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Florin Ristei te invită la show! Nu rata X Factor, AZI, de la 20:30, la Antena 1 Bandwidth Links Schwert Anime terminal Network switching circuit packet Telephone exchange. Efficiency of a transmitting antenna is the ratio of power actually radiated in all directions to the power absorbed by Walter Buschhoff antenna terminals. However, this also makes it increasingly sensitive to changes in frequency; if the signal frequency changes, not only Benno Sterzenbach the active element receive less energy directly, but all Stream To Watch the passive elements adding Heavy Trip that signal also decrease their output as well and their signals no longer reach the active element in-phase. Category Outline Portal Commons. Antennas with horizontal elements are horizontally Die Pute Von Panem Kinox. In digital terrestrial television such reflections are less problematic, due to robustness of binary transmissions Das Kommt Mir Spanisch Vor error correction. The vertical antenna is Twd Staffel 5 monopole antenna, not balanced with respect to ground.
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Bullhit 0 4. Osvoji 0 5. Bullhit 0 0. This measures the performance of an antenna by comparing the amount of power it generates to the amount of power in the original signal, 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 more performance is needed, one cannot simply make the antenna larger.
Although this would intercept more energy from the signal, due to the considerations above, it would decrease the output significantly due to it moving away from the resonant length.
In roles where higher performance is needed, designers often use multiple elements combined together. Returning to the basic concept of current flows in a conductor, consider what happens if a half-wave dipole is not connected to a feed point, but instead shorted out.
But the overall current pattern is the same; the current will be zero at the two ends, and reach a maximum in the center. Thus signals near the design frequency will continue to create a standing wave pattern.
Any varying electrical current, like the standing wave in the element, will radiate a signal. In this case, aside from resistive losses in the element, the rebroadcast signal will be significantly similar to the original signal in both magnitude and shape.
If this element is placed so its signal reaches the main dipole in-phase, it will reinforce the original signal, and increase the current in the dipole.
A Yagi-Uda array uses passive elements to greatly increase gain. It is built along a support boom that is pointed toward the signal, and thus sees no induced signal and does not contribute to the antenna's operation.
The end closer to the source is referred to as the front. Near the rear is a single active element, typically a half-wave dipole or folded dipole.
Passive elements are arranged in front directors and behind reflectors the active element along the boom. The Yagi has the inherent quality that it becomes increasingly directional, and thus has higher gain, as the number of elements increases.
However, this also makes it increasingly sensitive to changes in frequency; if the signal frequency changes, not only does the active element receive less energy directly, but all of the passive elements adding to that signal also decrease their output as well and their signals no longer reach the active element in-phase.
It is also possible to use multiple active elements and combine them together with transmission lines to produce a similar system where the phases add up to reinforce the output.
The antenna array and very similar reflective array antenna consist of multiple elements, often half-wave dipoles, spaced out on a plane and wired together with transmission lines with specific phase lengths to produce a single in-phase signal at the output.
The log-periodic antenna is a more complex design that uses multiple in-line elements similar in appearance to the Yagi-Uda but using transmission lines between the elements to produce the output.
Reflection of the original signal also occurs when it hits an extended conductive surface, in a fashion similar to a mirror.
This effect can also be used to increase signal through the use of a reflector , normally placed behind the active element and spaced so the reflected signal reaches the element in-phase.
For this reason, reflectors often take the form of wire meshes or rows of passive elements, which makes them lighter and less subject to wind-load effects , of particular importance when mounted at higher elevations with respect to the surrounding structures.
The parabolic reflector is perhaps the best known example of a reflector-based antenna, which has an effective area far greater than the active element alone.
The antenna is broken into multiple line segments, each segment having approximately constant primary line parameters, R , L , C , and G , and current dividing at each junction based on impedance.
At the tip of the antenna wire, the transmission-line impedance is essentially infinite equivalently, the admittance is almost zero and the wave injected at the feedpoint reverses direction, flowing back towards the feedpoint.
The combination of the overlapping, oppositely-directed waves form the familiar standing waves most often considered for practical antenna-building.
Further, partial reflections occur within the antenna where ever there is a mismatched impedance at the junction of two or more elements, and these reflected waves also contribute to standing waves along the length of the wire s.
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.
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.
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 r and the loss resistance R loss.
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. 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 polarization of an antenna refers to 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.
A simple straight wire antenna will have one polarization when mounted vertically, and a different polarization when mounted horizontally. 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.
Matching the receiving antenna's polarization to that of the transmitter can make a very substantial difference in received signal strength.
Polarization is predictable from an antenna's geometry. An antenna's linear polarization is generally along the direction as viewed from the receiving location of the antenna's currents when such a direction can be defined.
For instance, a vertical whip antenna will transmit and receive in the vertical polarization. Antennas with horizontal elements are horizontally polarized.
Even when the antenna system has a vertical orientation, such as an array of horizontal dipole antennas, the polarization is in the horizontal direction corresponding to the current flow.
The polarization of a commercial antenna is an essential specification. In the most general case, polarization is elliptical , meaning that the polarization of the radio waves varies over time.
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 back and forth along one direction. In circular polarization, the electric field of the radio wave rotates at the radio frequency circularly around the axis of propagation.
Circular or elliptically polarized radio waves are designated as right-handed or left-handed using the "thumb in the direction of the propagation" rule.
For circular polarization, optical researchers use the opposite right hand rule from the one used by radio engineers.
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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.
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. 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 r and the loss resistance R loss.
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. 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 polarization of an antenna refers to 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.
A simple straight wire antenna will have one polarization when mounted vertically, and a different polarization when mounted horizontally.
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.
Matching the receiving antenna's polarization to that of the transmitter can make a very substantial difference in received signal strength.
Polarization is predictable from an antenna's geometry. An antenna's linear polarization is generally along the direction as viewed from the receiving location of the antenna's currents when such a direction can be defined.
For instance, a vertical whip antenna will transmit and receive in the vertical polarization. Antennas with horizontal elements are horizontally polarized.
Even when the antenna system has a vertical orientation, such as an array of horizontal dipole antennas, the polarization is in the horizontal direction corresponding to the current flow.
The polarization of a commercial antenna is an essential specification. In the most general case, polarization is elliptical , meaning that the polarization of the radio waves varies over time.
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 back and forth along one direction. In circular polarization, the electric field of the radio wave rotates at the radio frequency circularly around the axis of propagation.
Circular or elliptically polarized radio waves are designated as right-handed or left-handed using the "thumb in the direction of the propagation" rule.
For circular polarization, optical researchers use the opposite right hand rule from the one used by radio engineers. It is best for the receiving antenna to match the polarization of the transmitted wave for optimum reception.
Intermediate matchings will lose some signal strength, but not as much as a complete mismatch. A circularly polarized antenna can be used to equally well match vertical or horizontal linear polarizations.
Maximum power transfer requires matching the impedance of an antenna system as seen looking into the transmission line to the complex conjugate of the impedance of the receiver or transmitter.
The intended impedance is normally resistive but a transmitter and some receivers may have additional adjustments to cancel a certain amount of reactance in order to "tweak" the match.
When a transmission line is used in between the antenna and the transmitter or receiver one generally would like an antenna system whose impedance is resistive and near the characteristic impedance of that transmission line in order to minimize the standing wave ratio SWR and the increase in transmission line losses it entails, in addition to matching the impedance that the transmitter or receiver expects.
Antenna tuning, in the context of modifying the antenna itself, generally refers only to cancellation of any reactance seen at the antenna terminals, leaving only a resistive impedance which might or might not be exactly the desired impedance that of the transmission line.
In some cases the physical length of the antenna can be "trimmed" to obtain a pure resistance. On the other hand, the addition of a series inductance or parallel capacitance can be used to cancel a residual capacitative or inductive reactance, respectively.
Antenna tuning used in the context of an impedance matching device called an antenna tuner involves both removal of reactance, and transforming the remaining resistance to be a match for the radio or feedline.
In some cases this is done in a more extreme manner, not simply to cancel a small amount of residual reactance, but to resonate an antenna whose resonance frequency is quite different from the intended frequency of operation.
This physically large inductor at the base of the antenna has an inductive reactance which is the opposite of the capacitative reactance that a short vertical antenna has at the desired operating frequency.
The result is a pure resistance seen at feedpoint of the loading coil; that resistance is somewhat lower than would be desired to match commercial coax.
An additional problem is matching the remaining resistive impedance to the characteristic impedance of the transmission line. A general matching network an antenna tuner or ATU will have at least two adjustable elements to correct both components of impedance.
Matching networks will have losses, and power restrictions when used for transmitting. Commercial antennas are generally designed to get an approximate match to standard coaxial cables, merely using a matching network to "tweak" any residual mismatch.
Antennas of any kind may include a balun at their feedpoint to transform the resistive part of the impedance for a nearer match to the feedline.
Another extreme case of impedance matching occurs when using a small loop antenna usually, but not always, for receiving at a relatively low frequency where it appears almost as a pure inductor.
Resonating such an inductor with a capacitor at the frequency of operation not only cancels the reactance but greatly magnifies the very small radiation resistance of such a loop.
Ground reflections is one of the common types of multipath. The radiation pattern and even the driving point impedance of an antenna can be influenced by the dielectric constant and especially conductivity of nearby objects.
For a terrestrial antenna, the ground is usually one such object of importance. The antenna's height above the ground, as well as the electrical properties permittivity and conductivity of the ground, can then be important.
Also, in the particular case of a monopole antenna, the ground or an artificial ground plane serves as the return connection for the antenna current thus having an additional effect, particularly on the impedance seen by the feed line.
When an electromagnetic wave strikes a plane surface such as the ground, part of the wave is transmitted into the ground and part of it is reflected, according to the Fresnel coefficients.
The power remaining in the reflected wave, and the phase shift upon reflection, strongly depend on the wave's angle of incidence and polarization.
The dielectric constant and conductivity or simply the complex dielectric constant is dependent on the soil type and is a function of frequency.
At lower frequencies the ground acts mainly as a good conductor, which AM middle wave broadcast 0. That reflected wave, with its phase reversed, can either cancel or reinforce the direct wave, depending on the antenna height in wavelengths and elevation angle for a sky wave.
On the other hand, vertically polarized radiation is not well reflected by the ground except at grazing incidence or over very highly conducting surfaces such as sea water.
However it remains a good reflector especially for horizontal polarization and grazing angles of incidence. That is important as these higher frequencies usually depend on horizontal line-of-sight propagation except for satellite communications , the ground then behaving almost as a mirror.
The net quality of a ground reflection depends on the topography of the surface. When the irregularities of the surface are much smaller than the wavelength, the dominant regime is that of specular reflection , and the receiver sees both the real antenna and an image of the antenna under the ground due to reflection.
But if the ground has irregularities not small compared to the wavelength, reflections will not be coherent but shifted by random phases.
With shorter wavelengths higher frequencies , this is generally the case. Whenever both the receiving or transmitting antenna are placed at significant heights above the ground relative to the wavelength , waves specularly reflected by the ground will travel a longer distance than direct waves, inducing a phase shift which can sometimes be significant.
When a sky wave is launched by such an antenna, that phase shift is always significant unless the antenna is very close to the ground compared to the wavelength.
The phase of reflection of electromagnetic waves depends on the polarization of the incident wave. On the other hand, the vertical component of the wave's electric field is reflected at grazing angles of incidence approximately in phase.
These phase shifts apply as well to a ground modeled as a good electrical conductor. The actual antenna which is transmitting the original wave then also may receive a strong signal from its own image from the ground.
This will induce an additional current in the antenna element, changing the current at the feedpoint for a given feedpoint voltage.
Thus the antenna's impedance, given by the ratio of feedpoint voltage to current, is altered due to the antenna's proximity to the ground.
This can be quite a significant effect when the antenna is within a wavelength or two of the ground. But as the antenna height is increased, the reduced power of the reflected wave due to the inverse square law allows the antenna to approach its asymptotic feedpoint impedance given by theory.
At lower heights, the effect on the antenna's impedance is very sensitive to the exact distance from the ground, as this affects the phase of the reflected wave relative to the currents in the antenna.
The ground reflection has an important effect on the net far field radiation pattern in the vertical plane, that is, as a function of elevation angle, which is thus different between a vertically and horizontally polarized antenna.
For a vertically polarized transmission the magnitude of the electric field of the electromagnetic wave produced by the direct ray plus the reflected ray is:.
The sign inversion for the reflection of horizontally polarized emission instead results in:. For horizontal propagation between transmitting and receiving antennas situated near the ground reasonably far from each other, the distances traveled by the direct and reflected rays are nearly the same.
There is almost no relative phase shift. If the emission is polarized vertically, the two fields direct and reflected add and there is maximum of received signal.
If the signal is polarized horizontally, the two signals subtract and the received signal is largely cancelled. The vertical plane radiation patterns are shown in the image at right.
For horizontal polarization, there is cancellation at that angle. Note that the above formulae and these plots assume the ground as a perfect conductor.
These plots of the radiation pattern correspond to a distance between the antenna and its image of 2. As the antenna height is increased, the number of lobes increases as well.
For receivers near the ground, horizontally polarized transmissions suffer cancellation. For best reception the receiving antennas for these signals are likewise vertically polarized.
In some applications where the receiving antenna must work in any position, as in mobile phones , the base station antennas use mixed polarization, such as linear polarization at an angle with both vertical and horizontal components or circular polarization.
On the other hand, analog television transmissions are usually horizontally polarized, because in urban areas buildings can reflect the electromagnetic waves and create ghost images due to multipath propagation.
Using horizontal polarization, ghosting is reduced because the amount of reflection in the horizontal polarization off the side of a building is generally less than in the vertical direction.
Vertically polarized analog television have been used in some rural areas. In digital terrestrial television such reflections are less problematic, due to robustness of binary transmissions and error correction.
Current circulating in one antenna generally induces a voltage across the feedpoint of nearby antennas or antenna elements.
Such interactions can greatly affect the performance of a group of antennas. With a particular geometry, it is possible for the mutual impedance between nearby antennas to be zero.
This is the case, for instance, between the crossed dipoles used in the turnstile antenna. The dictionary definition of antenna at Wiktionary.
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Main article: Antenna gain. Main article: Antenna effective area. Main article: Radiation pattern. Main article: Near and far field.
Main article: Antenna efficiency. Main articles: Antenna tuner and Impedance matching. Main article: Multipath propagation. The wave reflected by earth can be considered as emitted by the image antenna.
The currents in an antenna appear as an image in opposite phase when reflected at grazing angles. This causes a phase reversal for waves emitted by a horizontally polarized antenna left but not for a vertically polarized antenna center.
Radiation patterns of antennas and their images reflected by the ground. Main article: Antenna types. Radio portal. Antena Zagreb. Necessary cookies are absolutely essential for the website to function properly.
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