ntenna Theory & Techniques(Lecture-1)

Dr.K.Krishna NaikAssistant ProfessorDepartment of Electronics EngineeringDEFENCE INSTITUTE OF ADVANCED TECHNOLOGY (DU), PUNE

IN ASSOCIATION WITH

• Defence Institute of Advanced Technology (DU), Pune

• Dr.K.Krishna Naik, Assistant Professor, Department of EE

• LRDE, Bangalore• Dr.D.C.Pandey, Sc ‘H’

• RCI, Hyderabad• Dr.Borasker, Sc’H’

SYLLABUSINTRODUCTION OF ANTENNA

Fundamental of AntennaLOW FREQUENCY ANTENNA

Formulation of radiation integrals and its application to analysis of wire, Dipole and monopole, Folded Dipoles, Yagi-Uda Arrays and Loop antenna

ANTENNA ARRAYS AND TECHNIQUE FOR LOW SIDE LOBE Two element arrays – different cases, Principle of Pattern Multiplication, N element Uniform Linear Arrays – Broadside, End fire Arrays, Binomial Arrays, Effects of Uniform and Non-uniform Amplitude Distributions

SPREAD SPECTRUM TECHNIQUESAPERTURE ANTENNAS Horn antenna: Open ended waveguide, Design of E plane and H- Plane Antenna, Design of Pyramidal Horn Antenna, efficiency of aperture antenna and Waveguide slot antennaReflector Antennas for Radar : Parabolic Reflectors, Cassegrainian reflector, Flat Sheet and Corner Reflectors, Space-Fed array Antennas, tracking antenna.

LOW PROFILE ANTENNAS Microstrip Antenna and arrays: Rectangular and circular patch; Feed to microstrip antenna: probe feed, microstrip line feed, aperture feed, electromagnetically fed microstrip patch and arraysBroadband/Wide band antennas for EW: Principle of Frequency Independent antenna, Babinet principle, Frequency independent antenna, Archimeadian antenna, Spiral Antenna, conical log spiral, Log Periodic antenna, planar broad band antenna EMI/EMC Antenna: Loop antenna, Rod Antenna and Low frequency Active Antennan

IMPEDANCE MATCHING AND BROADBANDING

ANTENNA MEASUREMENTS Patterns Required, Set Up, Distance Criterion, Gain Measurements. Open ranges. Anechoic Chamber, Compact ranges, Near field and Far field Measurements

BOOKS

Antenna Theory, C. A. Balanis, John Wiley & Sons, 2nd ed., 2001

Antennas or Antennas for all applications, John D. Kraus, McGraw-Hill, Second Edition,1988

Antennas and Wave Propagation – K.D. Prasad, Satya Prakashan, Tech India Publications, New Delhi, 2001.

Microwave Antenna Measurement, Hollis, Lyon and Clayton, Scientific Atlanta,Inc.,1970

Antenna Theory and Design, R.E. Elliott, Wiley-IEEE Press, 2003.

Antenna Theory and Design, W. L. Stutzman, G. A. Thiele, John Wiley and Sons, New York, 1997.

INTRODUCTION Definition:

– ANY piece of conducting material will work as an antenna on any frequency.

– An antenna is an electrical device which converts electric currents into radio waves, and vice versa.

– An electrical device used to send or receive electromagnetic waves.

– An antenna is a specialized transducer that converts radio-frequency (RF) fields into alternating current (AC) or vice-versa.

– In electronics, system of wires or other conductors used to transmit or receive radio or other electromagnetic waves.

– The American Heritage Dictionary: A metallic apparatus for sending and receiving electromagnetic waves.

– Webster’s Dictionary: A usually metallic device (as a rod or wire) for radiating or receiving radio waves.

– Balanis; Antenna Theory: An antenna is a transitional structure between free-space and a guiding structure.

– Kraus “The structure associated with the region of transition between a guided wave and free space wave or vice versa”.

INTRODUCTIONAn antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver. In transmission, a radio transmitter applies an oscillating radio frequency electric current to the antenna's terminals, and the antenna radiates the energy from the current as electromagnetic waves (radio waves). In reception, an antenna intercepts some of the power of an electromagnetic wave in order to produce a tiny voltage at its terminals, that is applied to a receiver to be amplified. An antenna can be used for both transmitting and receiving.Antennas are essential components of all equipment that uses radio. They are used in systems such as

radio broadcasting, broadcast television, two-way radio, communications receivers, radar, cell phones, and satellite communications

Other devices such as wireless microphones, Bluetooth enabled devices, wireless computer networks, baby monitors, and RFID tags

INTRODUCTIONTypically an antenna consists of an arrangement of metallic conductors ("elements"), electrically connected (often through a transmission line) to the receiver or transmitter. An oscillating current of electrons forced through the antenna by a transmitter will create an oscillating magnetic field around the antenna elements, while the charge of the electrons also creates an oscillating electric field along the elements. These time-varying fields radiate away from the antenna into space as a moving electromagnetic field wave. Conversely, during reception, the oscillating electric and magnetic fields of an incoming radio wave exert force on the electrons in the antenna elements, causing them to move back and forth, creating oscillating currents in the antenna.Antennas may also contain reflective or directive elements or surfaces not connected to the transmitter or receiver, such as parasitic elements, parabolic reflectors or horns, which serve to direct the radio waves into a beam or other desired radiation pattern. Antennas can be designed to transmit or receive radio waves in all directions equally (omnidirectional antennas), or transmit them in a beam in a particular direction, and receive from that one direction only (directional or high gain antennas).The first antennas were built in 1888 by German physicist Heinrich Hertz in his pioneering experiments to prove the existence of electromagnetic waves predicted by the theory of James Clerk Maxwell.

INTRODUCTION

1. Antenna is a region of transition between a wave guided by transmission line and a free space wave.

2. The transmission line conductor separation is a small fraction of wavelength while separation at the open end of the transition region or antenna may be many wavelengths.

3. An antenna interfaces between electrons on conductor and photons in space.

ntenna Theory & Techniques(Lecture-2)

Dr.K.Krishna NaikAssistant ProfessorDepartment of Electronics EngineeringDEFENCE INSTITUTE OF ADVANCED TECHNOLOGY (DU), PUNE

INTRODUCTIONTopics to be covered

• Basic parameters• Patterns• Beam Area• Beam Efficiency• Directivity and gain• Physical and Effective apertures• Scattering aperture and radar cross section• The radio link• Aperture of dipoles and λ/2 antennas• Radiation resistance• Antenna impedance• Antenna duality• Sources of radiation• Field zones• Shape –impedance considerations• Polarization

• The dipole acts an antenna because it launches free-space wave. • It exhibits many of the characteristics of a resonator, since energy reflected

from the ends of the dipole gives rise to a standing wave and energy storage near the antenna.

• A single device, in this case the dipole, exhibits simultaneously properties characteristics of an antenna, a transmission line and a resonator.

INTRODUCTION:Basic parameters

• The antenna appears from the transmission line as a two terminal circuit element having an impedance Z with a resistive component called the Radiation Resistance Rr,

• While from space, the antenna is characterized by its radiation pattern or patterns involving field quantities.

• Fields and Radiation: An electromagnetic wave consists of electric and magnetic fields propagating through space. • A Field being a region where electric or magnetic forces act. • The electric and magnetic fields in a free- space wave travelling out ward

at a large distance from an antenna convey energy called Radiation.• The radiation resistance Rr is not associated with any resistance in the antenna

proper but is a resistance coupled from the antenna and its environment to the antenna terminals.

• Associated with the radiation resistance is also an antenna temperature TA.

• Both radiation resistance and antenna temperature are single valued scalar quantities.

• The radiation patterns, on the other hand, involve the variation of field or power as a function of two spherical coordinates θ and φ .

INTRODUCTION:Basic parameters

• A radio antenna may be defined as the structure associated with the region of transition between a guided wave and a free-space wave, or vice versa. Antennas convert electrons to photons, or vice versa.

• Regardless of antenna type, all involve the same basic principle that radiation is produced by accelerated (or decelerated) charge.

• The basic equation of radiation may be expressed simply as

• Thus, time-changing current radiates and accelerated charge radiates. • For steady state harmonic variation, we usually focus on current. • For transients or pulses, we focus on charge.• The radiation is perpendicular to the acceleration, and the radiated power is proportional to the

square of ˙IL or Q˙v.

INTRODUCTION:Basic parameters

• The two-wire transmission line in Fig.a is connected to a radio-frequency generator (or transmitter).

• Along the uniform part of the line, energy is guided as a plane Transverse Electro Magnetic Mode (TEM) wave with little loss. The spacing between wires is assumed to be a small fraction of a wavelength.

• Further on, the transmission line opens out in a tapered transition. As the separation approaches the order of a wavelength or more, the wave tends to be radiated so that the opened-out line acts like an antenna which launches a free-space wave.

• The currents on the transmission line flow out on the antenna and end there, but the fields associated with them keep on going.

INTRODUCTION:Basic parameters

• An antenna is a transition device, or transducer, between a guided wave and a free-space wave, or vice-versa. The antenna is a device which interfaces a circuit and space.

• From the circuit point of view, the antennas appear to the transmission lines as a resistance Rr , called the radiation resistance. It is not related to any resistance in the antenna itself but is a resistance coupled from space to the antenna terminals.

• In the transmitting case, the radiated power is absorbed by objects at a distance: trees, buildings, the ground, the sky, and other antennas.

• In the receiving case, passive radiation from distant objects or active radiation from other antennas raises the apparent temperature of Rr.

• For lossless antennas this temperature has nothing to do with the physical temperature of the antenna itself but is related to the temperature of distant objects that the antenna is “looking at,” as suggested in Fig above.

• In this sense, a receiving antenna (and its associated receiver) may be regarded as a remote-sensing temperature-measuring device.

• The radiation resistance Rr may be thought of as a “virtual” resistance that does not exist physically but is a quantity coupling the antenna to distant regions of space via a “virtual” transmission line.

INTRODUCTION:Basic parameters

• The radiation patterns are three-dimensional quantities involving the variation of field or power as a function of the spherical coordinates θ and φ.

• Figure opposite shows a three-dimensional field pattern with pattern radius r (from origin to pattern boundary at the dot) proportional to the field intensity in the direction θ and φ.

• The pattern has its main lobe (maximum radiation) in the z direction (θ = 0) with minor lobes (side and back) in other directions.

To completely specify the radiation pattern with respect to field intensity and polarization requires three patterns:

1.The θ component of the electric field as a function of the angles θ and φ or Eθ (θ, φ)(V m−1) as in Fig.

2.The φ component of the electric field as a function of the angles θ and φ or Eφ(θ, φ) (V m−1).

3.The phases of these fields as a function of the angles θ and φ or δ θ (θ, φ) and δφ(θ, φ) (rad or deg).

INTRODUCTION:Patterns

INTRODUCTION:Patterns

• Any field pattern can be presented in three-dimensional spherical coordinates or by plane cuts through the main-lobe axis.

• Two such cuts at right angles called the principal plane patterns (as in the xz and yz planes in Fig.) may be required but if the pattern is symmetrical around the z axis, one cut is sufficient.

• Figures a and b are principal plane field and power patterns in polar coordinates. • The same pattern is presented in Fig. c in rectangular coordinates on a logarithmic, or decibel, scale which gives

the minor lobe levels in more detail.• The angular beam width at the half-power level or half-power beam width (HPBW) (or −3-dB beam width) and

the beam width between first nulls (FNBW) are important pattern parameters.• Dividing a field component by its maximum value, we obtain a normalized or relative field pattern which is a

dimensionless number with maximum value of unity• The normalized field pattern (Fig a) for the electric field is given by

• The half-power level occurs at those angles θ and φ for which Eθ (θ, φ)n = 1/√2=0.707• At distances that are large compared to the size of the antenna and large compared to the wavelength, the

shape of the field pattern is independent of distance. • Usually the patterns of interest are for this far-field condition.

INTRODUCTION:Patterns

• Patterns may also be expressed in terms of the power per unit area [or Poynting vector S(θ, φ)]. • Normalizing this power with respect to its maximum value yields a normalized power pattern as a function of

angle which is a dimensionless number with a maximum value of unity.• the normalized power pattern (Fig.b) is given by

• where– S(θ, φ) = Poynting vector =[E2

θ (θ, φ) + E2φ(θ, φ)]/Z0, Wm−2

– S(θ, φ)max = maximum value of S(θ, φ),Wm−2

– Z0 = intrinsic impedance of space = 376.7 Ω

• The decibel level is given by dB = 10 log10 Pn(θ, φ)

INTRODUCTION:Patterns

INTRODUCTION:Patterns

• E(θ) at half power = 0.707. • Thus 0.707 = cos2 θ • so cos θ = √0.707 and θ = 33• HPBW = 2θ = 66 Ans

An antenna has a field pattern given by E(θ) = cos2 θ for 0 ≤ θ ≤ 90Find the half-power beam width (HPBW).

An antenna has a field pattern given by E(θ) = cos θ cos 2θ for 0 ≤ θ ≤ 90. Find (a) the half-power beam width (HPBW) and (b) the beam width between first nulls (FNBW).

• In polar two-dimensional coordinates an incremental area dA on the surface of a sphere is the product of the length r dθ in the θ direction (latitude) and r sin θ dφ in the φ direction (longitude).

Thus,– dA = (r dθ)(r sin θ dφ) = r 2 d Ω

where– d = solid angle expressed in steradians (sr) or square degrees– d = solid angle subtended by the area dA

a. Polar coordinates showing incremental solid angle dA = r 2 dΩ on the surface of a sphere of radius Ωr where d = solid angle subtended by the area dA.

b. Antenna power pattern and its equivalent solid angle or beam area Ω A.

BEAM AREA (OR BEAM SOLID ANGLE) ΩA

• The area of the strip of width r dθ extending around the sphere at a constant angle θ is given by (2πr sin θ)(r dθ). Integrating this for θ values from 0 to π yields the area of the sphere.

Thus,

• Where 4π = solid angle subtended by a sphere, srThus, 1 steradian = 1 sr = (solid angle of sphere)/(4π)

Therefore,– 4π steradians = 3282.8064 × 4π = 41,252.96 = ∼ 41,253 square degrees

• The beam area or beam solid angle or ΩA of an antenna (Fig.b) is given by the integral of the normalized power pattern over a sphere (4π sr)

• and

• Where dΩ = sin θ dθ dφ, sr.• The beam area ΩA is the solid angle through which all of the power radiated by the antenna would stream if P(θ, φ)

maintained its maximum value over ΩA and was zero elsewhere. Thus the power radiated = P(θ, φ) ΩA watts.

• The beam area of an antenna can often be described approximately in terms of the angles subtended by the half-power points of the main lobe in the two principal planes.

Thus,• Where θHP and φHP are the half-power beam widths (HPBW) in the two principal planes, minor lobes being

neglected.

BEAM AREA (OR BEAM SOLID ANGLE) ΩA

Find the number of square degrees in the solid angle on a spherical surface that is between θ = 20 and θ = 40 (or 70 and 50 north latitude) and between φ = 30 and φ = 70 (30 and 70 east longitude).

An antenna has a field pattern given by E(θ) = cos2 θ for 0 ≤ θ ≤ 90. Has the pattern shown below. Find the beam area of this pattern.

• The power radiated from an antenna per unit solid angle is called the radiation intensity U(watts per steradian or per square degree).

• The normalized power pattern can also be expressed in terms of this parameter as the ratio of the radiation intensity U(θ, φ), as a function of angle, to its maximum value.

• Whereas the Poynting vector S depends on the distance from the antenna (varying inversely as the square of the distance), the radiation intensity U is independent of the distance, assuming in both cases that we are in the far field of the antenna.

RADIATION INTENSITY

•The (total) beam area ΩA (or beam solid angle) consists of the main beam area (or solid angle) Ω M plus the minor-lobe area (or solid angle) Ωm.

•The ratio of the main beam area to the (total) beam area is called the (main) beam efficiency εM. Thus,

•The ratio of the minor-lobe area (Ω m) to the (total) beam area is called the stray factor.εm = Ω m/ Ω A = stray factor

It follows that εM + εm = 1

BEAM EFFICIENCY

DIRECTIVITY D• The directivity D and the gain G are

probably the most important parameters of an antenna.

• The directivity of an antenna is equal to the ratio of the maximum power density P(θ, φ)max (watts/m2) to its average value over a sphere as observed in the far field of an antenna.

• The directivity is a dimensionless ratio ≥1.

• The average power density over a sphere is given by

•Where Pn(θ, φ) d = P(θ, φ)/P(θ, φ)max = normalized power pattern•The idealized isotropic antenna (A = 4π sr) has the lowest possible directivity D = 1. •All actual antennas have directivities greater than 1 (D > 1). •The simple short dipole has a beam area ΩA = 2.67π sr and a directivity D = 1.5 (= 1.76 dBi).

• The gain G of an antenna is an actual or realized quantity which is less than the directivity D due to ohmic losses in the antenna.

• In transmitting, these losses involve power fed to the antenna which is not radiated but heats the antenna structure. A mismatch in feeding the antenna can also reduce the gain.

• The ratio of the gain to the directivity is the antenna efficiency factor [ G=kD ], – where k = efficiency factor (0 ≤ k ≤ 1), dimensionless.

• In many well-designed antennas, k may be close to unity. In practice, G is always less than D, with D its maximum idealized value.

• Gain can be measured by comparing the maximum power density of the Antenna Under Test (AUT) with a reference antenna of known gain, such as a short dipole.

• If the half-power beam widths of an antenna are known, its directivity• Where

– 41,253 square degrees = number of square degrees in sphere = 4π(180/n)2 square degrees– ΘHP= half-power beam width in one principal plane– ΦHP= half-power beam width in other principal plane

• Since D neglects minor lobes, a better approximation is a

GAIN G

RESOLUTION• The resolution of an antenna may be defined as equal to half the beam width between first nulls (FNBW)/2• an antenna whose pattern FNBW = 2 has a resolution of 1

– Half the beam width between first nulls is approximately equal to the half-power beam width (HPBW) or

– FNBW/2 =HPBW∼

• The product of the FNBW/2 in the two principal planes of the antenna pattern is a measure of the antenna beam area

ANTENNA APERTURESThe concept of aperture is most simply introduced by considering a receiving antenna.• Suppose that the receiving antenna is a rectangular electromagnetic horn immersed in the field of a uniform plane

wave• The Poynting vector(S), or power density, of the plane wave be S watts per square meter and the area, or physical

aperture of the horn, be Ap square meters.• If the horn extracts all the power from the wave over its entire physical aperture, then the total power P absorbed

from the wave is

• The horn may be regarded as having an aperture, the total power it extracts from a passing wave being proportional to the aperture or area of its mouth.

• But the field response of the horn is NOT uniform across the aperture A because E at the sidewalls must equal zero. Thus, the effective aperture Ae of the horn is less than the physical aperture Ap

• For horn and parabolic reflector antenna, aperture efficiencies are commonly in the range of 50 to 80% (0.5 ≤ εap ≤ 0.8).

• Large dipole or patch arrays with uniform field to the edges of the physical aperture may attain higher aperture efficiencies approaching 100%.

• However, to reduce side lobes, fields are commonly tapered toward the edges, resulting in reduced aperture efficiency.

• Plane wave incident on electromagnetic horn of physical aperture Ap

• Radiation over beam area ΩA from aperture Ae.

• Consider now an antenna with an effective aperture Ae, which radiates all of its power in a conical pattern of beam area Ω A, assuming a uniform field Ea over the aperture, the power radiated is ation over beam area ΩA from aperture Ae.

• where Z0 =intrinsic impedance of medium.

• Assuming a uniform field Er in the far field at a distance r , the power radiated is also given by

• The effective height h (meters) of an antenna is another parameter related to the aperture.• Multiplying the effective height by the incident field E (volts per meter) of the same polarization

gives the voltage V induced.V = hE

• Accordingly, the effective height may be defined as the ratio of the induced voltage to the incident field

h = V /E (m)• Effective height is to consider the transmitting case and equate the effective height to the physical

height (or length l) multiplied by the (normalized) average current.

EFFECTIVE HEIGHT

• Gain of Directional Antenna with Three-Dimensional Field Pattern of Fig. belowThe antenna is a lossless end-fire array of 10 isotropic point sources spaced λ/4 and operating with increased

directivity. The normalized field pattern is

Since the antenna is lossless, gain = directivity.(a) Calculate the gain G.(b) Calculate the gain from the approximate equation .(c) What is the difference?

For field pattern HPBW

The normalized field pattern of an antenna is given by En = sin θ sin φ, where θ = zenith angle (measured from z axis) and φ = azimuth angle (measured from x axis) (see figure). En has a value only for 0 ≤ θ ≤ π and 0 ≤ φ ≤ π and is zero elsewhere (pattern is unidirectional with maximum in +y direction). Find (a) the exact directivity, (b) the approximate directivity and (c) the decibel difference.

Solution

--------------------------------------------------------------------------------------------------------------------------------------------------------• Where • Since the antenna is lossless, gain = directivity.

Directivity and apertures.Show that the directivity of an antenna may be expressed as

where E(x, y) is the aperture field distribution. Solution: If the field over the aperture is uniform, the directivity is a maximum (= Dm) and the power radiated is

For an actual aperture distribution, the directivity is D and the power radiated is P. Equating effective powers

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Effective aperture and beam area.What is the maximum effective aperture (approximately) for a beam antenna having half-power widths of 30 and 35

in perpendicular planes intersecting in the beam axis? Minor lobes are small and may be neglected.Assume wavelength is 57.3

Solution:

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