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More complex antennas increase the directivity of the antenna. Additional elements in the antenna structure, which need not be directly connected to the receiver or transmitter, increase its directionality. Antenna "gain" describes the concentration of radiated power into a particular solid angle of space.
In contrast, for antenna "gain", the power increased in the desired direction is at the expense of power reduced in undesired directions. A phased array consists of two or more simple antennas which are connected together through an electrical network. This often involves a number of parallel dipole antennas with a certain spacing.
Depending on the relative phase introduced by the network, the same combination of dipole antennas can operate as a "broadside array" directional normal to a line connecting the elements or as an "end-fire array" directional along the line connecting the elements. Antenna arrays may employ any basic omnidirectional or weakly directional antenna type, such as dipole, loop or slot antennas. These elements are often identical. Log-periodic and frequency-independent antennas employ self-similarity in order to be operational over a wide range of bandwidths.
The most familiar example is the log-periodic dipole array which can be seen as a number typically 10 to 20 of connected dipole elements with progressive lengths in an endfire array making it rather directional; it finds use especially as a rooftop antenna for television reception. On the other hand, a Yagi—Uda antenna or simply "Yagi" , with a somewhat similar appearance, has only one dipole element with an electrical connection; the other parasitic elements interact with the electromagnetic field in order to realize a highly directional antenna but with a narrow bandwidth.
Even greater directionality can be obtained using aperture antennas such as the parabolic reflector or horn antenna. Since high directivity in an antenna depends on it being large compared to the wavelength, highly directional antennas thus with high antenna gain become more practical at higher frequencies UHF and above.
At low frequencies such as AM broadcast , arrays of vertical towers are used to achieve directionality  and they will occupy large areas of land. For reception, a long Beverage antenna can have significant directivity.
For non directional portable use, a short vertical antenna or small loop antenna works well, with the main design challenge being that of impedance matching. With a vertical antenna a loading coil at the base of the antenna may be employed to cancel the reactive component of impedance ; small loop antennas are tuned with parallel capacitors for this purpose.
An antenna lead-in is the transmission line , or feed line , which connects the antenna to a transmitter or receiver. A microwave antenna may also be fed directly from a waveguide in place of a conductive transmission line. An antenna counterpoise , or ground plane , is a structure of conductive material which improves or substitutes for the ground. It may be connected to or insulated from the natural ground.
In a monopole antenna, this 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 a structure is normally connected to the return connection of an unbalanced transmission line such as the shield of a coaxial cable. An electromagnetic wave refractor in some aperture antennas is a component which due to its shape and position functions to selectively delay or advance portions of the electromagnetic wavefront passing through it.
The refractor alters the spatial characteristics of the wave on one side relative to the other side. It can, for instance, bring the wave to a focus or alter the wave front in other ways, generally in order to maximize the directivity of the antenna system. This is the radio equivalent of an optical lens.
An antenna coupling network is a passive network generally a combination of inductive and capacitive circuit elements used for impedance matching in between the antenna and the transmitter or receiver. This may be used to minimize losses on the feed line, by reducing transmission line's standing wave ratio , and to present the transmitter or receiver with a standard resistive impedance needed for its optimum operation. The feed point location s is selected, and antenna elements electrically similar to tuner components may be incorporated in the antenna structure itself, to improve the match.
It is a fundamental property of antennas that the electrical characteristics of an antenna described in the next section, such as gain , radiation pattern , impedance , bandwidth , resonant frequency and polarization , are the same whether the antenna is transmitting or receiving.
This is a consequence of the reciprocity theorem of electromagnetics. A necessary condition for the aforementioned reciprocity property is that the materials in the antenna and transmission medium are linear and reciprocal. Reciprocal or bilateral means that the material has the same response to an electric current or magnetic field in one direction, as it has to the field or current in the opposite direction.
Most materials used in antennas meet these conditions, but some microwave antennas use high-tech components such as isolators and circulators , made of nonreciprocal materials such as ferrite. The majority of antenna designs are based on the resonance principle. This relies on the behaviour of moving electrons, which reflect off surfaces where the dielectric constant changes, in a fashion similar to the way light reflects when optical properties change.
In these designs, the reflective surface is created by the end of a conductor, normally a thin metal wire or rod, which in the simplest case has a feed point at one end where it is connected to a transmission line. The conductor, or element , is aligned with the electrical field of the desired signal, normally meaning it is perpendicular to the line from the antenna to the source or receiver in the case of a broadcast antenna.
The radio signal's electrical component induces a voltage in the conductor. This causes an electrical current to begin flowing in the direction of the signal's instantaneous field. When the resulting current reaches the end of the conductor, it reflects, which is equivalent to a degree change in phase.
That means it has undergone a total degree phase change, returning it to the original signal. The current in the element thus adds to the current being created from the source at that instant. This process creates a standing wave in the conductor, with the maximum current at the feed.
The ordinary half-wave dipole is probably the most widely used antenna design. The physical arrangement of the two elements places them degrees out of phase, which means that at any given instant one of the elements is driving current into the transmission line while the other is pulling it out. Monopoles, which are one-half the size of a dipole, are common for long-wavelength radio signals where a dipole would be impractically large.
Another common design is the folded dipole which consists of two or more half-wave dipoles placed side by side and connected at their ends but only one of which is driven. The standing wave forms with this desired pattern at the design operating frequency, f o , and antennas are normally designed to be this size.
This allows some flexibility of design in terms of antenna lengths and feed points. Antennas used in such a fashion are known to be harmonically operated. Antennas that are required to be small compared to the wavelength sacrifice efficiency and cannot be very directional. Since wavelengths are so small at higher frequencies UHF , microwaves trading off performance to obtain a smaller physical size is usually not required.
The quarter-wave elements imitate a series-resonant electrical element due to the standing wave present along the conductor. At the resonant frequency, the standing wave has a current peak and voltage node minimum at the feed. In electrical terms, this means the element has minimum reactance , generating the maximum current for minimum voltage. This is the ideal situation, because it produces the maximum output for the minimum input, producing the highest possible efficiency.
Contrary to an ideal lossless series-resonant circuit, a finite resistance remains corresponding to the relatively small voltage at the feed-point due to the antenna's radiation resistance as well as any actual electrical losses. Recall that a current will reflect when there are changes in the electrical properties of the material. In order to efficiently transfer the received signal into the transmission line, it is important that the transmission line has the same impedance as its connection point on the antenna, otherwise some of the signal will be reflected backwards into the body of the antenna; likewise part of the transmitter's signal power will be reflected back to transmitter, if there is a change in electrical impedance where the feedline joins the antenna.
This leads to the concept of impedance matching , the design of the overall system of antenna and transmission line so the impedance is as close as possible, thereby reducing these losses. Impedance matching is accomplished by a circuit called an antenna tuner or impedance matching network between the transmitter and antenna.
The impedance match between the feedline and antenna is measured by a parameter called the standing wave ratio SWR on the feedline. Consider a half-wave dipole designed to work with signals with wavelength 1 m, meaning the antenna would be approximately 50 cm from tip to tip. If the element has a length-to-diameter ratio of , it will have an inherent impedance of about 63 ohms resistive.
Using the appropriate transmission wire or balun, we match that resistance to ensure minimum signal reflection. Feeding that antenna with a current of 1 Ampere will require 63 Volts, and the antenna will radiate 63 Watts ignoring losses of radio frequency power. Now consider the case when the antenna is fed a signal with a wavelength of 1. Electrically this appears to be a very high impedance. The antenna and transmission line no longer have the same impedance, and the signal will be reflected back into the antenna, reducing output.
This could be addressed by changing the matching system between the antenna and transmission line, but that solution only works well at the new design frequency. The result is that the resonant antenna will efficiently feed a signal into the transmission line only when the source signal's frequency is close to that of the design frequency of the antenna, or one of the resonant multiples. This makes resonant antenna designs inherently narrow-band: Only useful for a small range of frequencies centered around the resonance s.
Sometimes the resulting lower electrical resonant frequency of such a system antenna plus matching network is described using the concept of electrical length , so an antenna used at a lower frequency than its resonant frequency is called an electrically short antenna .
Then it may be said that the coil has lengthened the antenna to achieve an electrical length of 2. For ever shorter antennas requiring greater "electrical lengthening" the radiation resistance plummets approximately according to the square of the antenna length , so that the mismatch due to a net reactance away from the electrical resonance worsens.
Or one could as well say that the equivalent resonant circuit of the antenna system has a higher Q factor and thus a reduced bandwidth,  which can even become inadequate for the transmitted signal's spectrum. Resistive losses due to the loading coil, relative to the decreased radiation resistance, entail a reduced electrical efficiency , which can be of great concern for a transmitting antenna, but bandwidth is the major factor [ dubious — discuss ] [ dubious — discuss ] that sets the size of antennas at 1 MHz and lower frequencies.
The radiant flux as a function of the distance from the transmitting antenna varies according to the inverse-square law , since that describes the geometrical divergence of the transmitted wave. For a given incoming flux, the power acquired by a receiving antenna is proportional to its effective area.
This parameter compares the amount of power captured by a receiving antenna in comparison to the flux of an incoming wave measured in terms of the signal's power density in Watts per square metre. A half-wave dipole has an effective area of 0. If higher gain is needed one cannot simply make the antenna larger. Due to the constraint on the effective area of a receiving antenna detailed below , one sees that for an already-efficient antenna design, the only way to increase gain effective area is by reducing the antenna's gain in another direction.
If a half-wave dipole is not connected to an external circuit but rather shorted out at the feedpoint, then it becomes a resonant half-wave element which efficiently produces a standing wave in response to an impinging radio wave. Because there is no load to absorb that power, it retransmits all of that power, possibly with a phase shift which is critically dependent on the element's exact length.
Thus such a conductor can be arranged in order to transmit a second copy of a transmitter's signal in order to affect the radiation pattern and feedpoint impedance of the element electrically connected to the transmitter.
Antenna elements used in this way are known as passive radiators. A Yagi-Uda array uses passive elements to greatly increase gain in one direction at the expense of other directions. A number of parallel approximately half-wave elements of very specific lengths are situated parallel to each other, at specific positions, along a boom; the boom is only for support and not involved electrically. Only one of the elements is electrically connected to the transmitter or receiver, while the remaining elements are passive.
The Yagi produces a fairly large gain depending on the number of passive elements and is widely used as a directional antenna with an antenna rotor to control the direction of its beam. It suffers from having a rather limited bandwidth, restricting its use to certain applications.
Rather than using one driven antenna element along with passive radiators, one can build an array antenna in which multiple elements are all driven by the transmitter through a system of power splitters and transmission lines in relative phases so as to concentrate the RF power in a single direction. What's more, a phased array can be made "steerable", that is, by changing the phases applied to each element the radiation pattern can be shifted without physically moving the antenna elements.
Another common array antenna is the log-periodic dipole array which has an appearance similar to the Yagi with a number of parallel elements along a boom but is totally dissimilar in operation as all elements are connected electrically to the adjacent element with a phase reversal; using the log-periodic principle it obtains the unique property of maintaining its performance characteristics gain and impedance over a very large bandwidth.
When a radio wave hits a large conducting sheet it is reflected with the phase of the electric field reversed just as a mirror reflects light. Placing such a reflector behind an otherwise non-directional antenna will insure that the power that would have gone in its direction is redirected toward the desired direction, increasing the antenna's gain by a factor of at least 2.
Likewise, a corner reflector can insure that all of the antenna's power is concentrated in only one quadrant of space or less with a consequent increase in gain. Practically speaking, the reflector need not be a solid metal sheet, but can consist of a curtain of rods aligned with the antenna's polarization; this greatly reduces the reflector's weight and wind load.
Specular reflection of radio waves is also employed in a parabolic reflector antenna, in which a curved reflecting surface effects focussing of an incoming wave toward a so-called feed antenna ; this results in an antenna system with an effective area comparable to the size of the reflector itself. Other concepts from geometrical optics are also employed in antenna technology, such as with the lens antenna. The antenna's power gain or simply "gain" also takes into account the antenna's efficiency, and is often the primary figure of merit.
Antennas are characterized by a number of performance measures which a user would be concerned with in selecting or designing an antenna for a particular application. A plot of the directional characteristics in the space surrounding the antenna is its radiation pattern. The frequency range or bandwidth over which an antenna functions well can be very wide as in a log-periodic antenna or narrow as in a small loop antenna ; outside this range the antenna impedance becomes a poor match to the transmission line and transmitter or receiver.
Use of the antenna well away from its design frequency affects its radiation pattern , reducing its directive gain. Generally an antenna will not have a feed-point impedance that matches that of a transmission line; a matching network between antenna terminals and the transmission line will improve power transfer to the antenna.
A non-adjustable matching network will most likely place further limits the usable bandwidth of the antenna system. It may be desirable to use tubular elements, instead of thin wires, to make an antenna; these will allow a greater bandwidth.
Or, several thin wires can be grouped in a cage to simulate a thicker element. This widens the bandwidth of the resonance. Amateur radio antennas that operate at several frequency bands which are widely separated from each other may connect elements resonant at those different frequencies in parallel.
Most of the transmitter's power will flow into the resonant element while the others present a high impedance. Another solution uses traps , parallel resonant circuits which are strategically placed in breaks created in long antenna elements. When used at the trap's particular resonant frequency the trap presents a very high impedance parallel resonance effectively truncating the element at the location of the trap; if positioned correctly, the truncated element makes a proper resonant antenna at the trap frequency.
At substantially higher or lower frequencies the trap allows the full length of the broken element to be employed, but with a resonant frequency shifted by the net reactance added by the trap. The bandwidth characteristics of a resonant antenna element can be characterized according to its Q where the resistance involved is the radiation resistance , which represents the emission of energy from the resonant antenna to free space.
The Q of a narrow band antenna can be as high as On the other hand, the reactance at the same off-resonant frequency of one using thick elements is much less, consequently resulting in a Q as low as 5. These two antennas may perform equivalently at the resonant frequency, but the second antenna will perform over a bandwidth 3 times as wide as the antenna consisting of a thin conductor.
Antennas for use over much broader frequency ranges are achieved using further techniques. Adjustment of a matching network can, in principle, allow for any antenna to be matched at any frequency. Thus the small loop antenna built into most AM broadcast medium wave receivers has a very narrow bandwidth, but is tuned using a parallel capacitance which is adjusted according to the receiver tuning.
On the other hand, log-periodic antennas are not resonant at any frequency but can be built to attain similar characteristics including feedpoint impedance over any frequency range. These are therefore commonly used in the form of directional log-periodic dipole arrays as television antennas. Gain is a parameter which measures the degree of directivity of the antenna's radiation pattern.
A high-gain antenna will radiate most of its power in a particular direction, while a low-gain antenna will radiate over a wide angle. This dimensionless ratio is usually expressed logarithmically in decibels , these units are called "decibels-isotropic" dBi. Since the gain of a half-wave dipole is 2. High-gain antennas have the advantage of longer range and better signal quality, but must be aimed carefully at the other antenna. An example of a high-gain antenna is a parabolic dish such as a satellite television antenna.
Low-gain antennas have shorter range, but the orientation of the antenna is relatively unimportant. An example of a low-gain antenna is the whip antenna found on portable radios and cordless phones. Antenna gain should not be confused with amplifier gain , a separate parameter measuring the increase in signal power due to an amplifying device placed at the front-end of the system, such as a low-noise amplifier. The effective area or effective aperture of a receiving antenna expresses the portion of the power of a passing electromagnetic wave which the antenna delivers to its terminals, expressed in terms of an equivalent area.
Since the receiving antenna is not equally sensitive to signals received from all directions, the effective area is a function of the direction to the source. Due to reciprocity discussed above the gain of an antenna used for transmitting must be proportional to its effective area when used for receiving. Therefore, the effective area A eff in terms of the gain G in a given direction is given by:. Therefore, the above relationship between gain and effective area still holds.
These are thus two different ways of expressing the same quantity. A eff is especially convenient when computing the power that would be received by an antenna of a specified gain, as illustrated by the above example. The radiation pattern of an antenna is a plot of the relative field strength of the radio waves emitted by the antenna at different angles in the far-field. It is typically represented by a three-dimensional graph, or polar plots of the horizontal and vertical cross sections.
The pattern of an ideal isotropic antenna , which radiates equally in all directions, would look like a sphere. Many nondirectional antennas, such as monopoles and dipoles , emit equal power in all horizontal directions, with the power dropping off at higher and lower angles; this is called an omnidirectional pattern and when plotted looks like a torus or donut.
The radiation of many antennas shows a pattern of maxima or " lobes " at various angles, separated by " nulls ", angles where the radiation falls to zero. This is because the radio waves emitted by different parts of the antenna typically interfere , causing maxima at angles where the radio waves arrive at distant points in phase , and zero radiation at other angles where the radio waves arrive out of phase.
In a directional antenna designed to project radio waves in a particular direction, the lobe in that direction is designed larger than the others and is called the " main lobe ". The other lobes usually represent unwanted radiation and are called " sidelobes ". The axis through the main lobe is called the " principal axis " or " boresight axis ".
The polar diagrams and therefore the efficiency and gain of Yagi antennas are tighter if the antenna is tuned for a narrower frequency range, e. Similarly, the polar plots of horizontally polarized yagis are tighter than for those vertically polarized.
The space surrounding an antenna can be divided into three concentric regions: The reactive near-field also called the inductive near-field , the radiating near-field Fresnel region and the far-field Fraunhofer regions. These regions are useful to identify the field structure in each, although the transitions between them are gradual, and there are no precise boundaries.
The far-field region is far enough from the antenna to ignore its size and shape: It can be assumed that the electromagnetic wave is purely a radiating plane wave electric and magnetic fields are in phase and perpendicular to each other and to the direction of propagation.
This simplifies the mathematical analysis of the radiated field. Efficiency of a transmitting antenna is the ratio of power actually radiated in all directions to the power absorbed by the antenna terminals. The power supplied to the antenna terminals which is not radiated is converted into heat. This is usually through loss resistance in the antenna's conductors, or loss between the reflector and feed horn of a parabolic antenna.
Antenna efficiency is separate from impedance matching , which may also reduce the amount of power radiated using a given transmitter. If an SWR meter reads W of incident power and 50 W of reflected power, that means W have actually been absorbed by the antenna ignoring transmission line losses. How much of that power has actually been radiated cannot be directly determined through electrical measurements at or before the antenna terminals, but would require for instance careful measurement of field strength.
The loss resistance and efficiency of an antenna can be calculated once the field strength is known, by comparing it to the power supplied to the antenna. The loss resistance will generally affect the feedpoint impedance, adding to its resistive component. That resistance will consist of the sum of the radiation resistance R rad and the loss resistance R loss. If a current I is delivered to the terminals of an antenna, then a power of I 2 R rad will be radiated and a power of I 2 R loss will be lost as heat.
According to reciprocity , the efficiency of an antenna used as a receiving antenna is identical to its efficiency as a transmitting antenna, described above. The power that an antenna will deliver to a receiver with a proper impedance match is reduced by the same amount.
In some receiving applications, the very inefficient antennas may have little impact on performance. At low frequencies, for example, atmospheric or man-made noise can mask antenna inefficiency. Consequently, an antenna with a 20 dB loss due to inefficiency would have little impact on system noise performance.
Antennas which are not a significant fraction of a wavelength in size are inevitably inefficient due to their small radiation resistance. AM broadcast radios include a small loop antenna for reception which has an extremely poor efficiency. This has little effect on the receiver's performance, but simply requires greater amplification by the receiver's electronics.
Contrast this tiny component to the massive and very tall towers used at AM broadcast stations for transmitting at the very same frequency, where every percentage point of reduced antenna efficiency entails a substantial cost. The definition of antenna gain or power gain already includes the effect of the antenna's efficiency. Therefore, if one is trying to radiate a signal toward a receiver using a transmitter of a given power, one need only compare the gain of various antennas rather than considering the efficiency as well.
This is likewise true for a receiving antenna at very high especially microwave frequencies, where the point is to receive a signal which is strong compared to the receiver's noise temperature. However, in the case of a directional antenna used for receiving signals with the intention of rejecting interference from different directions, one is no longer concerned with the antenna efficiency, as discussed above.
In this case, rather than quoting the antenna gain , one would be more concerned with the directive gain , or simply directivity which does not include the effect of antenna in efficiency. The directive gain of an antenna can be computed from the published gain divided by the antenna's efficiency. The orientation and physical structure of an antenna determine the polarization of the electric field of the radio wave transmitted by it.
For instance, an antenna composed of a linear conductor such as a dipole or whip antenna oriented vertically will result in vertical polarization; if turned on its side the same antenna's polarization will be horizontal. Reflections generally affect polarization. Radio waves reflected off the ionosphere can change the wave's polarization.
For line-of-sight communications or ground wave propagation, horizontally or vertically polarized transmissions generally remain in about the same polarization state at the receiving location. Using a vertically polarized antenna to receive a horizontally polarized wave or visa-versa results in relatively poor reception.
An antenna's polarization can sometimes be inferred directly from its geometry. When the antenna's conductors viewed from a reference location appear along one line, then the antenna's polarization will be linear in that very direction.
In the more general case, the antenna's polarization must be determined through analysis. For instance, a turnstile antenna mounted horizontally as is usual , from a distant location on earth, appears as a horizontal line segment, so its radiation received there is horizontally polarized. But viewed at a downward angle from an airplane, the same antenna does not meet this requirement; in fact its radiation is elliptically polarized when viewed from that direction.
In some antennas the state of polarization will change with the frequency of transmission. The polarization of a commercial antenna is an essential specification. In the most general case, polarization is elliptical , meaning that over each cycle the electric field vector traces out an ellipse.
Two special cases are linear polarization the ellipse collapses into a line as discussed above, and circular polarization in which the two axes of the ellipse are equal. In linear polarization the electric field of the radio wave oscillates along one direction. It has a shape of a disc with a straight edge, with a vertical pillar with a… … Wikipedia. Microstrip antenna — In telecommunication, there are several types of microstrip antennas also known as printed antennas the most common of which is the microstrip patch antenna or patch antenna.
A patch antenna is a narrowband, wide beam antenna fabricated by… … Wikipedia. Kurzwellenantenne, f rus. Dipole antenna — A schematic of a half wave dipole antenna connected to an unbalanced coaxial cable. Better practice is to connect the balanced dipole to the unbalanced line with a balun. A dipole antenna is a radio antenna that can be made of a simple wire, with … Wikipedia. Rubber Ducky antenna — on a transceiver The Rubber Ducky antenna or Rubber Duck aerial is an electrically short monopole antenna which functions somewhat like a base loaded whip antenna and is sealed in a rubber or plastic jacket to protect the antenna.
Whip antenna — on car A whip antenna is an antenna consisting of a single straight flexible wire or rod, often mounted above some type of conducting surface called a ground plane. Since the radiation intensity of an isotropically radiated power is… … Wikipedia Antenna diversity — Antenna diversity, also known as space diversity, is one in a superset of wireless diversity schemes that utilizes two or more antennas to improve the quality and reliability of a wireless link.
View credits, reviews, tracks and shop for the Vinyl release of "Life Is Too Short (Extended Version)" on Discogs. The short dipole antenna is a small version of the popular dipole antenna, typically less than a tenth of a wavelength in its size. Dipole Antennas Include. A short backfire antenna is a type of a directional antenna, characterized by high gain, relatively small size, and narrow band.