Antenna Fundamentals. An antenna is a device for converting electromagnetic radiation in space into electrical currents in conductors or. Thus the prime instrument of a radio telescope is the antenna. In this chapter we shall explain the basic concepts of antenna theory from the engineering point of. Antenna Fundamentals. ▫ An antenna is a device that provides a transition between guided electromagnetic waves in wires and electromagnetic waves in free.
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Antenna Basics. Introduction. Antennas are a very important component of communication systems. By definition, an antenna is a device used to transform an. Antenna Basics. White Paper. This white paper describes the basic functionality of antennas. Starting with Hertz's Antenna model followed by a short introduction . Antenna Fundamentals. Basics antenna theory and concepts. M. Haridim. Brno University of Technology, Brno. February
In linear polarization the electric field of the radio wave oscillates back and forth along one direction. Enter the email address you signed up with and we'll email you a reset link. The horn antennas are used in the frequency range of At lower frequencies the ground acts mainly as a good conductor, which AM middle wave broadcast. Antenna Theory and Design, 3rd Ed. 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. The effective area or effective aperture of a receiving antenna expresses the portion of the power of a passing electromagnetic wave which it delivers to its terminals, expressed in terms of an equivalent area.
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 is essentially two dipoles placed side-by-side and connected at their ends to make a single one-wavelength antenna.
The standing wave forms with this desired pattern at the design frequency, f 0 , 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. 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 send the signal into the transmission line, it is important that the transmission line has the same impedance as the elements, otherwise some of the signal will be reflected back into 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 between antennas and transmission lines is commonly handled through the use of a balun , although other solutions are also used in certain roles. An important measure of this basic concept is the standing wave ratio , which measures the magnitude of the reflected signal.
Using the appropriate transmission wire or balun, we match that resistance to ensure minimum signal loss. Feeding that antenna with a current of 1 ampere will require 63 volts of RF, 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 end 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 narrowband, useful for a small range of frequencies. 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.
The amount of signal received from a distant transmission source is essentially geometric in nature due to the inverse-square law , and this leads to the concept of effective area. 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. Elements used in this way are known as passive elements.
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'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.
The matching network may also limit 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 along each antenna element.
When used at one particular frequency band the trap presents a very high impedance parallel resonance effectively truncating the element at that length, making it a proper resonant antenna. At a lower frequency the trap allows the full length of the element to be employed, albeit with a shifted resonant frequency due to the inclusion of the trap's net reactance at that lower frequency.
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 wider 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 it 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 space surrounding an antenna can be divided into three concentric regions: These regions are useful to identify the field structure in each, although 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. 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.
If a current I is delivered to the terminals of an antenna, then a power of I 2 R r 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 the efficiency as defined 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.
In the case of a transmitter, however, the desired matching impedance might not correspond to the dynamic output impedance of the transmitter as analyzed as a source impedance but rather the design value typically 50 ohms required for efficient and safe operation of the transmitting circuitry. 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 supplying a good match at the transmitter or receiver itself. Antenna tuning generally refers 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.
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 such a 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 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. Antennas based on the dipole may include a balun. 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. 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. The horn antennas are used in the frequency range of The impedance of wave guides will be than Transmission lines. Micro strip antenna was first introduced by ………….
Marconi B. Hertz C. Munson D. Cassegrain 2. The widely used shape for patch antennas is …………. Rectangular B. Circular C. Elliptical D. Parabolic 3. The efficiency of Micro strip antenna is …………………. High B. Very high C. Low 4. For square corner reflector the flaring angle is……………………. The no. A parabola is a three dimensional curve.
A paraboloid is a three dimensional curve. In any dish antenna arrangement the parabolic reflector will acts as primary antenna [ ] The directivity of the paraboloid is The generally used feed antenna for paraboloids is The horn and hyperbola are used infeed of dish antennas. The disadvantage draw back of parabolic reflector is Lens Antennas 1.
Electrical path length is increased inlens antennas.
Electrical path length is decreased in lens antennas. E plane metal-plate lens are of type. H plane metal-plate lens are of type. The following is the example of dielectric material. All dielectric materials will have the refractive index of [ ] a unity. The design equation of lens antennas is given by [ ] a b c Both a and b. All metals will have the refractive index of [ ] a unity. By zoning the lens refractive index will…… [ ] a change. In dielectric lens, difference in electrical path length may be caused due to ………… [ ] a Length.
All antenna measurements are accurate. All antenna measurements will be done in the far field [ ] Cylindrical coordinate system will be choosed for all antenna measurements.
For better antenna measurements the tolerance value must be as large as possible. Anechoic chamber can be preferred for antenna measurements. Comparision method for measurement of antenna gain is also called as…………………. The formula for gain using comparision method is……………………. In two antennas method the assumption is……………………. FALSE 2. TRUE 4. FALSE 5. FALSE 7. TRUE 8. Gain-Transfer Page Related Papers. Antenna theory analysis and Design-Balanis.
By Felipe Valle. By Meganiqui Rocha. By jave abrigo. Antenna Theory-Balanis.