Principles of Naval Weapons Systems Edited by CDR Joseph Hall pot

So for now we will stay with sine waves, which have the following parameters: Wavelength: the distance between parts of the wave that repeat themselves.. Chapter 1 Waves17For electromagn

Principles of Naval Weapons Systems

Edited by CDR Joseph Hall, USN

weapons system, which is the complete set of interrelating pieces that function together to

achieve the goal of destroying a target

The complete description of a weapons system must include all of the means of

exchanging information between sub-systems, called communication systems; all means used to locate the target, called sensors; all means used to store, launch and deliver the weapon to the target, called delivery sub-systems; and all means used to inflict damage upon the target, called destruction sub-systems.

In this book, we will discuss how the various sub-systems function The goal is to

understand the principles of operation of many different weapons systems It is expected

that the reader will supplement this material with one of the many fine books describingthe arsenal of weapons currently in use by the major militaries of the world

To understand how a complicated device such as a weapons system works, it isoften helpful to perform some level of abstraction first In some ways, this is also howweapons systems are designed The abstraction is simply to ask “what are the inputs andoutputs?” This question can be applied at many different levels To illustrate this concept,consider the overall weapons system The inputs come from the target and/or an operator.The output is the destructive force that damages the target

At the next level of abstraction, the roles of the major sub-sections can beprescribed The sensor sub-section takes the signals from the target and outputs thelocation and direction of movement of the target to the delivery system The deliverysystem’s output is to put the weapon in close proximity to the target Finally, given somesmall separation from the weapon to the target, the destructive system outputs thedestructive force to the target Of course, most of this is obvious However, when thismethod of breaking systems down into smaller functional sub-systems is applied to smallerand smaller parts, it turns out to be a very useful way to understand how complicatedsystems work Once the roles of the various sub-systems are understood, then the details

of its operation can be put into context This is the approach taken here

Chapter 2 _21

Basic Phenomena _ 21

Reflection 23 Refraction 24 Interference _25 Diffraction 26

Antennas _ 27

The Dipole Antenna 27 Polarization _28 Antenna Beam-forming _30

Modes of Propagation in Air _ 32

The Electromagnetic Spectrum _32 Ground Waves 32 Sky Waves 33 Line of Sight 36

Chapter 3 _39

Basic Components 39

Transmitter _40 The Transmission Channel _40 The Receiver 41

Modulation 41 Noise _ 42

Broadband (White) Noise 42 Narrowband (Interference) Noise 42

Chapter 4 _45

Amplitude Modulation (AM) _ 46

An AM system _ 48 The AM Spectrum 49

Bandwidth 49 Efficiency 51

Frequency Modulation (FM) _ 54 Bandwidth of FM 55 Immunity to Static 56 Phase Modulation (PM) _ 56 Single Side-band (SSB) 57

Chapter 5 _61

The Binary Representation _ 62

The Binary number system _62 Bits, bytes and words _64

Basic Components 64

Central Processing Unit (CPU) 65 Bus _68 Memory 69 Input and Output (I/O) 71

Chapter 6 _73

Digital Data with Analog Signals 74

Amplitude Shift-Keying (ASK) _74 Frequency Shift-Keying (FSK) 75 Phase Shift-Keying (PSK) _75 M-ary Frequency/Phase Keying _75 Amplitude-Phase Keying 76 Capacity _76 Minimum Shift Keying (MSK) _77

Analog Data with Digital Signals 77

Sampling _78 Encoding _78

Local Area Networks (LAN) _ 83

Topology _83 Protocols _84

Wide Area Networks (WANs) 84

Fixed Wide Area Networks _84 Cellular Networks 85 Satellite Networks 85

Chapter 8 _87

Principles of Operation 87 Mechanization _ 89 Radar performance _ 95

Pulse Width _95 Pulse Repetition Frequency (PRF) _97 Radar Frequency _98

Theoretical Maximum Range Equation _ 99

Chapter 9 101

Principle of Operation 101 Frequency Modulated Continuous Wave (FMCW) radar _ 104

Chapter 10 _109

Radial Velocity Discrimination _ 109

Differentiation 109 Moving Target Indicator (MTI) 110 Pulse Doppler Radar _112 Limitations 113

High Resolution Radar _ 115

Pulse Compression 115 Synthetic Aperture Radar (SAR) _116 Inverse Synthetic Aperture Radar (ISAR) 118

Phased Array Radar _ 119

Chapter 11 _123

Radar Servo Tracking System _ 123 Range Tracking _ 128 Track-While-Scan (TWS) _ 129 Phased-Array Tracking 132 Tracking Networks 133

Chapter 12 _135

The Electromagnetic Spectrum 135

Sources of Electro-Optical Radiation _ 137

Thermal Radiators 137 Selective Radiators 141

Chapter 13 _145

Spreading 146

Point Source _146 Lambertian Source 147

Maximum Range Equation 154

Chapter 14 _157

The Field of View (FOV) _ 157 Depth of Focus 160 Scanning vs Staring Sensors 161 Resolution _ 163

Spatial Resolution _163 Thermal Resolution _164 Spatial and Thermal Resolution, MRTD _164

Infrared Search and Tracking (IRST) Systems _ 166

Triangulation (passive) _166 Laser range-finder (passive-active combination) _167

Visible Band Imaging Systems 167

Magnification 167 Stadimeter Ranging _170 Light Amplification _171 Laser Target Illumination _172

Chapter 15 _175

Acoustic Waves _ 176

Propagation Speed 177 Sound Pressure Level (SPL) _178 Combining Sound Pressure Levels 179

Propagation Paths _ 181

The Sound Velocity Profile (SVP) 181

Detection of Acoustic Energy 197

Beam-forming 197 Detection Criterion 198

Transmission Loss Formula _ 199

Range Effect _199 Absorption/Scattering 202 Other losses 203

Figure of Merit _ 204

Chapter 17 _209

Active Sonar _ 209

Transmitter 209 Transducer Array _210 Beamforming Processor 210 Duplexer 211 Synchronizer _211 Receiver _211 Display _212

Passive Sonar Systems _ 213

Hydrophone Array 213 Beamforming Processor 214 Broadband Display 216 Frequency Analyzer _216 Narrowband Display _217

Variable Depth Sonar (VDS) 219 Towed Array Sonar Systems (TASS) _ 219 Sonobuoys _ 221 Bistatic Sonar 222 Non-Acoustic Detection _ 223

Visual 223 Radar _223 Infrared Detection _223

Chapter 19 _233

Initial (Boost) Phase _ 234 Mid-course Phase _ 236 Terminal Phase _ 236

Chapter 20 _241

Global Positioning System 241

Principle of Operation 241 System Components _244

Inertial Navigation Systems _ 246

Principle of Operation 246

Chapter 21 _251

Interior Ballistics 251

Propellant Types 252 Rifling 255

Exterior Ballistics _ 257

Gravity _257 Aerodynamic Forces _257 Aerodynamic Stability _258 Drift 260 Coriolis Force 260

Aiming Errors 261

Chapter 22 _263

Explosive Reactions 263 Strength of Explosives 266 Categories of Explosives 268 Initiation of the Explosive Reaction _ 268

Chapter 23 _271

Warhead Construction _ 271

Blast Effects 272 Predicting Blast Effects _ 274 Fragmentation Warheads _ 276

Reduction in Velocity with Range 278

Chapter 24 _281

Warhead Reliability _ 281 Probability of Kill (P k ) _ 284 Circular Error Probable 285 Levels of Damage _ 285 Damage Criteria for Blast Effect Warheads 286 Damage Criteria for Fragmentation Warheads _ 289

Personnel 289 Aircraft _290 Armored Vehicles _290 Probable Number of Fragments Hitting the Target _290

Chapter 27 _307

Principles of Operation _ 307

The Difference Between Fission and Fusion Reactions 308

Mechanization of a Fission Warhead 310

The simplest warhead design: gun-type _311 Improvements of fission warhead design _312

Mechanization of a Fusion Warhead 313

Air Burst 322 High Altitude Burst 322 Surface Burst _322 Underground Burst 323 Underwater Burst _323

Nuclear Attack Defensive Measures _ 323

Sequence of Events 323 Defensive Measures: Priority of Actions _323

Chapter 29 _325

Types of Radiation 326

Gamma _326 Neutron _326 Beta 327 Alpha _327

Measuring Radiation _ 328

Units of measure 329 Somatic Effects 330 Sub-Clinical Dose, 0 to 100 rem 331 Therapeutic Dose, 100 to 1000 rem _331 Palliative Dose, > 1000 rem _332

Probable Health Effects resulting from Exposure to Ionizing Radiation _ 332 Protection Standards _ 333

Chapter 30 _341

Damage Criterion _ 341

Diffraction Loading _341 Drag Loading 342 Fires/Burns 342 Radiation 343

Predicting Nuclear Blast Effects 343

Thermal Radiation Prediction _ 347 Radiation Dose Prediction 348

Chapter 1 Waves11

Chapter 1 Waves

Figure 1-1 Energy for this wave comes form work done by the wind and gravity on the water offshore.

Basic to the process of communication is the ability to transmit or transfer

information This transfer is accomplished by propagating waves of energy Therefore, tounderstand how communication systems work, we must first review the basics of waves.The wave itself is some disturbance in a parameter that varies over its spatial dimensions.The movement of the disturbance that maintains its shape (or changes slowly) is known aspropagation The information that is to be exchanged is carried in one or more parameters

of the wave

The Parameters of a Wave

If we were to take a “snapshot” image of a simple wave and capture its shape at a

particular instant in time, we could make a plot of the value, or amplitude, of the

disturbance as it varies over some distance

Chapter 1

Figure 1-2 Sine Wave.

In this example, the disturbance is periodic, meaning it repeats itself at regularintervals This is just a plot of the Sine function, y = Sin(x) Sometimes, this is called asine wave or sinusoidal wave A wave need not be periodic or smoothly varying like thesine wave, but as we will discuss later, the set of all sine waves forms a basis for thedescription of any wave So for now we will stay with sine waves, which have the

following parameters:

Wavelength: the distance between parts of the wave that repeat themselves In ourexample, the wavelength would be 1 unit on the horizontal axis Wavelength is almostalways represented by the symbol λ (lambda)

Amplitude: the magnitude of the peak deviation from the average value of the

disturbance In this case, the value of the disturbance is plotted as “y” on the vertical axis.Since the wave varies about the value y=0, the amplitude is 0.5 units You can also seethat this is one-half of the peak-to-peak difference The units of amplitude depend on thenature of the wave A disturbance may be a physical displacement, like the height of thewater in an ocean wave The disturbance may be an electric or magnetic field, in whichcase they are called electromagnetic, or e-m waves If the disturbance is a change in thelocal pressure, the waves are called acoustic, or sound waves In the following figure theamplitude is labeled as “A.”

Chapter 1 Waves13

Figure 1-3 The parts of a sine wave.

Figure 1-4 Phase difference between two waves.

There is one more parameter necessary to completely describe this wave, butwhich cannot be readily understood from the current picture Suppose we look at thesetwo waves with the same wavelength and amplitude Although they have the same shape,they differ in the starting point This parameter is called the phase For a single wave, it isirrelevant because it corresponds to the arbitrary choice for the location of the horizontal

positive phase shift ⇔ advanced wavenegative phase shift ⇔ retarded wave

As with any physical quantity, the phase shift must have units We might betempted to use the same units as wavelength, but this would not be particularly useful.What we really want to know, is the shift in the wave relative to the complete cycle.Phase shift therefore has the units of an angle, because the sine wave is based on a circleand repeats every 3600 This is a natural choice, since the argument of the sine functionmust already have units of angle So phase shift may vary between 00 and 3600 Thephase shift of 00 is no shift, and 3600 corresponds to a shift of exactly one wavelength,

which is again equivalent to no shift at all At 3600, we start over with 00

You might also note that a negative phase shift of -φ degrees is the same as thepositive phase shift of 3600-φ These are just two equivalent ways to describe the

direction on a circle, one going counter-clockwise and the other clockwise

Chapter 1 Waves15

Figure 1-5 The equivalence of positive and negative phase shifts.

Finally, the units of phase are sometimes expressed in radians, which actually have nounits To convert between degrees and radians you only need remember that there are 2π

radians in a 3600 circle So:

radians = (2π/3600) x degreesdegrees = (3600/2π) x radians

The conversion factor is about 57.30 per radian, or if you are in a hurry, ≈600

As we will see, the concept of phase shift is key to the operation of antennas and sonararrays The last parameter of the sine wave is:

Phase: the angular argument (in degrees or radians) at which the sine wave begins, asdefined relative to some arbitrary starting point The symbol for phase is either φ or θ.Phase shift is sometimes represented by ∆φ or ∆θ (the Greek symbol ∆ , capital “delta,” isused to denote change in a quantity)

− φ

at a particular location and observe the wave, the disturbance would vary in time, identical

to the way it varied over distance in our “snapshot” view Distance and time are related

by the speed of propagation, v The wavelength now has an equivalent parameter,

corresponding to the interval of time between cycles, known as the period The period isoften given the symbol T A related parameter is the frequency

Frequency: the number of complete cycles of a periodic wave that pass a stationary point

in one second The frequency, f, is simply the inverse of the period, f = 1/T Frequencyhas units of Hertz (Hz), where 1 Hz = 1 cycle per second

Since the wave is propagating with speed v, the frequency (or period) and

wavelength are related to one another If the speed of propagation is independent ofwavelength of frequency, we have the following relationship:

fλ = cThis is also known as the dispersion relationship, because it describes how the speed ofpropagation varies with frequency For instance, a prism disperses light into separatecolors (frequencies) because they propagate at different speeds Components with lowerspeeds of propagation bend by a larger amount when they enter a new medium (this isSnell’s law which is discussed in a later chapter) This causes the different frequencycomponents (colors) to be separated or dispersed

prism white (all colors) light

red yellow green blue

Figure 1-6 How a prism disperses light.

Chapter 1 Waves17

For electromagnetic (e-m) waves (light also being an electromagnetic wave) the speed ofpropagation in space (or a perfect vacuum) is constant Its velocity is the speed of light,designated by the constant c, and has the following value for our purposes:

c = 3 x 108 m/s

The speed of light does not change greatly in most materials, especially air, and thereforeunless otherwise stated this value will also be used for the propagation of e-m waves inair

Frequency vs Time Domain

When we plot the wave’s amplitude as a function of time (at a fixed location), we areusing the time domain description This means nothing more than the fact that the

horizontal axis has units of time The wave has parameters of amplitude and period

(disregard phase for now) As an alternate way to describe the very same wave, you onlyneed specify the amplitude and frequency Clearly, these two descriptions are equivalentfor the simple sine wave we have been using as an example

Now, real waves often have very complicated shapes (in the “snapshot” view) Itcan be shown that most periodic waves can be constructed from a combinations of sineand cosine waves with different frequencies In order to construct the wave, one only needknow the relative amounts of each frequency component In other words, the amounts ofeach frequency are like a recipe card for constructing wave forms The reverse operation

of finding the frequency components that make up a particular wave is called the spectraldecomposition or frequency analysis

The description of a wave by specifying the value at points in time is the

representation in the time domain If you specify the amount of each frequency

component (for sine and cosine waves), then the representation is in the frequency

domain They should be thought of as two completely equivalent methods of representingthe same thing Now, let’s look at a simple cosine wave in both representations

Figure 1-7 Time and Frequency Domain representation of cosine wave.

In the time domain, the cosine wave completes 8 cycles in 1 second In the frequencydomain, we have a single component at 8 Hz

The real value of the frequency domain will come with signals that appear

complicated in the time domain, but are simple combinations of sinusoidal waves

Consider another example, but this time the signal is a simple combination of sine wavesbut with different frequencies

2

1.5 2.5

0.5

Figure 1-8 A more complicated wave.

Chapter 1 Waves19

The signal on the left appears slightly more complicated, and it would be difficult to

determine the nature of the signal In the spectrum we see that it is a simple combination

of three frequency components The strongest component is still at 8 Hz, with additionalcontributions from 16 and 32 Hz Because these higher frequencies are multiples of theoriginal frequency they are known as harmonics The relative power levels are the square

of the relative contributions to the amplitude In this case, the 16 Hz contribution is 1/3 aslarge as original at 8 Hz Likewise, the 32 Hz contribution is 2/3 as large Here’s howthese numbers where obtained:

Chapter 2 Propagation of Waves21

Chapter 2 Propagation of Waves

Figure 2-1 Communications satellite.

The process of communication involves the transmission of information from onelocation to another using modulation to encode the information onto a carrier wave It isonly the characteristics of the carrier wave that determine how the signal will propagateover any significant distance This chapter describes the different ways that

electromagnetic waves can propagate

Basic Phenomena

An electromagnetic wave is created by a local disturbance in the electric andmagnetic fields From its origin, the wave will propagate outwards in all directions If themedium in which it is propagating (air for example) is the same everywhere, the wave will

spread out uniformly in all directions, called a spherical wave.

Chapter 2

Figure 2-2 Spherical wave.

Far from its origin, the wave will have spread out far enough that it will appear have thesame amplitude everywhere on the plane perpendicular to its direction of travel (in the

near vicinity of the observer) This type of wave is called a plane wave A plane wave is

an idealization that allows one to think of the entire wave traveling in a single direction,instead of spreading out like a spherical wave

Figure 2-3 Plane wave.

Chapter 2 Propagation of Waves23

Electromagnetic waves propagate at the speed of light In a vacuum, the speed ofpropagation is about 3 x 108 m/s In other mediums, like air or glass, the speed of

propagation is slower If the speed of light in a vacuum is given the symbol c, and itsspeed in some medium is c0, we can define the index of refraction, n as:

is reflected has a very simple rule governing its behavior, the angle of reflection = theangle of incidence

Figure 2-4 Reflection.

Chapter 2

As seen in Figure 2-4, the angles are defined as:

♦ Angle of Incidence the angle between the direction of propagation and a line

perpendicular to the boundary, on the same side of the surface; and

♦ Angle of Reflection the angle between the direction of propagation of the reflected

wave and a line perpendicular to the boundary, also on the same side of the surface

Incoming and outgoing waves are 180 0 out of

phase

Figure 2-5 Phase shift on reflection.

If the incident medium has a lower index of refraction, then the reflected wave willhave an 1800 phase-shift upon reflection Conversely, if the incident medium has a largerindex of refraction the reflected wave has no phase-shift

Refraction

When the wave enters the new medium, the speed of propagation will change Inorder to match the incident and transmitted waves at the boundary, the transmitted wavewill have to change its direction of propagation For example, if the new medium has ahigher index of refraction, the wavelength must be shorter (the frequency must stay thesame due to the boundary conditions) The direction of propagation in the new mediummust be closer to perpendicular The angle of transmission, shown in Figure 2-6, will beless than the angle of incidence For the general case, the relationship between the angles

Chapter 2 Propagation of Waves25

of incidence and transmission, called Snell’s Law, will depend only on the relative indices

When the direction of propagation changes, the wave is said be refracted The transmitted

wave will always bend towards the perpendicular when entering a medium that has ahigher index of refraction

Example: Why a pool is deeper than it looks

When you look into a pool, the light from the bottom is refracted away from the

perpendicular, because the index of refraction in air is less than in water To the observer

at the side of the pool, the light appears to come from a shallower depth For the samereason, when you look at objects underwater through a mask, they will appear to be largerthan they really are The light from the object is spread outwards at the water-air interface

of your mask To you it will appear the object is closer or larger

Chapter 2

strength has increased, or destructive, meaning that the field strength has decreased The

type of interference at any point depends on the phase difference between the two waves

at that point It can be shown that constructive interference occurs when the phase

difference is between 0 and 1200, or between 240 and 3600 By elimination, destructiveinterference occurs when the phase difference is between 120 and 2400 For two identical

waves, if there is no phase difference, there will be total constructive interference,

meaning the field strength will be at its maximum value If the phase difference is 1800,

there will be total destructive interference, where two waves completely cancel each other

out

The phase difference between two waves at any point can be due to either a phasedifference at the sources or a difference in distance each wave travels from its origin,

called the path length, ∆x The phase difference, ∆φ, caused by a difference in path length

is given by, ∆φ = 2π∆x/λ and conversely )x = )N8/2B

Example: Omega is an old radio navigation system that used the phase difference in thesame signal from two fixed transmitters to determine a line-of-position The same amount

of phase difference from the two transmitters corresponds to a unique line-of-position.However, since the phase shift can only have values between 0 and 360o, there is anambiguity among lines-of-position Each phase shift actually corresponds to multiple lineseach separated by a distance equivalent to 3600 of phase shift Since the frequency was10.2 kHz, the wavelength corresponding to 3600 phase shift was 16 miles, which is theseparation between ambiguous lines-of-position This can be confirmed by viewing anOmega overprinted chart Loran-C, which is another radio navigation system, also has aphase-difference mode In phase-difference Loran-C, the spacing between lines is onlyabout 3000 m, since it operates at the higher frequency of 100 kHz

Diffraction

If a wave passes through an opening, called an aperture, it will diffract, or spread

out from the opening The degree by which the wave will spread out depends on the size

of the aperture relative to the wavelength In the extreme case where the aperture is verylarge compared to the wavelength, the wave will see no effect and therefore will notdiffract at all The wave will pass directly through, but will be cropped to match the size

of the opening At the other extreme, when the opening is very small, the wave willbehave as if it were again at its origin The wave will spread out uniformly in all directionsfrom the center of the aperture In between the extremes, there will be some amount ofdiffraction

Chapter 2 Propagation of Waves27

Figure 2-7 Diffraction.

As an example, consider a circular aperture If a wave with wavelength λ

encounters an opening with diameter D, the amount of diffraction as measured by theangle, θ (in radians), at which the new wave diverges from the opening, measured fromone edge to the other, is θ≈ 2.4 λ/D If the opening was a vertical slit, the factor in frontwould be 2.0 instead of 2.4 The exact value of this factor is not important, other than it issomething on the order of two, and it varies with the exact configuration

Antennas

Antennas couple the current flowing in conductors to electromagnetic waves

propagating in air The most basic form of the antenna is called the dipole antenna.

The Dipole Antenna

A center-fed dipole antenna is constructed from two straight pieces of wire in aline When an alternating voltage is applied between the wires, current flows and theelectrical charges pile up in either piece If the top piece has a net positive charge, theother piece will have a net negative charge The separation between the charges creates

an electric field, which is oscillating in time The oscillating electric field induces an

oscillating magnetic field around the antenna The magnetic field in turn creates anotheroscillating electric field, this time further from the antenna The electric and magneticfields induce each other at greater and greater distances and therefore propagate outwards

as an electromagnetic wave The same antenna can be used to receive electromagneticwaves When the electromagnetic wave passes over a conducting material in the antenna,

an oscillating current will be induced

Figure 2-8 Dipole antenna.

For a center-fed dipole antenna to work most effectively, it should be exactly half wavelength long Receiving antennas, which do not require high sensitivity, need notfollow this rule Transmitting antennas on the other generally do, except at very lowfrequencies When the antenna is placed in the ground, called a ground-plane antenna, theoptimum size is reduced by half again, due to signal reflection at the ground plane Thisappears to make an image antenna of equal size below the ground that reduces the actualantenna requirement So for ground-plane antennas, the optimum size is one-quarterwavelength

one-Example: Find the optimum antenna size for a ground-plane dipole used to broadcastcommercial AM radio (approximately 1 MHz) The wavelength at 1 MHz is 300m , sothe optimum antenna should be about 75 m tall

Polarization

The orientation of the fields in the wave is called the polarization In a dipole

antenna, the original electric field is oriented along the axis of the antenna and thereforethe induced magnetic field will be perpendicular to both the electric field and the direction

of travel As the wave propagates outwards, the electric and magnetic fields will remainperpendicular to each other They will also be perpendicular to the direction of

propagation

Chapter 2 Propagation of Waves29

E

B

Figure 2-9 Polarized fields.

When the field remains in a particular direction, as in the case of waves from the

dipole antenna, the wave is considered to be linearly polarized The direction will be

aligned to the antenna A vertical antenna will create a vertically, linearly polarized

electromagnetic wave A receiving antenna that is aligned with the polarization will havethe greatest sensitivity

Example: Commercial radio broadcasts come from large vertically oriented antennas.Therefore they are linearly vertically polarized signals and are best received by a verticalantenna So, to maximize reception of a radio signal, hold the antenna upright

Linear polarization is not the only possibility Another type is circular

polarization The best way to visualize this is like a corkscrew The electric field rotates

as it travels along If the rotation is clockwise as seen looking in the direction of

propagation, it is called right-hand circular polarization (RHCP) The other possibility isLHCP Antennas for circular polarization may look like corkscrews or pairs of dipoles atright angles Circular polarization is often used in satellite communications because iteliminates the need to try and match the receiving antenna to the orientation of the

satellite’s antenna

Some waves are not polarized at all For example, sunlight is homogeneous

mixtures of waves will all orientations It is said to be un-polarized It can however,become polarized either by filtering or upon reflection from a flat surface

Example: polarized sunglasses

Chapter 2

When sunlight is reflected off the road it appears as glare In the process of

reflection the light becomes horizontally polarized Sunglasses with vertical polarizationblock this component and therefore reduce glare These glasses can easily be checked tosee if they are polarized by holding two pairs at right angles In this case, all possibleorientations of linear polarization will be blocked and the lenses will appear opaque

Antenna Beam-forming

The dipole antenna we have been discussing radiates its energy in all directionsperpendicular to its axis There can be no signal coming from the ends, however In thissense, the dipole antenna has some directionality, or preferred direction In cases wherehigh sensitivity is required or when it is necessary to exclude transmission or receptionfrom unnecessary directions, antennas can be made even more directional The process of

creating and controlling directionality in antennas is called beam-forming It has

wide-spread applications in communications, radar and sonar

Beam-forming should be understood as the exploitation of interference Forexample, consider two identical receiving dipole antennas, both oriented vertically in theground, whose output is combined Since the dipole is only sensitive perpendicular to itsaxis, neither antenna will receive signals at any significant vertical angle Each antenna byitself, has no preferred direction in the horizontal plane Suppose now that they areseparated by exactly one-half of a wavelength of the signal they are receiving If theyreceive a signal coming from a direction along the line that connects them, there will be a

1800 phase-shift between their outputs, which will cause complete cancellation Thereforethey cannot receive signals along the line connecting them

out of phase:

destructive interference

λ/2

Figure 2-10 Two-antenna linear array.

Chapter 2 Propagation of Waves31

If the signal is coming from a direction perpendicular to the line connecting them, therewill be equal path lengths and therefore no phase-shift Their outputs will combine attwice the strength The combination of the two antennas will be more sensitive to signalscoming from a direction perpendicular to the line connecting them Therefore, the twoantenna array will have directionality in the horizontal plane As it turns out, the three-dipole antenna linear array is even more directional There is no reception from the

directions along its axis, and a more narrow region perpendicular to the array from which

it receives strongly The width of good reception is called its beamwidth.

For a many-dipole linear array, the beamwidth gets smaller proportionally as thenumber of elements increases If the overall array length is L, the beamwidth can bepredicted theoretically:

Region of constructive interference

Region of destructive interference

θ

Figure 2-11 Beams formed by two-antenna linear array.

Chapter 2

Example: Direct Satellite TV This system uses an 18” reflecting dish to receive signalsfrom geo-synchronous satellites, located at 101 W and near the equator The signal is Ku-band at about 12.5 GHz (2.4 cm wavelength), and is circularly polarized The beamwidth

of the 18” (44 cm) is ≈ 2 x 2.4/44 = 0.11 radians or ~60 This would imply that the

antenna should be positioned within less than six degrees of the line of sight to the

satellite The beamwidth is made as small as possible to maximize the sensitivity of theantenna A larger dish would have smaller beamwidth and therefore would be moresensitive but would require a more accurate aim

Modes of Propagation in Air

The Electromagnetic Spectrum

Frequency Range Band Designation

other The ionosphere is the region above the troposphere (where the air is), from about

50 to 250 miles above the earth It is a collection of ions, which are atoms that have some

of their electrons stripped off leaving two or more electrically charged objects The sun’srays cause the ions to form, after which they slowly recombine The propagation of radiowaves in the presence of ions is drastically different from propagation in air This is whythe ionosphere plays an important role in most modes of propagation Ground wavestravel between two limits, the earth and the ionosphere, which acts like a duct Since theduct curves with the earth, the ground wave will follow Therefore very long rangepropagation is possible using ground waves

Chapter 2 Propagation of Waves33

Figure 2-12 Ground wave propagation.

Sky Waves

Radio waves in the LF and MF ranges may also propagate as ground waves, butsuffer significant losses, or are attenuated, particularly at higher frequencies As the

ground wave mode fades out, a new mode develops, the sky wave Sky waves are

reflections from the ionosphere While the wave is in the ionosphere, it is strongly bent, orrefracted, ultimately back to the ground From a long distance away this appears as areflection Long ranges are possible in this mode also, up to hundreds of miles Skywaves in this frequency band are usually only possible at night, when the concentration ofions is not too great since the ionosphere also tends to attenuate the signal However, atnight, there are just enough ions to reflect the wave but not reduce its power too much

Chapter 2

Sky Waves Ionosphere

Figure 2-13 Sky wave propagation.

The HF band operates almost exclusively with sky waves The higher frequencieshave less attenuation and less refraction in the ionosphere as compared to MF At thehigh end, the waves completely penetrate the ionosphere and become space waves At thelow end, they are always reflected The HF band operates with both these effects almostall of the time The characteristics of the sky wave propagation depend on the conditions

in the ionosphere, which in turn are dependent on the activity of the sun

The ionosphere has several well-defined regions as shown in Figure 2-14

100 200 300 400 500

D-region E-region F-region

Figure 2-14 Regions of the ionosphere.

Chapter 2 Propagation of Waves35

D-region: about 75-95 km Relatively weak ionization Responsible for strong absorption

of MF during daylight

E-region: 95-150 km An important player in ionospheric scattering of VHF

F-region: 150-400 km Has separate F1 and F2 layers during the day The strongestconcentration of ions Responsible for reflection of HF radio waves

Since the propagation characteristics depend on frequency, several key frequenciescan de defined

Critical frequency

Critical frequency is the minimum frequency that will penetrate the ionosphere at

vertical incidence The critical frequency increases during the daylight and decrease atnight At other angles, the wave will be reflected back At frequencies above the criticalfrequency, some range of waves from vertical incidence and down will become space

waves This will cause a gap in coverage on the ground known as a skip zone In Figure

2-13, the skip zone extends to about 1400 miles The transmitted frequency is 5 MHz andthe critical frequency is 3 MHz

Maximum Useable Frequency (MUF)

Maximum useable frequency is defined for two stations It is the maximum frequencythat will reflect back to the receiving station from the transmitter When the frequency

exceeds the MUF, the wave will completely penetrate the ionosphere and become a space

wave When the frequency equals the MUF, the skip zone extends to just short of the

receiver In Figure 2-13, the MUF for a receiver at 1400 miles is 5 MHz

Lowest Useable Frequency (LUF)

The lowest useable frequency is again defined only between two stations If the

frequency is too low, the signal will be attenuated before it can be reflected The LUFincreases with sunlight and is a maximum near noon

Optimum Frequency for Traffic (OFT)

For two stations, taking into account the exact conditions in the ionosphere, there will bethe perfect frequency that gives the strongest signal This can be predicted by powerful

Chapter 2

modeling programs and is the best guarantee of success in HF This frequency is called

the optimum frequency for traffic Because the ionosphere is affected by exposure to

sunlight, there is a diurnal variation in propagation The diurnal variation in HF

propagation is characterized by a simple rule-of-thumb, OFT follows the sun Forexample, at noon the OFT is generally higher than at night

Line of Sight

In the VHF band and up, the propagation tends to straighten out into line-of-sight (LOS)waves However the frequency is still low enough for some significant effects

Ionospheric scatter

In this mode, the signal is scattered in all directions by the E-region of the

ionosphere Some of the energy makes its way back to the earth’s surface and can bereceived effectively up to ranges of 600-1000 miles

Chapter 2 Propagation of Waves37

Figure 2-16 Tropospheric scatter propagation.

Tropospheric ducting

Electromagnetic waves travel slower in cold dense air than in warm air Whenever

inversion conditions exist, the wave is naturally bent back to the ground When the

refraction from the inversion matches the curvature of the earth, long ranges can be

achieved The wave acts like it is trapped in a duct just above the surface of the earth.This ducting always occurs to some extent and improves the range over the true line-of-sight by about ten percent

Maximum range for LOS propagation

Beyond VHF, all the propagation is line-of-sight Excluding space waves,

communications at these frequencies are limited by the earth’s horizon The LOS rangecan be found from the height of the transmitting and receiving antennas by:

R = 17h t + 17h R

Where ht and hr are the heights of the antennas in meters, and R will be in km (the

conversion factor is already taken into account in the factor 17) This range is slightlylarger than the visual horizon, which can be found by a similar formula, using a factor of

13 instead of 17 The difference is due to ducting