Antennas-Different types KTUNOTES.IN

Downloaded from Ktunotes.in Classifications • Many Types of antennas . The choice of depends on • Frequency of operation • • Polarization, • Gain requirements • Application KTUNOTES.IN I. Isotropic Directional Omnidirectional Point Source Dipole Half wave Dipole Star antenna Horn Marconi Parabolic Circular Loop Downloaded from Ktunotes.in Classifications II. Resonant Non Resonant • Length is in exact multiples of /2. • Length is other than in multiples of /2. • Open at both ends • Open end is excited • Not terminated in any resistance • The other end is terminated in characteristic impedance • Used at a fixed frequency • Used at a range of frequencies and has a wider • Forward/incident and backward/reflected waves bandwidth exist • No reflected waves exist • Standing waves exist • No Standing waves exist • Radiation patterns are multi directionalKTUNOTES.IN • Radiation patterns are uni directional • Voltage and current are not in phase • Pattern is not symmetric about =90 • Has distributed L and C and act as resonant circuit • It is a travelling wave antenna • Known as periodic antennas • Also called directional antennas • Known as aperiodic antennas • Egs: Long wire, V antenna, Rhombic antenna

Downloaded from Ktunotes.in Classifications III. Standing Wave Travelling Wave • Nothing but a resonant Antenna • Nothing but a non resonant antenna • Standing wave is defined as the wave • An antenna that is associated with in which the ratio of the instantaneous radiation from a continuous source value of any component of the wave at • Travelling wave is defined as a wave one point to that at any other point whose frequency component have does not vary with time exponential variation of amplitude and KTUNOTES.INlinear variation of phase with distance. • A progressive phase pattern is associated with current and voltage distributions • Aperture antennas like Horn and Reflector antennas can be treated as Travelling Wave antennas

Downloaded from Ktunotes.in Classifications IV. LF Antenna HF,VHF,UHF Antenna • Operate at 3-300KHz • Dipole arrays • Inductance loaded Vertical • Folded dipoles antennas • V antennas • Inductance loaded Horizontal • Inverted V antenna dipoles • Rhombic antennas • Tower Antenna KTUNOTES.IN• Yagi-Uda antenna • Inverted L antenna • Log periodic antennas • Vertical monopole with top capacitance • Helical antennas

Downloaded from Ktunotes.in Travelling Wave antennas

• A travelling wave may be classified as a slow wave if its phase velocity vp, is equal or smaller than the velocity of light c in free-space • A fast wave is one whose phase velocity is greater than the speed of light • Two types of travelling wave antennas. • Surface wave antenna defined as “An antenna which radiates power flow from discontinuities in the structure that interrupt a bound wave on the antenna surface.” • A surface wave antenna is, in general, a slow wave structure whose phase velocity of the traveling wave is equal to orKTUNOTES.INless than the speed of light in free-space • For slow wave structures radiation takes place only at non uniformities, curvatures, and discontinuities • Most of the surface wave antennas are end-fire or near-end-fire radiators. • Practical configurations include line, planar surface, curved, and modulated structures.

Downloaded from Ktunotes.in Travelling Wave antennas • Another traveling wave antenna is a Leaky-wave antenna defined as “an antenna that couples power in small increments per unit length, either continuously or discretely, from a traveling wave structure to free space” • Leaky-wave antennas continuously lose energy due to radiation • The fields decay along the structure in the direction of wave travel and increase in others. • Most of them are fast wave structures. • An example of a slow wave traveling antenna is a long wire, as shown in Figure • A long wire antenna is one if it is a straight conductor with a length from one to many wavelengths. • A long wire antenna has the distinctionKTUNOTES.INof being the first traveling wave antenna.

Downloaded from Ktunotes.in V antenna

• An antenna in which the conductors are arranged in V shape. • Balanced –fed at the apex • The included angle, length and elevation are chosen to obtain the desired directional properties • Structure is as shown. • The excitation to each wire is out of phase. • Radiation pattern can be unidirectional or bidirectional • Easy to construct and inexpensive. • By adjusting the included angle of a V antenna,KTUNOTES.IN the directivity can be made greater and sidelobes smaller. • Designs for maximum directivity usually require smaller included angles for longer V’s.

• Most V antennas are symmetrical (θ1 = θ2 = θ0 and l1 = l2 = l).

Downloaded from Ktunotes.in V antenna

• Radiation pattern can be unidirectional or bidirectional

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Downloaded from Ktunotes.in V antenna • To achieve the unidirectional characteristics, the wires of the V antenna must be nonresonant • This can be accomplished by minimizing or eliminating reflections from the ends of the wire. • The reflected waves can be reduced by making the inclined wires of the V relatively thick • Reflections can also be eliminated by properly terminating the open ends of the V leading to a purely traveling wave antenna. KTUNOTES.IN • One way of terminating the V antenna will be to attach a load, usually a resistor equal in value to the open end characteristic impedance of the V- wire • The terminating resistance can also be divided in half and each half connected to the ground leading to the termination Downloaded from Ktunotes.in V antenna • The patterns of the individual wires of the V antenna are conical in form and are inclined at an angle from their corresponding axes. • The angle of inclination is determined by the length of each wire. • For the patterns of each leg of a symmetrical V antenna to add in the direction of the line bisecting the angle of the V and to form one major lobe, the total included angle 2θ0 of the V should be equal to 2θm, which is twice the angle that the cone of maximum radiation of each wire makes with its axis. • When this is done, beams 2 and 3 of Figure below are aligned and add constructively for unidirectional pattern

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Downloaded from Ktunotes.in V antenna

• For optimum operation, typically the included angle is chosen to be approximately θ0  0.8θm. • When this is done, the reinforcement of the fields from the two legs of the V lead to a total directivity for the V of approximately twice the directivity of one leg of the V. • For a symmetrical V antenna with legs each of length l, there is an optimum included angle which leads to the largest directivity. • Design data for optimum included angles of V dipoles were computed using Moment Method techniques and are given as:

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Downloaded from Ktunotes.in Rhombic Antenna • This is an antenna in the shape of a rhombus • Two V antennas can be connected at their open ends to form a diamond or rhombic antenna • It is terminated in a resistance to reduce reflections. • Side of the rhombus, Angle between the sides, Elevation, Termination and height above the earth are chosen to obtain the desired radiation characteristicsKTUNOTES.IN • To achieve the single , beams 2, 3, 6, and 7 are aligned and add constructively. • The other end is used to feed the antenna • In an inverted V antenna ,the inverted V is connected to the ground through a resistor as shown in the figure. The image is shown in dashed lines. Downloaded from Ktunotes.in Rhombic Antenna • Rhombic antennas are usually preferred over V’s for non resonant and unidirectional pattern applications because they are less difficult to terminate. • Additional directivity and reduction in side lobes can be obtained by stacking, vertically or horizontally, a number of rhombic and/or V antennas to form arrays • The field radiated by a rhombus can be found by adding the fields radiated by its four legs. • For a symmetrical rhombusKTUNOTES.INwith equal legs, this can be accomplished using array theory and pattern multiplication

Downloaded from Ktunotes.in Rhombic Antenna Design Equations: • It is desired to design a rhombus such that the maximum of the main lobe of the pattern, in a plane which bisects the V of the rhombus, is directed at an angle ψ0 above the ground plane. • The design can be optimized if the height h is selected according to 퐡 퐦 • 퐦 = 퐦 = ퟏ, ퟑ, ퟓ, . . ퟎ ퟒ 퐜퐨퐬(ퟗퟎ−ퟎ) With m=1 representing the minimumKTUNOTES.INheight. • The minimum optimum length of each leg of a symmetrical rhombus must be selected according to 퐥 ퟎ.ퟑퟕퟏ • = ퟎ ퟏ−퐬퐢퐧 ퟗퟎ−ퟎ 퐜퐨퐬ퟎ • The best choice for the included angle of the rhombus is selected to satisfy −ퟏ ퟎ = 퐜퐨퐬 [sin(90-ퟎ] Downloaded from Ktunotes.in • One of the simplest and probably the most widely used microwave antenna • The horn is widely used as a feed element for large radio astronomy, satellite tracking, and communication dishes found installed throughout the world. • Used as a feed for reflectors and lenses and is a common element of phased arrays • It serves as a universal standard for calibration and gain measurements of other high gain antennas. • Advantages are simplicity in construction, ease of excitation, versatility, large gain, and preferred overall performance KTUNOTES.IN • An electromagnetic horn can take many different forms • It consists of a hollow pipe of different cross sections, which has been tapered (flared) to a larger opening. • The type, direction, and amount of taper (flare) can have a profound effect on the overall performance of the element as a radiator. • The Horn antenna is a waveguide terminated by a gradual transformation or a waveguide with one end flared out. Downloaded from Ktunotes.in Horn antenna • Operation • In a waveguide ,propagation is restricted by conducting walls and waves will not spread. • After reaching the mouth, waves spread laterally and wavefronts becomes spherical. • At the mouth of the waveguide a transition region exists where a change of propagation from waveguide to free space takes place. • To match the waveguide impedance and free space impedance, the walls of the waveguide are flared out. • The flaring provides impedance KTUNOTES.INmatching, more directivity and narrow beam width • Horn produces a uniform plane wave front with a larger aperture in comparison with a waveguide.

Downloaded from Ktunotes.in Horn antenna • Horn antenna is a type of Aperture antenna • Types • Sectoral Horn o E plane Sectoral Horn o H Plane Sectoral Horn • Pyramidal Horn • Conical Horn • Ridge Horn • Septum Horn KTUNOTES.IN • Corrugated Horn • E plane Horn-RWG flared in E field direction only • H plane Horn-RWG flared in H field direction only • Pyramidal horn-RWG flared in both directions • Conical Horn-Circular waveguide is flared out Downloaded from Ktunotes.in Horn antenna

Salient Features: • Horn becomes small if flare angle is small. • Radiation pattern is directive • Wavefront is spherical • Aperture area is small • Directivity is small • Directivity of a pyramidal horn isKTUNOTES.INmore as the flare is in more than one direction • Used as primary antenna for paraboloid.

Downloaded from Ktunotes.in Horn antenna ퟒ푨 • The directivity of a lossless Horn antenna is given by 푫 = 풆 where ퟐ 푨풆 • Aperture efficiency is 풂 = 푨풑

• Ae is the effective aperture area

• Ap is the physical area

• For a Rectangular horn, 푨풑 = 풅푬풅푯 where

• dE is aperture size in E plane Horn

• dH is aperture size in H plane KTUNOTES.IN

• For a Conical horn, 푨풑 = 풅 where • d is the aperture diameter

Downloaded from Ktunotes.in Horn antenna • Design equations 퐝 퐥 •  = ퟐ퐭퐚퐧−ퟏ = ퟐ퐜퐨퐬−ퟏ ퟐ퐥 퐥+ 퐝ퟐ and 퐥 = ퟖ • HPBW of optimum flared horn is ퟓퟔ • 푬 = degrees 푬is HPBW in E plane 풅푬 ퟔퟕ • 푯 = degrees 푯 is HPBW in H plane 풅푯 KTUNOTES.IN ퟕ.ퟓ푨 • 푫 = 풑 Directivity ퟐ ퟒ.ퟓ푨 • Power Gain G= ퟐ • A=w x H is the area of the horn mouth

• 푨풑 = 풅푬풅푯

Downloaded from Ktunotes.in Reflector Antennas • Reflector antennas, in one form or another, have been in use since the discovery of electromagnetic wave propagation in 1888 by Hertz. • Used in radio astronomy, microwave communication, and satellite tracking • Reflector antennas take many geometrical configurations • Some of the most popular shapes are the plane, corner, and curved reflectors (especially the paraboloid) • The simplest type of reflectorKTUNOTES.INis a Plane reflector introduced to direct energy in a desired direction. • To better collimate the energy in the forward direction, and to prohibit radiation in the back and side directions the is employed. • This consists of two plane reflectors joined so as to form a corner, as shown.

Downloaded from Ktunotes.in Parabolic Reflector Antenna • The overall radiation characteristics (antenna pattern, antenna efficiency, polarization discrimination, etc.) of a reflector can be improved if the structural configuration of its surface is upgraded • In geometrical optics if a beam of parallel rays is incident upon a reflector whose geometrical shape is a parabola, the radiation will converge (focus) at a spot which is known as the focal point. • In the same manner, if a point source is placed at the focal point, the rays reflected by a parabolic reflector will emerge as a parallel beam. ThisKTUNOTES.INis one form of the principle of reciprocity. • The symmetrical point on the parabolic surface is known as the vertex. • Rays that emerge in a parallel formation are usually said to be collimated. • As the transmitter (receiver) is placed at the focal point of the parabola, the configuration is usually known as front fed. Downloaded from Ktunotes.in Parabolic Reflector Antenna

• The disadvantage of the front-fed arrangement is that the transmission line from the feed must usually be long enough to reach the transmitting or the receiving equipment, which is usually placed behind or below the reflector. • This may necessitate the use of long transmission lines whose losses may not be tolerable in many applications, especially in low-noise receiving systems. • In some applications, the transmitting or receiving equipment is placed at the focal point to avoid the need for long transmission lines. • In some of these applications,KTUNOTES.INespecially for transmission that may require large amplifiers, cooling and weatherproofing the equipment may be too heavy and bulky and will provide undesirable blockage. • An arrangement that avoids placing the feed (transmitter and/or receiver) at the focal point is known as the Cassegrain feed.

Downloaded from Ktunotes.in Parabolic Reflector Antenna • A parabolic reflector can take two different forms. • The paraboloid

Paraboloid is the three dimensional surface obtained by rotating a parabola about its axis. The paraboloid is called the parabolic reflector or dish antenna. The energy is collimated at a line that is parallel to the axis of the cylinder through the focal point of the reflector. The most widely used feed for this type of a reflector is a linear dipole, a linear array, or a slotted waveguide. • The parabolic right cylinder KTUNOTES.IN This is formed by rotating the parabola around its axis, and is referred to as a paraboloid or parabola of revolution A pyramidal or a conical horn has been widely utilized as a feed

• Paraboloidal reflectors are the most widely used large aperture ground-based antennas

Downloaded from Ktunotes.in Parabolic Reflector Antenna

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Downloaded from Ktunotes.in Parabolic Reflector Antenna • If the primary antenna is non-directional or isotropic, ퟕퟎ • HPBW= = 푫풂 ퟏퟒퟎ • BWFN=ퟎ = ퟐ = 푫풂 푫 • 푫 = ퟗ. ퟖퟕ[ 풂]ퟐ Directivity  푫 • 푮 = ퟔ. ퟒ[ 풂]ퟐ Power gain 풑  • Da is the aperture diameter • For a large uniformly illuminated rectangular aperture, ퟓퟕ.ퟓ KTUNOTES.IN • HPBW= = 푳 ퟏퟏퟓ • BWFN= = ퟐ = ퟎ 푳 ퟒ푨 • Directivity 푫 = ퟐ

• L-Length of aperture • A-Area of aperture Downloaded from Ktunotes.in Parabolic Reflector Antenna • The parabolic reflector converts a spherical wave front into a plane wave front • The directional beam has a sharp main lobe surrounded by several side lobes • The feed antenna is called the primary antenna and the reflector is called the Secondary antenna. Disadvantages of paraboloid reflectors • The side lobes create electromagnetic interference in low noise receivers • Diffraction around the edges is another cause of sidelobes • The finite size of the primary antennaKTUNOTES.INalso influences bandwidth • As the feed antenna is not a true point source it cannot be located exactly at the focal point. • Defects like aberrations cause the main lobe to be broadened • Deviations from the true shape of a paraboloid must not exceed one sixteenth of .

Downloaded from Ktunotes.in Cassegrain Reflector Antenna • Cassegrain Reflector Antenna is named after Cassegrain, a famous astronomer who showed that incident parallel rays can be focused to a point by utilizing two reflectors. • The main(primary) reflector must be a parabola • The secondary reflector (sub reflector) is a hyperbola • The feed placed along the axis of the parabola usually at or near the vertex • Cassegrain used this scheme to construct optical telescopes. KTUNOTES.IN • The rays that emanate from the feed illuminate the sub reflector and are reflected by it in the direction of the primary reflector, as if they originated at the focal point of the parabola (primary reflector). • The rays are then reflected by the primary reflector and are converted to parallel rays

Downloaded from Ktunotes.in Cassegrain Reflector Antenna • One focus of the hyperbolic reflector coincides with the focus of the paraboloid. • When rays from the feed antenna are incident on the hyperbolic reflector, they are reflected back and are then incident on the paraboloid. • These incident rays are reflected and propagate as plane wave front.

Cassegrain arrangement provides a variety of benefits, such as the • Ability to place the feed in a convenient location • Reduction of spillover and minor lobe radiation • Ability to obtain an equivalent focalKTUNOTES.INlength much greater than the physical length • Capability for scanning and/or broadening of the beam by moving one of the reflecting surfaces • With the Cassegrain-feed arrangement, the transmitting and/or receiving equipment can be placed behind the primary reflector. • This scheme makes the system relatively more accessible for servicing and adjustments • Cassegrain designs, employing dual-reflector surfaces, are used in applications where pattern control is essential, such as in satellite ground-based systems Downloaded from Ktunotes.in Cassegrain Reflector Antenna • The concept of virtual feed can be utilized to understand the performance of a Cassegrain. • By this principle, the real feed and the sub reflector are replaced by an equivalent system which consists of a virtual feed located at the focal point of the main reflector, as shown by the dashed lines • The new system is a single-reflectorKTUNOTES.IN arrangement with the original main dish, a different feed, and no sub reflector.

Downloaded from Ktunotes.in Cassegrain Reflector Antenna Disadvantages • Diffractions occur at the edges of the sub reflector and primary reflector, and they must be taken into account to accurately predict the overall system pattern, especially in regions of low intensity • Some of the electromagnetic energy is obstructed by the secondary reflector. This is tolerable when the size of the paraboloid is large when compared to the hyperboloid. • Large paraboloid is expensive KTUNOTES.IN • When vertically polarized waves are radiated by the feed antenna, they are reflected back to the main reflector by the hyperboloid. • Polarization of the waves is twisted by 90 when they are reflected by the paraboloid.

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