description of operation section above describes; it is presented separately from this text. *Try:* in the [[Radar]] sketch, compare its narrow rotating beam, swept to search every bearing in turn, with the fixed, all-round azimuth pattern a vertical monopole radiates without needing to turn at all. *Try:* in the [[Sonar]] sketch, watch the ping's wavefront expand as a widening arc in the vertical plane, a side-on view of the same kind of spreading a monopole's own pattern shows above its ground plane. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Monopole_antenna) : [Wikitube](https://en.wikitube.io/wiki/Monopole_antenna) Skeleton mirrored at revision 1373389233. Prose, emphasis and the microsims are Wikitube's own. ## See also - [[Antenna_(radio)]] - [[Dipole_antenna]] - [[Yagi–Uda_antenna]] - [[Ground_wave]] - [[Standing_wave_ratio]] - [[Guglielmo_Marconi]] ## Notes The elementary formulas above assume an infinitely large, perfectly conducting ground plane and an infinitesimally thin conductor; real monopoles depart from both idealisations, which is part of why measured gain falls short of the ideal directivity derived above. ## Footnotes Page numbers below, where given, are PDF pages of the open editions. The historical and record-setting claims in Types and uses and History are flagged citation needed rather than guessed, since no source in this article's approved list documents them by page. ## References [^cn-rubberduck]: Citation needed: who coined the informal "rubber duck" name for helically wound flexible monopole antennas, and when. [^cn-warsawmast]: Citation needed: the engineering record or contemporary report confirming the height, construction date and collapse date of the Konstantynów radio mast. [^cn-marconi]: Citation needed: a primary account or historical analysis fixing the exact antenna configuration used in Marconi's earliest wireless transmitters. [^cn-broadcaststandard]: Citation needed: a broadcast-engineering history source confirming when the top-loaded and mast-radiator monopole became the standard tall-tower configuration for medium-wave and longwave broadcasting. ## Further reading - Steven Ellingson. *Radio Systems Engineering - Revised First Edition*. 2023. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/radio-systems-engineering . CC BY-NC. ## External links No independent external links accompany this article; its three.js companion sketch is embedded in the Microsims section above.