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Antennas-Different Types KTUNOTES.IN Antennas-Different types KTUNOTES.IN Downloaded from Ktunotes.in Classifications • Many Types of antennas . The choice of antenna depends on • Frequency of operation • Radiation Pattern • 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 KTUNOTES.IN 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 transmission line • 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 KTUNOTES.IN 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: KTUNOTES.IN 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 main lobe, 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 퐜퐨퐬ퟎ • The best choice for the included angle of the rhombus is selected to satisfy −ퟏ ퟎ = 퐜퐨퐬 [sin(90-ퟎ] Downloaded from Ktunotes.in Horn antenna • 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.
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