AWP
Unit-3Wire Antenna Q.1 Define and explain in detail Infinitesimal DipoleAns – 1. Definition“A dipole whose length is far less than wavelength is infitesimal dipole.”This antenna is actually impractical. Here, the length of the dipole is less than even fiftieth part of the wavelength.2. The length of the dipole, Δl << λ. Where, λ is the wavelength.Δl=λ/50Hence, this is the infinitely small dipole, as the name implies.3. As the length of these dipoles is very small, the current flow in the wire will be dI. These wires are generally used with capacitor plates on both sides, where low mutual coupling is needed. Because of the capacitor plates, we can say that uniform distribution of current is present. Hence the current is not zero here.4. The capacitor plates can be simply conductors or the wire equivalents. The fields radiated by the radial currents tend to cancel each other in the far field so that the far fields of the capacitor plate antenna can be approximated by the infinitesimal dipole.5. Radiation PatternThe radiation pattern of infinitesimal dipole is similar to a half wave dipole. If the dipole is vertical, the pattern will be circular. The radiation pattern is in the shape of “figure of eight” pattern, when viewed in two-dimensional pattern.The following figure shows the radiation pattern of infinitesimal antenna, which is in omni-directional pattern.
Figure: Radiation pattern of Infinitesimal Dipole6. Advantages● Ease of construction, due to small size7. DisadvantagesThe following are the disadvantages of infinitesimal dipole antenna −● High resistive losses● High power dissipation● Low Signal-to-noise ratio● Radiation is low● Not so efficient8. ApplicationsThe following are the applications of infinitesimal dipole antenna −● Used in narrow band applications.● Used as an antenna for tuner circuits Q.2 Give the construction and working of Half wavelength dipole antenna.Ans – Construction & Working of Half-wave Dipole1. It is a normal dipole antenna, where the frequency of its operation is half of its wavelength. Hence, it is called as half-wave dipole antenna.2. The edge of the dipole has maximum voltage. This voltage is alternating (AC) in nature. 3. At the positive peak of the voltage, the electrons tend to move in one direction and at the negative peak, the electrons move in the other direction. This can be explained by the figures given below.
Figure: working of a half-wave dipole.4. Fig 1 shows the dipole when the charges induced are in positive half cycle. Now the electrons tend to move towards the charge.5. Fig 2 shows the dipole with negative charges induced. The electrons here tend to move away from the dipole.6. Fig 3 shows the dipole with next positive half cycle. Hence, the electrons again move towards the charge.7. The cumulative effect of this produces a varying field effect which gets radiated in the same pattern produced on it.8. Hence, the output would be an effective radiation following the cycles of the output voltage pattern. Thus, a half-wave dipole radiates effectively.Q.3 Explain in detail Ground EffectAns – 1. Waves radiated from the antenna directly downward reflect vertically from the ground and, in passing the antenna on their upward journey, induce a voltage in it. The magnitude and phase of the current resulting from this induced voltage depends on the height of the antenna above the reflecting surface. 2. The total current in the antenna consists of two components. The amplitude of the first is determined by the power supplied by the transmitter and the free-space feed-point resistance of the antenna.3. The second component is induced in the antenna by the wave reflected from the ground. This second component of current, while considerably smaller than the first at most useful antenna heights.4. At some heights, the two components will be in phase, so the total current is larger than is indicated by the free-space feed-point resistance. At other heights, the two components are out of phase, and the total current is the difference between the two components. 5. Changing the height of the antenna above ground will change the amount of current flow, assuming that the power input to the antenna is constant. A higher current at the same power input means that the effective resistance of the antenna is lower, and vice versa.6. In other words, the feed-point resistance of the antenna is affected by the height of the antenna above ground because of mutual coupling between the antenna and the ground beneath it. 7. Different impedance values may be encountered when an antenna is erected at identical heights but over different types of earth.
Figure: Effect of ground in Reactive near field8. Above Fig shows the way in which the radiation resistance of horizontal and vertical half-wave antennas vary with height above ground (in λ, wavelengths). For horizontally polarized half-wave antennas, the differences between the effects of perfect ground and real earth are negligible.9. If the antenna height is greater than 0.2 λ. At lower heights, the feed-point resistance over perfect ground decreases rapidly as the antenna is brought closer to a theoretically perfect ground, but this does not occur so rapidly for actual ground. 10. Over real earth, the resistance begins increasing at heights below about 0.08 λ. The reason for the increasing resistance at very low heights is that more and more of the reactive (induction) field of the antenna is absorbed by the lossy ground in close proximity. Q.4 Write a note on Monopole AntennaAns- 1. A monopole antenna is one half of a dipole antenna, almost always mounted above some sort of ground plane.2. The case of a monopole antenna of length L mounted above an infinite ground plane is shown in Figure below (a).
Figure : Monopole above a PEC (a), and the equivalent source in free space (b).3. Using image theory, the fields above the ground plane can be found by using the equivalent source (antenna) in free space as shown in Figure 1(b).4. This is simply a dipole antenna of twice the length. The fields above the ground plane in Figure 1(a) are identical to the fields in Figure 1(b), which are known and presented in the dipole antenna section. 5. The monopole antenna fields below the ground plane in Figure 1(a) are zero.6. Radiation PatternThe radiation pattern of monopole antennas above a ground plane are also known from the dipole result. The only change that needs to be noted is that the impedance of a monopole antenna is one half of that of a full dipole antenna. 7. For a quarter-wave monopole (L=0.25* ), the impedance is half of that of a half-wave dipole, so Zin = 36.5 + j21.25 Ohms. 8. This can be understood since only half the voltage is required to drive a monopole antenna to the same current as a dipole. Since Zin = V/I, the impedance of the monopole antenna is halved.
Q.5. Explain Folded Dipole AntennaAns-1. The folded dipole antenna consists of a basic dipole, but with an added conductor connecting the two ends together. This makes a ‘loop’ of wire that is a short circuit to DC. As the ends appear to be folded back, the antenna is called a folded dipole antenna.2. The basic format for the folded dipole aerial is shown below. Like the basic dipole, the folded dipole antenna is a balanced antenna, and needs to be fed with a balanced feeder. 3. Unbalanced feeders can be used provided that a balun (unbalanced to balanced transformer) is used.
Figure 3.5. Basic dipole antenna4. One of the main reasons for using a folded dipole antenna is the increase in feed impedance that it provides. If the conductors in the main dipole and the second or "fold" conductor are the same diameter, then it is found that there is a fourfold increase (i.e. two squared) in the feed impedance. 5. In free space, this gives an increase in feed impedance from 73Ω to around 300Ω ohms. Additionally the RF antenna has a wider bandwidth.6. Folded dipole advantagesThere are two main advantages for using a folded dipole antenna over a standard dipole:
Figure : Helical Antenna● The above image shows a helical antenna system, which is used for satellite communications. These antennas require wider outdoor space.● It consists of a helix of thick copper wire or tubing wound in the shape of a screw thread used as an antenna in conjunction with a flat metal plate called a ground plate. ● One end of the helix is connected to the centre conductor of the cable and the outer conductor is connected to the ground plate.● The image of a helix antenna detailing the antenna parts is shown above.The radiation of helical antenna depends on the diameter of helix, the turn the pitch angle.4. Pitch angle is the angle between a line tangent to the helix wire and plane normal to the helix axis.α=tan−1(S/πD)where,● D is the diameter of helix.● S is the turn spacing (centre to centre).● α is the pitch angleQ.8 Explain different Modes of Operation of helical antenna.Ans-The predominant modes of operation of a helical antenna are −● Normal or perpendicular mode of radiation.● Axial or end-fire or beam mode of radiation.1. Normal Mode● In normal mode of radiation, the radiation field is normal to the helix axis. The radiated waves are circularly polarized. ● This mode of radiation is obtained if the dimensions of helix are small compared to the wavelength. ● The radiation pattern of this helical antenna is a combination of short dipole and loop antenna.
Figure: Radiation pattern for normal mode of radiation in helical antenna● It depends upon the values of diameter of helix, D and its turn spacing, S. Drawbacks of this mode of operation are low radiation efficiency and narrow bandwidth. Hence, it is hardly used.2. Axial Mode● In axial mode of radiation, the radiation is in the end-fire direction along the helical axis and the waves are circularly or nearly circularly polarized. ● This mode of operation is obtained by raising the circumference to the order of one wavelength (λ) and spacing of approximately λ/4.● The radiation pattern is broad and directional along the axial beam producing minor lobes at oblique angles. Q.9 Draw and explain radiation Pattern of V AntennaAns – 1.The figure given below is representing the radiation pattern for the resonant type of V antenna:
Figure: Radiation pattern of resonant v antenna 2. Now, have a look at the radiation pattern of the non-resonant type of antenna, showing unidirectional radiation pattern:
Figure: Radiation pattern of non-resonant v antenna
Figure: Radiation pattern for axial mode of radiation in helical antenna● If this antenna is designed for right-handed circularly polarized waves, then it will not receive left-handed circularly polarized waves and vice versa.● This mode of operation is generated with great ease and is more practically used.Q.10 Give brief description of Yagi-uda AntennaAns- 1. The Yagi antenna or Yagi-Uda antenna or aerial is a particularly popular form of antenna where directivity and gain are required.2. Although the Yagi has become particularly popular for television reception, it is also used in many other applications, both domestic and commercial or professional.3. The gain and directivity of the Yagi antenna enable improved reception by enabling better levels of signal to noise ratio to be achieved, and by reducing interference levels by only picking up signals from a given direction.4. For transmitting much better use of the available power is made because it is possible to focus the transmitted power on areas where it is needed. 5. Similarly levels of general interference can be reduced to other users because the signal is not transmitted to areas where it is not needed.6. Construction
Figure: Yagi antenna componentsThere are three types of element within a Yagi antenna:
Typically a reflector will add around 4 or 5 dB of gain in the forward direction.
.Figure : Radiation Pattern of Yagi- Uda Antenna1. The antenna can be optimised to either reduce radiation in the reverse direction by altering the length and spacing of the reflector or it can be optimised to produce the maximum level of forward gain.2. Unfortunately the two conditions do not coincide exactly and a compromise on the performance has to be made depending upon the application. It is necessary to choose either maximum front to back ratio or maximum forward gain.
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Q.5. Explain Folded Dipole AntennaAns-1. The folded dipole antenna consists of a basic dipole, but with an added conductor connecting the two ends together. This makes a ‘loop’ of wire that is a short circuit to DC. As the ends appear to be folded back, the antenna is called a folded dipole antenna.2. The basic format for the folded dipole aerial is shown below. Like the basic dipole, the folded dipole antenna is a balanced antenna, and needs to be fed with a balanced feeder. 3. Unbalanced feeders can be used provided that a balun (unbalanced to balanced transformer) is used.
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● Increase in impedance
When higher impedance feeders need to be used, or when the impedance of the dipole is reduced by factors such as parasitic elements, a folded dipole provides a significant increase in impedance level that enables the antenna to be matched more easily to the feeder available.● Wide bandwidth
The folded dipole antenna has a flatter frequency response - this enables it to be used over a wider bandwidth with many transmissions utilising a variety of different selectable channels, e.g. television and broadcast radio, a wide bandwidth antenna is needed. The standard dipole antenna does not always provide the required bandwidth and the additional bandwidth of the folded dipole meets the requirements. Q.6 Give the applications of folded dipole antenna.Ans-There are many ways in which folded dipoles can be used. They find uses in many applications:1. On their ownFolded dipole antennas are sometimes used on their own, but they must be fed with a high impedance feeder, typically 300 ohms. This on its own can be very useful in certain applications where balanced feeders may be used.2. As part of another antennaHowever folded dipoles find more uses when a dipole is incorporated in another RF antenna design with other elements nearby. The issue is that incorporating a dipole into an antenna such as a Yagi where elements are closely coupled reduces the feed impedance. If a simple dipole was used, then the feed impedance levels of less than 20 Ω or less can easily be experienced.3. Using a folded dipole enables the impedance to be increased by a factor of four or whatever is required by having multiple wires in the folded dipole.4. Increased bandwidthSometimes folded dipoles may be employed purely to give a greater bandwidth. When used to increase bandwidth, folded dipoles may be used on their own or within another antenna system.Q.7 Write a note on Helical AntennaAns-1. Helical antenna is an example of wire antenna and itself forms the shape of a helix. This is a broadband VHF and UHF antenna.2. Frequency RangeThe frequency range of operation of helical antenna is around 30MHz to 3GHz. This antenna works in VHF and UHF ranges.3. Construction & Working of Helical Antenna● Helical antenna or helix antenna is the antenna in which the conducting wire is wound in helical shape and connected to the ground plate with a feeder line. ● It is the simplest antenna, which provides circularly polarized waves. It is used in extra-terrestrial communications in which satellite relays etc., are involved.
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● In the case of a vertical arrangement, the two or more antennas can be stacked one above the other. While in a horizontal manner these can be arranged in broadside.
● Here is no reflector is used in case of the array with elements as V antenna.
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● Driven element: The driven element is the Yagi antenna element to which power is applied. It is normally a half wave dipole or often a folded dipole.
● Reflector: The reflector element is made to be about 5% longer than the driven element. The Yagi antenna will generally only have one reflector. This is behind the main driven element, i.e. the side away from the direction of maximum sensitivity.
● Further reflectors behind the first one makes no noticeable different to the antenna performance. However, many designs use reflectors consisting of a reflecting plate, or a series of parallel rods simulating a reflecting plate. This gives a slight improvement in performance, reducing the level of radiation or pick-up from behind the antenna, i.e. in the backwards direction. Tis can help
in reducing the levels of interference received.Typically a reflector will add around 4 or 5 dB of gain in the forward direction.
● Director: The director or directors are made to be shorter than the driven element. There may be none, one of more reflectors in the Yagi antenna. The director or directors are placed in front of the driven element, i.e. in the direction of maximum sensitivity. Typically, each director will add around 1 dB of gain in the forward direction, although this level reduces as the number of directors increases.
Q.11 Draw and explain radiation Pattern of yagi-uda antenna.Ans-
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Q.12 Give the Advantages and Disadvantages of yagi-uda antenna.Ans- Advantages1. Directivity The Yagi antenna is directional enabling interference levels to be minimised for receiving and transmitting.2. Gain The Yagi antenna has gain allowing lower strength signals to be received.3. Straightforward construction The Yagi antenna is mechanically relatively straightforward when compared to other designs. It can be constructed using straight rods which are simple to use and robust for most instances.4. Polarisation The construction enables the antenna to be mounted easily on vertical and other poles with standard mechanical fixings Disadvantages 1. Max gain ~20dB Gain is limited to around 20dB or so for a single antenna otherwise it becomes too large and beamwidth narrows. For low frequency antennas the physical size means that the maximum number of elements and hence the gain is much lower than 20dB.2. Long for high gain: For high gain levels the antenna becomes very long.
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