Antenna Lab Manual 6i6u5z

Sri Satya Sai Institute Of Science & Technology, Sehore

DEPARTMENT OF ELECTRONICS & COMMUNICATION ENGG.

ANTENNA & WAVE PROPOGATION LAB

LIST OF EXPERIMENT 1. To study and plot the radiation pattern of simple dipole antenna . 2. To study and plot the radiation pattern of Half Wave dipole antenna . 3. To study and plot the radiation pattern of folded dipole antenna . 4. To study and plot the radiation pattern of 5 Element Yagi Uda antenna . 5. To study and plot the radiation pattern of Log Periodic antenna . 6. To study and plot the radiation pattern of Helical antenna . 7. To study and plot the radiation pattern of cut parabolic antenna with simple dipole feed . 8. To study various types of parabolic reflectors and their feed systems . 9. To study and plot the radiation pattern of broad side antenna array . 10.To study and plot the radiation pattern of end fire antenna array .

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Experiment 1 AIM:- To study and plot the radiation pattern of simple dipole antenna . Apparatus Required:- antenna trainer kit, BNC connecting chords,RF detector, Transmitting and receiving antenna

Theory:A dipole antenna is a radio antenna that can be made of a simple wire, with a center-fed driven element. It consists of two metal conductors of rod or wire, oriented parallel and collinear with each other (in line with each other), with a small space between them. The radio frequency voltage is applied to the antenna at the center, between the two conductors. These antennas are the simplest practical antennas from a theoretical point of view. They are used alone as antennas, notably in traditional "rabbit ears" television antennas, and as the driven element in many other types of antennas, such as the Yagi. Dipole antennas were invented by German physicist Heinrich Hertz around 1886 in his pioneering experiments with radio waves.

A short dipole is a physically feasible dipole formed by two conductors with a total length very small compared with the wavelength . The two conducting wires are fed at the centre of the dipole. We assume the hypothesis that the current is maximal at the centre (where the dipole is fed) and that it decreases linearly to be zero at the ends of the wires. Note that the direction of the current is the same in both the dipole branches - to the right in both or to the left in both. The far field of the electromagnetic wave radiated by this dipole is:

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Emission is maximal in the plane perpendicular to the dipole and zero in the direction of wires which is the direction of the current. The emission diagram is circular section torus shaped (right image) with zero inner diameter. In the left image the doublet is vertical in the torus centre

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Procedure:1. Mount the antenna on transmitting mast. 2. Bring the detector assembly near to the main unit and adjust the height of both Tx and Rx antenna for same. 3. Keep detector assembly away from the main unit about 1 mts and align both of them.Ensure that there is no reflector sort of thing in the vicinity of the experiment such as steel structure , mobile phones etc. 4. Adjust the RF detector so that deflection in detector is approx. 30-35 microampere 5. Align arrow mark of the disk with zero of the goniometer scale 6. Start taking reading at the interval of 10 degree . 7. Convert the reading into dB with the help of formula 8. Plot polar plot with all the readings and find HPBW of antenna

Observation Table:-

Sno.

Angle (in degree)

Detector current (in micro ampere)

Power (in dBs)

Result: The radiation pattern is studied and plot is attached Precaution:1. Connection and alignment of both antennas should be make carefully. 2. Reading must be taken carefully.

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Experiment 2 AIM:- To study and plot the radiation pattern of Half wave dipole antenna . Apparatus Required:- antenna trainer kit, BNC connecting chords,RF detector, Transmitting and receiving antenna

Theory:The half-wave dipole antenna is just a special case of the dipole antenna, but its important enough that it will have its own section. Note that the "half-wave" term means that the length of this dipole antenna is equal to a halfwavelength at the frequency of operation. One wavelength at 600 MHz is = c / f = 0.5 meters. Hence, the halfwavelength dipole antenna's length is 0.25 meters.The half-wave dipole antenna is as you may expect, a

simple half-wavelength wire fed at the center as shown in Figure 1:

The input impedance of the half-wavelength dipole antenna is given by Zin = 73 + j42.5 Ohms. The fields from the half-wave dipole antenna are given by:

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The directivity of a half-wave dipole antenna is 1.64 (2.15 dB). The HPBW is 78 degrees.In viewing the impedance as a function of the dipole length in the section on dipole antennas, it can be noted that by reducing the length slightly the antenna can become resonant. If the dipole's length is reduced to 0.48 , the input impedance of the antenna becomes Zin = 70 Ohms, with no reactive component. This is a desirable property, and hence is often done in practice. The radiation pattern remains virtually the sam


Observation Table:Sno.

Angle (in degree)


Power (in dBs)

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Result: The radiation pattern is studied and plot is attached Precaution:1. Connection and alignment of both antennas should be make carefully. 2. Reading must be taken carefully

Experiment 3 AIM:- To study and plot the radiation pattern of folded dipole antenna . Apparatus Required:- antenna trainer kit, BNC connecting chords,RF detector, Transmitting and receiving antenna

Theory:The use of parasitic elements and various stacking arrangements causes a reduction in the radiation resistance of a center-fed, half-wave antenna. Under these conditions obtaining a proper impedance match between the radiator and the transmission line is often difficult. A convenient method of overcoming these difficulties is to use a FOLDED DIPOLE in place of the center-fed radiator. .A FOLDED DIPOLE is an ordinary half-wave antenna that has one or more additional conductors connected across its ends. Additional conductors are mounted parallel to the dipole elements at a distance equal to a very small fraction of a wavelength. Spacing of several inches is common. The feed-point impedance can be further increased by using three or four properly spaced parallel conductors.

Standard feed-line SPREADERS are used to maintain this spacing when required. In any folded dipole, the increase of impedance is the square of the number of conductors used in the radiator. Thus, a three8

wire dipole has nine times (32) the feed-point impedance of a simple center-fed dipole. Second method of stepping up the impedance of a folded dipole is to use two conductors with different radii, as shown in view B. The directional characteristics of a folded dipole are the same as those of a simple dipole. However, the reactance of a folded dipole varies much more slowly as the frequency is varied from resonance. Because of this the folded dipole can be used over a much wider frequency range than is possible with a simple dipole. A folded dipole is a half-wave dipole with an additional wire connecting its two ends. If the additional wire has the same diameter and cross-section as the dipole, two nearly identical radiating currents are generated. The resulting far-field emission pattern is nearly identical to the one for the single-wire dipole described above; however, at resonance its input (feedpoint) impedance Rdf is four times the radiation resistance of a single-wire dipole. This is because for a fixed amount of power, the total radiating current I0 is equal to twice the current in each wire and thus equal to twice the current at the feed point. Equating the average radiated power to the average power delivered at the feedpoint, we may write

It follows that

The folded dipole is therefore well matched to 300-Ohm balanced transmission lines

Procedure:1. Mount the antenna on transmitting mast.

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2. Bring the detector assembly near to the main unit and adjust the height of both Tx and Rx antenna for same. 3. Keep detector assembly away from the main unit about 1 mts and align both of them.Ensure that there is no reflector sort of thing in the vicinity of the experiment such as steel structure , mobile phones etc. 4. Adjust the RF detector so that deflection in detector is approx. 30-35 microampere 5. Align arrow mark of the disk with zero of the goniometer scale 6. Start taking reading at the interval of 10 degree . 7. Convert the reading into dB with the help of formula 8. Plot polar plot with all the readings and find HPBW of antenna


Angle (in degree)


Power (in dBs)


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Experiment 4 AIM:- To study and plot the radiation pattern of Yagi Uda antenna . Apparatus Required:- antenna trainer kit, BNC connecting chords,RF detector, Transmitting and receiving antenna

Theory:- Yagi-Uda antennas are directional along the axis perpendicular to the dipole in the plane of the elements, from the reflector through the driven element and out via the director(s). Typically all elements are spaced about a quarter-wavelength apart.. All elements usually lie in the same plane, ed on a single boom or crossbar; however, they do not have to assume this coplanararrangement: for example, some commercially available Yagi-Uda antennas for television reception have several reflectors arranged to form acorner reflector behind the dipole.

The bandwidth of a Yagi-Uda antenna, which is usually defined as the frequency range for which the antenna provides a good match to the transmission line to which it is attached, is determined by the length, diameter and spacing of the elements. For most designs bandwidth is typically only a few percent of the design frequency.Yagi-Uda antennas can be designed to operate on multiple bands. Such designs are more complicated, using pairs of resonant parallel coiland capacitor combinations (called a "trap" or LC) in the elements. The trap serves to isolate the outer portion of an element from the inner portion at the trap design frequency. In practice the higher frequency traps are located closest to the boom of the antenna. Typically, a triband beam will have two pairs of traps per element. For example, a triband design covering the 10, 15 and 20 meter bands would have traps for the 10 and 15 meter bands. The use of traps is not without disadvantages, as they reduce the bandwidth of the antenna on each band and reduce its overall efficiency. In order to understand the operation of a Yagi-Uda, a simple antenna consisting of a reflector, driven element and a single director as discussed in the previous section will be studied. The driven element is typically a λ/2 dipole and is the only member of the structure that is excited. With all the other elements being parasitic, the antenna can be thought.

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The reflector being longer than λ/2 has an inductive reactance which means the current phase lags that of the voltage. The directors on the other hand, being shorter than λ/2 has a capacitive reactance with the voltage phase lagging that of the current . With the voltage across the driven element being in phase with the induced voltages across the parasitic elements, the current in the reflector lags the current in the driven element which in turn lags the current in the director. The antenna can therefore be thought of as a phased array with each of the elements being excited by a current with a progressive phase shift, starting from the reflector. By tuning the lengths of the reflector and directors and the distance between the elements, the antenna beam is directed towards one angle increasing the gain.



Angle (in degree)


Power (in dBs)

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Experiment 5 AIM:- To study and plot the radiation pattern of Log Periodic antenna . Apparatus Required:- antenna trainer kit, BNC connecting chords,RF detector, Transmitting and receiving antenna

Theory:-

In telecommunication, a log-periodic antenna (LP, also known as a log-periodic array) is a broadband, multi-element, unidirectional, narrow-beam antenna that has impedance and radiation characteristics that are regularly repetitive as a logarithmic function of the excitation frequency. The individual components are often dipoles, as in a log-periodic dipole array (LPDA). Log-periodic antennas are designed to be self-similar and

are thus also fractal antenna arrays.

The log periodic antenna is used in a number of applications where a wide bandwidth is required along with directivity and a modest level of gain. It is sometimes used on the HF portion of the spectrum where operation is required on a number of frequencies to enable communication to be maintained. It is also used at VHF and UHF for a variety of applications, including some uses as a television antenna. It is possible to explain the operation of a log periodic array in straightforward . The feeder polarity is reversed between successive elements. Take the condition when this RF antenna is approximately in the middle of its operating range. When the signal meets the first few elements it will be found that they are spaced quite close together in of the operating wavelength. This means that the fields from these elements will cancel one another out as the feeder sense is reversed between the elements. Then as the signal progresses down the antenna a point is reached where the feeder reversal and the distance between the elements gives a total phase shift of about 360 degrees. At this point the effect which is seen is that of 13

two phased dipoles. The region in which this occurs is called the active region of the RF antenna. Although the example of only two dipoles is given, in reality the active region can consist of more elements. The actual number depends upon the angle [greek letter alpha] and a design constant. The elements outside the active region receive little direct power. Despite this it is found that the larger elements are resonant below the operational frequency and appear inductive. Those in front resonate above the operational frequency and are capacitive. These are exactly the same criteria that are found in the Yagi. Accordingly the element immediately behind the active region acts as a reflector and those in front act as directors. This means that the direction of maximum radiation is towards the feed point


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Angle (in degree)


Power (in dBs)


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]

Experiment 6 AIM:- To study and plot the radiation pattern of Helical antenna . Apparatus Required:- antenna trainer kit, BNC connecting chords,RF detector, Transmitting and receiving antenna

Theory :-A helical antenna is an antenna consisting of a conducting wire wound in the form of a helix. In most cases, helical antennas are mounted over a ground plane. The feed line is connected between the bottom of the helix and the ground plane. Helical antennas can operate in one of two principal modes: normal mode or axial mode.In the normal mode or broadside helix, the dimensions of the helix (the diameter and the pitch) are small compared with the wavelength. The antenna acts similarly to an electrically short dipole or monopole, and the radiation pattern, similar to these antennas is omnidirectional, with maximum radiation at right angles to the helix axis. The radiation is linearly polarized parallel to the helix axis.In the axial mode or end-fire helix, the dimensions of the helix are comparable to a wavelength. The antenna functions as a directional antenna radiating a beam off the ends of the helix, along the antenna's axis. It radiates circularly polarized radio waves.

Broadside helical

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Radiating at 90 degrees from the axis of the helix this design is efficient as a practical reduced-length radiator when compared with the operation of other types such as base-loaded, top-loaded or center-loaded whips. They are typically used for applications where reduced size is a critical operational factor. End-fire helical In the axial mode, the helix dimensions are at or above the wavelength of operation. The antenna then falls under the class of waveguide antennas, and produces a true and consistent circular polarization. The main lobes of the radiation pattern are along the axis of the helix, off both ends. Since in a directional antenna only radiation in one direction is wanted, the other end of the helix is terminated ina a flat metal sheet or screen reflector to reflect the waves forward. Terminal impedance in axial mode ranges between 100 and 200 ohms. The resistive part is approximated by:

where R is resistance in ohms, C is the circumference of the helix, and λ is the wavelength. Impedance matching to the cable C is often done by a short stripline section between the helix and the cable termination.


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Angle (in degree)


Power (in dBs)


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Experiment 7 AIM:- To study and plot the radiation pattern of Parabolic reflector antenna with simple dipole. Apparatus Required:- antenna trainer kit, BNC connecting chords,RF detector, Transmitting and receiving antenna

Theory :-

A parabolic antenna is an antenna that uses a parabolic reflector, a surface with the cross-sectional shape of a parabola, to direct the radio waves. The most common form is shaped like a dish and is popularly called a dish antenna or parabolic dish. The main advantage of a parabolic antenna is that it is highly directive; it functions analogously to a searchlight or flashlight reflector to direct the radio waves in a narrow beam, or receive radio waves from one particular direction only. Parabolic antennas have some of the highestgains, that is they can produce the narrowest beam width angles, of any antenna type. They are used as high-gain antennas for point-topoint radio, television and data communications, and also for radiolocation (radar), on the UHF and microwave (SHF) parts of the radio spectrum. The relatively short wavelength of electromagnetic radiation at these frequencies allows reasonably sized reflectors to exhibit the desired highly directional response.

The directive qualities of an antenna are measured by a dimensionless parameter called its gain, which is the ratio of the power received by the antenna from a source along its beam axis to the power received by a hypothetical isotropic antenna. The gain of a parabolic antenna is:

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where: A is the area of the antenna aperture, that is, the mouth of the parabolic reflector d is the diameter of the parabolic reflector λ is the wavelength of the radio waves. eA is a dimensionless parameter called the aperture efficiency. The aperture efficiency of typical parabolic antennas is 0.55 to 0.70.    

\

Procedure:1. Mount the antenna on transmitting mast. 2. Bring the detector assembly near to the main unit and adjust the height of both TX and Rx antenna for same.

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3. Keep detector assembly away from the main unit about 1 mts and align both of them. Ensure that

4. 5. 6. 7. 8.

there is no reflector sort of thing in the vicinity of the experiment such as steel structure , mobile phones etc. Adjust the RF detector so that deflection in detector is approx. 30-35 microampere Align arrow mark of the disk with zero of the goniometer scale Start taking reading at the interval of 10 degree . Convert the reading into dB with the help of formula Plot polar plot with all the readings and find HPBW of antenna


Angle (in degree)


Power (in dBs)


21

Experiment 8 AIM:- To study various feed systems of Parabolic reflector antenna Apparatus Required:- antenna trainer kit, BNC connecting chords,RF detector, Transmitting and receiving antenna

Theory :- A parabolic antenna is an antenna that uses a parabolic reflector, a surface with the crosssectional shape of a parabola, to direct the radio waves. The most common form is shaped like a dish and is popularly called a dish antenna or parabolic dish. The main advantage of a parabolic antenna is that it is highly directive Parabolic antennas are distinguished by their shapes: Paraboloidal or dish - The reflector is shaped like a paraboloid. This is the most common type. It radiates a narrow pencil-shaped beam along the axis of the dish.  Shrouded dish - Sometimes a cylindrical metal shield is attached to the rim of the dish.The shroud shields the antenna from radiation from angles outside the main beam axis, reducing the sidelobes. It is sometimes used to prevent interference in terrestrial microwave links, where several antennas using the same frequency are located close together. The shroud is coated inside with microwave absorbent material. Shrouds can reduce back lobe radiation by 10 dB.  Cylindrical - The reflector is curved in only one direction and flat in the other. The radio waves come to a focus not at a point but along a line. The feed is often a dipole antenna located along the focal line. It radiates a fan-shaped beam, narrow in the curved dimension, and wide in the uncurved dimension. The curved ends of the reflector are sometimes capped by flat plates, to prevent radiation out the ends, and this is called a pillbox antenna.  "Orange peel" - Another type is very long and narrow, shaped like the letter "C". This is called an orange peel design, and radiates an even wider fan beam. It is often used for radar antennas. 

They are classified by the type of feed; how the radio waves are supplied to the antenna:

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Axial or front feed - This is the most common type of feed, with the feed antenna located in front of the dish at the focus, on the beam axis. A disadvantage of this type is that the feed and its s block some of the beam, which limits the aperture efficiency to only 55 - 60%.[2]  Offset or off-axis feed - The reflector is an asymmetrical segment of a paraboloid, so the focus, and the feed antenna, are located to one side of the dish. The purpose of this design is to move the feed structure out of the beam path, so it doesn't block the beam. It is widely used in home satellite television dishes, which are small enough that the feed structure would otherwise block a significant percentage of the signal.  Cassegrain - In a Cassegrain antenna the feed is located on or behind the dish, and radiates forward, illuminating a convex hyperboloidalsecondary reflector at the focus of the dish. The radio waves from the feed reflect back off the secondary reflector to the dish, which forms the main beam. An advantage of this configuration is that the feed, with its waveguides and "front end" electronics does not have to be suspended in front of the dish, so it is used for antennas with complicated or bulky feeds, such as large satellite communication antennas and radio telescopes. Aperture efficiency is on the order of 65 - 70%  Gregorian - Similar to the Cassegrain design except that the secondary reflector is concave, (ellipsoidal) in shape. Aperture efficiency over 70% can be achieved 

The radiation pattern of the feed antenna has a strong influence on the aperture efficiency, which determines the antenna gain . Radiation from the feed that falls outside the edge of the dish is called "spillover" and is wasted, reducing the gain and increasing the backlobes, possibly causing interference or (in receiving antennas) increasing susceptibility to ground noise. However, maximum gain is only achieved when the dish is uniformly "illuminated" with a constant field strength to its edges. So the ideal radiation pattern of a feed antenna would be a constant field strength throughout the solid angle of the dish, dropping abruptly to zero at the edges. However, practical feed antennas have radiation patterns that drop off gradually at the edges, so the feed antenna is a compromise between acceptably low spillover and adequate illumination

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Result:- Different types of parabolic reflector and there feed system are studied.

Experiment 9 AIM:- To study and plot the radiation pattern of Broad side antenna array. Apparatus Required:- antenna trainer kit, BNC connecting chords,RF detector, Transmitting and receiving antenna

Theory :- When α = 0°, all the elements are in phase and pattern maxima occur at φ = 0° and φ = 180°, that is, in the directions perpendicular to the line of the array. This configuration is called a broadside array. The pattern will be maximum in these directions regardless of the element spacing, d. These will be the only primary maxima if d < λ. If d = λ, additional maxima occur at φ = 90° and φ = 270°. As d is increased still further, additional maxima occur as cones of radiation about the axis of the array. They are known as grating lobes, analogous to the lobes observed in the optical study of a diffraction or reflection grating. Ordinarily, therefore, the spacing of elements in a broadside array is kept less than a wavelength. However, there is an advantage in spacing the elements of an array by more than half a wavelength. For a two-element array the optimum spacing is about 0.7λ, for a four-element array it is about 0.8λ and for a large number of elements the optimum is about 0.95λ. The directivity increases gradually as the spacing is 24

increased until the optimum is reached, then drops rather sharply with further increase. The advantage of the wider spacing is in the directivity obtainable with a given number of elements. In of the ratio of the directivity to the total length of the array, there is no advantage in the wider spacing. These observations can be seen in the applet. Although the above discussion is based on a uniform linear broadside array of point-source isotropic radiators, they apply also to a similar array of dipoles. These dipoles have their centres on the array line and their axes perpendicular to the array line and parallel to each other. The pattern is in the plane perpendicular to the dipole axes, that is, the xy planeas shown in Figure .

The polarization of the array follows the direction of the dipoles. If the array line is horizontal and the dipole axes are vertical, the radiation is vertically polarized. If the array line is horizontal and the dipole axes are horizontal, as shown in Figure , the radiation is horizontally polarized .The in-phase currents in the individual dipoles, required for a broadside pattern, may be obtained by properly connecting a branched transmission line to the feed point of each dipole. That is, if the total line length from the transmitter to each dipole is the same the dipoles will be fed in phase. It is important to ensure that the same side of the line is connected to the same side of each dipole. Reversing this connection reverses the phase. Broadside arrays may also b e formed from other types of elements, such as horns, slots, helixes and polyrods. If the elements are unidirectional radiators, such as sectoral or pyramidal horns, waveguide slots, axial mode helixes and polyrods, a unidirectional broadside array results.

Procedure:1. Mount the antenna on transmitting mast. 2. Bring the detector assembly near to the main unit and adjust the height of both Tx and Rx antenna for same.

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3. Keep detector assembly away from the main unit about 1 mts and align both of them.Ensure that there is no reflector sort of thing in the vicinity of the experiment such as steel structure , mobile phones etc. 4. Adjust the RF detector so that deflection in detector is approx. 30-35 microampere 5. Align arrow mark of the disk with zero of the goniometer scale 6. Start taking reading at the interval of 10 degree . 7. Convert the reading into dB with the help of formula 8. Plot polar plot with all the readings and find HPBW of antenna


Angle (in degree)


Power (in dBs)


Experiment 10 AIM:- To study and plot the radiation pattern of End fire antenna array. Apparatus Required:- antenna trainer kit, BNC connecting chords,RF detector, Transmitting and receiving antenna

Theory :-

the phase-difference α between adjacent elements is equal to 2πdλ radians, the condition for a maximum of radiation is satisfied when φ = 90° and for this value only, provided that d < λ/2. The maximum field intensity is radiated in a direction along the line of the array, “off the end” rather than off the side. Hence the name endfire array. The maximum is toward only one of the ends of the array, rather than in both the endfire directions. In of Fig, if the progressive phase change is a retardation going in the direction of the positive y-axis by the amount α per element, the beam will be in the φ = 90° direction. If the

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sign of α is changed, or its amount is increased by 180 degrees without changing the element spacing, the beam will be in the direction φ = 270°. Although the condition stated above for the value of α results in an endfire array, it does not result in an endfire pattern with the maximum possible directivity and narrowest possible beam.


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Angle (in degree)


Power (in dBs)


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