Prepared By:Engr. Nuzhat Madina

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Lecture 3-10Fundamental Parameters of

Antenna

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Parameters of Antenna are:

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Beam AreaBeam widthHalf power beam

width Full Null beam

width PolarizationRadiation IntensityBeam Efficiency Antenna field zonesTransmission

formulaDirectivity

Directive GainRadiation ResistanceRadiation efficiencyResolutionAntenna aperture –

physical and effectiveEffective heightTransmission loss as a

function of frequencyAntenna temperature

and signal to noise ratio

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Co-ordinate system

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Radiation Pattern

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A mathematical or graphical representation of the radiation properties of antenna such asAmplitude Phase Polarization, etcAs a function of the angular space co-ordinates

is called as radiation pattern.

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FIELD PATTERN: The plot of field either electric |E| or magnetic |H| on a linear scale is called as field pattern.

POWER PATTERN : A plot of the power ( proportional to either |E|2 or magnetic |H|2

) on linear or decibel (dB) scale .

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Directional pattern of Horn antenna

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Omni-directional pattern

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Isotropic Radiators

Independent of the use of a given antenna for transmitting or receiving, an important characteristic of this antenna is the gain. Some antennas are highly directional; that is, more energy is propagated in certain directions than in others. The ratio between the amount of energy propagated in these directions compared to the energy that would be propagated if the antenna were not directional (Isotropic Radiation) is known as its gain. When a transmitting antenna with a certain gain is used as a receiving antenna, it will also have the same gain for receiving.

Isotropic Radiators

An isotropic radiator is a theoretical point source of electromagnetic or sound waves which radiates the same intensity of radiation in all directions. It has no preferred direction of radiation. It radiates uniformly in all directions over a sphere centred on the source. Isotropic radiators are used as reference radiators with which other sources are compared.

Radiation pattern

Most radiators emit (radiate) stronger radiation in one direction than in another. A radiator such as this is referred to as anisotropic. However, a standard method allows the positions around a source to be marked so that one radiation pattern can easily be compared with another.

The energy radiated from an antenna forms a field having a definite radiation pattern. A radiation pattern is a way of plotting the radiated energy from an antenna. This energy is measured at various angles at a constant distance from the antenna. The shape of this pattern depends on the type of antenna used.

Radiation pattern

To plot this pattern, two different types of graphs, rectangular-and polar-coordinate graphs are used. The polar-coordinated graph has proved to be of great use in studying radiation patterns. In the polar-coordinate graph, points are located by projection along a rotating axis (radius) to an intersection with one of several concentric, equally-spaced circles. The polar-coordinate graph of the measured radiation is shown in Figure 1.

Radiation Lobes

The main beam (or main lobe ) is the region around the direction of maximum radiation (usually the region that is within 3 dB of the peak of the main beam). The main beam in Figure 1 is northbound.

The sidelobes are smaller beams that are away from the main beam. These sidelobes are usually radiation in undesired directions which can never be completely eliminated. The sidelobe level (or sidelobe ratio) is an important parameter used to characterize radiation patterns. It is the maximum value of the sidelobes away from the main beam and is expressed in Decibels. One sidelobe is called backlobe. This is the portion of radiation pattern that is directed opposing the main beam direction.

Major and Side Lobes (Minor Lobes)

The pattern shown in the upper figures has radiation concentrated in several lobes. The radiation intensity in one lobe is considerably stronger than in the other. The strongest lobe is called major lobe; the others are (minor) side lobes. Since the complex radiation patterns associated with arrays frequently contain several lobes of varying intensity, you should learn to use appropriate terminology. In general, major lobes are those in which the greatest amount of radiation occurs. Side or minor lobes are those in which the radiation intensity is least.

Beam Width

The angular range of the antenna pattern in which at least half of the maximum power is still emitted is described as a „Beam With”. Bordering points of this major lobe are therefore the points at which the field strength has fallen in the room around 3 dB regarding the maximum field strength. This angle is then described as beam width or aperture angle or half power (- 3 dB) angle - with notation Θ (also φ). The beamwidth Θ is exactly the angle between the 2 red marked directions in the upper pictures. The angle Θ can be determined in the horizontal plane (with notation ΘAZ) as well as in the vertical plane (with notation ΘEL).

Front-to-Back Ratio

The front-to-back ratio of an antenna is the proportion of energy radiated in the principal direction of radiation to the energy radiated in the opposite direction. A high front-to-back ratio is desirable because this means that a minimum amount of energy is radiated in the undesired direction.

RECIPROCITY

RECIPROCITY is the ability to use the same antenna for both transmitting and receiving.

The electrical characteristics of an antenna apply equally, regardless of whether you use the antenna for transmitting or receiving.

The more efficient an antenna is for transmitting a certain frequency, the more efficient it will be as a receiving antenna for the same frequency.

This is illustrated by figure 2-1, view A. When the antenna is used for transmitting, maximum radiation occurs at right angles to its axis. When the same antenna is used for receiving (view B), its best reception is along the same path; that is, at right angles to the axis of the antenna.

RECIPROCITY

Radiation Pattern

A radiation pattern defines the variation of the power radiated by an antenna as a function of the direction away from the antenna. This power variation as a function of the arrival angle is observed in the antenna's far field.

As an example, consider the 3-dimensional radiation pattern in Figure 1, plotted in decibels (dB) .

Radiation Pattern

This is an example of a donut shaped or toroidal radiation pattern. In this case, along the z-axis, which would correspond to the radiation directly overhead the antenna, there is very little power transmitted. In the x-y plane (perpendicular to the z-axis), the radiation is maximum. These plots are useful for visualizing which directions the antenna radiates.

Radiation Pattern

A pattern is "isotropic" if the radiation pattern is the same in all directions. Antennas with isotropic radiation patterns don't exist in practice, but are sometimes discussed as a means of comparison with real antennas.

Some antennas may also be described as "omnidirectional", which for an actual antenna means that the radiation pattern is isotropic in a single plane

Examples of omnidirectional antennas include the dipole antenna and the slot antenna.

Radiation Pattern

The third category of antennas are "directional", which do not have a symmetry in the radiation pattern. These antennas typically have a single peak direction in the radiation pattern; this is the direction where the bulk of the radiated power travels. These antennas are very common; examples of antennas with highly directional radiation patterns include the dish antenna

Field Regions The fields surrounding an antenna are divided into 3

principle regions: Reactive Near Field Radiating Near Field or Fresnel Region Far Field or Fraunhofer Region The far field region is the most important, as this

determines the antenna's radiation pattern. Also, antennas are used to communicate wirelessly from long distances, so this is the region of operation for most antennas. We will start with this region.

Far Field (Fraunhofer) Region

The far field is the region far from the antenna, as you might suspect. In this region, the radiation pattern does not change shape with distance (although the fields still die off as 1/R, the power density dies off as 1/R^2). Also, this region is dominated by radiated fields, with the E- and H-fields orthogonal to each other and the direction of propagation as with plane waves.

If the maximum linear dimension of an antenna is D, then the following 3 conditions must all be satisfied to be in the far field region:

Far Field (Fraunhofer) Region

Far Field (Fraunhofer) Region The first and second equation above ensure that the

power radiated in a given direction from distinct parts of the antenna are approximately parallel (see Figure 1). This helps ensure the fields in the far-field region behave like plane waves. Note that >> means "much much greater than" and is typically assumed satisfied if the left side is 10 times larger than the right side.

Figure 1. The Rays from any Point on the Antenna are Approximately Parallel in the Far Field.

Far Field (Fraunhofer) Region Finally, where does the third far-field equation come

from? Near a radiating antenna, there are reactive fields (see reactive near field region, below), that typically have the E-fields and H-fields die off with distance as 1/R2 and1/R3 . The third equation above ensures that these near fields are gone, and we are left with the radiating fields, which fall off with distance as 1/R.

The far-field region is sometimes referred to as the Fraunhofer region, a carryover term from optics.

Reactive Near Field Region In the immediate vicinity of the antenna, we have the

reactive near field. In this region, the fields are predominately reactive fields, which means the E- and H- fields are out of phase by 90 degrees to each other (recall that for propagating or radiating fields, the fields are orthogonal (perpendicular) but are in phase).

The boundary of this region is commonly given as:

Radiating Near Field (Fresnel) Region The radiating near field or Fresnel region is the region

between the near and far fields. In this region, the reactive fields are not dominate; the radiating fields begin to emerge. However, unlike the Far Field region, here the shape of the radiation pattern may vary appreciably with distance.

The region is commonly given by:

Note that depending on the values of R and the wavelength, this field may or may not exist.

Finally, the above can be summarized via the following diagram:

Figure 2. Illustration of the Field Regions for an Antenna of Maximum Linear Dimension D.

Field Regions

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Types :

1. Reactive Near-Field2. Radiating Near-Field :

Fresnel3. Far- field : Fraunhofer

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Field Regions

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Reactive FieldPhases of electric and magnetic field are often near

quadrature.Highly reactive wave impedanceHigh content of non propagating stored energy near

antennaRadiating Near-Field: Fresnel

Fields are predominant in phaseFields do not yet display a spherical wave front; thus

pattern varies with distance.Regions where near field measurements are made.

Far- Field : FraunhoferField exist spherical wave front ( ); thus

pattern ideally does not vary with distanceElectric and magnetic fields are in phaseWave impedance is, ideally, realPower predominantly real, propagating energy

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Evolution of pattern from near to Far Field

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Pattern of Paraboloid Reflector

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Radian & Steradian

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Beam Area

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

The Beam Area of an antenna can often be describe approximately in terms of the angles subtended by the half-power points of the main lobe in the two principal planes.

Are the HPBW in the two principal planes, minor lobes being neglected.

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Radiation intensity

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

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

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

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Beamwidth

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The angular separation between two identical points on opposite side of pattern maximum is called as beamwidth.

Importance: It is very important figure of merit It is often used as a trade off between it and side

lobe level. As beam width increases side lobe level decreases. & vice versa.

It is also used to describe the resolution capabilities of the antenna to distinguish between two adjacent radiating sources or radar targets.

The most important resolution criterion states that the “Resolution capability of an antenna to distinguish between two sources is equal to half of the first-null beamwidth”. 2 sources separated by angular distance ≥ of

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Half Power Beamwidth (HPBW): In a plane containing the direction of the

maximum beam, the angle between the two directions in which the radiation intensity is ½ value of beam is called as HPBW.

First Null Beamwidth (FNBW):The angular separation between the first

nulls of the patterns is referred to as the FNBW.

Beamwidth

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Polarization

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Polarization is the direction of the electric field and is the same as the physical attitude of the antennaA vertical antenna will transmit a vertically polarized

waveThe receive and transmit antennas need to possess the

same polarization

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Beam Efficiency

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The (total) beam area ΩA (or beam solid angle) consists of the main beam area (or solid angle) ΩM plus the minor lobe area Ωm.

The ratio of the main beam area to the (total) beam area is called the (main) beam efficiency.

The ratio of the minor lobe area to the (total) beam area is called the Stray Factor.

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Directivity

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The directivity of an antenna is equal to the ratio of the maximum power density to its average value over a sphere as observed in the far field of an antenna.

Directivity from pattern :

The directivity is also the ratio of the area of a sphere (4π sr) to the beam area ΩA of the antenna.

Directivity from beam area:

The smaller the beam area, the larger the directivity D.

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For an antenna that radiates over only half a sphere the beam area ΩA = 2π sr,

dBi = decibels over isotropic.

For, Ideal isotropic antenna

Radiation pattern of dipole λ=0.5

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GAIN

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Used as a figure of merit.The ability of an antenna or antenna system to

concentrate the radiated power in a given direction or conversely to absorb effectively the incident power from that direction is specified by various antenna terms i.e. antenna gain or simply gain or directive gain or power gain or directivity.

Definition of GAIN is:Gain of antenna without involving the antenna

efficiency is defined as:

Reference antenna may be an isotropic antenna or lossless antenna.

Often gain of an antenna is expressed in decibel ratio i.e.

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Directive Gain

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The extent to which a practical antenna concentrates its radiated energy relative to that of some standard antenna is termed as directive gain.

Directive gain is the ratio of the radiation intensity in that direction to the average radiated power.

Directive Gain solely depends on the distribution of radiated power in space. It does not depend upon the power input to the antenna, antenna losses or the power consumed in a terminating resistance.

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Comparison between Directivity and Gain

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The ratio of the gain to the directivity is the antenna efficiency factor.

G=kD.k = efficiency factor (0 ≤ k ≤ 1).

Dimensionless.If an antenna has not any losses like ohmic,

dielectric mismatch i.e. 100% efficient, then directivity and gain are same.

For an antenna with losses, gain will be less than directivity by factor which corresponds to efficiency.

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Resolution

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Resolution of antenna may be defined as equal to the half the beam width between first null (FNBW)/2.

When the antenna beam maximum is aligned with one satellite, the first null coincides with the adjacent satellite.

Half the beam width between first nulls is approximately equal to the half-power beam width,

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Antenna Apertures

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Total power absorbedAp = physical aperture

As, at sidewalls E=0. Thus, the effective aperture Ae < Ap of horn.

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Effective height

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Effective height may be defined as the ratio of the induced voltage to the incident field

Represents the effectiveness of an antenna as radiator or collector of electromagnetic wave energy.

It indicates how for an antenna is effective in transmitting or receiving the electromagnetic wave energy.

lh

E

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Radiation Resistance

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The radiation resistance (Rr) is thus defined as that fictitious resistance which, when substituted in series with the antenna will consume the same power as is actually radiated.

The radiation resistance represents,Total energy radiating form transmitting antennaCurrent flowing in the antenna

The value of radiation resistance depends onConfiguration of antennaThe point where radiation resistance is consideredLocation of antenna w.r.t. grounds and other

objects, andRatio of length of diameter of the conductor used.Corona discharge – a luminous discharge round the

surface of antenna due to ionization of air etc.

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Antenna Apertures

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Physical apertureEffective aperture

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For horn, parabolic reflector, mattress multi element antennas,

For short dipole antennaFor ideal size when there is no thermal

losses and field is in phaseRelation between directivity and aperture is

Absorption ratio

Antenna Apertures

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Self & Mutual Impedance Of Antenna

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In general, any antenna is usually fed with a transmission line. So it is important to know the impedance of the antenna at the terminal where transmission line is connected. Such impedance is very important in the analysis of the antenna.

In general any antenna can be used either as transmitting antenna or as receiving antenna .

The transmitter and receiver are used along with transmitting antenna and receiving antenna respectively .

So in order to obtain maximum power available from transmitter or to extract maximum received power from antenna at receiver, the impedance of the antenna must be known

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POLARIZATION

Energy from an antenna is radiated in the form of an expanding sphere.

A small section of this sphere is called a wavefront. positioned perpendicular to the direction of the radiation field (fig. 2-2).

Within this wavefront. all energy is in phase. Usually, all points on the wavefront are an equal distance from the antenna.

The farther from the antenna the wave is, the less curved it appears. At a considerable distance, the wavefront can be considered as a plane surface at right angles to the direction of propagation.

POLARIZATION

POLARIZATION

The radiation field is made up of magnetic and electric lines of force that are always at right angles to each other.

Most electromagnetic fields in space are said to be linearly polarized. The direction of polarization is the direction of the electric vector. That is, if the electric lines of force (E lines) are horizontal, the wave is said to be horizontally polarized (fig. 2-2), and if the E lines are vertical, the wave is said to be vertically polarized. Since the electric field is parallel to the axis of the dipole, the antenna is in the plane of polarization.

POLARIZATION

A horizontally placed antenna produces a horizontally polarized wave, and a vertically placed antenna produces a vertically polarized wave.

In general, the polarization of a wave does not change over short distances. Therefore, transmitting and receiving antennas are oriented alike, especially if they are separated by short distances.

POLARIZATION

Over long distances, polarization changes. The change is usually small at low frequencies, but quite drastic at high frequencies. (For radar transmissions, a received signal is actually a wave reflected from an object. Since signal polarization varies with the type of object, no set position of the receiving antenna is correct for all returning signals). Where separate antennas are used for transmitting and receiving, the receiving antenna is generally polarized in the same direction as the transmitting antenna.

POLARIZATION

When the transmitting antenna is close to the ground, it should be polarized vertically, because vertically polarized waves produce a greater signal strength along the earth’s surface. On the other hand, when the transmitting antenna is high above the ground, it should be horizontally polarized to get the greatest signal strength possible to the earth’s surface.

Transmission between two antennasThe Friis Transmission Equation is used to

calculate the power received from one antenna (with gain G1), when transmitted from another antenna (with gain G2), separated by a distance R, and operating at frequency f or wavelength lambda.

To begin the derivation of the Friis Equation, consider two antennas in free space (no obstructions nearby) separated by a distance R:

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Transmission between two antennasAssume that PT Watts of total power are delivered to the

transmit antenna. For the moment, assume that the transmit antenna is omni directional, lossless, and that the receive antenna is in the far field of the transmit antenna. Then the power density p (in Watts per square meter) of the plane wave incident on the receive antenna a distance R from the transmit antenna is given by:

If the transmit antenna has an antenna gain GT in the direction of the receive antenna given by , then the power density equation above becomes:

The gain term factors in the directionality and losses of a real antenna. Assume now that the receive antenna has an effective aperture given by

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Transmission between two antennas Then the power received by this antenna (PR) is given by:

Since the effective aperture for any antenna can also be expressed as:

The resulting received power can be written as: This is known as the Friis Transmission Formula. It relates

the free space path loss, antenna gains and wavelength to the received and transmit powers. This is one of the fundamental equations in antenna theory, and should be remembered (as well as the derivation above).

Another useful form of the Friis Transmission Equation is given in Equation [2]. Since wavelength and frequency f are related by the speed of light c , we have the Friis Transmission Formula in terms of frequency:

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