select and erect terrestrial antennas.pdf

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Select and Erect Terrestrial Antennas

Rev Date: 20-Oct-06 - 1 - NSW DET 2006

A Learning Object in support of the

Certificate II in Antennae Equipment

UEE21205 from the National

Electrotechnology Training Package

UEE05

7. Select and Erect

Terrestrial Antennas


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NSW Department of Education andTraining (DET) 2006

All rights reserved. This work is

copyright to NSW Department ofEducation and Training.

Permission is given to trainers andteachers to make copies byphotocopying or other duplicatingprocesses for use within their owntraining organisations, or in aworkplace where training is beingconducted.

This permission does not extend tothe resale of this material to thirdparties, the making of copies for use

outside the immediate trainingenvironment for which they are made,and the use of the materials for hire.

Outside these guidelines, all materialis subject to copyright under theCopyright Act. 1968 (Commonwealth)and permission must be obtained inwriting from the NSW Department ofEducation and Training.

Disclaimer

The views expressed in this work do

not necessarily represent the views ofthe NSW Department of Educationand Training. The NSW Departmentof Education and Training does notgive warranty nor accept any liabilityin relation to the content of this work.

Resource Development Team

David Neyle, Lightship PeopleSystems

Product Advisory Committee

Mike Horne, ElectroSkills Centre

Peter Bowd, NSW TAFE

Naomi Dinnen, NSW UEITAB

Deborah Griffin, NSW DET

Giselle Mawer, Giselle Mawer andAssociates

Technical Reference Committee

George Kozak

Stephen Creese, Matchmaster

Peter OConnor, Foxtel

Graham La Motte, NSW TAFEOTEN

Copyright Acknowledgements

The assistance of Matchmaster isacknowledged in the production ofthis learning object.

Acknowledgment

This work has been produced initiallywith the assistance of fundingprovided by the NSW Department ofEducation and Training, TrainingDevelopment Unit, through theTraining Resources & SupportProgram with advice from the ProductAdvisory Committee.

Further copies of this resource areavailable from:

Website:www.skillsonline.net.au

For further information,contact:

Email:

[email protected]


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Table of Contents

INTRODUCTION ............................................................................. 4

LEARNING OUTCOMES ................................................................ 5

MAPPING TO TRAINING P ACKAGE............................................. 6

ELECTROMAGNETIC RADIATION................................................ 7

FREQUENCY ....................................................................................................7

WAVELENGTH ..................................................................................................8

SIGNAL PROPAGATION......................................................................................8

SIGNAL POLARISATION....................................................................................11

BASIC ANTENNAS....................................................................... 12

THE DIPOLE...................................................................................................12

PRACTICAL ANTENNAS ....................................................................................14

ANTENNA CHARACTERISTICS ...........................................................................15

COMMERCIAL ANTENNAS.................................................................................16

FREE-TO-AIR (FTA) TELEVISION SIGNAL STANDARDS ......... 20

ANALOG TELEVISION STANDARDS ....................................................................20

DIGITAL TELEVISION STANDARDS .....................................................................21

RADIO FREQUENCY ........................................................................................23

TV BANDS AND FREQUENCIES IN AUSTRALIA.....................................................24

DIGITAL COMPATIBILITY WITHANALOG INSTALLATIONS .......................................26

CARRIER TO NOISE RATIO ...............................................................................31

FREE AIR ATTENUATION..................................................................................32

REQUIRED SIGNAL LEVEL AT ANTENNA................................. 34

ANALOG SIGNAL LEVELS .................................................................................34

DIGITAL SIGNAL LEVELS ..................................................................................34SUMMARY OF REQUIRED TV SIGNAL FACTORS ..................................................36

BALUNS........................................................................................ 37

COMBINING ANTENNAS ............................................................. 38

ANTENNA SEPARATION ...................................................................................39

INCREASING GAIN...........................................................................................41

SITE SURVEY .................................................................................................42

CONCLUSION............................................................................... 45

SELF ASSESSMENT.................................................................... 46


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Introduction

When you look at the rooftop

level of any suburb you willsee TV antennas mounted on

a variety of styles of hardware.

As you move from place to

place, between city and rural

areas, you will notice that the design of the antenna varies.

Sometimes the elements are longer or shorter. Sometimes the

boom length will be very long, or the antenna will be mounted very

high on a guyed m ast. In some regions the antenna is mounted

sideways, in others it lies parallel with the ground. There are

specific and good reasons why various styles and designs of TV

antennas are used throughout Australia.

The careful selection and installation of the correct antenna (or

antennas) for the location you are working in will ensure a

satisfactory TV picture, whether it is for the older analog system, or

the newer digital TV system. It is also important that you

understand some basic concepts about how television signals are

transmitted and received, as this will determine the types of

antenna system components you will use to ensure a satisfactory

installation.

This Learning Object will introduce you to the different factors that

influence the design of TV antennas so that you can correctly select

or specify the requirements for antennas that you use in

installations.


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LEARNING OUTCOMES

When you have completed this Learning Object you will be able to:

Describe in simple terms the concepts of frequency and

wavelength.

Name different types of TV antenna.

Describe the characteristics and differences between analog

and digital free-to-air television signals.

State the frequency bands used for television transmission inAustralia and the lower and upper frequencies that a TV

installation must be designed for.

Perform simple calculations using decibels.

Describe how RF travels through free space and the causes of

signal attenuation.

State the required signal levels at the output of the antenna

necessary to receive a high quality picture.

Perform an antenna site survey.

Calculate the required antenna gain based upon the results of a

site survey.

Select an appropriate antenna for the site at which it is to be

installed.

Describe the use of diplexers and baluns.

Position multiple antennas.


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Self Assessment

After reading through this Learning Object you will find a series of

questions that you should attempt. This will enable you to check

your level of understanding and knowledge before progressing ontoyour workplace assessment.

You should not request a workplace assessment unless you have

attempted these questions, checked your answers, and are

confident that you can achieve a satisfactory outcome.

Recognition of Prior Learning

If you believe you are already competent in the content of this

Learning Object then you should check yourself out with the Self

Assessment questions and then request a workplace assessment.

Mapping to Training Package

This learning object provides knowledge and skills for parts/aspectsof the following Unit of Competence:

UEENEEH008A Assemble and erect reception antennae andsignal distribution equipment

In particular, it addresses the following Essential Knowledge and

Skills from the Training Package UEE05:

2.10.14(a) The common difficulties associated with TVreception.

2.10.15(a) Propagation of radio waves from a transmitter to areceiver.

2.10.15(b) Characteristics of antenna systems.

2.10.15(c) Selection of antennae systems for variousapplications.

2.10.15(d) Installation techniques for antenna to receiver andantennae to transmitter transmission and distributionsystems.


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Electromagnetic Radiation

A television signal transmitted from any site is a form of

electromagnetic radiation; this radiation has both an electric and

magnetic field component and as such is induced into most metallic

objects. However a receiving antenna that is tuned to the

frequency of the transmitter will be much more effective in the

reception of signal able to be used by a receiver or processed by a

distribution system.

It is for this reason that the elements of antennas are of different

lengths, depending upon the frequency they must receive.

Frequency

Electromagnetic radiation is measured according to the number of

times the signal changes per second; one cycle change per second

is known as one Hertz (Hz), one thousand changes per second is

one Kilohertz (kHz), one million changes per second is one

Megahertz (MHz) and one billion changes per second is oneGigahertz (GHz).

Fig 7.1: 1 Hz wavelength

A one Hertz s ignal: a singlesine wave cycle in onesecond.

1 sec Time


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Wavelength

The distance an electromagnetic wave travels in the course of one

cycle is known as the wavelength. Since electromagnetic radiation

travels at the speed of light (300,000,000metres/second) thewavelength can be calculated as follows:

Wavelength (metres) = 300 divided by the frequency in MHz

(You may also see this formula written as: = 300 / f )

This means that the wavelength of a 300MHz signal is 1 metre

It follows that the lower the frequency, the longer the wavelength

and the higher the frequency, the shorter the wavelength.

Example

Channel 28 has a vision carrier frequency of 527.25MHz. The

wavelength is therefore 300 527.25 = 0.57 metres.

Task 7.1

What is the wavelength of Digital Ch 9a, which has a centre

frequency of 205.5MHz?

________________________

Signal Propagation

Electromagnetic signals radiated from a transmitter can be

reflected, refracted, and diffracted as the signal travels through the

atmosphere.

Reflection

Reflection occurs when the electromagnetic energy hits an object

and some of the energy is then reflected away at an angle.


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Fig 7.2: Reflection

Large buildings (skyscrapers), mountains and hills, and even

passing aircraft, can cause reflections of TV signals, and are the

major cause of ghosting.

Ghosting

When a TV signal is reflected the TV antenna may receive two of

the same signals, the second arriving slightly later (because of the

longer path length) and at a lower signal strength. The more

correct term for ghosting is multipathing. On an analog TV this

causes a second image to appear slightly beside the main picture.

(Digital TV is not affected by ghosting, and in fact a multipath signal

can actually enhance digital reception.)

The selection of an appropriate model of TV antenna that has a

high front-to-back ratio (for ghost signals coming in from the rear of

the antenna) or an antenna with a narrow acceptance angle (forghost signals coming in from the sides) will help to deal with

ghosting problems.

Refraction

Refraction occurs when the signal passes through a change of

density/temperature of the atmosphere. This results in a bending or

refraction of the signal as it passes through that layer. Lower

frequencies (HF) are more susceptible to this than higher

frequencies (VHF and UHF).

Solid


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Fig 7.3: Refraction

On rare occasions refraction can cause TV signals to travel much

further than the intended coverage area, causing interference to

local TV stations. (TV channel allocations in different regions have

been carefully selected to minimise the problems caused by

refraction.)

Diffraction

Diffraction occurs when the signal grazes the edge of an object

(e.g. the tops of mountains) and tends to bend around that edge.

(The same effect explains how you can see a cars headlights over

the crest of a hill before you see the main headlight beam come into

view.)

Fig 7.4: Diffraction

Even though VHF and UHF signals travel in straight lines (line ofsight) it also explains why hilly areas can still receive some signal,


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even though the receiver is somewhat shadowed from the

transmitter.

Signal Polarisation

During particular atmospheric conditions (usually caused by

refraction, also known as ducting) a TV signal can travel much

further than normally intended. If the different TV signals are on the

same frequency they will cause a distorted picture on the analog

receiver. To help avoid this problem some signals are transmitted

with horizontal polarisation and others are transmitted with vertical

polarisation. A correctly polarised TV antenna will reject the

unwanted signal by approximately 20dB.

Capital cities in Australia usually have horizontally polarised signals,

and many regional areas have vertically polarised signals. (In

regional Australia it is quite common to see a combination of

vertical and horizontally polarised antennas used at the one location

to receive the full complement of available signals.)

Polarisation is defined by the plane that the elements of the

antenna are in. If the elements are horizontal, then the antenna is

receiving a horizontally polarised signal. The opposite is true for

vertically polarised signals.

The three issues of reflection, refraction and diffraction can have an

impact on antenna selection for television reception. TV antenna

designs and characteristics are covered in the following sections.


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require connection to a balanced 70 ohm feeder cable. If the

antenna is folded, as shown below, the impedance at the centre of

the dipole is approximately 300 ohms (balanced).

Fig 7.7: Basic Folded Dipole

In most cases, connection to a standard 75 coaxial cable requires

the use of a balun (covered later in this Learning Object).

Characteristics of the Dipole Antenna

Directivity

A simple dipole will receive signals from either direction (at right

angles to the dipole). This receiving performance (or the strength of

the received signal at different angles of incidence) is known as

directivity and is specified by a polar diagram.

The receiving characteristic of a simple half wave dipole is used as

standard and is used to compare the directivity of more complicated

receiving antennas.

Transmission line to receiver.

wavelength foldeddipole.

300 ohms impedance

Fig 7.8: Simple dipole antenna and a graphic representationshowing which direction it receives signal from. (A polardia ram.


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Practical antennas

Parasitic Elements

The receiving performance of a simple dipole can be modified by

the presence of additional elements adjacent to the dipole. The

dipole connected to the feeder cable is called the driven element,

while elements not connected to the feeder are called parasitic

elements. A parasitic element slightly shorter than the driven

element and placed in front of it will reinforce the pick-up from the

forward direction; it is known as a director. A parasitic element

slightly longer than the driven element and placed behind it will also

help to reinforce the pick-up from the forward direction and reduce

the pick-up from the rear. This element is known as a reflector.

An antenna such as this combining a driven element with a director

and reflector is known as a Yagi antenna; the name is applied to

many variations of this basic design. Any number of directors can

be used but only one reflector or reflecting system is used for any

one frequency.

D

irector

Re

flector

Driven

Element

Fig 7.9: Changing the directivity of a basic dipole

by the addition of a director and reflector.


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If a parasitic element is positioned very close to the driven element,

variations of its length and its distance from the driven element will

modify the current distribution in the driven element. By doing this,

the driven element can be made resonant over a particular range of

frequencies and its impedance can be changed. A parasitic

element used in this way is referred to as a resonator or phasing

element.

Antenna Characteristics

As well as the frequency range covered (the channels that the

antenna can receive) there are two other characteristics of the

antenna that you, as an installer, will be interested in. These are:

Gain, and

Front-to-Back ratio

Gain

The gain of an antenna is the power of the signal received by the

antenna compared to that received by a reference dipole; this is

Example of a Yagi antenna,showing one rear reflector and a

number of directors.

Fig 7.10: Shown here is a typical polar diagram for a Yagi antenna. It hasa large forward receiving lobe and a small rear lobe. This gives itincreased output signal power compared to a basic dipole. It also gives itan ability to reject undesired signals coming from the rear of the antenna.


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expressed as a ratio in dB. The gain of the antenna determines the

amount of signal captured by the antenna from the atmosphere.

Front-to-Back Ratio.

The Front-to-Back ratio of an antenna is the ratio of the signalreceived when the back of the antenna is facing the transmitter,

compared to the signal received when the front of the antenna is

facing the transmitter. Front-to-Back ratio is measured in dB; it is a

measure of the antennas ability to reject unwanted reflected signals

being received from the rear. A good (high) front-to-back ratio is

important if you are experiencing problems with ghosting.

Commercial Antennas

Commercial antennas are available to receive VHF/UHF signals in

a variety of forms. Common types are:

Yagi,

Collinear or Phased array, and

Log Periodic.

Yagi

Yagis always comprise reflectors (longest element at the rear), the

driven element or dipole (the element where the connection is

made), and directors (short elements at the front). The more

elements incorporated into the antenna, the more signal received

by the driven element. When fitted with several directors theseantennas are very directional. Yagi antennas are available for use

on VHF and UHF frequencies. UHF Yagi antennas can be

recognised by their short elements usually 140mm to 300mm long.

Some models are combo antennas with VHF and UHF yagis

combined on a single boom (although the interaction of the

antennas does reduce the gain somewhat). These are common in

metropolitan domestic use where both VHF and UHF channels are

required, signal strength is good, and they reduce the installation

cost.


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Phased Array

A phased array is based on two collinear half wave dipoles

arranged as a pair in phase (so that the signals add together). (The

term collinear means arranged in one line; this means that a

collinear element may comprise several half-wave elements all in

one line.) Phased arrays are available for VHF and UHF bands and

when stacked provide high gain and good anti-ghosting properties.

VHF Yagi UHF Yagi

Combination VHF/UHF Yagi


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A typical 4 bay phased array is shown below left.

Log Periodic

A log periodic antenna is recognisable by the tapered length of the

elements and the cross connection of the closely spaced dipoles.

The gain of log periodics is relatively low, however they offer

wideband performance and relatively constant gain over the

frequencies they are designed to receive.

A combination VHF-UHF Log Periodic

Log periodics exhibit relatively flat gain and have become popular

for digital TV reception, especially in regional and rural areas. (The

flatness in gain helps ensure good multiplex flatness covered later

in this Learning Object.)

VHF Horizontally PolarisedPhased Array

UHF Vertically Polarised PhasedArray


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Task 7.2

Ask your Workplace Trainer to demonstrate to you a range of TV

antennas used in your locality. Identify whether they are Yagis,

Phased Arrays, or Log Periodics. Then identify whether they are

VHF, UHF, or combination antennas.

Variations on the Theme

Yagis, phased arrays, and log periodics all have variations that

designers have developed and are marketed by the various

antenna manufacturers. Sometimes the reflector is of a more solid

style, or is angled (as in the corner reflector style).

It can be difficult at times to classify these more unusual designs,

however they will almost always be one of the three basic types

described in this Learning Object.

Corner reflectorCorner reflector UHF Yagi


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FREE-TO-AIR (FTA) TELEVISION SIGNAL

STANDARDS

There are currently two standards for television broadcasting in

Australia: Analog PAL, and Digital DVB standards.

Analog Television Standards

Analog television has been the regular standard for television

transmission and reception that we have used since TV was

introduced into Australia in 1956. The analog signal was slightly

modified with the addition of colour transmission in 1975.

At the television studio, the luminance signal (black/white level), the

chrominance signal (colour), and the audio signal (stereo FM

sound) are mixed together to make the raw baseband signal. If

you looked at this signal on a spectrum analyser it would look like

the picture shown below:

Important points to note about this analog signal are:

The bandwidth of the total signal is restricted to 7MHz

The Vision Carrier (luminance) is at 1.25MHz

The Sound Carrier (shown here as Stereo Carriers) is

transmitted at a level of13dB less than the Vision Carrier.

Fig. 7.11: 7 MHz Analog TV signal


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This is the standard for Australia. Other countries have different

standards.

A disadvantage of analog television is that it can suffer from

interference that degrades the picture. An advantage is that even

with a low or poor signal you may still receive an image on the TV

screen, although it will be grainy or lacking in colour quality.

Digital Television Standards

Analog television is being phased out in Australia and replaced by

Digital services.

The baseband signal of digital television looks very different to an

analog signal. It is composed of many individual carriers (6817 in

total) with integral error correction, and the ability to carry extra

services other than just the vision, colour and sound for the TV

picture. It is beyond the scope of this course to explain how the

coded digital signal is generated or converted back to a TV picture.

However you should understand some of the basic characteristics

of the signal. If you looked at this signal on a spectrum analyser itwould look like the picture shown below:

The bandwidth of the signal remains 7MHz. This is the same

bandwidth as an analog signal.

7 MHz Wide

6817 individual carriers(too many to be able to see individually)

Fig 7.12: 7 MHz Digital TV Signal


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Digital television has the advantage that it produces a virtually

perfect image and sound if the quality of the signal is acceptable.

Poor signal will result in no picture (or sometimes cause freezing of

the picture or pixilation, which is actually an inbuilt feature of the

digital decoder as it tries to maintain something on the screen). A

(potential) disadvantage of digital TV is that you basically either

have a perfect picture or no picture.

Impulse Noise

On an analog picture impulse noise appears as a series of regular

sparkles on the screen. You m ay be familiar with interference that

can appear when kitchen appliances such as electric blenders are

being used. The arcing of the brushes in the motor causes this

interference. Another source of impulse noise is poorly designed

switchmode power supplies in consumer appliances.

Impulse noise can cause dropout of digital TV reception. Well-

shielded components and coaxial cable can help reduce this

problem. If you can see significant impulse noise on an analog

picture it is worthwhile tracking down the source, otherwise

customers are likely to complain about poor digital TV reception.

Digital Cliff

As the signal strength reduces, the effect of the background

electrical noise becomes worse. (This is more correctly known as

the Carrier to Noise Ratio and is covered in more detail later.) On

an analog TV picture we see this as snow (noise) and colour drop

out. However on a digital picture we do not see this degradation in

picture quality the picture simply freezes or presents a blank

screen. The signal level at which this complete loss of picture

occurs is called the digital cliff. Just because you have a

watchable digital picture this does not mean that the signal strength

is adequate. The digital receiver may be operating right on the

threshold of the digital cliff. Anything that introduces m inor signal

loss (e.g. rain) can cause the signal level to fall over the digital cliff.

Digital signals need to be measured with a signal strength meter,


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just as analog signals do, to ensure satisfactory long-term

performance.

Experiment:

Your Workplace Trainer may demonstrate the Digital Cliff Effect toyou if the necessary equipment is available at your site.

Radio Frequency

Both analog and digital TV signals are upconverted to higher

frequencies so that a number of TV signals can be transmitted

simultaneously. In Australia, free-to-air (FTA) TV can be

transmitted at frequencies ranging from 45MHz through to

820MHz.

Radio Frequency (RF) signals behave very differently to direct

current or 50Hz AC. W hat may appear to be a short circuit at DC

can behave very differently at RF. Tight bends in coaxial cable can

act as inductors and capacitors, degrading the signal. Poorly

shielded components become transmitters, causing further signal

loss.

Because UHF Bands are increasingly being used for free-to-air TVin Australia it is important that you learn and follow the principles

contained in this Learning Object and the companion learning

Fig 4.13:


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materials if you are to ensure a trouble-free high quality TV antenna

installation.

Frequencies between 30MHz and 300MHz are known as Very High

Frequencies (VHF). These are TV Bands I, II and III, and TVantennas for these Bands are relatively large, with long elements.

Frequencies between 300MHz and 3,000MHz (3GHz) are known as

Ultra High Frequency (UHF). These are TV bands IV and V, and

TV antennas for these Bands have short elements (although they

may have long booms in order to maximise the signal received).

TV Bands and Frequencies in Australia

The following table may assist you in your role as a TV antenna

installer.

Channel No. Vision MHz Sound MHz Freq Limits(MHz)

DigitalCentre Freq

Band I

0 46.25 51.75 45 - 52 -

1 57.25 62.75 56 - 63 -

2 64.25 69.75 63 - 70 -Band II

3 86.25 91.75 85 - 92 -

4 95.25 100.75 94 - 101 -

5 102.25 107.75 101 - 108 -Out-of-Band

5a 138.25 143.75 137 - 144 -Band III

6 175.25 180.75 174 - 181 177.507 182.25 187.75 181 - 188 184.50

8 189.25 194.75 188 - 195 191.50

9 196.25 201.75 195 - 202 198.50

9a digital - - 202 - 209 205.50

10 analog 209.25 214.75 208 - 215 -10 digital - - 209 - 216 212.50

11 analog 216.25 221.75 215 - 222 -

11 digital 216 - 223 219.50

12 analog 223.25 228.75 222 - 229 -

12 digital - - 223 - 230 226.50


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Channel No. Vision MHz Sound MHz Freq Limits(MHz)

DigitalCentre Freq

Band IV

28 527.25 532.75 526 - 533 529.5029 534.25 539.75 533 - 540 536.5030 541.25 546.75 540 - 547 543.50

31 548.25 553.75 547 - 554 550.50

32 555.25 560.75 554 - 561 557.50

33 562.25 567.75 561 - 568 564.50

34 569.25 574.75 568 - 575 571.5035 576.25 581.75 575 - 582 578.50

36 583.25 588.75 582 - 589 585.50

37 590.25 595.75 589 - 596 592.50

38 597.25 602.75 596 - 603 599.50

(Band V39 604.25 609.75 603 - 610 606.50

40 611.25 616.75 610 - 617 613.50

41 618.25 623.75 617 - 624 620.50

42 625.25 630.75 624 - 631 627.5043 632.25 637.75 631 - 638 634.50

44 639.25 644.75 638 - 645 641.50

45 646.25 651.75 645 - 652 648.50

46 653.25 658.75 652 - 659 655.50

47 660.25 665.75 659 - 666 662.50

48 667.25 672.75 666 - 673 669.5049 674.25 679.75 673 - 680 676.50

50 681.25 686.75 680 - 687 683.50

51 688.25 693.75 687 - 694 690.50

52 695.25 700.75 694 - 701 697.5053 702.25 707.75 701 - 708 704.50

54 709.25 714.75 708 - 715 711.50

55 716.25 721.75 715 - 722 718.50

56 723.25 728.75 722 - 729 725.5057 730.25 735.75 729 - 736 732.50

58 737.25 742.75 736 - 743 739.50

59 744.25 749.75 743 - 750 746.50

60 751.25 756.75 750 - 757 753.50

61 758.25 763.75 757 - 764 760.5062 765.25 770.75 764 - 771 767.50

63 772.25 777.75 771 - 778 774.50

64 779.25 784.75 778 - 785 781.50

65 786.25 791.75 785 - 792 788.50

66 793.25 798.75 792 - 799 795.5067 800.25 805.75 799 - 806 802.50

68 807.25 812.75 806 - 813 809.50

69 814.25 819.75 813 - 820 816.50


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Digital Compatibility with Analog Installations

In the capital city areas the digital channels are being allocated next

to the existing analog channels. For example, Ch9 analog is

broadcast on Ch8 digital. Therefore, in this case, a well designedand installed antenna and distribution system will work for digital if it

works for analog. (N.B. The ABC is being transmitted on CH12

digital in most capital cities, so the existing antenna must have a

bandwidth wide enough to receive this channel.)

In rural and regional areas the new digital

channels may not necessarily be in the

same band or even transmitted from the

same location. The website for Digital

Broadcasting Australia (DBA) [www.dba.org.au] can provide details

of the frequencies and transmitter locations for digital broadcast

channels.

The introduction of digital television into your area may require an

upgrade of the customers existing TV antenna installation. Many

older antenna and distribution systems will require upgrading due to

poor cable quality, poor connections, and low quality distribution

components. Good installations deliver a quality signal to the digital

decoder, which should ensure trouble-free viewing.

Signal Strength Levels and Attenuation (dBV anddBs)

It is common practice in telecommunications to use logarithmic

numbers to simplify calculations of signal strength. This practice

has also been adopted in the allied industry of television

transmission and reception.

A Bit of Mathematics

Logarithms are handy because we can use addition and subtraction

to make calculations (rather than multiplication and division if the

numbers are expressed as their true value). dBs are a logarithmic

ratio.


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dBs for Voltage

In the TV industry we use a logarithmic ratio of voltage because the

power levels are so small. The formula for calculating dBs when

working with voltage ratios is:

x dB = 20 logVin

Vo

In the following example we have an amplifier, with the voltages

shown at its input and output. W e can calculate the gain of the

amplifier.

In the next example we have a circuit that has an attenuator in the

line (it could be a length of coaxial cable). When we measure the

input and output voltages we calculate a loss of 6dB.

Exercise 7.3

Calculate the gain in dB of the following systems:

300 V input, 2 mV output = ? .dB

5mV input, 2 mV output = ? ..dB

You will need a calculator with a logarithm function to answer thesequestions. There is a scientific calculator available in MicrosoftWindows if you need one.

0.1 mV 2 mV

20 log (2 mV 0.1mV) = 26dB gain

1 mV 500 V

20 log (500V 1mV) = - 6dB gain

or 6 dB loss


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For interest only:

When working with power, the formula for calculating dBs is

different. It is: x dB = 10 log PinPo

Regardless of whether working with voltages or power, the gain (or

loss) still gives the same value in dB.

Adding and Subtracting Gains

When we have a distribution system, such as a TV antenna

distribution network made up of:

o Antenna with gain

o Masthead amplifier with gain

o Cable with loss

o Splitters with loss

o Other active and passive devices,

we can easily just add and subtract the gains of the component that

make up the network.

Fig 7.14: Example of calculating gain in a system.

This is why using dBs is preferable to using actual values the

calculations become simple arithmetic.

Gain = 10 + 20 -8 -4 = 18dB


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dBV

By referencing the actual voltage back to 1 microvolt we can

continue to use numbers that are easily added and subtracted.

1 millivolt (the minimum analogue signal at the wall plate) is now

known as 60dBV.

20 log (1000V 1V) = 60dBV

Signal level meters give readouts as dBVs. Addition and

subtraction is used to calculate the signal levels throughout our

installation if we know the losses and gains of our components in

the installation.

For example, if we have a system such as shown in Fig 7.15, and

we are receiving a signal of 45dBV out of the antenna, we can

calculate the signal level at the output from the splitter.

Fig 7.15: An example of gains and losses in an antenna distribution system.

Output of splitter = 45dBV + 20dB -8dB -4dB = 53dBV, which isinsufficient to drive a TV receiver (needs minimum 60dBV).


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Exercise 7.4

Calculate the output signal level for the following system:

Output signal level = .dbV. Is this level sufficient to

drive a TV receiver? Yes / No

(This topic of calculating signal levels in a system is covered further

in the Learning Module: Master Distribution Systems.)


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Carrier to Noise Ratio

Signal strength is not the only parameter that will ensure a good

picture (whether analog or digital). Just as important is the concept

known as carrier to noise ratio (sometimes abbreviated to CNR).

Electrical Noise

All electrical components (both active and passive devices,

including coaxial cables) produce a wide spectrum of random

electrical noise. If you fully turn up the volume control on an audio

amplifier with no signal being fed in you will hear a hiss. We call

this white noise, because it is equally distributed across all

frequencies, just as white light is made up of all frequencies at

once. This electrical noise extends right through into the radio

frequency spectrum, and if our desired signal is small it can be

swamped by the noise.

C/N ratio

Some other texts on this topic use the term Signal to Noise Ratio.

For our purposes you can consider them to be the same, although

strictly speaking there is a difference. (Because the signal is RF it

is more appropriate to think in terms of the carrier power, hence

CNR.)

In an analog transmission we are measuring the vision carrier.

Most modern signal level meters allow a channel to be directly

selected and will automatically tune to the vision carrier when

performing CNR measurements. Older meters and spectrumanalysers may need to be tuned to the frequency, and hence the

use of the Frequency Allocation Tables included earlier in this

Learning Object.

Digital transmission actually measures the average sum of the

power across all the 6817 carriers.

Noise Power in a Distribution SystemYou can never achieve a better CNR than that received at the

antenna. Using masthead or distribution amplifiers will not improve


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the CNR, even though the signal strength may increase. This is a

common mistake made by novice antenna installers; thinking that

an amplifier will solve low received signal strength problems.

It is also the reason why received signals are amplified as close aspossible to the antenna. A clean signal that is amplified will swamp

any noise introduced further downline. However, if a signal is

amplified too far down the distribution chain e.g. just before the TV

receiver, the CNR will be already degraded by electrical noise

introduced by the cable, splitters or other passive devices, and

amplification will not be able to improve the CNR.

CNR for Analog and Digital Transmission

Analog television requires a minimum CNR of 48dB for an

acceptable picture.

Digital television requires a minimum CNR of 34dB to ensure that

it is sufficiently far away from the digital cliff.

Free Air Attenuation

The greater the distance between the transmitter and the receiving

antenna the less signal that will be received. Each time the

distance from the transmitter is doubled the power received by the

antenna is reduced by , or -6dB.

In fringe areas (especially rural areas) this can be compensated for

by doubling the height of the tower, which theoretically increases

the signal by 6dB, however in practice it will be less.

Other Causes of Free Air Signal Attenuation

VHF and UHF signals essentially travel in line-of-sight. Any

obstructions between the transmitter and the receiving antenna will

reduce the signal strength level. Obstructions can include

undulating ground, skyscrapers or trees.


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Case Study 7.1

The owners of an apartment block in the Eastern Suburbs ofSydney had noticed that their analog TV reception had degradedover a number of years, to the point where it was unwatchable. Thecause was a very large eucalyptus tree planted as a seedling a fewyears earlier, and which had grown up directly in the path of thesignal.

Day-Night Fade can cause as much as 3dB change in signal level.

Rain Fade can also result in 3dB change in signal level.

(The addition of these two types of attenuation is the reason why

sometimes you will see a figure of 66dBV quoted for the level at

the outlet plate (or even rounded off to 65dBV), and other times

60dBV. It is the margin that is allowed for attenuation, because

ideally we want a minimum of 1mV [60 dBV] signal at the

receiver.)


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Required Signal Level at Antenna

[Measuring signal levels requires the use of a signal level meter.

This is covered in the Learning Object: RF Signal Testing.]

The critical factor that will determine a high quality viewable picture

is the signal level received at the antenna for the channel of

interest. Do not be fooled into believing that a low level signal can

be boosted by an amplifier, because when you do this you also

boost the noise in the signal, which still results in a degraded

picture. (See the previous topic on Carrier to Noise Ratio.)

Analog Signal Levels

Ideally you need 65dBV of signal or better at the output of the

antenna. (This will provide a minimum 45dB carrier/noise ratio

resulting in a virtually perfect picture so long as the distribution

system is correctly designed.) The absolute minimum signal level

should be 60dBV. In an analog TV signal this is actually the

signal level of the vision carrier. Some older signal strength metersmay need to be manually set to the required frequency, and for this

purpose the table included in this Learning Object showing TV

channel numbering and associated vision carrier frequency can be

used.

A/V Ratio

Looking at the 7MHz analog TV spectrum (refer to diagram on page

17) you will see that the audio carrier is transmitted at 13dB down

from the level of the vision carrier. This is a fixed value, and most

signal strength meters will measure this A/V ratio. If the value is

anything other than13dB it can indicate ghosting problems.

Digital Signal Levels

To minimise adjacent channel interference, the digital TV signal (in

Australia) is transmitted at 6dB less than analog TV signals. The

specification is for54dBV or better signal strength for the specifiedchannel at the outlet plate.


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Bit Error Ratio (BER)

A second common measurement you may make with a digital

signal strength meter is the bit error ratio (BER). This should be

less than one error in a million, which is usually shown as 106. If

the signal strength is at least 54dBV then typically the BER will be

acceptable.

Most meters simply approximate the BER from the signal strength

level. As an installer you do not need to be overly concerned with

BER; if you have sufficient signal with a good CNR then the BER

will look after itself.

Mulitplex Flatness

This is a measure of how much difference in level there is across

the 6817 carriers in the 7MHz digital signal. There must be no

more than than 5dB variation. Some digital signal strength meters

will measure multiplex flatness, abbreviated to MFlat.

Fig 7.16: Example of Spectrum for a Digital Channel showing variation in Digital

Carriers.

You probably cant do too much about the quality of multiplex

flatness received at the antenna. However, if multiplex flatness is

acceptable at the antenna, but out-of-specification at the wall plate

then it is an indication of a poorly installed distribution system.

Multiplex flatness problems result in poor BER, and loss of digital

picture (despite having adequate signal and good CNR).

7 MHz Wide

6817 individual carriers

5dB


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Summary of Required TV Signal Factors

Analog

Signal Level = 60dBV (minimum allow 6dB more for

fading)

CNR = 48dB

A/V Ratio = 13dB

Digital

Signal Level = 54dBV (minimum preferable to aim for 57

58 dBV better

CNR = 34dB

BER = 106

MFlat = 5dB


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Baluns

At the antenna connection point it is necessary to correctly match

the coaxial cable (which is 75 ohms impedance) to the antenna

folded dipole (also called the driven element, which is 300 ohms

impedance). This matching is done through a special type of RF

transformer known as a balun (which is a combination of the two

words Balanced and Unbalanced).

If you do not use a balun, a mismatch will occur, resulting in

significant loss of signal and multiplex flatness problems. Good

quality Baluns are of the printed circuit board style, with F-connector

outputs and protective UV stabilised ABS plastic casing. It is

recommended that you do not use the cheaper styles of balun

commonly available at hardware stores and electronics shops (see

examples below) as the insertion losses in these can be very high,

and the life expectancy is low due to weathering problems.

Example of a high quality F-connector balun attached

to a UHF dipole.


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Combining Antennas

In some regional and rural areas it is necessary to use a number of

antennas pointing in different directions to pick up the range of

channels available. Even in city areas the terrain m ay require a

number of antennas pointing in different directions to receive all

channels. (The Northern Beaches region in Sydney is an example

where this is a common practice.) The gain from a combination

VHF/UHF antenna may be insufficient, requiring separate VHF and

UHF antennas.

You cannot simply parallel connect antennas together. If you doyou will cause ghosting problems and mismatch (resulting in signal

loss). You must use a device called a diplexer (or a triplexer if

three antennas are combined together). (Diplexers should also be

of high quality with F-connectors.)

Fig 7.17: Diplexer used to combine a VHF and UHF antenna.

Diplexers filter the desired channels before joining them together at

the output. There are different models available depending upon

the channels that are received at the location and the choice of

band antenna used. If you have a complex antenna installation

(such as in rural and regional areas) then you are advised to

contact the technical support staff at your antenna distributor foradvice regarding the appropriate selection.


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It is common for separate VHF and UHF antennas to be installed at

a location. In this case a diplexer is used that has an input for

channels 0-12 and a separate input for channels 28-69.

Some models of masthead amplifier have the diplexer integrated

into the design. This will be covered in more detail in the Learning

Object Master Distribution Systems.

Task 7.5

Ask your Workplace Trainer to show you the range of diplexers

commonly used in your region of installations and operation.

Antenna Separation

When you are mounting multiple antennas there needs to be some

physical separation between them so as to avoid having the

metalwork of one antenna electrically interact with another. When

mounting multiple antennas, the UHF antenna should be mounted

at the top of the mast. The minimum distance to keep antennas

separated is 600mm, however the greater the distance that can be

achieved the better.

Example of a diplexer used

in a domestic application.


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Example of a correctly mounted

VHF vertical polarised phasedarray mounted above a UHF

vertical polarised phased array.

Another example of a correctly

stacked mast, with a UHF vertical

polarised Yagi mounted above a

VHF vertical polarised phased

array.

Incorrect example. The UHF

antenna is lower than the VHF

antenna. (This may be

acceptable if you are receiving a

local UHF and distant VHF

signal.)


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Increasing Gain

It is possible to increase the signal strength by combining the

signals received from two similar antennas. This is beyond the

scope of this learning object, however it is an option worth

considering if you are struggling to achieve the required signal

strength. The two signals need to be phased together so that the

signals add together (out-of-phase signals will actually give you less

signal than a single antenna) and this requires careful engineering

design.

A poor installation, where the

antennas are far too close

together, causing interaction, loss

of signal strength, and ghosting

(due to poor front-to-back ratio).

To achieve the required

separation you might

prefer to use separate

masts.


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Here are examples of yagis phased together to increase the gain

Task 7.6

As you move around your area look for masts that have combined

antennas, and identify correctly and incorrectly installed systems.

Write down the locations and the problems in your notebook.

Site Survey

In order to determine the best location to erect the antenna at the

customers premises you will likely need to conduct a site survey.

This will also tell you how much signal is available at the location,

and therefore the gain requirements for the antenna you select to

use. The signal strength will vary greatly as you move around

different points on the roof, and the site survey will also help

determine the optimum location for the antenna.

When choosing a final location for the antenna consider:

The need to keep 3 metres away from aerial power line feeders.

Access for coaxial cable to enter the premises.

The cost of mounting hardware. (For example, theres little point

struggling for an extra 2dB in a difficult to mount location if an

antenna with more gain will do the job.)

Reference AntennaThe reference antenna you use needs to be appropriate for the TV

channel you are interested in. It should be of a manageable size for


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you to use safely at heights. You also need to know the gain of the

antenna at the required frequency so that you calculate the required

gain of the antenna to be installed.

The major TV antenna manufacturers can recommend suitableantennas for use as reference antennas.

A test reference antenna needs to have broad bandwidth (covering

all the required channels) and be

physically small enough to use safely

when working at heights. The

characteristics of the log periodic

tend to make it suitable for this

purpose.

Example:

You need to set up an antenna to receive Ch 8 in a rural area of

New South Wales. The test antenna you are using has a gain of

5dB on Band III. The highest signal level you measure as you

move about the roof is 58dBV. You need 65dBV. Therefore youneed an antenna with at least 7dB more than your test antenna,

that is, a gain of better than 12dB (5dB + 7dB).

Watch DVD

Watch Chapter 2 to see a technician conducting a site survey.

Fig 4.18: Log periodic test antenna.


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Task 7.7

Assuming you want 65dbV out of the antenna, calculate the gain

of antenna required if these are the site survey results you

measure:

Required Channel: 28

Band IV Test Antenna Gain: 8dB

Measured Ch28 Signal Strength out of antenna: 59dBV

Required gain of antenna: _____ dB

Task 7.8

Under the supervision of your workplace trainer, conduct a site

survey for an antenna installation at a customers premises, or at

any site suitable for this purpose. Recommend a suitable antenna/s

to receive the TV channels for your area.


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Conclusion

A well designed and installed antenna system will provide a clean

signal of sufficient strength to drive the distribution system for the

premises or location you have been contracted to service. The use

of quality components will provide trouble free operation for many

years. Short-cuts and cheap components may provide some

reduction in initial equipment costs or labour time, however in the

longer term it will cause warranty callouts and damage to your

reputation as a TV antenna installer.

A very poorly installed antenna system!


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SELF ASSESSMENT

The following set of questions will assist you to determine whether you

have learnt and understood the content of this Learning Object. Your

Workplace Assessor may ask questions that are different from those here.

Circle the correct answer.

1. The bandwidth of a TV channel is:

a) 1MHz

b) 5MHz

c) 7 MHz

d) Varies depending upon the frequency of the transmission.

2. In an analog system the vision carrier is:

a) 0.5 MHz above the bottom lowest frequency of the allocated bandwidth

b) 1.25 MHz above the bottom lowest frequency of the allocated bandwidth

c) in the middle of the allocated bandwidth for the Channel

d) at the uppermost frequency allocated for the Channel bandwidth.

3. The channel bandwidth for digital television is:

a) greater than for analog television

b) the same as for analog television

c) much more than for analog television

4. Comparing analog and digital television: (circle all correct answers)

a) a digital TV picture will almost always be of higher quality than an analog

picture.

b) in areas where ghosting is a problem, digital television provides a goodsolution.

c) you will always need to install a new TV antenna and distribution systemif your customer requires digital television reception.

d) if you can see a good picture on a digital TV receiver you have a goodquality TV antenna installation.

5. What is the upper and lower frequency range of free-to-air television

transmissions in Australia?


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6. What difference would we notice between an antenna designed for Band III and

an antenna designed for Band IV?

7. What are the minimum signal strengths for analog and digital reception that you

require at the TV antenna wall plate (in dBV) to ensure good quality pictures?

8. Ghosting is caused by:

a) excessive signal entering the TV tuner.

b) the TV signal bouncing off the ionosphere.

c) faulty components inside the TV receiver

d) multipathing (reflections) of the TV signal off buildings, mountains, and

other such structures.

9. TV signals essentially travel in line-of-sight. True / False

10. It doesnt matter if a TV antenna is installed with its elements horizontal orvertical. True / False. Explain why.

11. The name of the device that combines multiple antennas together is:

a) a tap

b) a diplexer

c) a filter

d) a splitter

12. A balun is only necessary if you are in a fringe area with poor signal strength.

True / False?13. What is the wavelength of Ch 2? (Show your calculations.)

14. What is the impedance of a folded dipole? How is the antenna impedance

matched to the impedance of the coaxial cable in order to minimise signal loss?


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15. What addition is made to a simple dipole to make a directional antenna with

gain?

16. Name three types of TV antenna design.

17. When mounting a UHF and VHF antenna on the same mast what is therecommended minimum separation distance, and which antenna should go atthe top of the mast?

18. Calculate the dB gain (or loss) for the following systems:-

a) Vin = 300V and Vout 2mV

b) Vin = 10mV and Vout 0.5mV

19. In the following network there is 55dBV into the amplifier. What level of signal

will appear out of the splitter? ..dBV

Signal In =

55dBV

Cable loss = 10dB

Splitter loss = 8.5dB

G = 8dB


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Answers t o S elf Assessment Questions

1. (c)

2. (b)

3. (b)

4. (a), (b). You may not need to install a new TV antenna and distribution

system for digital television if the previous installation is within

specifications. A digital TV picture may be received even with a poor

installation, however the problem is that it may be very close to the digital

cliff, and any further minor degradation in signal could cause total loss of

picture.

5. 45MHz to 820MHz.

6. Band III is VHF, requiring a VHF antenna, recognised by its longer

elements. Band IV is UHF, requiring a UHF antenna, recognised by its

shorter elements.

7. Analog = 60dBV; Digital = 54dBV. (These are regarded as absolute

minimums, and it is recommended that 3 6 dB more should be the

adopted practice.)

8. (d)

9. True

10. False. A TV antenna must be installed with the same polarisation as the

transmitted signal.

11. (b)

12. False.

13. 300 divided by 64.25 = 4.67 metres wavelength.

14. 300. Baluns are required to change the 300 impedance of the folded

dipole to 75 of the coax cable.


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15. Reflectors are added and a director, making a Yagi antenna.

16. Yagi, Phased Array, Log Periodic.

17. The recommended minimum distance between two antennas is 600mm.

The UHF antenna should be mounted above the VHF antenna.

18. (a) 20 log (2 / 0.3) = 16.5dB (b) 20 log (0.5 / 10) = -26dB

19. 55dBV + 8dB -10dB -8.5dB = 44.5dBV

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