текст - antenna (текст - antenna)

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Contents

  • 1 Terminology

  • 2 Overview

  • 3 Parameters

    • 3.1 Resonant frequency

    • 3.2 Gain

    • 3.3 Radiation pattern

    • 3.4 Impedance

    • 3.5 Efficiency

    • 3.6 Bandwidth

    • 3.7 Polarization

    • 3.8 Transmission and reception

  • 4 Basic antenna models

  • 5

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    Practical antennas


Antenna (radio)

From Wikipedia, the free encyclopedia

An antenna is a transducer designed to transmit or receive electromagnetic waves. In other words, antennas convert electromagnetic waves into electrical currents and vice versa. Antennas are used in systems such as radio and television broadcasting, point-to-point radio communication, wireless LAN, radar, and space exploration. Antennas usually work in air or outer space, but can also be operated under water or even through soil and rock at certain frequencies for short distances.

Physically, an antenna is an arrangement of conductors that generate a radiating electromagnetic field in response to an applied alternating voltage and the associated alternating electric current, or can be placed in an electromagnetic field so that the field will induce an alternating current in the antenna and a voltage between its terminals. Some antenna devices (parabolic antenna, Horn Antenna) just adapt the free space to another type of antenna.

Thomas Edison used antennas by 1885. Edison patented his system in U.S. Patent 465,971 . Antennas were also used in 1888 by Heinrich Hertz (1857-1894) to prove the existence of electromagnetic waves predicted by the theory of James Clerk Maxwell. Hertz placed the emitter dipole in the focal point of a parabolic reflector. He published his work and installation drawings in Annalen der Physik und Chemie (vol. 36, 1889).

Terminology

The words antenna (plural: antennas[1]) and "aerial" are used interchangeably; but usually a rigid metallic structure is termed an antenna and a wire format is called an aerial. In the United Kingdom and other British English speaking areas the term aerial is more common, even for rigid types. The noun aerial is occasionally written with a diaresis mark — aërial — in recognition of the original spelling of the adjective aërial from which the noun is derived.

The origin of the word antenna relative to wireless apparatus is attributed to Guglielmo Marconi. In 1895, while testing early radio apparatus in the Swiss Alps at Salvan, Switzerland in the Mont Blanc region, Marconi experimented with early wireless equipment. A 2.5 meter long pole, along which was carried a wire, was used as a radiating and receiving aerial element. In Italian a tent pole is known as l'antenna centrale, and the pole with a wire alongside it used as an aerial was simply called l'antenna. Until then wireless radiating transmitting and receiving elements were known simply as aerials or terminals. Marconi's use of the word antenna (Italian for pole) would become a popular term for what today is uniformly known as the antenna.[2] .

A Hertzian antenna is a set of terminals that does not require the presence of a ground for its operation (versus a Tesla antenna which is grounded. [3]) A loaded antenna is an active antenna having an elongated portion of appreciable electrical length and having additional inductance or capacitance directly in series or shunt with the elongated portion so as to modify the standing wave pattern existing along the portion or to change the effective electrical length of the portion. An antenna grounding structure is a structure for establishing a reference potential level for operating the active antenna. It can be any structure closely associated with (or acting as) the ground which is connected to the terminal of the signal receiver or source opposing the active antenna terminal, (i.e., the signal receiver or source is interposed between the active antenna and this structure).

Overview

Antennas have practical uses for the transmission and reception of radio frequency signals (radio, TV, etc.). In air, those signals travel close to the speed of light in vacuum and with a very low transmission loss. The signals are absorbed when propagating through more conducting materials, such as concrete walls, rock, etc. When encountering an interface, the waves are partially reflected and partially transmitted through.

The vast majority of antennas are simple vertical rods a quarter of a wavelength long. Such antennas are simple in construction, usually inexpensive, and both radiate in and receive from all horizontal directions (omnidirectional). One limitation of this antenna is that it does not radiate or receive in the direction in which the rod points. This region is called the antenna blind cone or null.

There are two fundamental types of antennas, which, with reference to a specific three dimensional (usually horizontal or vertical) plane are either:

  1. Omni-directional (radiates equally in all directions), such as a vertical rod or

  2. Directional (radiates more in one direction than in the other).

In colloquial usage omni-directional usually refers to all horizontal directions with reception above and below the antenna being reduced in favor of better reception (and thus range) in other directions. Also directional antennas are usually meant to refer to one targeting a single specific direction such as a telescope, satellite dish, or possibly a 120° horizontal reception and transmission area.

All antennas radiate some energy in all directions in free space but careful construction results in substantial transmission of energy in a preferred direction and negligible energy radiated in other directions.

By adding additional conducting rods or coils (called elements) and varying their length, spacing, and orientation (or changing the direction of the antenna beam), an antenna with specific desired properties can be created, such as a Yagi-Uda Antenna (often abbreviated to "Yagi").

An antenna array is two or more antennas coupled to a common source or load to produce a specific directional radiation pattern. The spatial relationship between individual antennas contributes to the directivity of the antenna.

The term active element is intended to describe an element whose energy output is modified due to the presence of a source of energy in the element (other than the mere signal energy which passes through the circuit) or an element in which the energy output from a source of energy is controlled by the signal input.

An antenna lead-in is the medium, for example, a transmission line or feed line for conveying the signal energy between the signal source or receiver and the antenna. The antenna feed refers to the components between the antenna and an amplifier.

An antenna counterpoise is a structure of conductive material most closely associated with ground that may be insulated from or capacitively coupled to the natural ground. It aids in the function of the natural ground, particularly where variations (or limitations) of the characteristics of the natural ground interfere with its proper function. Such structures are usually connected to the terminal of a receiver or source opposite to the antenna terminal.

An antenna component is a portion of the antenna performing a distinct function and limited for use in an antenna, as for example, a reflector, director, or active antenna.

Parasitic elements are usually metallic conductive structures which reradiate into free space impinging electromagnetic radiation coming from or going to the active antenna.

An electromagnetic wave refractor is a structure which is shaped or positioned to delay or accelerate transmitted electromagnetic waves, passing through such structure, an amount which varies over the wave front. The refractor alters the direction of propagation of the waves emitted from the structure with respect to the waves impinging on the structure. It can alternatively bring the wave to a focus or alter the wave front in other ways, such as to convert a spherical wave front to a planar wave front (or vice versa). The velocity of the waves radiated have a component which is in the same direction (director) or in the opposite direction (reflector) that of the velocity of the impinging wave.

A director is usually a metallic conductive structure which reradiates into free space impinging electromagnetic radiation coming from or going to the active antenna, the velocity of the reradiated wave having a component in the direction of velocity of the impinging wave. The director modifies the radiation pattern of the active antenna and there is no significant potential relationship between the active antenna and this conductive structure.

A reflector is usually a metallic conductive structure (e.g., screen, rod or plate) which reradiates back into free space impinging electromagnetic radiation coming from or going to the active antenna. The velocity of the returned wave having a component in a direction opposite to the direction of velocity of the impinging wave. The reflector modifies the radiation of the active antenna. There is no significant potential relationship between the active antenna and this conductive structure.

An antenna coupling network is a passive network (which may be any combination of a resistive, inductive or capacitive circuit(s)) for transmitting the signal energy between the active antenna and a source (or receiver) of such signal energy.

Typically, antennas are designed to operate in a relatively narrow frequency range. The design criteria for receiving and transmitting antennas differ slightly, but generally an antenna can receive and transmit equally well. This property is called reciprocity.

Антенна (радио)

От Wikipedia, свободной энциклопедии

Антенна - преобразователь, разработанный, чтобы передать или получить электромагнитные волны. Другими словами, антенны преобразовывают электромагнитные волны в электрические потоки и наоборот. Антенны используются в системах, таких как радио и радиовещание телевидения, двухточечная радио-коммуникация, беспроводная ЛВС, радар, и исследование космоса. Антенны обычно работают в воздухе или космосе, но могут также управляться под водой или даже через почву и скалу в определенных частотах для коротких расстояний.

Физически, антенна - договоренность проводников, которые производят исходящую электромагнитную область в ответ на прикладное переменное напряжение и связанный переменный электрический ток, или могут быть помещены в электромагнитную область так, чтобы область вызвала переменный ток в антенне и напряжении между его терминалами. Некоторые устройства антенны (параболическая антенна, Роговая Антенна) только приспосабливают свободное место к другому типу антенны.

К 1885 Томас Edison использовал антенны. Edison запатентовал свою систему в американских Доступных 465 971. Антенны также использовались в 1888 Heinrich Hertz (1857-1894), чтобы доказать существование электромагнитных волн, предсказанных в соответствии с теорией Джеймса Clerk Maxwell. Герц поместил диполь эмитента в фокус параболического отражателя. Он издал свою работу и инсталляционные рисунки в Annalen der Physik und Chemie (издание 36, 1889).

Терминология

Антенна слов (множественное число: антенны [1]), и "антенна" используются попеременно; но обычно твердую металлическую структуру называют антенной, и проводной формат называют антенной. В Великобритании и другой британский вариант английского языка, говоря области термин антенна больше характерен, даже для твердых типов. Антенна существительного иногда пишется с маркой diaresis - aërial - с учетом оригинального правописания прилагательного aërial, из которого получено существительное.

Происхождение антенны слова относительно беспроводного аппарата приписано Guglielmo Marconi. В 1895, проверяя ранний радио-аппарат в швейцарских Альпах в Salvan, Швейцария в Монбланской области, Marconi экспериментировал с ранним беспроводным оборудованием. Полюс 2.5 метра длиной, вдоль которого несся провод, использовался как излучение и получение воздушного элемента. На итальянском языке столб для палатки известен как l'antenna centrale, и полюс с проводом рядом с используемым, поскольку антенну просто назвали l'antenna. До тех пор излучение радио передавшие и получающие элементы было известно просто как антенны или терминалы. Использование Marconi's антенны слова (итальянский язык для полюса) стало бы популярным сроком для того, что сегодня однородно известно как антенна. [2].

Антенна Hertzian - ряд терминалов, который не требует присутствия основания для его операции (против антенны Тесла, которая основана. [3]), нагруженная антенна - активная антенна, имеющая удлиненную часть заметной электрической длины и имеющая дополнительную индуктивность или емкость непосредственно последовательно или шунт с удлиненной частью, чтобы изменить постоянный образец волны, существующий вдоль части или изменить эффективную электрическую длину части. Антенна, основывающая структуру, является структурой для того, чтобы основать уровень потенциала ссылки для того, чтобы управлять активной антенной. Это может быть любая структура, близко связанная с (или действующий как) основание, которое связано с терминалом приемника сигнала или источника, выступающего против активного терминала антенны, (то есть, приемник сигнала или источник вставлены между активной антенной и этой структурой).

Краткий обзор

У антенн есть практическое использование для передачи и приема сигналов радиочастоты (радио, телевидение, и т.д.). В воздухе те сигналы едут близко к скорости света в вакууме и с очень низкой потерей передачи. Сигналы поглощены, размножаясь через большее количество материалов проведения, таких как конкретные стены, скала, и т.д. Сталкиваясь с интерфейсом, волны частично отражены и частично переданы через.

Огромное большинство антенн - простые вертикальные пруты четверть длины волны долго. Такие антенны просты в строительстве, обычно недороги, и оба исходят в и получают от всех горизонтальных (всенаправленных) указаний. Одно ограничение этой антенны - то, что она не излучает или получает в руководстве, в котором указывает прут. Эту область называют антенной слепым конусом или пустым указателем.

Есть два фундаментальных типа антенн, которые, в отношении определенного трехмерного (обычно горизонтальный или вертикальный) самолет также:

1. Всенаправленный (исходит одинаково во всех указаниях), таких как вертикальный прут или

2. Направленный (исходит больше в одном руководстве чем в другом).

В разговорном использовании, всенаправленном обычно, обращается ко всем горизонтальным указаниям с приемом выше и ниже антенны, уменьшаемой в пользу лучшего приема (и таким образом расположитесь) в других указаниях. Также направленные антенны обычно предназначаются, чтобы обратиться к одному планированию для единственного определенного руководства, такого как телескоп, спутниковая антенна, или возможно 120 ° горизонтальных приемов и область передачи.

Все антенны излучают немного энергии во всех указаниях в свободных космических, но осторожных строительных результатах в существенной передаче энергии в привилегированном руководстве и незначительной энергии, излученной в других указаниях.

Добавляя дополнительные пруты проведения или катушки (названный элементами) и изменение их длины, интервал, и ориентация (или изменение руководства луча антенны), антенна с определенными желательными свойствами может быть создана, такие как Антенна Яги-Uda (часто сокращенный к "Яги").

Множество антенны - две или больше антенны, соединенные к общему источнику или грузу, чтобы произвести определенный направленный радиационный образец. Пространственные отношения между индивидуальными антеннами способствуют директивности антенны.

Активный элемент срока предназначен, чтобы описать элемент, продукция энергии которого изменена из-за присутствия источника энергии в элементе (кроме простой энергии сигнала, которая проходит через кругооборот), или элемент, в котором продукцией энергии из источника энергии управляет вход сигнала.

Ввод антенны - среда, например, линия передачи или линия подачи для того, чтобы передать энергию сигнала между источником сигнала или приемником и антенной. Подача антенны обращается к компонентам между антенной и усилителем.

Баланс антенны - структура проводящего материала, наиболее близко связанного с основанием, которое может быть изолировано от или емкостно соединено к естественному основанию. Это помогает в функции естественного основания, особенно где изменения (или ограничения) особенностей естественного основания сталкиваются с его надлежащей функцией. Такие структуры обычно связываются с терминалом приемника или источника напротив терминала антенны.

Компонент антенны - часть антенны, выполняющей отличную функцию и ограниченный для использования в антенне, что касается примера, отражателя, директора, или активной антенны.

Паразитные элементы - обычно металлические проводящие структуры, которые повторно исходят в свободное место, посягающее электромагнитная радиация, прибывающая из или идущая в активную антенну.

Электромагнитный линзовый телескоп волны - структура, которая сформирована или помещена, чтобы задержать или ускорить переданные электромагнитные волны, проходя через такую структуру, количество, которое изменяется по фронту волны. Линзовый телескоп изменяет руководство распространения волн, испускаемых от структуры относительно волн, посягающих на структуру. Это может альтернативно принести волну в центр или изменить фронт волны другими способами, например, преобразовать сферический фронт волны в плоский фронт волны (или наоборот). У скорости излученных волн есть компонент, который находится в том же самом руководстве (директор) или в противоположном руководстве (отражатель) та из скорости посягающей волны.

Директор обычно - металлическая проводящая структура, которая повторно исходит в свободное место, посягающее электромагнитная радиация, прибывающая из или идущая в активную антенну, скорость повторно излученной волны, имеющей компонент в направлении скорости посягающей волны. Директор изменяет радиационный образец активной антенны и нет никаких существенных потенциальных отношений между активной антенной и этой проводящей структурой.

Отражатель обычно - металлическая проводящая структура (например, экран, прут или пластина), который повторно исходит назад в свободное место, посягающее электромагнитная радиация, прибывающая из или идущая в активную антенну. Скорость возвращенной волны, имеющей компонент в руководстве напротив руководства скорости посягающей волны. Отражатель изменяет радиацию активной антенны. Нет никаких существенных потенциальных отношений между активной антенной и этой проводящей структурой.

Сеть сцепления антенны - пассивная сеть (который может быть любой комбинацией индуктивного или емкостного кругооборота (ов) имеющего сопротивление) для того, чтобы передать энергию сигнала между активной антенной и источником (или приемник) такой энергии сигнала.

Как правило, антенны разработаны, чтобы работать в относительно узком частотном диапазоне. Критерии проекта для получения и передачи антенн отличаются немного, но вообще антенна может получить и передать одинаково хорошо. Эту собственность называют взаимностью.



Parameters

There are several critical parameters that affect an antenna's performance and can be adjusted during the design process. These are resonant frequency, impedance, gain, aperture or radiation pattern, polarization, efficiency and bandwidth. Transmit antennas may also have a maximum power rating, and receive antennas differ in their noise rejection properties. All of these parameters can be measured through various means.

Resonant frequency

The "resonant frequency" and "electrical resonance" is related to the electrical length of the antenna. The electrical length is usually the physical length of the wire divided by its velocity factor (the ratio of the speed of wave propagation in the wire to c0, the speed of light in a vacuum). Typically an antenna is tuned for a specific frequency, and is effective for a range of frequencies usually centered on that resonant frequency. However, the other properties of the antenna (especially radiation pattern and impedance) change with frequency, so the antenna's resonant frequency may merely be close to the center frequency of these other more important properties.

Antennas can be made resonant on harmonic frequencies with lengths that are fractions of the target wavelength. Some antenna designs have multiple resonant frequencies, and some are relatively effective over a very broad range of frequencies. The most commonly known type of wide band aerial is the logarithmic or log periodic, but its gain is usually much lower than that of a specific or narrower band aerial.

Gain

Gain as a parameter measures the directionality of a given antenna. An antenna with a low gain emits radiation with about the same power in all directions, whereas a high-gain antenna will preferentially radiate in particular directions. Specifically, the Gain, Directive gain or Power gain of an antenna is defined as the ratio of the intensity (power per unit surface) radiated by the antenna in a given direction at an arbitrary distance divided by the intensity radiated at the same distance by an hypothetical isotropic antenna.

The gain of an antenna is a passive phenomenon - power is not added by the antenna, but simply redistributed to provide more radiated power in a certain direction than would be transmitted by an isotropic antenna. If an antenna has a greater than one gain in some directions, it must have a less than one gain in other directions since energy is conserved by the antenna. An antenna designer must take into account the application for the antenna when determining the gain. High-gain antennas have the advantage of longer range and better signal quality, but must be aimed carefully in a particular direction. Low-gain antennas have shorter range, but the orientation of the antenna is inconsequential. For example, a dish antenna on a spacecraft is a high-gain device (must be pointed at the planet to be effective), while a typical WiFi antenna in a laptop computer is low-gain (as long as the base station is within range, the antenna can be in an any orientation in space). It makes sense to improve horizontal range at the expense of reception above or below the antenna. Thus most antennas labeled "omnidirectional" really have some gain.[4]

Sometimes, the half-wave dipole is taken as a reference instead of the isotropic radiator. The gain is then given in dBd (decibels over dipole):

0 dBd = 2.15 dBi

Radiation pattern

The radiation pattern of an antenna is the geometric pattern of the relative field strengths of the field emitted by the antenna. For the ideal isotropic antenna, this would be a sphere. For a typical dipole, this would be a toroid. The radiation pattern of an antenna is typically represented by a three dimensional graph, or polar plots of the horizontal and vertical cross sections. The graph should show sidelobes and backlobes, where the antenna's gain is at a minima or maxima.

Impedance

As an electro-magnetic wave travels through the different parts of the antenna system (radio, feed line, antenna, free space) it may encounter differences in impedance (E/H, V/I, etc). At each interface, depending on the impedance match, some fraction of the wave's energy will reflect back to the source[5], forming a standing wave in the feed line. The ratio of maximum power to minimum power in the wave can be measured and is called the standing wave ratio (SWR). A SWR of 1:1 is ideal. A SWR of 1.5:1 is considered to be marginally acceptable in low power applications where power loss is more critical, although an SWR as high as 6:1 may still be usable with the right equipment. Minimizing impedance differences at each interface (impedance matching) will reduce SWR and maximize power transfer through each part of the antenna system.

Complex impedance of an antenna is related to the electrical length of the antenna at the wavelength in use. The impedance of an antenna can be matched to the feed line and radio by adjusting the impedance of the feed line, using the feed line as an impedance transformer. More commonly, the impedance is adjusted at the load (see below) with an antenna tuner, a balun, a matching transformer, matching networks composed of inductors and capacitors, or matching sections such as the gamma match.

Efficiency

"Efficiency" is the ratio of power actually radiated to the power put into the antenna terminals. A dummy load may have an SWR of 1:1 but an efficiency of 0, as it absorbs all power and radiates heat but not RF energy, showing that SWR alone is not an effective measure of an antenna's efficiency. Radiation in an antenna is caused by radiation resistance which can only be measured as part of total resistance including loss resistance. Loss resistance usually results in heat generation rather than radiation, and reduces efficiency. Mathematically, efficiency is calculated as radiation resistance divided by total resistance.

Bandwidth

The "bandwidth" of an antenna is the range of frequencies over which it is effective, usually centered on the resonant frequency. The bandwidth of an antenna may be increased by several techniques, including using thicker wires, replacing wires with cages to simulate a thicker wire, tapering antenna components (like in a feed horn), and combining multiple antennas into a single assembly and allowing the natural impedance to select the correct antenna. Small antennas are usually preferred for convenience, but there is a fundamental limit relating bandwidth, size and efficiency.

Polarization

The "polarization" of an antenna is the orientation of the electric field (E-plane) of the radio wave with respect to the Earth's surface and is determined by the physical structure of the antenna and by its orientation. It has nothing in common with antenna directionality terms: "horizontal", "vertical" and "circular". Thus, a simple straight wire antenna will have one polarization when mounted vertically, and a different polarization when mounted horizontally. "Electromagnetic wave polarization filters" are structures which can be employed to act directly on the electromagnetic wave to filter out wave energy of an undesired polarization and to pass wave energy of a desired polarization.

Reflections generally affect polarization. For radio waves the most important reflector is the ionosphere - signals which reflect from it will have their polarization changed unpredictably. For signals which are reflected by the ionosphere, polarization cannot be relied upon. For line-of-sight communications for which polarization can be relied upon, it can make a large difference in signal quality to have the transmitter and receiver using the same polarization; many tens of dB difference are commonly seen and this is more than enough to make the difference between reasonable communication and a broken link.

Polarization is largely predictable from antenna construction but, especially in directional antennas, the polarization of side lobes can be quite different from that of the main propagation lobe. For radio antennas, polarization corresponds to the orientation of the radiating element in an antenna. A vertical omnidirectional WiFi antenna will have vertical polarization (the most common type). An exception is a class of elongated waveguide antennas in which vertically placed antennas are horizontally polarized. Many commercial antennas are marked as to the polarization of their emitted signals.

Polarization is the sum of the E-plane orientations over time projected onto an imaginary plane perpendicular to the direction of motion of the radio wave. In the most general case, polarization is elliptical (the projection is oblong), meaning that the antenna varies over time in the polarization of the radio waves it is emitting. Two special cases are linear polarization (the ellipse collapses into a line) and circular polarization (in which the ellipse varies maximally). In linear polarization the antenna compels the electric field of the emitted radio wave to a particular orientation. Depending on the orientation of the antenna mounting, the usual linear cases are horizontal and vertical polarization. In circular polarization, the antenna continuously varies the electric field of the radio wave through all possible values of its orientation with regard to the Earth's surface. Circular polarizations, like elliptical ones, are classified as right-hand polarized or left-hand polarized using a "thumb in the direction of the propagation" rule. Optical researchers use the same rule of thumb, but pointing it in the direction of the emitter, not in the direction of propagation, and so are opposite to radio engineers' use.

In practice, regardless of confusing terminology, it is important that linearly polarized antennas be matched, lest the received signal strength be greatly reduced. So horizontal should be used with horizontal and vertical with vertical. Intermediate matchings will lose some signal strength, but not as much as a complete mismatch. Transmitters mounted on vehicles with large motional freedom commonly use circularly polarized antennas so that there will never be a complete mismatch with signals from other sources. In the case of radar, this is often reflections from rain drops.

Transmission and reception

All of the antenna parameters are expressed in terms of a transmission antenna, but are identically applicable to a receiving antenna, due to reciprocity. Impedance, however, is not applied in an obvious way; for impedance, the impedance at the load (where the power is consumed) is most critical. For a transmitting antenna, this is the antenna itself. For a receiving antenna, this is at the (radio) receiver rather than at the antenna. Tuning is done by adjusting the length of an electrically long linear antenna to alter the electrical resonance of the antenna.

Antenna tuning is done by adjusting an inductance or capacitance combined with the active antenna (but distinct and separate from the active antenna). The inductance or capacitance provides the reactance which combines with the inherent reactance of the active antenna to establish a resonance in a circuit including the active antenna. The established resonance being at a frequency other than the natural electrical resonant frequency of the active antenna. Adjustment of the inductance or capacitance changes this resonance.

Antennas used for transmission have a maximum power rating, beyond which heating, arcing or sparking may occur in the components, which may cause them to be damaged or destroyed. Raising this maximum power rating usually requires larger and heavier components, which may require larger and heavier supporting structures. This is a concern only for transmitting antennas, as the power received by an antenna rarely exceeds the microwatt range.

Antennas designed specifically for reception might be optimized for noise rejection capabilities. An "antenna shield" is a conductive or low reluctance structure (such as a wire, plate or grid) which is adapted to be placed in the vicinity of an antenna to reduce, as by dissipation through a resistance or by conduction to ground, undesired electromagnetic radiation, or electric or magnetic fields, which are directed toward the active antenna from an external source or which emanate from the active antenna. Other methods to optimize for noise rejection can be done by selecting a narrow bandwidth so that noise from other frequencies is rejected, or selecting a specific radiation pattern to reject noise from a specific direction, or by selecting a polarization different from the noise polarization, or by selecting an antenna that favors either the electric or magnetic field.

For instance, an antenna to be used for reception of low frequencies (below about ten megahertz) will be subject to both man-made noise from motors and other machinery, and from natural sources such as lightning. Successfully rejecting these forms of noise is an important antenna feature. A small coil of wire with many turns is more able to reject such noise than a vertical antenna. However, the vertical will radiate much more effectively on transmit, where extraneous signals are not a concern.

Basic antenna models

There are many variations of antennas. Below are a few basic models. More can be found in Category:Radio frequency antenna types.

  • The isotropic radiator is a purely theoretical antenna that radiates equally in all directions. It is considered to be a point in space with no dimensions and no mass. This antenna cannot physically exist, but is useful as a theoretical model for comparison with all other antennas. Most antennas' gains are measured with reference to an isotropic radiator, and are rated in dBi (decibels with respect to an isotropic radiator).

  • The dipole antenna is simply two wires pointed in opposite directions arranged either horizontally or vertically, with one end of each wire connected to the radio and the other end hanging free in space. Since this is the simplest practical antenna, it is also used as reference model for other antennas; gain with respect to a dipole is labeled as dBd. Generally, the dipole is considered to be omnidirectional in the plane perpendicular to the axis of the antenna, but it has deep nulls in the directions of the axis. Variations of the dipole include the folded dipole, the half wave antenna, the ground plane antenna, the whip, and the J-pole.

  • The Yagi-Uda antenna is a directional variation of the dipole with parasitic elements added with functionality similar to adding a reflector and lenses (directors) to focus a filament light bulb.

  • The random wire antenna is simply a very long (greater than one wavelength) wire with one end connected to the radio and the other in free space, arranged in any way most convenient for the space available. Folding will reduce effectiveness and make theoretical analysis extremely difficult. (The added length helps more than the folding typically hurts.) Typically, a random wire antenna will also require an antenna tuner, as it might have a random impedance that varies nonlinearly with frequency.

  • The Horn is used where high gain is needed, the wavelength is short (microwave) and space is not an issue. Horns can be narrow band or wide band, depending on their shape. A horn can be built for any frequency, but horns for lower frequencies are typically impractical.

Practical antennas

Although any circuit can radiate if driven with a signal of high enough frequency, most practical antennas are specially designed to radiate efficiently at a particular frequency. An example of an inefficient antenna is the simple Hertzian dipole antenna, which radiates over wide range of frequencies and is useful for its small size. A more efficient variation of this is the half-wave dipole, which radiates with high efficiency when the signal wavelength is twice the electrical length of the antenna.

One of the goals of antenna design is to minimize the reactance of the device so that it appears as a resistive load. An "antenna inherent reactance" includes not only the distributed reactance of the active antenna but also the natural reactance due to its location and surroundings (as for example, the capacity relation inherent in the position of the active antenna relative to ground). Reactance diverts energy into the reactive field, which causes unwanted currents that heat the antenna and associated wiring, thereby wasting energy without contributing to the radiated output. Reactance can be eliminated by operating the antenna at its resonant frequency, when its capacitive and inductive reactances are equal and opposite, resulting in a net zero reactive current. If this is not possible, compensating inductors or capacitors can instead be added to the antenna to cancel its reactance as far as the source is concerned.

Once the reactance has been eliminated, what remains is a pure resistance, which is the sum of two parts: the ohmic resistance of the conductors, and the radiation resistance. Power absorbed by the ohmic resistance becomes waste heat, and that absorbed by the radiation resistance becomes radiated electromagnetic energy. The greater the ratio of radiation resistance to ohmic resistance, the more efficient the antenna.






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