Slide 1

Antennas basic classificationsMost practical transmitting antennas are divided into two basic classifications, (1) HERTZ (half-wave) ANTENNAS and (2) MARCONI (quarter-wave) ANTENNAS.Hertz antennas are generally installed some distance above the ground and are positioned to radiate either vertically or horizontally. Hertz antennas are generally used for frequencies above 2 megahertz.

Marconi antennas operate with one end grounded and are mounted perpendicular to the Earth or to a surface acting as a ground. Marconi antennas are used for frequencies below 2 megahertz and may be used at higher frequencies in certain applications.

1 Antenna characteristicsThe characteristics that determine the design of the antenna are:

1. Antenna pattern or Radiation pattern2. Radiation Intensity3. Directive gain4. Power gain2Radiation properties1. Power density: a.Vertical Pattern b.Horizontal Pattern2. Radiation Intensity3. Field strength a.Far Field region b.Near field region4. Directivity (polarization)3Radiation patternRadiation pattern of antenna or antenna pattern is defined as a mathematical function or a graphic representation of the radiation properties of an antenna as function of space coordinates.

In most cases , the radiation pattern is determined in far field and is represented as function the directional coordinated.

4Pattern TypesPower pattern : A trace of received power at constant radius is called power pattern.

Field pattern : A graph of variation of electric ( or magnetic) field along a constant radius is called amplitude field pattern.

5ReciprocityTypically, antennas are designed to operate in a relatively narrow frequency range. The design criteria for receiving and transmitting antennas differ slightly, but generally an antenna can receive and transmit equally well. This property is called reciprocity.6Wavelength

An antenna size is referred , relative to wavelength. For example: a 1/2 wave dipole is approximately half a wavelength long. Wavelength is the distance a radio wave travels during one cycle. The formula for wavelength is:

Where: is the wavelength expressed in units of length, typically meters, feet or inchesis the speed of light (11,802,877,050 inches/second)is the frequency

For example, wavelength in air at 825 MHz is:11.803 X 109 in./sec = 14.307 inches825 x 106 cycles/sec.

Note: The physical length of a half-wave dipole is slightly less than a half-wavelength due to end effect. The speed of propagation in coaxial cable is slower than in air, so the wavelength in the cable is shorter. The velocity of propagation of electromagnetic waves in coax is usually given as a percentage of free space velocity, and is different for different types of coax.

7Impedance Matching

For efficient transfer of energy, the impedance of the radio, the antenna and the transmission line connecting the radio to the antenna must be the same.

Radios typically are designed for 50 Ohms impedance, and the coaxial cables (transmission lines) used with them also have a 50 Ohm impedance.

Efficient antenna configurations often have an impedance other than 50 Ohms. Some sort of impedance matching circuit is then required to transform the antenna impedance to 50 Ohms.8Antenna equivalent circuitIf input voltage and current are Vi and Ii then antenna impedance ZA= Vi/Ii

In case of an aperture antenna fed by waveguide, without terminals , antenna impedance is given in terms of reflection coefficient ZA= Z0 1+A 1-A

ZA= RA+jXA

9

Maximum Power Transfer and Impedance Matching

The average power delivered to the load ZL is PL = VR II = E/ZS+ZL = E/RS+j XS+RL+j XL = E/RS+RL+j (XS+XL)

I = E/ (RS+RL)2+(XS+XL)2VR=I RL=ERL/RS+RL+j (XS+XL) = ERL/(RS+RL)2+(XS+XL)2 PL = VR I

P = E RL X E (RS+RL)2+(XS+XL)2 (RS+RL)2+(XS+XL)2

P = E2 RL (RS+RL)2+(XS+XL)2 For maximum conditions XL= -XS OR XL XS = 0P = E2 RL (RS+RL)2 Under matched conditionsRS = RLTherefore for maximum and under matched conditions P = E2 = E2 4RS 4RL In terms of current P= I2 RS2 = I2 RS = I2 = I2 4RS 4 4Gs 4GL

10Transmitting antenna efficiencyRrad is a fictitious resistance termed the radiation resistance. If it carries the same current as the antenna, on transmission would dissipate the same amount of power as was radiated.A certain amount of power will be dissipated in the antenna as heat in Rloss, therefore the resistance Rloss represent the losses in antenna.If total resistance of antenna is RA thenRA =Rloss+Rrad

If total power supplied to the antenna is IRA and power radiated IRrad Antenna efficiency is therefore A= IRA = RA IRrad Rrad

11Receiving antenna efficiencyIn receiving mode, the efficiency is defined as the ratio of power delivered to a matched load from the actual antenna to the power delivered to matched load from the antenna with Rloss assumed equal to be zero.

For receiving antenna the maximum power of real antenna is Vs/4RA and for the lossless antenna it is Vs/4Rrad .

Thus receiving antenna efficiency is also given by A= IRA = RA IRrad Rrad

The antenna feeder should be matched at both ends for eliminating reflected waves and obtaining maximum power transfer.

12Matching efficiencyConsidering mismatch with source resistance Z0 feeding a load ZA,the current flowing will be I = V02/ Z0+ZA 2And power delivered to ZA in transmitting :P = I2R = RAV02/ Z0+ZA 2If power delivered under matched conditions:V02/4Z0 The matched efficiency = RAV02 x 4Z0 Z0+ZA 2 x V02 = 4RA x Z0 Z0+ZA 2 Power delivered to Z0 in receiving :P = Z0VA2/ Z0+ZA 2If the power delivered under matched conditions is:VA2/4ZA The matched efficiency in receiving case is also:

= Z0VA2 x 4RA Z0+ZA 2 x VA2 = 4RA Z0 (SAME IN BOTH CASES TRANSMISSION AND RECEPTION) Z0+ZA 2 Applying RA = X( ZA +ZA) and ZA= Z0 1+A 1-AMatching efficiency is the same for both transmitting and receiving conditions and given by: = 1-|A|

13VSWR and Reflected Power Voltage Standing Wave Ratio (VSWR) is an indication of the quality of the impedance match.

A high VSWR is an indication the signal is reflected prior to being radiated by the antenna. VSWR and reflected power are different ways of measuring and expressing the same thing.

A VSWR of 2.0:1 or less is often considered acceptable. Mostly the antennas are specified to be 1.5:1 or less over some bandwidth..

14Bandwidth Bandwidth can be defined in terms of radiation patterns or VSWR/reflected power. The definition used is based on VSWR. Bandwidth is often expressed in terms of percent bandwidth, because the percent bandwidth is constant relative to frequency.

15Decibels Decibels (dB) are the accepted method of describing a gain or loss relationship in a communication system. dB may be added and subtracted. A decibel relationship (for power) is calculated using the following formula. dB = 10 log Power A Power B A might be the power applied to the connector on an antenna, the input terminal of an amplifier or one end of a transmission line. B might be the power arriving at the opposite end of the transmission line, the amplifier output or the peak power in the main lobe of radiated energy from an antenna. If A is larger than B, the result will be a positive number or gain. If A is smaller than B, the result will be a negative number or loss.

It is convenient to remember these simple dB values which are handy when approximating gain and loss:Power Gain Power Loss 3 dB = 2X power -3 dB = 1/2 power 6 dB = 4X power -6 dB = 1/4 power10 dB = 10X power -10 dB = 1/10 power 20 dB = 100X power -20 dB = 1/100 powerIn the case of antennas, passive structures cannot generate power. dB is used to describe the ability of these structures to focus energy in a part of space.

16Directivity and Gain

Directivity is the ability of an antenna to focus energy in a particular direction when transmitting or to receive energy better from a particular direction when receiving. There is a relationship between gain and directivity. ( We see the phenomena of increased directivity when comparing a light bulb to a spotlight. A 100-watt spotlight will provide more light in a particular direction than a 100-watt light bulb and less light in other directions. We could say the spotlight has more "directivity" than the light bulb. The spotlight is comparable to an antenna with increased directivity.)

Gain is the practical value of the directivity. The relation between gain and directivity includes a new parameter which describes the efficiency of the antenna.

For example, an antenna with 3dB of directivity and 50% of efficiency will have a gain of 0 dB.

17Directivity and GainMore technically the directivity is defined as the ratio of the radiation intensity in a given direction from the antenna to the radiation intensity averaged over all directions. The average radiation intensity is equal to the total power radiated by the antenna divided by 4. If the direction is not specified, the direction of maximum radiation intensity is implied. Stated more simply, the directivity of a nonisotropic source is equal to the ratio of its radiation intensity in a given direction over that of an isotropic source. 18Directivity and GainIn mathematical form, the radiation intensity of an isotropic source is

where D = directivity (dimensionless)U = radiation intensity (W/unit solid angle)U0 = radiation intensity of isotropic source (W/unit solid angle)Prad = total radiated power (W)

The radiation intensity of an isotropic source is given by19Directivity and GainIf the direction is not specified, it implies the direction of maximum radiation intensity (maximum directivity), is expressed as

Where

Umax = maximum radiation intensity (W/unit solid angle)D0 = maximum directivity (dimensionless) For an isotropic source, the directivity is unity since U, Umax, and U0 are all equal to each other.

20Gain Measurement One method of measuring gain is to compare the antenna under test against a known standard antenna. This is known as a gain transfer technique. At lower frequencies, it is convenient to use a 1/2-wave dipole as the standard. At higher frequencies, it is common to use a calibrated gain horn as a gain standard with gain typically expressed in dBi.

Another method for measuring gain is the 3-antenna method. Transmitted and received powers at the antenna terminal are measured between three arbitrary antennas at a known fixed distance. The Friis transmission formula is used to develop three equations and three unknowns. The equations are solved to find the gain expressed in dBi of all three antennas.

Friis transmission formula F= F1+ F2-1+ F3-1 + --- (F is noise factor and G is gain) G1 G1G2 Use the following conversion factor to convert between dBd and dBi: 0 dBd = 2.15 dBi. Example: 3.6 dBd + 2.15 dB = 5.75 dBi

21Radiation PatternsRadiation or antenna pattern describes the relative strength of the radiated field in various directions from the antenna at a constant distance. The radiation pattern is a "reception pattern" as well, since it also describes the receiving properties of the antenna. The radiation pattern is three-dimensional, but it is difficult to display the three-dimensional radiation pattern in a meaningful manner. It is also time-consuming to measure a three-dimensional radiation pattern. Often radiation patterns measured are a slice of the three-dimensional pattern, resulting in a two-dimensional radiation pattern which can be displayed easily on a screen or piece of paper. These pattern measurements are presented in either a rectangular or a polar format.

22Radiation fieldsThe field originated from an antenna is complicate and consists of:1. An electric field component that lags the current by 900 and that decreases in amplitude as the cube of distance.2. An electromagnetic field(combined both electric and magnetic field) that is in phase with the current and that decreases in amplitude as the square of the distance.3. An electromagnetic field that leads the current by 900 and that decreases the amplitude directly as the distance increases.

Note: It only appears as plane TEM wave when reaches at the receiving antenna. 23

Radiation pattern

In the field of antenna design the term radiation pattern most commonly refers to the directional (angular) dependence of radiation from the antenna or other source (synonyms: antenna pattern-near-field pattern -, far-field pattern).

The near-field pattern is most commonly defined over a plane placed in front of the source, or over a cylindrical or spherical surface enclosing it.

The far field radiation pattern may be represented graphically as a plot of one of a number of related variables, including; the field strength at a constant (large) radius (an amplitude pattern or field pattern), the power per unit solid angle (power pattern) and the gain or directive gain 24

Near /Far field zonesThe antennas for which the largest dimension D is very much greater than wavelength being radiated, the far field zone becomes the only significant one for distances d greater than 2D2/. d 2D2/ 25

26Plotting radiation pattern / antenna pattern When the amplitude of a specified component of E field is plotted ,it is called the field pattern or voltage pattern.

When the square of the amplitude of E field is plotted ,it is called the power pattern.

A three dimensional plot of an antenna pattern is avoided by plotting separately the normalized IEsI versus for constant (called E plane pattern or vertical pattern), the normalized IEsI versus for =/2 (called H plane pattern or horizontal pattern).

Normalization of IEsI is with respect to maximum value of IEsI which is unity.

27Understanding and Using Antenna Radiation PatternsAll antennas have directional qualities. They do not radiate power equally in all directions.

Therefore, antenna radiation patterns or plots are a very important tool to both the antenna designer and the end user.

These plots show a quick picture of the overall antenna response. 28

"Most antenna users are interested in the directivity or beamwidth of the antenna. This is usually referred to as the "half-power" or 3 dB beamwidth, the points between which half the power is radiated or concentrated, and specified in degrees. As an example, the typical half-power beamwidths of a 3, 6 and 10 element Yagi are 60, 40 and 30 degrees respectively 29Antenna Radiation Patterns:

Antenna radiation plots can be quite complex because in the real world they are three-dimensional. However, to simplify them a Cartesian coordinate system (a two-dimensional system which refers to points in free space) is often used.

Radiation plots are most often shown in either the plane of the axis of the antenna or the plane perpendicular to the axis and are referred to as the azimuth or "E-plane" and the elevation or "H-plane" respectively 30Rectangular and polar coordinate systemsMany plotting formats or grids are in use. Rectangular grids as well as polar coordinate systems are in wide use. The principal objective to show a radiation plot that is representative of a complete 360 degrees in either the azimuth or the elevation plane. In the case of highly directional antennas, the radiation pattern is similar to a flashlight beam.Rectangular grids

31Polar coordinate systemNote that it shows the sidelobes of the antenna relative to the main beam in decibels. This type of plot is preferred when the exact level of the sidelobes is important.

32Types of plotting scalesThree types of plotting scales are in common usage; Linear, The linear scale emphasizes the main radiation beam

Linear logarithmic The linear logarithmic scale is preferred when the level of all side lobes is important

Modified logarithmic. The modified logarithmic scale emphasizes the shape of the major beam while compressing very low-level (>30 dB) side lobes towards the center of the pattern. This plotting scale is now becoming quite popular

33Radiation PatternAn antenna radiation pattern or antenna pattern is defined as a mathematical function or a graphical representation of the radiation properties of the antenna as a function of space coordinates. In most cases, the radiation pattern is determined in the far - field region and is represented as a function of the directional coordinates. Radiation properties include power flux densityradiation intensityfield strengthdirectivity,phase or polarizationThe radiation property of most concern is the two - or three - dimensional spatial distribution of radiated energy as a function of the observers position along a path or surface of constant radius.Radiation Pattern

Radiation Pattern

Coordinate system for antenna analysisRadiation PatternA trace of the received electric (magnetic) field at a constantradius is called the amplitude field pattern. On the other hand, a graph of the spatial variation of the power density along a constant radius is called an amplitude power pattern.Often the field and power patterns are normalized with respect to their maximum value, yielding normalized field and power patterns.

Power pattern is usually plotted on a logarithmic scale or more commonly in decibels (dB). This scale is usually desirable because a logarithmic scale can accentuate in more details those parts of the pattern that have very low values, which later we will refer to as minor lobes. Radiation PatternFor an antenna, the field pattern ( in linear scale) typically represents a plot of the magnitude of the electric or magnetic field as a function of the angular space.power pattern ( in linear scale) typically represents a plot of the square of the magnitude of the electric or magnetic field as a function of the angular space.power pattern ( in dB) represents the magnitude of the electric or magnetic field, in decibels, as a function of the angular space.Radiation Pattern

Two-dimensional normalized field pattern ( plotted in linear scale)power pattern ( plotted in linear scale)Power pattern (plotted on a logarithmic dB scale ) of a 10-element linear antenna array of isotropic sources, with a spacing of d = 0.25 between the elements

Plus (+) and minus () signs in the lobes indicate the relative polarization of the amplitude between the various lobes, which changes (alternates) as the nulls are crossedRadiation Pattern

Figure (a)To find the points where the pattern achieves its half-power (3 dB points), relative to the maximum value of the pattern, you set the value of the

field pattern at 0.707 value of its maximum, as shown in Figure (a)

power pattern (in a linear scale) at its 0.5 value of its maximum, as shown in Figure (b)

power pattern (in dB) at 3 dB value of its maximum, as shown in Figure (c)

Figure (b)

Figure (c)Radiation PatternAll three patterns yield the same angular separation between the two half-power points, 38.64, on their respective patterns, referred to as HPBW

In practice, the three-dimensional pattern is measured and recorded in a series of two-dimensional patterns.

However, for most practical applications, a few plots of the pattern as a function of for some particular values of , plus a few plots as a function of for some particular values of , give most of the useful and needed information.Radiation Pattern Lobes

Figure (a) Radiation lobes and beamwidths of an antenna pattern. Figure (b) Linear plot of power pattern and its associated lobes and beamwidthsRadiation Pattern LobesVarious parts of a radiation pattern are called lobes, which may be sub-classified into major or main, minor, side, and back lobes.

A radiation lobe is a portion of the radiation pattern bounded by regions of relatively weak radiation intensity.

Figure (a) demonstrates a symmetrical three dimensional polar pattern with a number of radiation lobes. Some are of greater radiation intensity than others, but all are classified as lobes.

Figure (b) illustrates a linear two-dimensional pattern [one plane of Figure (a)] where the same pattern characteristics are indicated.Radiation Pattern LobesA major lobe (also called main beam) is defined as the radiation lobe containing the direction of maximum radiation. The major lobe is pointing in the = 0 direction. In some antennas, such as split-beam antennas, there may exist more than one major lobe. A minor lobe is any lobe except a major lobe. All the lobes with the exception of the major can be classified as minor lobes.A side lobe is a radiation lobe in any direction other than the intended lobe. (Usually a side lobe is adjacent to the main lobe and occupies the hemisphere in the direction of the main beam.) A back lobe is a radiation lobe whose axis makes an angle ofapproximately 180 with respect to the beam of an antenna. Usually it refers to a minor lobe that occupies the hemisphere in a direction opposite to that of the major (main) lobe.Radiation Pattern LobesMinor lobes usually represent radiation in undesired directions, and they should be minimized. Side lobes are normally the largest of the minor lobes. The level of minor lobes is usually expressed as a ratio of the power density in the lobe in question to that of the major lobe. This ratio is often termed the side lobe ratio or side lobe level.Side lobe levels of 20 dB or smaller are usually not desirable in most applications.Attainment of a side lobe level smaller than 30 dB usually requires very careful design and construction. In most radar systems, low side lobe ratios are very important to minimize false target indications through the side lobes.Radiation Pattern LobesA normalized three-dimensional far-field amplitude pattern, plotted on a linear scale, of a 10-element linear antenna array of isotropic sources with a spacing of d = 0.25 and progressive phase shift = 0.6, between the elements is shown in Figure

It is evident that this pattern has one major lobe, five minor lobes and one back lobe. The level of the side lobe is about 9 dB relative to the maximum.

For an amplitude pattern of an antenna, there are, in general, three electric-field components (Er , E , E ) at each observation point on the surface of a sphere of constant radius r = rc.

Radiation Pattern LobesIn the far field, the radial Er component for all antennas is zero or vanishingly small compared to either one, or both, of the other two components.

Some antennas, depending on their geometry and also observation distance, may have only one, two, or all three components.

In general, the magnitude of the total electric field would be |E| = |Er |2 + |E |2 + |E|2

The radial distance represents the magnitude of |E|.Isotropic, Directional, and Omnidirectional Patterns An isotropic radiator is defined as a hypothetical lossless antenna having equal radiation in all directions.

Although it is ideal and not physically realizable, it is often taken as a reference for expressing the directive properties of actual antennas.

A directional antenna is one having the property of radiating or receiving electromagnetic waves more effectively in some directions than in others.

This term is usually applied to an antenna whose maximum directivity is significantly greater than that of a half-wave dipole. Isotropic, Directional, and Omnidirectional Patterns Examples of antennas with directional radiation patterns areShown on page Figures (a) and Figure (b).

It is seen that the pattern in Figure (b) is nondirectional in the azimuth plane [f (), = /2] and directional in the elevation plane [g(), = constant].

This type of a pattern is designated as omnidirectional, and itis defined as one having an essentially nondirectional pattern in a given plane (in this case in azimuth) and a directional pattern in any orthogonal plane (in this case in elevation).

An omnidirectional pattern is then a special type of a directional pattern.Isotropic, Directional, and Omnidirectional Patterns

Figure (a):Principal E- and H-plane patterns for a pyramidal horn antennaFigure (b):Omnidirectional antenna pattern

Principal PatternsFor a linearly polarized antenna, performance is often described in terms of its principal E- and H-plane patterns. The E-plane is defined as the plane containing the electric field vector and the direction of maximum radiation, and the H-plane as the plane containing the magnetic-field vector and the direction of maximum radiation. Although it is very difficult to illustrate the principal patterns without considering a specific example, it is the usual practice to orient most antennas so that at least one of the principal plane patterns coincide with one of the geometrical principal planes. An illustration is shown in Figure (a). For this example, the x-z plane (elevation plane; = 0) is the principal E-plane and the x-y plane (azimuthal plane; = /2) is the principal H-plane. Other coordinate orientations can be selected. The omnidirectional pattern of Figure (b) has an infinite number of principal E-planes (elevation planes; = c) and one principal H-plane (azimuthal plane; = 90).Radian and Steradian

Radian and SteradianThe measure of a plane angle is a radian. One radian is defined as the plane angle with its vertex at the center of a circle of radius r that is subtended by an arc whose length is r. A graphical illustration is shown in above Figure (a). Since the circumference of a circle of radius r is C = 2r, there are 2 rad (2r/r) in a full circle.The measure of a solid angle is a steradian. One steradian is defined as the solid angle with its vertex at the center of a sphere of radius r that is subtended by a spherical surface area equal to that of a square with each side of length r. A graphical illustration is shown in Figure (b). Since the area of a sphere of radius r is A = 4r2, there are 4 sr (4r2/r2) in a closed sphere.The infinitesimal area dA on the surface of a sphere of radius r, is given by dA = r2 sin d d (m2)Therefore, the element of solid angle d of a sphere can be written as d = dA / r2 = sin d d (sr)Example 2.1For a sphere of radius r, find the solid angle 2A (in square radians or steradian) of a spherical cap on the surface of the sphere over the north-pole region defined by spherical angles of 0 30 , 0 180. Do this a. exactly.b. using 2A 341 342, where 341 and 342 are two perpendicular angular separations of the spherical cap passing through the north pole.

Compare the two.

SolutionUsing the equation, d = dA / r2 = sin d d

b.It is apparent that the approximate beam solid angle is about 31.23% in error.55ARRAYS1. Driven arrays2. Broad side arrays3. End fire arrays4.Parasitic arraysa. Parasitic reflectorsb. Parasitic directorsc. Yagi-uda arrayd. Plane reflector array56Decibels-Recall** Decibels (dB) are the accepted method of describing a gain or loss relationship in a communication system. dB may be added and subtracted. A decibel relationship (for power) is calculated using the following formula. dB = 10 log Power A Power B A might be the power applied to the connector on an antenna, the input terminal of an amplifier or one end of a transmission line. B might be the power arriving at the opposite end of the transmission line, the amplifier output or the peak power in the main lobe of radiated energy from an antenna. If A is larger than B, the result will be a positive number or gain. If A is smaller than B, the result will be a negative number or loss.

It is convenient to remember these simple dB values which are handy when approximating gain and loss:Power Gain Power Loss 3 dB = 2X power -3 dB = 1/2 power** 6 dB = 4X power -6 dB = 1/4 power10 dB = 10X power -10 dB = 1/10 power 20 dB = 100X power -20 dB = 1/100 powerIn the case of antennas, passive structures cannot generate power. dB is used to describe the ability of these structures to focus energy in a part of space.

57How to interpret antenna radiation plots?

An antenna plot is like a road map. It tells you where the radiation is concentrated. Patterns are usually referenced to the outer edge of the plot which is the maximum gain of the antenna.

This makes it easy to determine other important antenna characteristics directly from the plot, the directivity or beamwidth of the antenna. It is usually referred to as the "half-power" or 3 dB beamwidth, the points between which half the power is radiated or concentrated, and specified in degrees.

Another popular antenna specification is the "front-to-back" (F/B) ratio. It is defined as the difference in dB between the maximum gain or front of the antenna (usually 0 degrees) and a point exactly 180 degrees behind the front. The problem with specifying only the F/B ratio is that it does not account for any lobes in the rear two quadrants.

Another important antenna parameter is the side and rear lobe levels (if any). In a well designed antenna they should typically be 10-15 dB below the main beam. This parameter is often important but seldom seen on data sheets. 58Beam widthAssociated with the pattern of an antenna is a parameter designated as beamwidth. The beamwidth of a pattern is defined as the angular separation between two identical points on opposite side of the pattern maximum. In an antenna pattern, there are a number of beamwidths. One of the most widely used beamwidths is the Half-PowerBeamwidth (HPBW ), which is defined by IEEE as: In a plane containing the direction of the maximum of a beam, the angle between the two directions in which the radiation intensity is one-half value of the beam. Another important beamwidth is the angular separation between the first nulls of the pattern, and it is referred to as the First-Null Beamwidth (FNBW ).

59Half-Power BEAMWIDTHFull Null BeamwidthBetween1st NULLS0dB-3dBPEAKSIDE LOBE LEVEL( SLL )~ -20dBMain lobe Side lobes Back lobesBeam widthBeam widthA wideband antenna will pick up all the channel in a band, while a narrowband antenna will receive a few channels well but most channels poorly. (note: The TV spectrum is in 3 bands: VHF low, VHF high, and UHF.) The beam width is normally measured to the half-power points. That is, the beam width is the number of degrees between the points where the gain is 3 dB less than for the antennas strongest direction The antennas maximum gain can be found from the beam width using the formula: G=41000/(A*B) whereG is the raw gain factor (relative to isotropic, not in dB)A is the beam width, in degrees, in the elevation planeB is the beam width, in degrees, in the azimuth planeThis is an approximate formula, but it tends to be highly accurate for common, one-directional TV antennas

61Effective length of an antennaThe concept of effective area of an antenna is useful particularly for microwave antennas. For lower frequencies where the structure of antenna is the form of linear conductor or an array of conductors, effective length of an antenna is more useful. If VA is the emf appearing on a receiving antenna E is the strength of electric field of wave sweeping over the antenna, then effective length leff is defined by :VA = E leff

The definition of effective length leff, in case of transmitting antenna is in terms of input terminal current I0 is: I0 leff = area current length curve 62

E fieldH fieldP(Poynting vector)Direction ofpropagationRadiation Power DensityRadiation Power DensityElectromagnetic waves are used to transport information through a wireless medium or a guiding structure, from one point to the other. It is then natural to assume that power and energy are associated with electromagnetic fields. The quantity used to describe the power associated with an electromagnetic wave is the instantaneous Poynting vector defined as

64Radiation Power DensitySince the Poynting vector is a power density, the total power crossing a closed surface can be obtained by integrating the normal component of the Poynting vector over the entire surface. In equation form

For applications of time-varying fields, it is often more desirable to find the average power density which is obtained by integrating the instantaneous Poynting vector over one period and dividing by the period. where65Radiation Power DensityFor time-harmonic variations of the form ejt , we define the complex fields E and H which are related to their instantaneous counterparts

Using the definitions of above equations and the identity

can be written as

66Radiation Power DensityThe first term of above equation is not a function of time, and the time variations of the second are twice the given frequency. The time average Poynting vector (average power density) can be written as

The factor appears in above equations because the E and H fields represent peak values, and it should be omitted for RMS values.

The real part of (E H )/2 in above equation represents the average (real) power density and the imaginary part represents the reactive (stored) power density associated with the electromagnetic fields.67Radiation Power DensityThe power density associated with the electromagnetic fields of an antenna in its far-field region is predominately real and is referred as radiation density.

Based upon the above definition, the average power radiated by an antenna (radiated power) can be written as

The power pattern of the antenna, is just a measure, as a function of direction, of the average power density radiated by the antenna.68Radiation Power DensityAn isotropic radiator is an ideal source that radiates equally in all directions. Although it does not exist in practice, it provides a convenient isotropic reference with which to compare other antennas. Because of its symmetric radiation, its Poynting vector will not be a function of the spherical coordinate angles and . In addition , it will have only a radial component. Thus the total power radiated by it is given by

and the power density bywhich is uniformly distributed over the surface of a sphere of radius r.69Example

70Radiation IntensityRadiation intensity in a given direction is defined as the power radiated from an antenna per unit solid angle. The radiation intensity is a far-field parameter, and it can be obtained by simply multiplying the radiation density by the square of the distance. In mathematical form it is expressed as

whereU = radiation intensity (W/unit solid angle)Wrad = radiation density (W/m2)

The radiation intensity is also related to the far-zone electric field of an antenna,71Radiation IntensityThe radiation intensity is also related to the far-zone electric field of an antenna, referred by

The radial electric-field component (Er ) is assumed, if present, to be small in the far zone. Thus the power pattern is also a measure of the radiation intensity.72Radiation IntensityThe total power is obtained by integrating the radiation intensity, over the entire solid angle of 4. Thus

where d = element of solid angle = sin d dFor an isotropic source, U will be independent of the angles and , as was the case for Wrad

Hence the above equation can be written as

or the radiation intensity of an isotropic source as

73TYPES OF ANTENNASResonant Antennasa. Hertzian dipoleb. Half wave dipolec. Vertical Antennasd. Loop and Ferrite Antennas

Non Resonant Antennasa.Microwave antennasb.Dielectric lens antennas

UHF-VHF AntennasMicrowave Antennas74Hertzian dipoleIt is a short linear antenna assumed to carry uniform current along its length. Such antenna cannot be realized in practice but longer antennas can be assumed to be made of number of dipoles connected in series.

Radiation properties of Hertzian dipole are readily calculated. Properties of longer antennas often be deduced by superimposing the results of the chain of Hertzian dipoles making up longer antenna.Directivity DM=1.5

Aeff=1.5 2 = 0.119 2 For unity efficiency.75Half wave dipoleHalf wave dipole is a resonant antenna ,total length of which is nominally /2 at the carrier frequency.

Standing wave of voltage and current exist along the antenna assuming the antenna to be opened-out /4 section of an open-circuited transmission line.

DM=1.64Aeff= 0.13 2

leff = / Rrad = 73 0hms

76Vertical AntennasGround reflections:

The ground acts as a perfect reflector for an antenna placed near the surface and an apparent mirror image of the antenna will appear to be placed immediate beneath the surface below the antenna. The interaction of direct and reflected waves, the radiation polar pattern drastically gets modified and appears to be the vector sum of radiation from two separate antennas.

The net effect depends on the height of the antenna from the surface. If the effective height is several wave lengths then no reflection effect and antenna will be considered in free space. If it is few wavelengths high then the reflection is to be considered which will be in phase with the radiated wave, antenna and its image act as phased array of two antennas.

77Grounded vertical Antenna Most of the medium-frequency(MF) and VHF mobile-whip antenna fall into this category. This type of antennas are called the Marconi Antenna. The vertical pole, mast or rod forms the main radiator. It may be free standing or supported by insulated guy wires and placed in a location good electrical ground.

Good location include marshy fields ,seacoast flats. Artificial ground plane may be created by burying a mat of heavy conductors extending radially from the mast far up to at least quarter wave length or preferably half wave length.78Non Resonant antennas1. LONG-WIRE ANTENNA : A LONG-WIRE ANTENNA is an antenna that is a wavelength or more long at the operating frequency. These antennas have directive patterns that are sharp in both the horizontal and vertical planes. The wire is driven at one end and has a resistive termination at the remote end that is matched to the characteristic impedance of the line at that end. This forms transmission line with a ground return and a matched termination. There exist no reflection end, no standing wave .Half of the energy fed is radiated where as remaining is either dissipated in the wire or the terminating resistor.

79Non Resonant antennas-Contd2. RHOMBIC ANTENNA uses four conductors joined to form a rhombus shape. Each of the four legs has the same length ranging from 2 to 4.The lengths and angle are chosen so that the main lobe lies along the main axis of the rhombus and side lobes get cancelled. Ground reflections cause the lobe to be tilted upward into the sky.Resistive termination chosen so that no reflection occur.It is highly directional and if tilt is chosen properly, is ideal for point to point sky wave propagation. This antenna has a wide frequency range, is easy to construct and maintain, and is non critical as far as operation and adjustment are concerned.

80UHF-VHF Antennas

1. Discone OmniThe Discone Antenna operates over broadband frequency range of 700 to 2000 MHz, cover VHF to UHF band. It is a very small antenna, but very effective. It is vertically polarized. and has an omni-directional radiation pattern.81UHF-VHF Antennas-contd 2. Helical Antenna - An antenna that has the form of a helix. When the helix circumference is much smaller than one wavelength, the antenna radiates at right angles to the axis of the helix. When the helix circumference is one wavelength, maximum radiation is along the helix axis.

82UHF-VHF Antennas-contd 3. Log Periodic Antenna is an ideal solution for radiated emissions and normalized site attenuation. Log periodic antennas operate over a broad frequency range. Generally log periodic antennas have a plurality of dipole elements in a planar spaced array. The length of the elements and the spacing between the elements are selected in accordance with a mathematical formula, with the shortest elements being near the top of the antenna. Feed conductors generally connect at the tip of the antenna. Electrical connections from feed conductors to opposed elements are alternated to provide a 180 degree phase shift between successive elements.

83Microwave AntennasParaboloidal Reflector Antenna

The parabolic reflector or dish antenna has been used far more widely in recent years with advent of satellite television (TV).

The RF antenna consists of a radiating system that is used to illuminate a reflector that is curved in the form of a paraboloid. This shape enables a very accurate beam to be obtained. In this way, the feed system forms the actual radiating section of the antenna, and the reflecting parabolic surface is purely passive.

When looking at parabolic reflector antenna systems there are a number of parameters and terms that are of importance:

a. Focus The focus or focal point of the parabolic reflector F is the point at which any incoming signals are concentrated. When radiating from this point the signals will be reflected by the reflecting surface and travel in a parallel beam and to provide the required gain and beamwidth. b.Vertex This is the innermost point at the centre of the parabolic reflector. c.Focal length The focal length of a parabolic antenna is the distance from its focus to its vertex. fd.Aperture The aperture of a parabolic reflector is what may be termed its "opening" or the area which it covers. For a circular reflector, this is described by its diameter. D

84Paraboloidal Reflector (Antenna-Microwave antennas)Aperture Efficiency I() =f/D Physical area A= D/4

Effective Area Aeff =AX I()

Gain G = I() x ( D/ )

Directivity D0 = 4 Aeff

BW -3dB = 70 /D

Null BW B= 140 /D

85Paraboloidal Reflector (Antenna-Microwave antennas)Aperture Efficiency I() =f/D Physical area A= D/4

Effective Area Aeff =AX I()

Gain G = I() x ( D/ )

Directivity D0 = 4 Aeff

BW -3dB = 70 /D

Null BW B= 140 /D

86Example:Find the directivity, beamwidth, and effective area for a praboloidal reflector diameter 6m and illumination efficiency is 0.65. The frequency of operation is 10 GHz.

Class activity!87Solution= c/f = 300x106 = .03m = 3 cm 10x109 A = D/4 = 3.14 x 62 = 28.26 m2 4

Aeff = AX I() = 0.65 A = 18.4 m2

D0 = 4 Aeff = 257000 ( 54.1dB) 2

BW(-3-dB) = 70 /D = 70x 0.03 = 0.350 6

BW (null) = 2x0.35 =0.700

88Slot antennas-MWWhen a slot in a large metallic plane is coupled to an RF source, it behaves like a dipole antenna mounted over reflected surface.

Slot antennas can be used for fixed stations, satellite ground stations and beacons. With proper mounting, a slot antenna can also be used for microwave mobile. With a 16-slot total, the antenna can have 10-12 dBi gain.Slot antennas can be built from surplus waveguide sections, which will give an omni-directional pattern and horizontal polarization

89Dielectric Lens Antennas-MWElectromagnetic wave is refracted when it passes through a surface separating a zone of lower fielectric constant like.The angles of incidence and refraction are related as under: Sin r/ Sin i = ri/ rr =1/ n

Where n is refractive index.

Dielectric lens antennas are applicable to the design of multiple-beam antenna (MBA) systems on EHF communication. Advantages include excellent wide angle scanning properties, vehicle speed monitoring.

90Microwave SystemMicrowave radio system operate at frequencies above 1 GHz in the light of sight or free space mode whether they are on ground or in satellite systems.Carrier frequencies range from 3 to 12 GHz are used, in telephone and TV channels.Repeater stations are provided at about 50 km as the microwaves travel in line of sight only.Repeater stations include separate transmitters and receivers.91Tropospheric PropagationThe troposheric is the region of the earths atmosphere immediately adjacent to the earths surface and extending upward for some tens of kilometers.

In this region ,the free space conditions are modified by:

(a) surface of earth

(b)earths atmosphere 92

surface of earth effect:

Considering the earth surface flat, there reach two components of wave at receiver end: i. Direct wave ii. Reflected wave: traveles longer distance s than direct path. Combination of two components makes a little difference in amplitude but it does introduce a phase difference s which is highly significant. Since a phase length of 2 radians corresponds to a path length of one wave length , the phase angle corresponding to s is:s = 2 s From geometry of the model for the purpose2d s=4ht hrs= 2 ht hr ds = 4 ht hr d p-543 Dennis Roddy

93 Earth surface effect on reflected waveReflected wave itself is effected both in amplitude and phase shift relative to direct wave. If E0 is field strength at unit distance and field strength at receiver ER, then

ER=E0 4 ht hr d2

Where E0= 30 PT GT94In a VHF mobile radio system, the base station transmits 100W at 150MHz,and the antenna is 20m above the ground. The transmitting antenna is a1/2 dipole for which the gain is 1.64. Calculate the field strength at a receiving antenna of height 2m at a distance of 40km.Solution: = 300x106 = 2m 15x106E0 = 30x100x1.64 =70 V/mER = 70x4xx20x2 2x(40x103)2 = 11 V/mExample: 95Plotting radiation pattern / antenna pattern When the amplitude of a specified component of E field is plotted ,it is called the field pattern or voltage pattern.

When the square of the amplitude of E field is plotted ,it is called the power pattern.

A three dimensional plot of an antenna pattern is avoided by plotting separately the normalized IEsI versus for constant (called E plane pattern or vertical pattern), the normalized IEsI versus for =/2 (called H plane pattern or horizontal pattern).

Normalization of IEsI is with respect to maximum value of IEsI which is unity.

96Coordinate systemThe directional characteristics of an antenna is described in terms of spherical coordinates.The surface of the sphere can be defined in relation to the antenna by the radius d and the angles and . These are shown in equatorial plane and meridian plane.

97Power Gain plottingAssociated with power gain is the directive gain of the antenna and is denoted by D(, ).The average power per unit solid angle is A Ps/4 where A is antenna efficiency and Ps is power input Thus the average power is A Pi Directivity is related to power gain D(, ) = G(, ) A The maximum value of D(, ) is termed directivity or directive gain, given by : DM = GM / A When the gain function is plotted, a three dimensional plot results.

98Power Gain plotting-Polar diagramsIn practice two dimensional are often used. One for equatorial plane and one for meridian plane.The function g (,) in equatorial plane is denoted by g (), since () is constant.In meridian plane it is denoted by g () , since is constant.Example : Plot polar diagram for: g () = sin2 and g ()=1

99Power Gain plotting-Polar diagramsIn practice two dimensional are often used. One for equatorial plane and one for meridian plane.The function g (,) in equatorial plane is denoted by g (), since () is constant.In meridian plane it is denoted by g () , since is constant.Example : Plot polar diagram for: g () = sin2 and g ()=1

100PolarizationPolarization of the wave is defined by the direction of the electric field vector in relation to the direction of propagation. If the E field component of the radiated wave travels in a plane perpendicular to the Earth's surface (vertical), the radiation is said to be VERTICALLY POLARIZED, If the E field propagates in a plane parallel to the Earth's surface (horizontal), the radiation is said to be HORIZONTALLY POLARIZED.Polarization power factorTo receive maximum signal, the polarization of the receiving antenna must be the same that of the transmitting antenna. If it is at some angleThen only the component of electric field parallel to the receiving antenna will induce a signal component Ecos and therefore

Polarization power factor plf = cos

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