AWP
Unit-4Antennas for Different Applications Q.1 Draw and Explain radiation pattern of E- plane Sectoral Horn.Ans- 1. Radiation pattern of E-Plane hornThe E pattern is much narrower than H plane because of the flaring and larger dimensions of the horn in the directions.
Figure: Radiation pattern of E-Plane horn2. It is assumed that, there exist a line source radiating cylindrical waves at the imaginary apex of the horn. As waves travel in the outward radial direction, the constant phase fronts are cylindrical which do not coincide with aperture plane.3. At any point y’ at the aperture of the horn, the phase of the field will not be the same as that at the origin. 4. The phase difference because the wave travelled different distances from the apex to the aperture.5. The difference in path of travel, designated as δ(y’),can be obtained as followsQ.2 Explain 1. Aperture phase distribution 2. Aperture Fields 3. DirectivityOf E- plane Sectoral Horn.Ans- 1. Aperture phase distribution
2. Aperture Fields
(
) = 
Amplitude distribution
= Phase distribution
(
) = 
3. Directivity The directivity of the E-plane sectoral horn is found in a manner analogous to the H-plane sectoral horn:
= 
= 
Where,
= 
, 
= 
, q =
Figure: Directivity of E planeMaximum Directivity occurs when
Q.3 Explain in detail H-plane sectoral horn.Ans- 1. The geometry and the respective parameters shown in the figure below are used in the subsequent analysis. The two required dimensions for the construction of the horn are A and 
.
= -
Figure : H-plane sectoral horn2. Directivity The directivity of the H-plane sectoral horn is maximum when,
Figure: Directivity of H sectoral hornMaximum Phase error occurs when x’ = a1 Δ max = 2πt, where, t=
Which gives‘t’ approximately equal to:
= 
at 
= 
Q.3 Write a note on Pyramidal horn Ans – 1. The pyramidal horn is probably the most popular antenna in the microwave frequency ranges (from ≈1 GHz up to ≈18 GHz). The feeding waveguide is flared in both directions, the E-plane and the H-plane. 2. The field distribution at the aperture is approximated as
3. The E-plane principal pattern of the pyramidal horn is the same as the E-plane principal pattern of the E-plane sectoral horn. The same holds for the H-plane patterns of the pyramidal horn and the H-plane sectoral horn. 4. The directivity of the pyramidal horn can be found by introducing the phase efficiency factors of both planes and the taper efficiency factor of the H-plane:
6. The gain of a horn is usually very close to its directivity because the radiation efficiency is very good (low losses). The above expression is a physical optics approximation, and it does not take into account only multiple diffractions, and the diffraction at the edges of the horn arising from reflections from the horn interior.7. These phenomena, which are unaccounted for, lead to only very minor fluctuations of the measured result. That is why horns are often used as gain standards in antenna measurements.8. The optimal directivity of an E-plane horn is achieved at q =1 , 
=0.8 The optimal directivity of an H-plane horn is achieved at t = 3 / 8)], 
=0.79 . Thus, the optimal horn has a phase aperture efficiency of
=0.632The total aperture efficiency includes the taper factor, too: 
=0.81 *0.632 =0.519. Therefore, the best achievable directivity for a rectangular waveguide horn is about half that of a uniform rectangular aperture. 10. Radiation pattern of Pyramidal horn
Figure: Radiation pattern of Pyramidal horn The radiation pattern of pyramidal horn is the combination of E plane and H plane horn.Q.4 Explain Conical HornAns-1. A horn in the shape of a cone, with a circular cross section. They are used with cylindrical waveguides.2. Conical horn is a truncated section of a right circular conical waveguide, and it is usually connected to a circular waveguide or a rectangular waveguide which is gradually transitioned into the circular waveguide.3. It is a basic and popular microwave antenna for many practical applications. Also, it is widely utilized as a gain standard for calibration and gain measurements.4. Characteristics
Figure: Radiation Pattern Q.5 Give the brief description of Corrugated Horn.Ans- 1. The corrugated horn antenna has parallel slots or grooves along the inside surface of the horn, transverse to the axis. These corrugations are small when compared to the wavelength. 2. Corrugated horns have several advantages including a wider bandwidth, and smaller side-lobes that other types. The corrugated horn provides a pattern that is nearly symmetrical, with the E and H plane beam-widths being nearly the same.3. As a result corrugated horn antenna are widely used as feed horns for satellite dishes and radio telescopes.4. These are popularly used as feed horns for dish reflector antennas due to their low side lobe and cross-polarization level. The excited TE mode from a circular waveguide passes along the corrugated inner surface of a circular horn antenna, where a TM mode is also generated. 5. When combined, these two modes give lower cross polarization at the antenna aperture. By using this application, the antenna radiation characteristics and aperture cross-polarization ratio can be improved by modifying the geometry of the antennaQ.6 What are the types of reflectors antenna. Explain plane antenna in detail.Ans- There are mainly four types of reflector antenna.Plane Reflector Corner Reflector Parabolic Reflector Spherical Reflector Plane Reflector
Figure: Plane Reflector1. The simplest type of reflector is a plane reflector. It consists of primary antenna and reflecting surface introduced to direct energy in a desired direction. 2. Position of radiating source and it’s relative to the reflecting surface can be used to control the radiating properties (pattern, impedance, directivity) of the overall system.3. Image theory has been used to analyze the radiating characteristics of such a system. 4. Disadvantage of this type that not possible to collimate energy in forward direction.Q.7 Write a note on Corner Reflector.Ans-1. To better collimate the energy in the forward direction, the geometrical shape of the plane reflector itself must be changed so as to prohibit radiation in the back and side directions.2. It consists of two plane reflectors joined so as to form a corner. The feed element for a corner reflector is almost always a dipole or an array of collinear dipoles placed parallel to the vertex a distance s away.
Figure: Corner Reflector3. Greater bandwidth is obtained when the feed elements are cylindrical or biconical dipoles instead of thin wires.4. In many applications, especially when the wavelength is large compared to tolerable physical dimensions, the surfaces of the corner reflector are frequently made of grid wires rather than solid sheet metal, one of the reasons for doing that is to reduce wind resistance and overall system weight.5. The spacing (g) between wires is made a small fraction of a wavelength (usually g ≤ λ/10). For wires that are parallel to the length of the dipole, the reflectivity of the grid-wire surface is as good as that of a solid surface.Q.8 Give the practical design for Corner Reflector.Ans – 1. The length of the sides for reflectors with smaller included angles, the sides are made larger.2. For each reflector, there is an optimum feed-to-vertex spacing. If the spacing becomes too small, the radiation resistance decreases and becomes comparable to the loss resistance of the system which leads to an inefficient antenna. For very large spacing, the system produces undesirable multiple lobes, and it loses its directional characteristics.3. Increasing the size of the sides does not greatly affect the beam width and directivity, but it increases the bandwidth and radiation resistance.4. The analysis for the field radiated by a source in the presence of a corner reflector is facilitated when the included angle (α) of the reflector is α = π/n,Where, n is an integer (α = π,π/2,π/3,π/4, etc.).For those cases (α = 180◦, 90◦, 60◦, 45◦,…)5. To maintain a given system efficiency, the spacing between the vertex and the feed element must increase as the included angle of the reflector decreases, and vice versa. For reflectors with infinite sides, the gain increases as the included angle between the planes decreases.6. The number of images, polarity, and position of each is controlled by the included angle of the corner reflector and the polarization of the feed element.
Figure: Corner reflector for different induced anglesQ.9. What is the feed mechanism of parabolic antenna?Ans – 1. The primary Parabolic Antenna is placed at the focus of the paraboloid for best results in transmission or reception. The direct radiation from the feed, which is not reflected by the paraboloid, tends to spread out in all directions and hence partially spoils the directivity.2. Several methods are used to prevent this, one of them being the provision of a small spherical reflector, as shown in Figure below, to redirect all such radiation back to the paraboloid. 3. Another method is to use a small dipole array at the focus, such as a Yagi-Uda or an end-fire array, pointing at the paraboloid reflector.
Figure: Centre fed paraboloid with spherical shell4. Figure above shows yet another way of dealing with the problem. A horn antenna pointing at the main reflector. 5. It has a mildly directional pattern, in the direction in which its mouth points. Direct radiation from the feed antenna is once again avoided. 6. It should be mentioned at this point that, although the feed antenna and its reflector obstruct a certain amount of reflection from the paraboloid when they are placed at its focus, this obstruction is slight indeed.
Figure: Paraboloid reflector with horn feed7. Another feed method, the Cassegrain feed, is named after an early-eighteenth-century astronomer and is adopted directly from astronomical reflecting telescopes.
Figure: Geometry of the Cassegrain feed8. One of its foci coincides with the focus of the paraboloid, resulting in the action shown (for transmission) in Figure 4.12.3. 9. The rays emitted from the feed horn antenna are reflected from the paraboloid mirror. The effect on the main paraboloid reflector being the same as that of a feed antenna at the focus.10. The main reflector then collimates (renders parallel) the rays in the usual manner.
Figure 4.12.4. Paraboloid reflector with cassegrain feed 11. The Cassegrain feed is used when it is desired to place the primary antenna in a convenient position and to shorten the length of the transmission line or waveguide connecting the receiver (or transmitter) to the primary. 12. This requirement often applies to low-noise receivers, in which the losses in the line or waveguide may not be tolerated, especially over lengths which may exceed 30 m in large antennas. Q.10.Explain Rectangular patch with its equivalent circuit.Ans-1.The figure below shows the rectangular patch. The radiation of the patch is perpendicular to the board on which it is placed. The dimension L is slightly less than half the free-space wavelength divided by the square root of the effective dielectric constant of the board. 2. The feedline , in this case a microstrip line, is etched alongside the patch at the center of its width.
Figure: Rectangular patch 3. The other two opposing edges that are W apart do not radiate so long as the feedline is at the centre of the radiating edges. Thus, it can be concluded that a radiating patch can be modelled by two slots separated by a transmission line. Each slot can be represented by a parallel circuit of susceptance X and conductance G as shown by the equivalent circuit below. 4. Note, however, that the circuit presented in Figure below does not model the mutual coupling present between the two radiating slots nor does it account for the radiation due to the non radiating edges of the patch. 5. Due to these limitations, this model becomes unsuitable to analyse nonrectangular shapes and hence it is very limited in its application.
Figure: Equivalent circuit for the microstrip feedline6. A rectangular microstrip radiator can be modeled as an open cavity bounded by the patch and the ground plane. 7. Assume the relative dielectric constant to be εr and the substrate thickness to be h,k then the total electric field in the leaky cavity can be expressed as
8. Where the constant Cm,n depends on the feed location and L and W are the patch's resonant length and nonresonant width.9. The resonant frequency of a rectangular patch in the fundamental mode can be predicted using its dimensions and the relative dielectric constant, that is
Where, c is the speed of light, and 
is the effective dielectric constant 
, where h/W > 110. Note that for a very small h, the relationship for the resonant frequency becomes,
Q.11 Write a note on Circular Patch.Ans- 1. The mode supported by the circular patch antenna can be found by treating the patch, ground plane and the material between the two as a circular cavity. The radius of the patch is the only degree of freedom to control the modes of the antenna.2. The antenna can be conveniently analyzed using the cavity model. The cavity is composed of two electric conductors at the top and the bottom to represent the patch and the ground plane and by a cylindrical perfect magnetic conductor around the circular periphery of the cavity.
Figure: Circular Patch3. The dielectric material of the substrate is assumed to be truncated beyond the extent of the patch. The field configuration within the cavity can be found using the vector potential. 4. Circular Patch Radius and Effective Radius Since the dimension of the patch is treated a circular loop, the actual radius of the patch is given by, a = 
F = 
= a 
Q.12 Explain in brief Bow Tie antenna.Ans- 1.A bowtie antenna uses triangular elements instead of straight rods as the antenna elements. The triangular elements sticking out on both sides of the antenna resemble a bow tie, hence the name.2. The “wings” of the antenna flare out symmetrically on both sides of the supporting beam. The two antennas nearly touch at the centre.
Figure : Bow Tie antenna3. A bowtie antenna is sometimes called a butterfly antenna because it looks like a butterfly with its wings held open. When the bow tie elements have a metal bar that closes the antenna, it may be called a cat’s whisker antenna.4. The bowtie antenna is a type of UHF fan dipole antenna. Bow tie antennas may resemble log periodic antennas, but they are not considered LP antennas.
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● When δ(y’) is multiplied by the phase constant k, the result is a quadratic phase variation between the constant phase surface and the aperture plane.
● So the aperture fields becomes…..
() = Amplitude distribution = Phase distribution() = 3. Directivity The directivity of the E-plane sectoral horn is found in a manner analogous to the H-plane sectoral horn: = = Where, = , = , q =
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Q.3 Explain in detail H-plane sectoral horn.Ans- 1. The geometry and the respective parameters shown in the figure below are used in the subsequent analysis. The two required dimensions for the construction of the horn are A and .= -
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Which gives‘t’ approximately equal to:= at = Q.3 Write a note on Pyramidal horn Ans – 1. The pyramidal horn is probably the most popular antenna in the microwave frequency ranges (from ≈1 GHz up to ≈18 GHz). The feeding waveguide is flared in both directions, the E-plane and the H-plane. 2. The field distribution at the aperture is approximated as
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Where, q =
t = |
=0.8 The optimal directivity of an H-plane horn is achieved at t = 3 / 8)], =0.79 . Thus, the optimal horn has a phase aperture efficiency of =0.632The total aperture efficiency includes the taper factor, too: =0.81 *0.632 =0.519. Therefore, the best achievable directivity for a rectangular waveguide horn is about half that of a uniform rectangular aperture. 10. Radiation pattern of Pyramidal horn
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● The gain of antenna ranges to 25 dBi, with 10 to 20 dBi being typical.
● The usable BW of antenna is in the order of 10:1 and can be upto 20:1 that is upto 1GHz to 20 GHz.
● Horns of widely used as antennas
5. Radiation Pattern
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Where, c is the speed of light, and is the effective dielectric constant , where h/W > 110. Note that for a very small h, the relationship for the resonant frequency becomes,Q.11 Write a note on Circular Patch.Ans- 1. The mode supported by the circular patch antenna can be found by treating the patch, ground plane and the material between the two as a circular cavity. The radius of the patch is the only degree of freedom to control the modes of the antenna.2. The antenna can be conveniently analyzed using the cavity model. The cavity is composed of two electric conductors at the top and the bottom to represent the patch and the ground plane and by a cylindrical perfect magnetic conductor around the circular periphery of the cavity.
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F = 5. Above equation does not take into consideration the fringing effect. Since fringing makes the patch electrically larger, the effective radius of patch is used and is given by,
= a Q.12 Explain in brief Bow Tie antenna.Ans- 1.A bowtie antenna uses triangular elements instead of straight rods as the antenna elements. The triangular elements sticking out on both sides of the antenna resemble a bow tie, hence the name.2. The “wings” of the antenna flare out symmetrically on both sides of the supporting beam. The two antennas nearly touch at the centre.
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Q.13. What are the Advantages and Disadvantages of Bow tie antenna?Ans- Advantages 1. The bandwidth of the antenna is increased by using triangular elements over straight ones.2. Furthermore, the antennas often receive signals from a 60 degree angle, perfect for receiving signals from various sources. This is why bowtie antennas were common used with televisions receiving over the air broadcasts. They could receive a wide range of UHF signals, and it will do much better than a thin wire dipole antenna in this regard.3. This type of antenna is lighter in weight than a fan dipole antenna that connects the horizontal elements.4. The design also makes it more wind resistant. That’s a plus when you don’t want to have to climb on the roof to restore the antenna so you can watch the news.5. The design is made stronger by the metal bar that closes the bow tie, though that does add to its weight.6. This antenna design is cheap and easy to construct. That’s why rabbit ear antennas were so common on old televisions.7.The mesh reflector in the bowtie antenna is more efficient than other types of antennas such as yagi antennas. Disadvantages1. Biconical antennas, including bowtie antennas, have poor transmitting efficiencies in the low end of their frequency range. A log periodic antenna is better in that regard.2. A bowtie antenna isn’t as well suited as a full biconical antenna in RF sniffing or RF signal detection.
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