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An antenna or '''aerial''' is an electronic component designed to transmit or receive Radio signals (and other Electromagnetic Wave s). The words "antenna" (plural: antennas ) and "aerial" are used interchangeably throughout this article. Physically, an antenna is an arrangement of Conductor s designed to radiate (transmit) an Electromagnetic Field in response to an applied alternating voltage and the associated alternating Electric Current , or to be placed into an electromagnetic field so that the field will Induce an alternating current in the antenna and a voltage between its terminals. The origin of the word ''antenna'' relative to wireless apparatus is due to 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 carried a wire, was used as a radiating and receiving aerial element. In Italian, a tent pole is known as ''l'antenna centrale'', and this pole with a wire alongside it used as an aerial was simply called ''l'antenna''. Until this time, wireless radiating transmitting and receiving elements were known simply as aerials, but Marconi's use of the word antenna, Italian for ''pole'', soon came to be the most popular term for what today is uniformly known as an antenna. "Salvan: Cradle of Wireless, How Marconi Conducted Early Wireless Experiments in the Swiss Alps", Fred Gardiol & Yves Fournier, Microwave Journal, February 2006, pp. 124-136.'' OVERVIEW There are two fundamental types of antennas, which, with reference to a specific three dimensional (usually horizontal or vertical) plane, are either omni-directional (radiate equally in the plane) or directional (radiates more in one direction than in the other). All antennas radiate some energy in all directions but careful construction results in large directivity in certain directions and negligible energy radiated in other directions. By adding additional conducting rods or coils (called ''elements'') and varying their length, spacing, and orientation, an antenna with specific desired properties can be created, such as a Yagi-Uda Antenna (often abbreviated to "Yagi"). 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 . 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 . Antennas have practical use for the Transmission and Reception of Radio Frequency signals (radio, TV, etc.), which can travel over great distances at the Speed Of Light , and pass through nonconducting walls (although often there is a variable signal reduction depending on the type of wall, and natural rock can be very reflective to radio signals). ANTENNA 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. Resonant frequency The Resonant Frequency is related to the Electrical Length of the antenna. The electrical length is usually the physical length of the wire multiplied by the ratio of the speed of wave propagation in the wire. 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 In antenna design, Gain is the logarithm of the ratio of the intensity of an antenna's radiation pattern in the direction of strongest radiation to that of a reference antenna. If the reference antenna is an Isotropic Antenna , the gain is often expressed in units of dBi (decibels over isotropic). For example, a Dipole Antenna has a gain of 2.14 dBi {Link without Title} . Often, the dipole antenna is used as the reference (since a perfect isotropic reference is impossible to build), in which case the gain of the antenna in question is measured in dBd (decibels over dipole). The gain of an antenna is a passive phenomena - 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 positive gain in some directions, it must have a negative gain in other directions as energy is conserved by the antenna. The gain that can be achieved by an Antenna is therefore trade-off between the range of directions that must be covered by an Antenna and the gain of the antenna. For example, a dish antenna on a spacecraft has a very large gain, but only over a very small range of directions - it must be accurately pointed at earth - but a radio transmitter has a very small gain as it is required to radiate in all directions. For dish-type antennas, gain is proportional to the Aperture (reflective area) and surface accuracy of the dish, as well as the frequency being transmitted/received. In general, a larger aperture provides a higher gain. Also, the higher the frequency, the higher the gain, but surface inaccuracies lead to a larger degradation of gain at higher frequencies. Aperture , and ''' Radiation Pattern ''' are closely related to gain. ''Aperture'' is the shape of the "beam" cross section in the direction of highest gain, and is two-dimensional. (Sometimes aperture is expressed as the radius of the circle that approximates this cross section or the angle of the cone.) ''Radiation pattern'' is the three-dimensional plot of the gain, but usually only the two-dimensional horizontal and vertical cross sections of the radiation pattern are considered. Antennas with high gain typically show side lobes in the radiation pattern. Side lobes are peaks in gain other than the main lobe (the "beam"). Side lobes detract from the antenna quality whenever the system is being used to determine the Direction of a signal, as in Radar systems and reduce gain in the main lobe by distributing the power. Bandwidth The Bandwidth of an antenna is the range of frequencies over which it is effective, usually centered around 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. Impedance ''') 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 Inductor s and Capacitor s, or matching sections such as the gamma match. Polarization The Polarization of an antenna, or more precisely the orientation of the electric field ( E-plane )of the radio wave with respect to the Earth's surface, is determined by physical structure of the antenna and by its orientation. It has nothing in common with antenna directionality, however confusing are the terms used: horizontal, vertical, circular. Thus, a simple straight wire antenna will have one polarization when mounted vertically, and a different polarization when mounted horizontally. Reflections generally affect polarization. For radio waves the most important reflector is the ionosphere, and so signals which reflect from it will have their polarization changed unpredictably, so for signals which are reflected by the ionosphere, polarization cannot be relied upon. However, 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. 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 exactly opposite to radio engineers' use. In actual practice, regardless of confusing terminology, it is important that linearly polarized antennas be matched, lest the received signal strength be very 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. Efficiency ''' which can only be measured as part of total Resistance including loss resistance. Loss resistance usually results in heat generation rather than radiation, and therefore, reduces efficiency. Overview of antenna parameters Except for polarization, the SWR is the most easily measured of the parameters above. Impedance can be measured with specialized equipment, as it relates to the Complex SWR. Measuring radiation pattern requires a sophisticated setup including significant clear space (enough to put the sensor into the antenna's Far Field , or an anechoic chamber designed for antenna measurements), careful study of experiment geometry, and specialized measurement equipment that rotates the antenna during the measurements. Bandwidth depends on the overall effectiveness of the antenna, so all of these parameters must be understood to fully characterize the bandwidth capabilities of an antenna. However, in practice, bandwidth is typically determined by looking only at SWR, i.e., by finding the frequency range over which the SWR is less than a given value. Bandwidth over which an antenna exhibits a particular radiation pattern is also important, for in practical use the performance of an antenna at the extremes of an assigned frequency band is important. Transmission and reception All of these 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. 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. Of course, this is only a concern for transmitting antennas; the power received by an antenna rarely exceeds the microwatt range. Antennas designed specifically for reception might be optimized for Noise rejection capabilities. This 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, but here are a few common models. More can be found in .
COMBINATION OF MULTIPLE ANTENNAS Multiple antennas can be combined in one physical device to save space and weight in mobile applications. E.g. airplanes need antennas for Radar , GPS , Radio , Beacon s, and voice radio.
HOW ANTENNAS WORK The reactive field Fundamentally, all electromagnetic fields are created by the existence or movement of Electrical Charge , and in normal electrical circuits, this charge is exclusively carried by Electron s and Proton s. Since protons tend to be confined within Atom s and move very little, it is usually only the movement of electrons that needs to be considered. Since an electric current in a wire consists of a moving cloud of electrons, it follows that every electric current induces a magnetic field. (Every electron also has its own permanent electric field called its ''coulomb field'', but this is not observable outside the circuit because it is canceled by the equal but opposite coulomb field of a nearby proton.) If the current is constant, it induces a constant magnetic field, and the magnetic field is proportional to current. Maxwell's Equations predict that a changing magnetic field induces a changing electric field, so we now have both magnetic and electric fields around the circuit, creating an electromagnetic field called the ''reactive field'' or ''inductive field''. However, when the current stops, these fields collapse, returning energy to the power supply. The circuit therefore behaves like a reactive component, either a Capacitor or an Inductor , which stores energy temporarily but periodically returns it to the source. The radiating field Now consider a current that periodically reverses direction: an Alternating Current . This consists of a flow of electrons that must therefore reverse direction, and a change of direction is an Acceleration . Because of the way that electromagnetic fields propagate through space at the speed of light, an accelerating electrical charge creates electromagnetic radiation. The result is that energy is continually radiated into space, and must be replenished from the circuit's power supply. The circuit is now behaving as an antenna, and is continually converting electrical energy into a ''radiating field'' that extends indefinitely outward. When the circuit is much shorter than the wavelength of the signal, the rate at which it radiates energy is proportional to the size of the current, the length of the circuit and the frequency of the alternations. In most circuits, the product of these three quantities is small enough that not much energy is radiated, and the result is that the reactive field dominates the radiating field. When the length of the antenna approaches the wavelength of the signal, the current along the antenna is no longer uniform and the calculation of power output becomes more complex. 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. 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. References for this section
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