Antennas wave pro frenzel

© 2008 The McGraw-Hill Companies

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Principles of ElectronicPrinciples of ElectronicCommunication SystemsCommunication Systems

Third Edition

Louis E. Frenzel, Jr.


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Chapter 14Chapter 14

Antennas and Wave Propagation


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Topics Covered in Chapter 14Topics Covered in Chapter 14

14-1: Antenna Fundamentals 14-2: Common Antenna Types 14-3: Radio-Wave Propagation


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14-1: Antenna Fundamentals14-1: Antenna Fundamentals

The interface between the transmitter and free space and between free space and the receiver is the antenna.

At the transmitting end the antenna converts the transmitter RF power into electromagnetic signals; at the receiving end the antenna picks up the electromagnetic signals and converts them into signals for the receiver.


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Radio Waves A radio signal is called an electromagnetic wave

because it is made up of both electric and magnetic fields.

Whenever voltage is applied to the antenna, an electric field is set up.

This voltage causes current to flow in the antenna, producing a magnetic field.

These fields are emitted from the antenna and propagate through space at the speed of light.


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Radio Waves: Magnetic Fields A magnetic field is an invisible force field created by a

magnet. An antenna is a type of electromagnet. A magnetic field is generated around a conductor when

current flows through it. The strength and direction of the magnetic field depend

upon the magnitude and direction of the current flow. The SI unit for magnetic field strength is ampere-turns

per meter.


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Figure 14-1: Magnetic field around a current-carrying conductor. Magnetic field strengthH in ampere-turns per meter = H = I I(2 πd).


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Radio Waves: Electric Field An electric field is an invisible force field produced by

the presence of a potential difference between two conductors.

For example, an electric field is produced between the plates of a charged capacitor.

An electric field exists between any two points across which a potential difference exists.

The SI unit for electric field strength is volts per meter. Permittivity is the dielectric constant of the material

between the two conductors.


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Figure 14-2: Electric field across the plates of a capacitor.


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Radio Waves: Magnetic and Electric Fields in a Transmission Line At any given time in a two-wire transmission line, the

wires have opposite polarities. During one-half cycle of the ac input, one wire is

positive and the other is negative. During the negative half-cycle, the polarity reverses. The direction of the electric field between the wires

reverses once per cycle. The direction of current flow in one wire is always

opposite that in the other wire. Therefore, the magnetic fields combine.


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Radio Waves: Magnetic and Electric Fields in a Transmission Line A transmission line is made up of a conductor or

conductors. Transmission lines do not radiate signals efficiently. The closeness of the conductors keeps the electric field

concentrated in the transmission line dielectric. The magnetic fields mostly cancel one another. The electric and magnetic fields do extend outward from

the transmission line, but the small amount of radiation that does occur is extremely inefficient.


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Figure 14-3: (a) Magnetic and electric fields around a transmission line. (b) Electric field. (c) Magnetic fields.


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Antenna Operation: The Nature of an Antenna If a parallel-wire transmission line is left open, the

electric and magnetic fields escape from the end of the line and radiate into space.

This radiation is inefficient and unsuitable for reliable transmission or reception.

The radiation from a transmission line can be greatly improved by bending the transmission-line conductors so they are at a right angle to the transmission line.


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Antenna Operation: The Nature of an Antenna The magnetic fields no longer cancel; they now aid one

another. The electric field spreads out from conductor to

conductor. Optimum radiation occurs if the segment of

transmission wire converted into an antenna is one quarter wavelength long at the operating frequency.

This makes an antenna that is one-half wavelength long.


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Figure 14-5: Converting a transmission line into an antenna. (a) An open transmission line radiates a little. (b) Bending the open transmission line at right angles createsan efficient radiation pattern.


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Antenna Operation The ratio of the electric field strength of a radiated wave

to the magnetic field strength is a constant and is called the impedance of space, or the wave impedance.

The electric and magnetic fields produced by the antenna are at right angles to one another, and are both perpendicular to the direction of propagation of the wave.


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Antenna Operation Antennas produce two sets of fields, the near field and

the far field. The near field describes the region directly around

the antenna where the electric and magnetic fields are distinct.

The far field is approximately 10 wavelengths from the antenna. It is the radio wave with the composite electric and magnetic fields.

Polarization refers to the orientation of magnetic and electric fields with respect to the earth.


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Antenna Reciprocity Antenna reciprocity means that the characteristics

and performance of an antenna are the same whether the antenna is radiating or intercepting an electromagnetic signal.

A transmitting antenna takes a voltage from the transmitter and converts it into an electromagnetic signal.

A receiving antenna has a voltage induced into it by the electromagnetic signal that passes across it.


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The Basic Antenna An antenna can be a length of wire, a metal rod, or a

piece of tubing. Antennas radiate most effectively when their length is

directly related to the wavelength of the transmitted signal.

Most antennas have a length that is some fraction of a wavelength.

One-half and one-quarter wavelengths are most common.


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14-2: Common Antenna Types14-2: Common Antenna Types

The Dipole Antenna One of the most widely used antenna types is the half-

wave dipole. The half-wave dipole, also called a doublet, is formally

known as the Hertz antenna. A dipole antenna is two pieces of wire, rod, or tubing

that are one-quarter wavelength long at the operating resonant frequency.

Wire dipoles are supported with glass, ceramic, or plastic insulators at the ends and middle.


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Figure 14-10: The dipole antenna.


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The Dipole Antenna The dipole has an impedance of 73 Ω at its center,

which is the radiation resistance. An antenna is a frequency-sensitive device. To get the dipole to resonate at the frequency of

operation, the physical length must be shorter than the one-half wavelength computed by λ = 492/f.

Actual length is related to the ratio of length to diameter, conductor shape, Q, the dielectric (when the material is other than air), and a condition known as end effect.


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The Dipole Antenna End effect is a phenomenon caused by any support

insulators used at the ends of the wire antenna and has the effect of adding capacitance to the end of each wire.

The actual antenna length is only about 95 percent of the computed length.

If a dipole is used at a frequency different from its design frequency, the SWR rises and power is lost.


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The Dipole Antenna: Antenna Q and Bandwidth The bandwidth of an antenna is determined by the

frequency of operation and the Q of the antenna according to the relationship BW = fr/Q.

The higher the Q, the narrower the bandwidth. For an antenna, low Q and wider bandwidth are

desirable so that the antenna can operate over a wider range of frequencies with reasonable SWR.

In general, any SWR below 2:1 is considered good in practical antenna work.


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The Dipole Antenna: Antenna Q and Bandwidth The Q and thus the bandwidth of an antenna are

determined by the ratio of the length of the conductor to the diameter of the conductor.

Bandwidth is sometimes expressed as a percentage of the resonant frequency of the antenna.

A small percentage means a higher Q, and a narrower bandwidth means a lower percentage.


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The Dipole Antenna: Conical Antennas A common way to increase bandwidth is to use a

version of the dipole antenna known as the conical antenna.

The center radiation resistance of a conical antenna is much higher than the 73 Ω usually found when straight-wire or tubing conductors are used.

The primary advantage of conical antennas is their tremendous bandwidth.

They can maintain a constant impedance and gain over a 4:1 frequency range.


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Figure 14-14: The conical dipole and its variation. (a) Conical antenna. (b) Broadside view of conical dipole antenna (bow tie antenna) showing dimensions. (c) Open-grillbow tie antenna.


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The Dipole Antenna: Dipole Polarization Most half-wave dipole antennas are mounted

horizontally to the earth. This makes the electric field horizontal to the earth and

the antenna is horizontally polarized. Horizontal mounting is preferred at the lower

frequencies because the physical construction, mounting, and support are easier.

This mounting makes it easier to attach the transmission line and route it to the transmitter or receiver.


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The Dipole Antenna: Radiation Pattern and Directivity The radiation pattern of any antenna is the shape of

the electromagnetic energy radiated from or received by that antenna.

Most antennas have directional characteristics that cause them to radiate or receive energy in a specific direction.

The radiation is concentrated in a pattern that has a recognizable geometric shape.

The measure of an antenna’s directivity is beam width, the angle of the radiation pattern over which a transmitter’s energy is directed or received.


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Figure 14-15: Three-dimensional pattern of a half-wave dipole.


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The Dipole Antenna: Antenna Gain A directional antenna can radiate more power in a given

direction than a nondirectional antenna. In this “favored” direction, it acts as if it had gain.

Antenna gain of this type is expressed as the ratio of the effective radiated output power Pout to the input power Pin.


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The Dipole Antenna: Antenna Gain Effective radiated power is the actual power that would

have to be radiated by a reference antenna (usually a nondirectional or dipole antenna) to produce the same signal strength at the receiver as the actual antenna produces.

The power radiated by an antenna with directivity and therefore gain is called the effective radiated power (ERP).

ERP = ApPt


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The Dipole Antenna: Folded Dipole A popular variation of the half-wave dipole is the folded

dipole. The folded dipole is also one-half wavelength long. It consists of two parallel conductors connected at the

ends with one side open at the center for connection to the transmission line.

The impedance of this antenna is 300 Ω. Folded dipoles usually offer greater bandwidth than

standard dipoles. The folded dipole is an effective, low-cost antenna that

can be used for transmitting and receiving.


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Figure 14-18: Folded dipole. (a) Basic configuration. (b) Construction with twin lead.


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Marconi or Ground-Plane Vertical Antenna The one-quarter wavelength vertical antenna, also

called a Marconi antenna, is widely used.

It is similar in operation to a vertically mounted dipole antenna.

The Marconi antenna offers major advantages because it is half the length of a dipole antenna.


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Marconi or Ground-Plane Vertical Antenna: Radiation Pattern Vertical polarization and omnidirectional

characteristics can be achieved using a one-quarter wavelength vertical radiator. This antenna is called a Marconi or ground-plane antenna.

It is usually fed with coaxial cable; the center conductor is connected to the vertical radiator and the shield is connected to earth ground.

The earth then acts as a type of electrical “mirror,” providing the other one-quarter wavelength making it equivalent to a vertical dipole.


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Figure 14-20: Ground-plane antenna. (a) One-quarter wavelength vertical antenna. (b) Using radials as a ground plane.


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Marconi or Ground-Plane Vertical Antenna: Ground Plane, Radials, and Counterpoise

When a good electrical connection to the earth has been made, the earth becomes what is known as a ground plane.

If a ground plane cannot be made to earth, an artificial ground can be constructed of several one-quarter wavelength wires laid horizontally on the ground or buried in the earth.

These horizontal wires at the base of the antenna are called radials, and the collection of radials is called a counterpoise.


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Marconi or Ground-Plane Vertical Antenna: Antenna Length For many applications, e.g., with portable or mobile

equipment, it is not possible to make the antenna a full one-quarter wavelength long.

To overcome this problem, shorter antennas are used, and lumped electrical components are added to compensate for the shortening.


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Marconi or Ground-Plane Vertical Antenna: Antenna Length The practical effect of this design is a decreased

inductance. The antenna no longer resonates at the desired operating frequency, but at a higher frequency.

To compensate for this, a series inductor, called a loading coil, is connected in series with the antenna coil.

The loading coil brings the antenna back into resonance at the desired frequency.


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Figure 14-22: Using a base leading coil to increase effective antenna length.


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Directivity Directivity refers to an antenna’s ability to send or

receive signals over a narrow horizontal directional range.

The physical orientation of the antenna gives it a highly directional response or directivity curve.

A directional antenna eliminates interference from other signals being received from all directions other than the desired signal.


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Directivity A highly directional antenna acts as a type of filter to

provide selectivity. Directional antennas provide greater efficiency of

power transmission. Directivity, because it focuses the power, causes the

antenna to exhibit gain, which is one form of amplification.


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Figure 14-25: Radiation pattern of a highly directional antenna with gain. (a) Horizontal radiation pattern. (b) Three-dimensional radiation pattern.


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Directivity To create an antenna with directivity and gain, two or

more antenna elements are combined to form an array.

Two basic types of antenna arrays are used to achieve gain and directivity:

1. Parasitic arrays.

2. Driven arrays.


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Parasitic Arrays A parasitic array consists of a basic antenna

connected to a transmission line plus one or more additional conductors that are not connected to the transmission line.

These extra conductors are referred to as parasitic elements and the antenna is called a driven element.

A Yagi antenna is made up of a driven element and one or more parasitic elements.


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Figure 14-26: A parasitic array known as a Yagi antenna.


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Driven Arrays A driven array is an antenna that has two or more

driven elements. Each element receives RF energy from the

transmission line. Different arrangements of the elements produce

different degrees of directivity and gain. The three basic types of driven arrays are the collinear,

the broadside, and the end-fire. A fourth type is the wide-bandwidth log-periodic

antenna.


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Driven Arrays: Collinear Antenna Collinear antennas usually consist of two or more half-

wave dipoles mounted end to end. Collinear antennas typically use half-wave sections

separated by shorted quarter-wave matching stubs which ensure that the signals radiated by each half-wave section are in phase.

Collinear antennas are generally used only on VHF and UHF bands because their length becomes prohibited at the lower frequencies.


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Figure 14-29: Radiation pattern of a four-element collinear antenna.


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Driven Arrays: Broadside Antenna A broadside array is a stacked collinear antenna

consisting of half-wave dipoles spaced from one another by one-half wavelengths.

This antenna produces a highly directional radiation pattern that is broadside or perpendicular to the plane of the array.

The broadside antenna is bidirectional in radiation, but the radiation pattern has a very narrow beam width and high gain.


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Figure 14-30: A broadside array.


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Driven Arrays: End-Fire Antenna The end-fire array uses two half-wave dipoles spaced

one-half wavelength apart. The end-fire array has a bidirectional radiation pattern,

but with narrower beam widths and lower gain. The radiation is in the plane of the driven elements. A highly unidirectional antenna can be created by

careful selection of the optimal number of elements with the appropriately related spacing.


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Figure 14-31: End-fire antennas. (a) Bidirectional. (b) Unidirectional.


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Driven Arrays: Log-Periodic Antennas A special type of driven array is the wide-bandwidth

log-periodic antenna. The lengths of the driven elements vary from long to

short and are related logarithmically. The spacing is also variable.

The great advantage of the log-periodic antenna over a Yagi or other array is its very wide bandwidth.

The driving impedance is constant over this range. Most TV antennas in use today are of the log-periodic

variety so that they can provide high gain and directivity on both VHF and UHF TV channels.


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Figure 14-32: Log-periodic antenna.


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Impedance Matching One of the most critical aspects of any antenna system

is ensuring maximum power transfer from the transmitter to the antenna.

When the characteristic impedance of the transmission line matches the output impedance of the transmitter and the impedance of the antenna, the SWR will be 1:1.

When SWR is 1:1, maximum power transfer will take place.


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Impedance Matching A Q section, or matching stub, is a one-quarter

wavelength of coaxial or balanced transmission line of a specific impedance that is connected between a load and source and is used to match impedances.

A balun is a transformer used to match impedances. An antenna tuner is a variable inductor, one or more

variable capacitors, or a combination of these components connected in various configurations.


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Figure 14-33: A one-quarter wavelength matching stub or Q section.


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Figure 14-34: A bifilar toroidal balun for impedance matching.


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Figure 14-36: An antenna tuner.


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14-3: Radio-Wave Propagation14-3: Radio-Wave Propagation

Once a radio signal has been radiated by an antenna, it travels or propagates through space and ultimately reaches the receiving antenna.

The energy level of the signal decreases rapidly with distance from the transmitting antenna.

The electromagnetic wave is affected by objects that it encounters along the way such as trees, buildings, and other large structures.

The path that an electromagnetic signal takes to a receiving antenna depends upon many factors, including the frequency of the signal, atmospheric conditions, and time of day.


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Optical Characteristics of Radio Waves Radio waves act much like light waves.

Light waves can be reflected, refracted, diffracted, and focused by other objects.

The focusing of waves by antennas to make them more concentrated in a desired direction is comparable to a lens focusing light waves into a narrower beam.


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Optical Characteristics of Radio Waves: Reflection Any conducting surface looks like a mirror to a radio

wave, and so radio waves are reflected by any conducting surface they encounter.

Radio-wave reflection follows the principles of light-wave reflection.

The angle of reflection is equal to the angle of incidence.

The direction of the electric field approaching the reflecting surface is reversed from that leaving the surface. This is equivalent to a 180° phase shift.


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Figure 14-37: How a conductive surface reflects a radio wave.


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Optical Characteristics of Radio Waves: Refraction Refraction is the bending of a wave due to the physical

makeup of the medium through which the wave passes. Index of refraction is obtained by dividing the speed of

a light (or radio) wave in a vacuum and the speed of a light (or radio) wave in the medium that causes the wave to be bent.


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Optical Characteristics of Radio Waves: Refraction The relationship between the angles and the indices of

refraction is given by a formula known as Snell’s law:

n1 sin Θ1 = n2 sin Θ2

where

n1 = index of refraction of initial medium

n2 = index of refraction of medium into which wave passes

Θ1 = angle of incidence

Θ2 = angle of refraction


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Figure 14-38: How a change in the index of refraction causes bending of a radio wave.


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Optical Characteristics of Radio Waves: Diffraction Diffraction is the bending of waves around an object. Diffraction is explained by Huygen’s principle:

Assuming that all electromagnetic waves radiate as spherical waveforms from a source, each point on a wave front can be considered as a point source for additional spherical waves.

When the waves encounter an obstacle, they pass around it, above it, and on either side.

As the wave front passes the object, the point sources of waves at the edge of the obstacle create additional spherical waves that penetrate and fill in the shadow zone.


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Figure 14-39: Diffraction causes waves to bend around obstacles.


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Radio-Wave Propagation Through Space The three basic paths that a radio signal can take

through space are: Ground wave Sky wave Space wave


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Radio-Wave Propagation Through Space: Ground Waves Ground or surface waves leave an antenna and

remain close to the earth. Ground waves actually follow the curvature of the earth

and can travel at distances beyond the horizon. Ground waves must have vertical polarization to be

propagated from an antenna. Ground-wave propagation is strongest at the low- and

medium-frequency ranges. AM broadcast signals are propagated primarily by

ground waves during the day and by sky waves at night.


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Figure 14-40: Ground or surface wave radiation from an antenna.


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Radio-Wave Propagation Through Space: Sky Waves Sky-wave signals are radiated by the antenna into the

upper atmosphere, where they are bent back to earth. When a radio signal goes into the ionosphere, the

different levels of ionization cause the radio waves to be gradually bent.

The smaller the angle with respect to the earth, the more likely it is that the waves will be refracted and sent back to earth.

The higher the frequency, the smaller the radiation angle required for refraction to occur.


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Figure 14-41: Sky wave propagation.


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Radio-Wave Propagation Through Space: Space Waves A direct wave, or space wave, travels in a straight line

directly from the transmitting antenna to the receiving antenna.

Direct-wave radio signaling is often referred to as line-of-sight communication.

Direct or space waves are not refracted, nor do they follow the curvature of the earth.

Line-of-sight communication is characteristic of most radio signals with a frequency above 30 MHz, particularly VHF, UHF, and microwave signals.


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Figure 14-42: Line-of-sight communication by direct or space waves.


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Radio-Wave Propagation Through Space: Space Waves Repeater stations extend the communication distance at

VHF, UHF, and microwave frequencies. A repeater is a combination of a receiver and a

transmitter operating on separate frequencies. The receiver picks up a signal from a remote transmitter,

amplifies it, and retransmits it (on another frequency) to a remote receiver.

Repeaters are widely used to increase the communication range for mobile and handheld radio units.


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Radio-Wave Propagation Through Space: Space Waves In a trunked repeater system, multiple repeaters are

under the control of a computer system that can transfer a user from an assigned but busy repeater to another, available repeater, thus spreading the communication load.

Communication satellites act as fixed repeater stations.

The receiver-transmitter combination within the satellite is known as a transponder.


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Common Propagation Problems: Fading Fading is the variation in signal amplitude at the

receiver caused by the characteristics of the signal path and changes in it.

Fading typically makes the received signal smaller. Fading is caused by four factors:

1. Variation in distance between transmitter and receiver. 2. Changes in the environmental characteristics of the

signal path. 3. The presence of multiple signal paths.

4. Relative motion between the transmitter and receiver.


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Common Propagation Problems: Diversity System A diversity system uses multiple transmitters,

receivers, or antennas to mitigate the problems caused by multipath signals.

With frequency diversity, two separate sets of transmitters and receivers operating on different frequencies are used to transmit the same information simultaneously.

Space or spatial diversity uses two receive antennas spaced as far apart as possible to receive the signals.

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Antennas wave pro frenzel