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ASSIGNMENT 02

ANTENNA

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OFFICER CADET

ABEYRATHNE MMLD

Contents

1.Basic antenna properties.............................................4

1.1. Reciprocity: ...............................................................................................................................4

1.2. Polarization: ...............................................................................................................................4

1.3. Wavelength: ...............................................................................................................................6

Gain (directivity): ............................................................................................................................6

1.4. Beam width: ..............................................................................................................................7

1.5. Impedance Matching: .................................................................................................................7

1.6. VSWR and Reflected Power: .....................................................................................................7

1.7. Near field (induction field): ......................................................................................................8

1.8. Near field region: .......................................................................................................................8

1.9. Far field (radiation field): ............................................................................................................8

1.10. Far field region: ........................................................................................................................8

1.11. Input Impedance: .....................................................................................................................8

1.12. Radiation resistance: .................................................................................................................8

1.13. Loss resistance: .........................................................................................................................8

1.14. Reactance: ................................................................................................................................9

1.15. Efficiency: ...............................................................................................................................9

1.16. Electrical length: .......................................................................................................................9

1.17. Bandwidth: ...............................................................................................................................9

1.18. Azimuth and Elevation: ............................................................................................................9

Basic antenna principles...............................................10

1.19. Antenna Parameters ...............................................................................................................10

1.20. Electromagnetic Energy Calculations ....................................................................................11

Types of antenna..........................................................13

1.21. Diploes and Monopoles ..........................................................................................................13

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1.22. Loop Antennas ........................................................................................................................15

1.23. Micro strip Antennas ...............................................................................................................16

1.24. Helical Antennas .....................................................................................................................17

1.25. Horn Antennas .......................................................................................................................18

References....................................................................19

Content of Figures

Figure 1.2-1 Polarization of an antenna.........................5

Figure 1.2-2 Elliptical Polarization................................5

Figure 1.3-3 Wave length...............................................6

Figure 1.4-4 Beam width................................................7

Figure 1.18-5 Azimuth and Elevation angle..................9

Figure 2.1-6 (a) Dipole Principal E-plane Pattern.......10

Figure 3.1-7 Dipole Antenna, 1/2 Wave ................13

Figure 3.1-8 Current and Voltage Distributions for a

Half-Wave Dipole Antenna..........................................14

Figure 3.1-9 Monopole Antenna, 1/4 Wave.................14

Figure 3.1-10 (a) Monopole Principal E-plane Pattern14

Figure 3.2-11 (a) Circular Loop Antenna....................15

Figure 3.2-12 (a) Loop Antenna Principal E-plane

Pattern..........................................................................16

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Figure 3.3-13 Rectangular Micro strip-Antenna Element

......................................................................................17

Figure 3.4-14 Helical Antenna.....................................18Figure 3.5-15 Rectangular Waveguide Horns............19

1. Basic antenna properties

1.1. Reciprocity:

An antennas electrical characteristics are the same whether it is used for transmitting

or receiving. Because this is always true, throughout this lecture, we will consider antennas as

transmitting antennas.

1.2. Polarization:

Polarization is the orientation of the electric field vector of the electromagnetic wave

produced by the antenna. For most antennas, the orientation of the antenna conductor

determines the polarization. Polarization may be vertical, horizontal or elliptical.

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Figure 1.2-1 Polarization of an antenna

The diagram above shows vertical and horizontal polarization. If the radio wave's

electric field vector points in some other direction, it is said to be obliquely polarized. If the

electric field rotates in space, such that its tip follows an elliptical path, it is elliptically

polarized.

Figure 1.2-2 Elliptical Polarization

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1.3. Wavelength:

This is the length of one RF wave. It can be computed by either of the following

formulas, depending on the units required: l (meters) =300/f (MHz) or l (feet) =984/f (MHz)

Figure 1.3-3 Wave length

Gain (directivity):

This is a measure of the degree to which an antenna focuses power in a given

direction, relative to the power radiated by a reference antenna in the same direction. Units of

measure are dBi (isotopic antenna reference) or dBd (half-wave dipole reference). The twogain measurements can be converted using the following formula:

If the directivity of the transmitting and receiving antennas is known, it is possible to

compute the power received by the receiving antenna using either of the formulas below:

When using dB:

Antenna gain should be expressed in dBi, wavelength and distances in m and powers

in dBm or dBW.

When using gain ratios and powers in W:

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Antenna gains should be expressed as a number, distances and wavelengths in m and

powers in W.

1.4. Beam width:

The angular separation between the half-point (-3dB) points in an antennas radiation

pattern. In general, the beam width of the main lobe of the radiation pattern decreases as the

directivity increases.

Figure 1.4-4 Beam width

1.5. Impedance 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 impedance other

than 50 Ohms. Some sort of impedance matching circuit is then required to transform the

antenna impedance to 50 Ohms. Radial/Larsen antennas come with the necessary impedance

matching circuitry as part of the antenna. We use low loss-components in our matching

circuits to provide the maximum transfer of energy between the transmission line and the

antenna.

1.6. VSWR and Reflected Power:

Voltage Standing Wave Ratio (VSWR) is an indication of the quality of the

impedance match. VSWR is often abbreviated as SWR. 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. Most commercial antennas,

however, are specified to be 1.5:1 or less over some bandwidth. Based on a 100-watt radio, a

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1.5:1 VSWR equates to a forward power of 96 watts and a reflected power of 4 watts, or the

reflected power is 4.2% of the forward power.

1.7. Near field (induction field):

Electromagnetic field created by an antenna that is only significant at distances of less

than 2D/l from the antenna, where D is the longest dimension of the antenna.

1.8. Near field region:

A spherical region of radius 2D/l centered on the antenna.

1.9. Far field (radiation field):

Electromagnetic field created by the antenna that extends throughout all space. At

distances greater than 2D/l from the antenna, it is the only field. It is the field used for

communications.

1.10. Far field region:

The region outside the near field region, at distances greater than 2D/l.

Where:

is the minimum distance from the antenna

D is the largest dimension of the antenna

is the wavelength

1.11. Input Impedance:

This is the impedance measured at the antenna input terminals. In general, it is

complex and has two real parts and one imaginary part:

1.12. Radiation resistance:

Represents conversion of power into RF waves (real)

1.13. Loss resistance:

Represents conductor losses, ground losses, etc. (real)

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1.14. Reactance:

Represents power stored in the near field (imaginary)

1.15. Efficiency:This is the ratio of radiation resistance to total antenna input resistance:

The loss resistances come from conductor losses and losses in the ground (the near

field of the antenna can interact with the ground and other objects near the antenna). The

efficiency of practical antennas varies from less than 1% types of low frequency antennas to

99% for some types of wire antennas.

1.16. Electrical length:

This came up in the section on transmission lines. It is the length or distance

expressed in terms of wavelengths.

1.17. Bandwidth:

Generally, the range of frequencies over which the antenna system is SWR remains

below a maximum value, typically 2.0

1.18. Azimuth and Elevation:

These are angles used to describe a specific position in an antenna's radiation pattern.

Azimuth is a horizontal angle, generally measured from true north. The elevation angle is a

vertical angle, ranging from 0 degrees (horizon) to 90 degrees (zenith).

Figure 1.18-5 Azimuth and Elevation angle

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Basic antenna principles

1.19. Antenna Parameters

There are five basic parameters that one must understand to determine how an antenna

will operate and perform. The first is the characteristic of radiation resistance. The

radiation resistance (Rrad) of an antenna relates the power supplied (Prad) to the antenna and

the current (I) flowing into the antenna. The equation for radiation resistance is given in (1).

As can be seen, the greater the radiation resistance, the more energy is radiated or received by

the antenna. When the radiation resistance of the antenna matches the resistance of the

transmitter or receiver, the system is optimized. Antennas also have ohmic or loss resistance

which decreases efficiency. It can be shown that an efficient antenna must be comparable to

a wavelength in size.

(1)

A second parameter of antennas is the antenna pattern. This gives a distribution of

radiated power as a function of direction in space. Generally, planar sections of the radiation

pattern will be shown instead of the complete three-dimensional surface. The most important

views are those of the principal E-plane and H-plane patterns. The E-plane pattern containsthe plane in which the electric field lies. Similarly, the H-plane pattern is a sectional view in

which the magnetic field lines. An example of each type is shown in Figure 1, which gives

the antenna pattern for a half-wave dipole orientated in the z-direction.

Figure 2.1-6 (a) Dipole Principal E-plane Pattern

Figure 2.1-1 (b) Dipole principle H-plane pattern

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Most antennas do not radiate uniformly, as seen in Figure 1a. This implies that there is

some directivity, which is the third parameter. Closely related to this is the gain of the

antenna. The directivity is a measure of the ability of an antenna to concentrate the radiated

power in a given direction. The gain is similarly defined. Usually given in decibels, it is the

ratio of power radiated to input power. Thus, using an antenna with a higher gain would

require less input power. Gain is a more significant parameter in practice than the directivity.

Bandwidth is another important antenna parameter. This refers to the inclusive

frequencies available outside the center frequency. For example, a 10 MHz transmitter with a

10% bandwidth could send information on frequencies from 9 MHz to 11 MHz This is

important to Frequency Modulated (FM) signals, as they require modulation about the carrier

frequency in order to send data.

The final antenna parameter is the signal-to-noise ratio. This is the relationship

between the desired information signal and the noise. If this ratio does not exceed unity,

information will not be transferred. Noise can be caused by obstructions, large distances

between antennas, and environmental RF noise, such as power supplies and digital switching

devices.

1.20. Electromagnetic Energy Calculations

An antennas electric and magnetic fields can be calculated at any point, but the

equations are not simple. A short current filament generates the fields shown in (2), (3), and

(4), given in spherical coordinates. Because of the complexity of these equations, most

problems involving antennas are solved by experimental rather than theoretical methods.

(2)

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(3)

(4)

Despite the complexity of these equations, some observations can be made. The

exponential factor in each equation is the same. This indicates wave propagation outward

from the origin in the positive rdirection. The wavelength is equal to , and d is the length

of the current filament. Also as expected, the strength of the fields is related directly to the

peak current, I0. The factor is equal to 120 in free space.

At radial distances 10 or more wavelengths away from the oscillating current element,

these equations can be simplified. All terms except the 1/r term may be neglected, and the

radiation fields are given in (5), (6), and (7). Assuming a vertical orientation of the current

element, plotting E s against for a constant rwill result in a graph similar to Figure 1a,

called the vertical pattern. Plotting H s shows the variation of field intensity with , often

referred to as the horizontal pattern, such as in Figure 1b.

(5)

(6)

(7)

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Types of antenna

1.21. Diploes and Monopoles

The worlds most popular antenna is the half-wave dipole. Shown in Figure 2, the total

length of the antenna is equal to half of the wavelength. The relationship between

wavelength and frequency is f= c / , where 8103=c m/s in free space. Dipoles may

be shorter or longer than half of the wavelength, but this fraction provides the best antenna

efficiency. The radiation resistance can be calculated as 73.1 .

Figure 3.1-7 Dipole Antenna, 1/2 Wave

A two-wire line feeds the dipole antenna, where the two currents in the conductors are

equal in amplitude but opposite in direction. Since the antenna ends are essentially an open

circuit, the current distribution along the length of the half-wave dipole is sinusoidal. This

pattern shows that when the antenna is vertical, it radiates the most in the horizontal direction

and very little out the ends of the antenna. A typical gain for a dipole antenna is 2dB, and the

bandwidth is generally around 10%.

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Figure 3.1-8 Current and Voltage Distributions for a Half-Wave Dipole Antenna

A monopole antenna is one-half a dipole plus a perfectly conducting plane. It behaves

in a similar way to the dipole, but most of its parameters are halved. Figure 4 shows aquarter-wave monopole, also known as the vertical whip antenna.

Figure 3.1-9 Monopole Antenna, 1/4 Wave

The radiation resistance is 36.5 , half that of a dipole. The total power radiated is also

half that of a dipole and the radiation pattern is shown in below. A typical gain for a

monopole antenna is 2 to 6dB, and the bandwidth is also around 10%.

Figure 3.1-10 (a) Monopole Principal E-plane Pattern

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Figure 3.1-4 (b) Monopole Principal H-plane Pattern

1.22. Loop Antennas

The loop antenna is a conductor bent into the shape of a closed curve, such as a circle or

square, with a gap in the conductor to form the terminals. Figure 6 shows a circular and a

square loop. These antennas may also be found as multiturn loops or coils, designed with a

series connection of overlaying turns. There are two sizes of loop antennas: electrically small

and electrically large. If the total conductor length is small compared with a wavelength, it is

considered small. An electrically large loop typically has a circumference approaching one

wavelength.

Figure 3.2-11 (a) Circular Loop Antenna

Figure 3.2-1 (b) Square Loop Antenna

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The current distribution on a small loop antenna is assumed uniform. This allows it to

be simply analyzed as a radiating inductor. Used as transmitters, loop antennas have a

pattern that follows below. Loop antennas can have a gain from 2dB to 3dB and a

bandwidth of around 10%.

Figure 3.2-12 (a) Loop Antenna Principal E-plane Pattern

Figure 3.2-2 (b) Loop Antenna Principal H-plane Pattern

Loop antennas have found to be very useful as receivers. For low frequencieswheredipoles would become very largeloop antennas can be used. While the efficiency of a

small loop antenna is not good, a high signal-to-noise ratio makes up for it. A common

method to increase loop antennas performance is to fill the core with a ferrite. This has the

effect of increasing the magnetic flux through the loop and increasing radiation resistance.

1.23. Micro strip Antennas

The micro strip or patch antenna is often manufactured directly on a printed circuit

board, where the patch is a rectangular element that is photo etched from one side of the

board. Most micro strip elements are fed by a coaxial conductor, which is soldered to the

back of the ground plane. Typically, the upper plate conductor is smaller than the ground

plane to allow fringing of the electric field.

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Figure 3.3-13 Rectangular Micro strip-Antenna Element

Despite its low profile, the micro strip antenna has an efficient radiation resistance. The

source of this radiation is the electric field that is excited between the edges of the micro strip

element and the ground plane. The equation for the radiation resistance is a function of the

desired wavelength ( ) and the width (W) of the micro strip:

(8)

Micro strip antennas are generally built for devices that require small antennas, which

lead to high frequencies, typically in the Gigahertz. Most micro strip elements are very

efficient, anywhere from 80 to 99 percent. Factors that affect efficiency are dielectric loss,

conductor loss, reflected power, and the power dissipated in any loads involved in the

elements. Using air as a substrate leads to very high efficiencies, but is not practical for

photo-etched antennas.

1.24. Helical Antennas

The basic helical antenna consists of a single conductor wound into a helical shape,

shown in Figure 9. Helical antennas are circularly polarized, that is, the radiated

electromagnetic wave contains both vertical and horizontal components. This is unlike the

dipole, which only radiates normal to its axis. Like the monopole, a ground plane must be

present.

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Figure 3.4-14 Helical Antenna

The antenna shown in Figure 9 has a gain of about 12dB. It operates in the 100 to 500

MHz range and is fed with a coax where the center conductor is fed through the center of the

ground plane. The spacing of the turns is 1/4-wave and the diameter of the turns is 1/3-wave.

This is just one example of a helical antenna; they can be scaled to other frequencies of

operation as well. Other modifications can be made; nonuniform-diameter helical structures

can widen the bandwidth and improve radiation performance.

1.25. Horn Antennas

Horn antennas are made for controlling one or more of the fundamental antenna

properties: gain, antenna pattern, and radiation resistance. A horn antenna works in

conjunction with a waveguidea tube that channels energy from one location to another.

Horn antennas can have several shapes, depending on their function. Several are shown in

below.

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Figure 3.5-15 Rectangular Waveguide Horns

(a) Pyramidal (b) Sectoral H-plane (c) Sectoral E-plane (d) Diagonal

The pyramidal horn in Figure 10a is used to maximize the gain, since the antenna is

flared in both the H-plane and E-plane. This obviously gives the antenna a fixed directivity,

and it will radiate principally in the direction of the horns axis. Figures 10b and 10c are

special cases of the pyramidal horn, where either the H-plane or E-plane is flared thus and

maximized.

References

1. Introduction to Antenna Types and Their Applications Engineering

Electromagnetics (pdf) by Justin Voogt.

2. Basic Antenna Theory and Concepts. ICS 620 Communication Technologies Class

#11. (ppt)

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