Solve wide-ranging technical and
`operational problems
`
`Install and service new types of cable,
`wiring, hubs, and transmission and
`control methods
`
`Clear coverage of new FCC regulations
`
`1
`
`Comcast, Ex. 1018
`
`

`

`- - - - ---------------------
`
`Library of Congress Cataloging-in-PubHcation Data
`
`Bartlett, Eugene R.
`Cable Television Handbook I Eugene R. Bartlett
`p.cm.
`Includes bibliographical references.
`ISN 0-07~006891-7 (book)
`1. Cable television-Handbook, manuals, etc. 2. High definition television(cid:173)
`Handbooks, manuals, etc. 3. Color television-Handbooks, manuals, etc. I.
`Title.
`TK6675.B36 1999
`621.388'57-dc21
`
`99-043784
`
`McGraw-Hill
`A Division ofTheMcGraw-HillCompanies
`
`'iZ
`
`Copyright© 2000 by The McGraw-Hill Companies, Inc. All Rights Reserved.
`Printed in the United States of America. Except as permitted under the United
`States Copyright Act of 1976, no part of this publication may be reproduced or
`distributed in any form or by any means, or stored in a data base or retrieval
`system, without the prior written permission of the publisher.
`
`1 2 3 4 5 6 7 8 9 0 DOC/DOC 9 0 9 8 7 6 5 4 3 2 1 0 9
`
`ISBN 0-07-006891-7
`
`This book was set in Century Schoolbook by D&G Limited LLC
`
`Printed and bound by R. R. Donnelley J\; Sons Company.
`
`McGraw-Hill books are available at special quantity discounts to use as premi(cid:173)
`ums and sales promotions, or for use in corporate training programs. For more
`information, please write to Director of Special Sales, McGraw-Hill, 11 West
`19th Street, New York, NY 10011. Or contact your local bookstore.
`
`Information contained in this work has been obtained by The McGraw-Hill
`Companies, Inc. ("McGraw-Hill") from sources believed to be reliable. However,
`neither McGraw-Hill nor its authors guarantees the accuracy or completeness
`of any information published herein and neither McGraw-Hill nor its authors
`shall be responsible for any errors, omissions, or damages arising out of use of
`this information. This work is published with the understanding that
`McGraw-Hill and its authors are supplying information but are not attempt(cid:173)
`ing to render engineering or other professional services. If such services are
`required, the assistance of an appropriate professional should be sought.
`
`This book is printed on recycled, acid-free paper containing a minimum
`of 50 percent recycled de-inked fiber.
`
`2
`
`

`

`CONTENTS
`
`Preface
`
`Acknowledgements
`
`Chapter 'II.
`
`Introduction
`
`Cable Television History
`The Television Reception Problem
`Early Cable Systems
`The Early Systems Evolve
`Changes in Plant Design and Development
`Development in Electronics
`Development of Coaxial Cables
`Construction
`Changes in CATV _Regulations, Requirements
`Federal Regulations, Communications Act of J 934
`Local Regulations
`Commercial and Consumer Electronic Development
`
`Chapter 2
`
`The Tree/Branch Cable System Network Topology
`
`Introduction
`A One-way System, Two-way Considerations
`Television Receivers
`The Head-end
`Antennas and Off-Air Reception
`Head-end Electronic Equipment
`Head-end Powering and Monitoring
`Satellite Systems
`The Cable Distribution Plant
`The Trunk-feeder System
`Cable System Electronics
`Construction Practices
`Subscriber Equipment and Signal Securify
`System Testing/Proof of Performance
`
`Chapter 3
`
`Fiber-Optic Technology in Cable Television Systems
`
`Introduction
`Fiber-Optic Development
`
`ix
`
`xi
`
`2
`3
`4
`6
`8
`9
`J3
`J8
`25
`25
`27
`28
`
`33
`
`34
`34
`36
`4J
`4J
`50
`55
`58
`68
`68
`74
`82
`87
`94
`
`99
`
`JOO
`JOO
`
`3
`
`

`

`------------------------------------------------~-----------~-
`
`vm
`
`Contents
`
`Light Sources and Their Development
`Optical Detectors
`Splicing and Connectorizing
`Fiber-Optic Trunking and Cable Television Applications
`Fiber-Optic Cable Overlay
`Fiber-Optic Super Trunking
`Benefits of Fiber-Optic Plant Addition
`Fiber-Optic Construction and Installations
`Handling Fiber-Optic Cable
`Admittance Testing
`Fiber-Optic Cable Installation
`Aerial Electronic Equipment
`Types of Electronic Equipment
`Optical Equipment Tests
`
`Chapter 4
`
`Digital Technology and Cable System Applications
`
`A Short History of Digital Communications
`Nature of Digital Technology
`Digital Computing and Data Storage
`Development of Data Communications
`Present Data Communication Systems
`LANs and Types Topology
`Fiber-Optic Systems
`Telephone Systems and Digital Technology
`Basic Telephone System
`Digital Telephone Methods
`Fiber-Optic Methods
`Digital Video & HDTV
`NTSC & Digital Techniques
`Digital TV Transmission
`
`Chapter 5
`
`Subscriber Installation and Terminal Devices
`
`The Subscriber Drop
`Early Installation Techniques
`Drop Aging & Deterioration
`Subscriber Converters
`Midband Block Converters
`The Selectable Converter
`The Programmable Converter
`The Addressable Converter
`The Interactive Subscriber Terminal
`
`108
`111
`112
`119
`l 19
`125
`128
`130
`131
`132
`135
`138
`138
`141
`
`143
`
`144
`144
`151
`153
`154
`155
`161
`163
`163
`170
`174
`178
`179
`184
`
`191
`
`192
`192
`205
`209
`210
`212
`213
`214
`216
`
`4
`
`

`

`Contents
`
`vii
`
`Chapter 6
`
`Cable Plant Testing and Maintenance
`
`Instruments and Measurements
`Signal Level Meters
`Spectrum Analyzers
`Cable & Passive Testing
`Cable System Tests & Measurements
`System Turn On & Balancing
`Proof of Performance Testing
`System Maintenance Measurements
`Head end Hub Testing
`Off Air Signals
`Microwave Connections
`Locally Generated Signals
`Fiber Optical Plant
`Fiber Cable
`Fiber Optical Testing
`Digital Signal Testing
`Instruments for Digital Testing
`Typical System Problems and Solutions
`System Powering & Power Supplies
`Coaxial Cable System Problems
`Fiber Optical System Problems
`Head end Problems & Maintenance
`
`Appendix A
`
`Appendix B
`
`Appendix C
`
`Appendix D
`
`Appendix E
`
`Appendix F
`
`Appendix G
`
`Appendix.H
`
`Bibliography
`
`Glossary
`
`Index
`
`219
`
`220
`220
`222
`227
`231
`231
`232
`240
`243
`243
`244
`245
`245
`245
`247
`250
`251
`257
`257
`266
`269
`271
`
`277
`
`281
`
`285
`
`359
`
`361
`
`363
`
`373
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`375
`
`383
`
`385
`
`389
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`5
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`CHAPTER
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`6
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`
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`
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`

`

`34
`
`Chapter2
`
`Introduction
`
`The basic idea of the cable television system was that the central antenna
`system would be shared among connected subscribers willing to pay a
`monthly fee. The head-end, as it was known and still is, was the source of
`the signals making up the basic service. The head-end was usually located
`on a hilltop, tall building, water tank, or steel tower located out of town
`away from the subscriber homes. The basic off-air received television sig(cid:173)
`nals were amplified, filtered, adjusted in level, and combined and delivered
`by the cable amplifier cascade to subscribers. This elementary topology
`was known as the tree-branch cable system. The main cable was the trunk
`system from the head-end to the town where the cable branched down the
`streets to the subscriber's homes.
`
`A One=way System, Two=way Considerations
`
`The basic tree-branch system consisted of cable-amplifier sections feeding
`signals one way from the receiving head-end tower site to subscribers. Many
`early cable operators were aware of possible two-way operations by splitting
`the service signal band between downstream and upstream service and
`installing upstream amplifiers. Such early thinking was basically wishful
`thinking mainly because of system problems such as temperature stability,
`signal level control, and amplifier noise and distortion. Basically, early sys(cid:173)
`tems were one-way systems and even today many systems are still essen(cid:173)
`tially one-way.
`
`Pr<JHd!.uct Delivery, Many Cll:umnels The bread and butter of cable sys(cid:173)
`tems operation was, and in many cases still is, the delivery of many cable
`television channels containing a variety of program choices to subscribers.
`When more channels were offered and cable system costs increased, sub(cid:173)
`scriber rates were subsequently increased. The concept of program tiering
`allowed subscribers a choice of programming packages such as news,
`sports, and movies: The source of these programs came about with the intro(cid:173)
`duction of satellite technology. Presently, the number of program channels
`available from various satellite systems is enormous.
`The two problems cable system operators have in today's market are (1)
`adequate system bandwidth needed for an increase in channels and (2)
`making the decision as to what programming will sell best. To repeat, the
`bread and butter of cable system operations is the delivery of television
`
`7
`
`

`

`able Systems and Network Topology
`
`35
`
`programming to subscribers. It was thought at one time that auxiliary ser(cid:173)
`vices such as electric meter reading and alarm systems would support two(cid:173)
`way cable operations. The cost of the reverse system activation is a
`significant expense compared to projected revenues for the services.
`
`Service Area Specifications The problem of identifying the service
`area of a cable television system is a difficult one. First and foremost is the
`fact that the centers of residential pop:uJation are the criteria for successful
`subscriber penetration. Unfortunately, the licensing or franchising of
`municipal authorities may impose requirements that non-productive areas
`still be served. Also, future areas of residential expansion may not be read(cid:173)
`ily identified, thus necessitating future plant expansions.
`Most cable system operators avoided building cable plants in industrial
`and commercial areas. Thus, many cable operators have had to build more
`plants to provide information services to these areas. The location of the
`subscriber office area, the technical maintenance area, and the head-end
`location can also be factors in planning and specifying the service area.
`Essentially, the service area has to be specified before financing and con(cid:173)
`struction plans can commence. Thus, as the saying says, do the homework
`carefully and diligently. Technical people as well as managerial people
`should be involved in this decision-making process. By estimating the miles
`of the plant, the feeder-to-trunk ratio, and the number of program channels,
`the cost per mile of plant and the head-end costs can be projected. Now the
`basic revenue can be projected, producing a cash-flow or financial pro-forma
`to offset debt and support the venture. This process is complicated and has
`to be carefully completed before any construction schedule can be made.
`
`Expansion of Feeder Criteria The proper plant design required to
`serve the more heavily populated area of many communities should not be
`so trunk-restrictive that businesses will box themselves in by making it dif(cid:173)
`ficult to extend the distribution cables to new areas. In short, a system does
`not want to run out of trunk that can restrict extending the feeder plant. It
`must be remembered it is the feeder or distribution plant that connects the
`cable system to subscribers. The trunk is considered as the transportation
`system bringing the service to the distribution plant.
`By now, it should be obvious that the feeder/distribution plant makes the
`money, while the trunk cable plant is a necessary cost. The extension of a
`feeder plant adds more subscribers, thus improving the cash flow. Since the
`cost of extending a feeder plant is significantly less than the trunk plant
`costs, a careful analysis has to be made when extending a plant. Often a
`
`8
`
`

`

`36
`
`c_::~J CC::J :··· .. : __ J
`Figure 2-'il
`Trunk extension
`required to extend
`service area
`
`Power
`Block
`
`\
`-----1
`
`<l--o~-
`
`/
`
`End of
`Trunk
`
`Chapter 2
`
`Note: Signal level
`has to be at least the
`minimum amplifier
`input level. If too
`large, amplifier 14
`can be placed further
`down.
`
`Extension
`__ _,fjji>.
`
`0-t>--
`Note: If more
`service is required
`beyond this point,
`trunk lines can be
`extended from
`Amplifier 13.
`
`(60% Full Load) I Power Supplies.
`
`__ :
`
`:_ __ _J
`
`:::~1
`
`Figure 2-2
`Added trunk requires
`added system power
`supplies
`
`12 Amp Supply
`
`Trunk Extension
`beyond his point
`requires more
`
`.
`l~A--~---B>-1
`
`Amps
`.
`
`2 Trunk
`Amps
`
`Extension
`---Ilia>-
`
`Added Power
`Supply
`
`significant extension of the feeder plant means adding some more trunk
`plants. This is shown in Figure 2-1, where feeder plant amplifier cascades
`are limited to two amplifiers in a cascade off the bridger trunk amplifier.
`Adding more cable amplifiers to a system also means that system pow(cid:173)
`ering may have to be restructured with the addition of more power supplies
`and resegmenting the powering system. Repowering such plant additions is
`shown in Figure 2-2.
`
`Television Receivers
`
`The main receiving device of a cable television system is the subscriber's
`television receiver. These receivers can range from new to almost museum-
`
`9
`
`

`

`Tree/Branch Cable Systems and Network Topology
`
`37
`
`piece television sets. However, since television sets have an average life of
`about 10 years, it is fairly safe to assume that most subscriber sets range
`from new to 10 years old. The median age range is estimated to be about
`four or five years old. Also, almost all of the television sets are color TV sets.
`Since cable systems carry the service in standard NTSC VSB format, the
`television receives the signals and makes the channel selection by
`frequency-synthesized tuners, the same as if it were an off-air TV station.
`Since cable system channels can be offset in channel frequencies in the
`HRC or IRC format, television sets made in the last 10 to 12 years have a
`switch often appearing on the rear cover or selected by the remote control
`labeled Cable 1 or Cable 2. This switch controls the action of the channel
`selector to tune to the desired channel settings.
`The present-day television set is devoid of horizontal and sometimes ver(cid:173)
`tical hold controls due to vastly improved circuitry that locks the picture
`synchronization. Automatic chroma controls and contrast controls keep the
`pictures at the same color and brightness. The remote control system can
`usually address all picture quality controls, channel selection, and video
`source selection, making most of us the so-called couch potatoes.
`Integrated circuit technology, circuit board construction, and modular
`tuning, along with the remote control system, have reduced the cost of tele(cid:173)
`vision sets drastically. Still the picture tube or the projection system deter(cid:173)
`mines the cost of the receiver. All of the electronic systems of a number of
`present-day television sets are valued at $40 to $50. The cabinet and
`screen/tube system results in most of the cost of a TV set. Since the sub(cid:173)
`scriber's television set is what produces the cable system's picture, it is most
`important that cable operators make sure the cable installer makes the
`proper connections to the TV set and instructs and advises the subscriber
`in getting the most out of the service.
`
`Channel Tuning As stated earlier, the television set has to tune pre(cid:173)
`cisely to the cable channels for best signal and sound quality. The noise
`characteristics of the television set tuner are most critical. The noise figures
`of early TV sets were quite poor and the UHF portion even worse. As the TV
`sets improved, the noise figures of the channel-selecting tuners did as well.
`The IF bandwidth characteristics improved also, which in turn gave better
`pictures with improvements in the quality of TV receiver design. The pic(cid:173)
`ture resolution improved from about 260 lines.to about 380 to 400. With the
`arrival of HDTV; picture resolution will indeed improve again.
`Since HDTV will be digital, as proposed and scheduled in the next cen(cid:173)
`tury, the consumer electronic manufacturers are going to be busy designing,
`developing, and testing TV sets for this new service. The broadcasting
`
`10
`
`

`

`38
`
`Chapter2
`
`companies are busy getting new studio and transmitting equipment capa(cid:173)
`ble of handling the digital signals, readying the public for HDTV service
`where no TV HDTV sets exist. The trials of off-air digital television services
`will be taking place soon. Then maybe the problem areas will be identified.
`As yet no signal strength parameters, signal-to-noise specifications, or the
`tolerance-to-multipath reception information has resulted. In short, no one
`knows if digital television broadcast stations can be received with rabbit(cid:173)
`ear antennas or will be back to roof-top antennas.
`As far as the cable operator is concerned, questions remain. First, will
`cable head-ends receive digitally modulated signals from a tower-mounted
`antenna? Second, will cable systems convert some of the signals to other
`modulation schemes or to NTSC VSB format? Third, what will be the must(cid:173)
`carry rules as set forth by the FCC for broadcast stations transmitting mul(cid:173)
`tiple channel digital signals? And the list seems to go on and on.
`
`Channel Converters The channel converter either rented or purchased
`from the cable operator has traditionally been required to provide cable
`television channels unable to be selected by standard TV sets. For some
`early cable systems that placed added programming channels in the so(cid:173)
`called mid-band, a simple UHF-up converter was used. This device simply
`converted the mid-band channels that were spaced from the FM band to
`VHF Channel 7 to the UHF band ( 4 70 to 890 MHz). This block converter, as
`it was known, connected between the cable and the TV set's UHF terminal,
`as shown in Figure 2-3.
`Since there were nine equivalent 6-MHz television channels spaced in
`the mid-band, the nine channels, in addition to the 12 VHF channels,
`resulted in what was known as the 21-channel cable system. At this point
`in time, available trunk and distribution cables had significant loss at UHF
`frequencies. Available amplifiers had upper frequency limits of 220 to 300
`MHz. The reception of UHF broadcast stations was carried on the cable sys(cid:173)
`tems by converting each one of them to one of these nine mid-band cable
`channels at the head-end. The blockup converter at the subscriber's televi(cid:173)
`sion set would then convert them back from mid-band to UHF.
`Back in the 1970s, the 21-channel cable system offered a decent selection
`of programming with better reception than most indoor or outdoor anten(cid:173)
`nas. The success of 21-channel systems encouraged the expansion of cable
`operators to increase the upper frequency limit to 300 MHz. Thus, another
`14 television channels were added, making the 35 channel system. This
`increase in channels beyond the frequency selection capabilities of the tele(cid:173)
`vision sets of the day made the tunable or frequency-agile set-top converter
`
`11
`
`

`

`Tree/Branch Cable Systems and Network Topology
`
`39
`
`r::::-=J
`
`Splitter
`
`[ - j 1 ~-·--J L ___ J
`figure 2-3
`The block-up
`converter
`
`TV VHF
`
`Biock up
`converter
`VHF-UHF
`
`TV UHF
`
`0
`
`0
`
`l--:::J c::-:1 [-::=-J C::~I
`figure 2-4
`Tuneable subscriber
`converter
`
`Gabie
`in
`
`TV Set
`fixed tuned
`to Ch 4
`
`TV UHF
`
`'O
`
`necessary. The method of connecting this device between the cable and the
`television set is shown in Figure 2-4.
`Converters of this type had to have good noise specifications, or low
`noise, simply because the cable signal has to go through the so-called front
`end of the converter and then through the front end of the television set,
`resulting in noise contributions from both sources. These first generation
`tunable converters required the subscriber to select the desired program
`on the converter, which translated to a fixed channel (usually channel 3 or
`4). This situation essentially defeated the use of the set's remote control.
`Thus, the first big problem with subscriber devices resulted. A natural pro(cid:173)
`gression to increase the upper frequency limit beyond 300 MHz was forth(cid:173)
`coming. Thus, the evolution from 52 channels at 400 MHz to 62 channels
`at 450 MHz to current state of the art systems offering more than 150
`channels resulted.
`
`12
`
`

`

`Chapter.2
`
`Subscriber Services With the tremendous increase in cable system
`channel capacity, subscriber service selections became enormous. Many
`subscribers only wanted a few basic channels covering local news, weather,
`and sports with nothing more. Therefore, cable operators tiered the pro(cid:173)
`gram selections according to their subscribers' viewing habits. Since all
`cable system programs were on the system, it was natural to use a filter
`installed at the service drop to prevent the undesired program bands to be
`fed to the subscribers' home. ·This was referred, in the industry, as negative
`trapping; that is, the unordered services were trapped out of the subscriber
`service drop.
`With the arrival of the satellite system that made available premium
`(pay) channels to cable operators, the negative traps could prevent these
`channels getting into homes not subscribing to these channels. System
`audits could be made simply by passing the aerial plant taps and looking
`for the presence or absence of the traps. This method was easily defeated by
`subscribers getting up to the tap usually by use of a ladder and removing
`the trap. More determined cheaters replaced the trap filter with a look(cid:173)
`alike device to fool the cable operator. Hence, the pilfering of cable services
`started and program security became an issue.
`Today cable operators have developed various programs to control the
`theft of cable services. From negative signal-trapping methods, the positive
`trap method was developed. This method employed an interfering carrier
`system in which an interfering carrier signal was injected in the band of the
`signal to be protected. At the subscriber tap, a trap would filter out the
`interfering carrier, allowing the signal to enter the subscriber drop without
`the interfering signal so the subscriber could receive the premium program.
`This method meant that only premium (pay) program subscribers would
`need this trap. The ratio of premium subscribers to non-premium sub(cid:173)
`scribers was an important factor for making the economic decision for the
`best method. In short, a system that does not have many pay program sub(cid:173)
`scribers needs a greater number of negative traps than positive-type traps.
`It must be remembered that the programming choices are the stock and
`trade of the cable television industry. In today's market where the computer
`Internet connection is a desired service, many telephone system seitvice
`providers have difficulty in delivering large data files because of the limit(cid:173)
`ing transfer speeds or low bit rates. A cable system's bandwidth can allow
`for faster downstream bit rates, but upstream ordering capabilities are in
`many cases not available. For cable systems operating in the present com(cid:173)
`puter environment, the expansion toward two services is a must. Bidirec(cid:173)
`tional plants with sufficient bandwidth are necessary for cable operators to
`become players in the telecommunications field.
`
`13
`
`

`

`Tree/Branch Cable Systems and Network Topology
`
`41
`
`The Head .. end
`
`The cable system head-end is the source for all the programs offered on a
`system. It is also the main source of control, often referred to the root of the
`trunk/branch-tree network. From this point, signals are delivered to sub(cid:173)
`scribers via the trunk/distribution cable system known as the limbs and
`branches of the tree network topology. The head-end system can vary from
`one central point to a system of other source points feeding a common
`source node. Head-end signal system networks can range from simple to
`several locations connected by a trunk cable system. Some head-end net(cid:173)
`work topology is shown in Figure 2-5.
`
`Antennas and Off=Air Reception
`
`One very important source of signals most subscribers watch and depend
`on is the local television broadcast stations. These stations are the local
`
`,-J c:=~ L-=i ==
`
`fig11JJre 2-5
`Examples of
`head/end-hub
`configuration
`
`Off-air
`Broadcast TV
`
`Satellite Sevices
`
`Hub - Office, Studio and service
`
`Cable System
`
`Off-air
`Broadcast TV
`
`Hub - Office, Studio and service
`
`·super Trunk
`
`Cable System
`
`'Super Trunk Method can limit Cable
`System Cascades. Present Day Systems
`use Fiber Optic Methods.
`
`14
`
`

`

`42
`
`Chapter 2
`
`network affiliate in the standard VHF band plus the independents and the
`public broadcast stations. Local news, weather, and sports are usually
`favored programming. After all, cable television got its start carrying the
`off-air broadcast stations to distant areas or areas where the signal was of
`poor quality. Usually cable companies receive the off-air broadcast stations
`from tower-mounted antennas feeding the signal to amplifiers or signal
`processors in the head-end electronic equipment building. Usually this
`building was placed near the foot of the tower. An ideal situation would be
`to have the satellite receiving antennas and equipment at this location
`as well.
`
`Antenna Types Log periodic antennas and Yagi antennas are the two
`most common types for VHF reception. Figure 2-6a-b shows these two
`types of antennas. The log periodic is a bit more broad-band and hence can
`be used for two or three adjacent channels, providing the stations are in
`the same pointing direction. This antenna has good gain, directivity, and
`high front-to-back ratio characteristics and is usually the antenna of
`choice for moderate to distant television stations.
`For UHF TV station applications, the bow-tie corner reflector can be
`used for stations not too far away and for distant stations either the wire
`type or solid-aluminum parabolic antenna is usually the best choice. These
`antenna types are shown in Figure 2-7.
`Placement on the antenna tower or mast is important and, unfortu(cid:173)
`nately in many instances, is overlooked by some cable systems. The close
`
`::_=.1 C_:] C.J c=.-:i
`Figure 2-6a
`Yagi pattern
`
`Beamwidth in Degrees
`
`(
`
`)
`
`Connections
`
`'"l
`6 Element Yagi
`
`Directivity Pattern
`
`15
`
`

`

`Tree/Branch Cable Systems and Network Topology
`
`r-1 r~.......-::i r-=-=i ~J
`figure 2-6b
`Log perodic antenna
`
`43
`
`0
`
`OdB
`
`------
`...... ____ _
`x...,
`x.
`
`x,
`
`' \
`
`L,=1/2 wavelength of lowest antena frequency
`L,.2 =0-38 wavelength of highest antena frequency
`
`\
`' \
`
`',,
`,
`
`',, ,_ , __
`
`' I
`' I
`
`/
`
`/
`
`,,,,."''
`
`, ; '
`
`------
`---
`
`angle 13 =antenna beamwidth in degrees (at -3 db points)
`Antenna radiation pattern
`
`1-----:I c::J c:=i [_ __ _J
`figure 2-'1'
`Corner reflector and
`parabolic antennas
`
`'¥=-1::!..=~=~= ..... .
`L2
`L3
`L4
`'¥= ~= x, =..&= ..... .
`X2
`X3
`X4
`l:z..==tan~
`20,
`2
`
`Parabolic Reflector,
`Driven element at
`focal point
`
`Low Gain Broad Band
`
`High Gain Narrow Band
`
`proximity of antennas can interfere with the antenna beam patterns of
`each, and proper spacing, depending on the wavelengths of the antennas, is
`required to overcome this problem.
`
`Placement on Tower Placement on the tower between VHF antennas
`is often the main consideration. Since UHF stations operate at shorter
`wavelengths, the separation between antennas can be closer. Also the loca-
`
`16
`
`

`

`Chapter2
`
`tion of antennas depends on the pointing direction. For example, antennas
`placed at directions on opposite tower legs can be spaced vertically closer.
`This case is shown in Figure 2-8.
`Notice that antennas pointing in a direction 180 degrees from each other
`can be placed on different· tower legs at the same level. When working on
`the antenna placement problems on a tower, the result will be the necessary
`amount of tower height needed for the required antennas. Once this mea-
`
`-. r=J c=J c--::i
`-
`Fiigull'e 2-8
`Antenna mounting
`location on tower
`
`A
`
`Channel 2 Lowest Frequency
`
`4
`A (ft)
`{MHz)
`freque::y
`A {ft)= 984 = 17.8 ft
`55.25
`
`1. The antenna for the weakest station should occupy the topmost sec(cid:173)
`tion of the tower.
`
`2. If the receiver signals are equal, place the highest frequency
`(smallest) on top so as not to weigh the top unnecessarily.
`
`3. Follow the procedures outlined for proper minimum allowed spacing.
`
`Vertical Separation of Antennas
`{a)
`
`top
`view
`
`Antennas back to back
`(180 degrees apart)
`(b)
`
`90 degree dif(cid:173)
`ference in bearing
`
`Arnio= A (1 - 1 ~0 )
`
`= 17.8 (1 - 19800)
`
`= 17.8(1-+)=17.8x1/2
`
`= 8.9 ft (or 1/2 what it was before)
`
`Antennas 90 degrees apart)
`(c)
`
`17
`
`

`

`Tree/Branch Cable Systems and Network Topology
`
`45
`
`surement is known, the next issue is the amount of added height needed for
`proper signal strength. The usual method involves an on-site signal survey,
`which involves some expense.
`First and foremost is obtaining the broadcast station signal contour
`maps, which are often available from either the antenna manufacturers or
`from a consulting firm specializing in performing such on-site signal sur(cid:173)
`veys. Data taken from these maps can tell whether a full signal survey is
`necessary. In many instances, the cable ~ystem technical staff can simply go
`to the site and erect a temporary wide-band rooftop antenna and take
`signal-level measurements and picture-quality measurements with a signal(cid:173)
`level meter and a television set. If any interference is evident, further sig(cid:173)
`nal tests with a spectrum analyzer may be necessary.
`
`Pre-amplification If some of the desired stations may be quite weak due
`to the distance from the station, a pre-amplifier may solve the problem. In
`some instances, the pre-amp can be placed in the head-end building before
`the signal processor, or if the signal is extremely weak, the pre-amp can be
`placed on the tower at the antenna. Tower-mounted pre-amps usually are
`fed their power through the coaxial cable downlead. Care must be taken to
`maintain good coaxial connections with no chance· of moisture ingress so
`corrosion problems can be prevented. The signal-to-noise ratio is the most
`important parameter for the pre-amp placement problem and is shown in
`an example in Figure 2-9.
`For UHF stations, the downlead loss can become a significant parameter
`for particularly tall towers or where the head-end off-air building is sepa(cid:173)
`rated a long distance from the tower. If this loss is significant, a tower(cid:173)
`mounted converter can be used that converts the UHF signal to a VHF
`signal, which now has a lower cable loss. Often these converters contain a
`pre-amp that improves the signal-to-noise ratio. An example comparing the
`results of the use of a converter and the results without one is shown in Fig(cid:173)
`ure 2-10.
`Cable systems requiring tower-mounted converters and/or pre-amps are
`usually at remote areas far from the broadcast stations. For metropolitan
`cable systems, often a tall building's roof can be used to circumvent the mul(cid:173)
`tipath reception problem. For such systems, signal level is not a problem so
`small, more elementary antennas are sufficient. Again, an on-site signal
`survey will tell the story and indicate the needed equipment.
`
`Tower Types Most cable systems use some form of tower or antenna
`mast to support the off-air antennas. Therefore, some space should be
`devoted to the discussion of towers and tower maintenance. Towers over the
`
`18
`
`

`

`46
`
`-
`
`__.:::__] [.~ t:_[._J
`Fiigure 2-9
`Pre-amp location at
`head end
`
`Chapter 2
`
`NF,;0 =Nfsp+downlead loss
`=10dB+10dB=20dB
`Nfsp =noise figure signal processor
`NF,;0 =noise figure out
`C/NOUT
`=C/N,,.-Nfsp=66dB-20dB=46dB
`
`C/N0, 1=46dB
`Processor
`Downlead loss=1 OdB
`._---------11--1Nfsp=10dB1-+--+-----P-
`
`(a) Preamplifier requirement
`
`-10dB
`
`(b) Preamplifier in head end
`
`C/N 0"'=52dB
`
`(c) Preamplifier on tower
`
`usual 200-foot height are considered obstructions and are a hazard to air(cid:173)
`plane traffic. Proper application should be made to the obstruction depart(cid:173)
`m~nt of the regional Federal Aviation Administration (FAA) office.
`Information pertaining to the location of the office can usually be found in
`the telephone book under U.S. Government or by calling the local regional
`airport. The FAA office will provide the necessary application forms and
`procedures to apply for permission to construct. Any required lighting and
`painting requirements will be specified by the FAA.
`
`19
`
`

`

`Tree/Branch Cable Systems and Network Topology
`
`7
`
`C~1 C::::-:J l._~ , __ _
`!Figure 2-10
`Tower mounted
`amplifier converter
`application and
`signal level
`comparison
`
`350'
`
`UHF
`Ch