Analog TV

INTRODUCTION TO THE HOME

TV RECEPTION SYSTEMS
When carried out the design and installation of a certain system of reception of TV signals is essential knowledge of the various modes of propagation which runs the TV signal and the units of measurement and evaluation of it.

In this sense and, before going into the types and elements of an installation should take a brief look at these points.

PROPAGATION IN FREE SPACE

ESPRECTO RADIOELECTRICAL
The frequency spectrum is the set of electromagnetic waves whose frequency is between 3 kHz and 3000 GHz.

The frequency spectrum is divided according to the Radio Regulations of the International Telecommunications Union.

Bands allocated to broadcasting services radio and TV are:

Longwave 0.15 to 0.285 MHZ
Medium wave from 0.52 to 1.605 MHz
Shortwave 2.30 to 26.100 MHz

VHF

Band I from 47 to 68 MHZ
Band II from 87 to 110 MHZ
Band III 174 to 230 MHZ

UHF
Band IV from 470 to 606 MHZ
Band V from 606 to 862 MHZ

KU

11.7 to 12.5 GHz DBS
FSS upper band 12.7 to 12.75 GHz
FSS lower band from 10.7 to 11.7 GHz

The bands that we are interested in this part are: BI, B II, B III, B IV and BV, later we will address the FSS and DBS bands.

Currently in Spain bands BI, BIII, BIV and BV are intended for terrestrial broadcasting service. Notwithstanding the BI and B III are intended to be abandoned for this service as recorded by the CNAF (National Table of Frequency Allocation).The transmitters and repeaters with power less than 100 W had to abandon these bands on 1.1.1995 and higher powers they have to leave the 1-1-2000.In Spain the television standard used is PAL BG, ie TV channels of 8 MHz of bandwidth in UHF (B IV and BV) and 7 MHz in VHF.

Propagation mechanismsThe radio and TV waves are electromagnetic odes transmitted at the speed of light 300,000 km / s.When an antenna radiates, creates around an electromagnetic field whose intensity is a function of current flowing through the antenna and to be softening as we move away from it. 
The value of the attenuation wave undergoes when it propagates is a direct function of its frequency, so that the higher the frequency the greater is its damping. 
Waves radiated from a transmitting antenna are of two types: 
a) ground or surface: spread over the surface of the earth. Are the cause of the effect "fade" when received with different phase space the waves.

 b) space: they are the waves radiated into space and form the basis of all communications. The buffer space waves is lower than in the surface. 
Depending on the type of emission, the spreading mechanism will occur in one form or another. 
• Emissions of long-wave (0.15 to 0,285 MHz): the spread usually occurs by the surface wave.• half wave emissions (0.52 to 1.6 MHZ): the propagation of these signals may take place by wave or by space surface. The attenuation in the surface wave is higher than in the space.• Emissions shortwave (2.3 to 26.1 MHz): the propagation of these signals hae through space due to wave attenuation experienced by the surface wave.• Emissions wave, VHF, UHF and higher: in waves BI VHF signals involved both the properties of the short waves as the higher frequency. This band BI can be considered as the transition between the shortwave and VHF and UHF.

The signals used in B III VHF, UHF and above propagate rectilinearly and if they find their way into a receiving antenna induces in it an emf which is grasped. Here the surface wave is not important.The set of waves radiated in TV are usable only those that constitute the optical beam or direct. Theoretically, the maximum range of a transmitter beam is given by the tangent to the surface of the earth TD, which is the limit of visibility between transmitter and receiver.This optical path has a value:Since H is usually much greater than h, an increase of equal height of the transmitting antenna or receiving, it is always more useful in the latter to increase further the scope.This leads to a practical consideration is that in marginal areas should limit or increase the height of the receiving antenna even if a few meters, it will improve significantly the signal.In practice it is observed that as the weather, time of year, etc.., The scope given by the above formula is multiplied by a factor varying between 1.25 and 2.5.Ello is due to the diffraction effect of troposferita waves.Double image effectThe propagation of waves of TV is greatly affected by the obstacles between transmitter and receiving antenna which attenuate the signal much (houses, forests, mountains, etc..) And also can act as reflective displays.The appearance of ghost images or echoes is due to this and his explanation is as follows: The signal reaches the receiver in two different ways one direct and one due to reflection, as the signal travels reflectada more space than the direct reaching the receiver with a time delay, which gives rise to the appearance of double or phantom signal on the receiver screen.As a TV the time it takes a line swept is 64 nanoseconds, it appears that for a television of 23 inches, the scanning is done at a rate of 40/54 = 0.74 cm / ns = 74 mm / nanoseconds.Thus in our case we obtain a second image to the right of the authentic and spaced therefrom a distance of 7.4 mm.Propagation in transmission linesA line is a power transmission medium.

Since lines are used for sending signals from different kinds and different distances, it must not radiate and also have the smallest possible losses.A parameter of great importance in any transmission line is the characteristic impedance (Zo).The characteristic impedance of a transmission line can be defined as the impedance that would be measured at one end of the line if this is of infinite length. This impedance depends on the nature of each particular line, and is a key parameter when considering the possible reflections on the same line:• if a transmission line is terminated or connected to an impedance equal to its characteristic impedance reflection will not occur at the end signal of said line and all the transmitted energy will be delivered to it.• If the end of a line is open or shorted, there will be a hundred percent of signal reflection at that end.The number that defines the energy that is reflected at a point on the transmission line is called reflection coefficient at that point. If the characteristic impedance Zo is the impedance Zr and closing, the reflection coefficient is given by:This will say that a transmission line is better suited when the impedance value of their closure (load impedance) is closer to its characteristic impedance.
The coaxial cableThe transmission line used for distribution of TV signals in collective antenna systems is the coaxial cable.The coaxial cable is constituted by two concentric conductors. Is called the inner conductor and outer conductor live that serves shield is called mesh, two conductors are separated by an insulating material called dielectric.
 
 The main advantage of this cable is that its losses are low, regardless of the external environment, also to be screened or receive radiation or parasites, a condition important for weak signal places as strong. 

There are various types of cables such as:

A. - Air dielectric coaxial cable. There are two types, in a support is used and separation between conductors spiral polyethylene and other channels or perforations along the cable so that the polyethylene is the minimum necessary for clamping the center conductor. These cables are showing very low attenuation.

2. - Cable with Foamed polyethylene dielectric or spongy. This type of cable has more consistency than the previous ones but also their losses are higher.

3. - Coaxial cable with solid polyethylene dielectric. It is the highest attenuation and is only recommended for short connections (10 to 15 m approximately).

The most important data cable from the viewpoint of the installer of antennas are:

characteristic impedance
attenuation

The characteristic impedance of a coaxial cable when one considers the diameter of the conductors and the dielectric is given by:


For polyethylene K = 2.3 for air and K = 1, D being the inside diameter of outer conductor and the outer diameter d of the inner conductor.

Considering the inductance and capacitance per unit length that the cable has its characteristic impedance will be:


Where: L: inductance in henrys (H)
              C: capacitance in farads (F)
             Zo: characteristic impedance (Omega)

Typically for this cable TV is manufactured with a characteristic impedance of Zo = 75 ohms.

The attenuation is the loss of the signal level that occurs in a given wire length and for a given frequency.

The calculation is done using the expression: 

Where: At (dB), attenuation expressed in decibels (dB).
              Vs, the voltage signal at the end of the cable
              Seen, the signal voltage at the beginning of the cable.

Units for use in collective antenna systems.

The more units used in antenna systems are those acontinuacion.

Expressions in decibels dB

The expressions in decibels dB are logarithmic base 10 comparisons between magnitudes of the same type, so they are dimensionless. Widely used in telecommunication practical reasons, it makes multiplication and division in addition and subtraction, thus simplifying numerical expressions.

On the power parameter, we will have the power gain will be defined by:


Where: Ps, the output power W
              Eg, the input power W
              P, the dimensionless power gain

If we express the same gain in dB, we have:


Where: Ap power gain in dB
              Ps and Pe are the same as the above expression

If Ap> 0, is called profit, if Ap <0 is called attenuation.

Moreover we have that if: 

We will obtain expressions for the gains in voltage and current expressed in dB.

And so:


Where: Av, the voltage gain in dB
              Ai, the current gain in dB
              Is, output A
              Ie, input current A
              Vs, the output voltage V
              Go, input voltage V

When we are interested represent a level of power, voltage or intensity in dB, always has to be compared with another level taken as reference. So we will take as a reference 1W, 1 Mw., 1 V, 1 nanoV, etc.. And we have:



Where dBm, expresses the power level existing at a point about 1 MW. and dBnanoV expresses the voltage level at a point about 1 nanoV, relative to a given impedance.

1. - INTRODUCTION

• A TV reception system is the set of devices, both mechanical and electronic, responsible for receiving TV signals and transmit the TV receiver in good condition.

                       Electromagnetic wave ------ Installation ------ TV Receiver

• The quality of picture and sound on the TV receiver depends critically on the facility.
• When there was only a TV antenna installation did not require highly specialized, it was enough to direct the transmitter and antenna, coaxial cable, connect it to the TV outlet.
• Over the years they have been introducing new complications:

• Collective antenna

• The color TV requires higher levels of quality 

• With the increasing number of issuers which can receive signals -  may be necessary to use several antennas oriented in different directions and since not all signals arrive at the same rate, some have to be preamp and other attenuated .

• Increased number of channels

• Satellite TV

• Types of systems: individual and collective

There are two major groups of facilities

• individual installations
• community facilities
With the advent of TV each user had its own antenna on the roof, regardless of the other neighbors, forming a single installation.

The appearance of the 2nd string forced the use of a second antenna for reception and the massive introduction of TV households -  overpopulation of antennas on the roofs.

The overpopulation had serious safety issues, aesthetics and interference between antennas.

The problem originated the law of collective antennas, which established the requirement for buildings of more than 10 dwellings or more than 4 plants comply therewith, in addition to the limits of quality.

Since the law began to settle collective antenna systems.

Currently the two systems coexist installation, using the individual and collective single-family buildings for multi-user buildings ...

• Blocks of a facility:

       Signal capturing system

• set of devices responsible for receiving the signals coming from the transmitters, transmitters and repeaters, are transmitted via terrestrial.
• Its location on the outside, usually on the roof or on the roof of the building.

It consists of the following devices:

• Antennas
• Preamps

Head-end

Responsible for receiving signals from the sensor system and adapt them for distribution to the user in terms of quality required.

Usually located within the building in a community area, close to the sensor system signals.

It is the core of the installation and is made up almost all of the active devices within it.

• broadband amplifiers
• Single-channel amplifiers
• Frequency Converters
• Filters
• Mixers
• Equalizers
• Attenuators

Distribution network

Is responsible for collecting the signals at the output of the header and distribute them to take user installation.

Its location is distributed throughout the building, from the head-up the last shot of the user.

It is made in addition to the coaxial cable and connectors for the following devices:

• Dispatchers
• Shunts
• Thomas and pull boxes

Keep in mind that the distribution of signals to the user different takes, there are numerous losses which forces a perfectly calculated distribution network to reach these shots with adequate signal levels.


2. - Terrestrial TV Antennas

Several definitions of antenna that can be used in the study of this device, since it depends on many cases the appearance of the antenna being treated.

One possible definition: 

Structure formed by conductor which forms the transition zone between the radiated power and the power guided.
 radiated power Antenna Power Tour
Unlike what happens in radio broadcasting, because:
• Levels of energy radiated• Frequency bands used• Scope of issue
In the broadcast type of antenna used in the reception largely determines the quality of the received signal.
Main types of antennas used at the reception:
• Dipole antenna
Antenna formed by two straight conductors of the same length, located next to each other.
The maximum efficiency is obtained at the frequency for which the total length of the dipole is equal to half the wavelength.
• folded dipole
Is obtained by folding the dipole antenna (joining the ends)
• Yagi Antenna
Antenna comprising:
• folded dipole• Passive elements: directors and reflectors
Reflectors can be single, double, or even a kind of screen to increase their effectiveness.
Reflect forward (dipole) the signal of interest
Block signals that eventually come from behind.
The elements directors increase the directivity of the antenna and the gain in the direction of the station, decreasing it in other directions.
Focus the main signal on the dipole.
The reflectors and directors generate a polar plot strongly asymmetric, with a predominance of the anterior lobe.
Common antenna for receiving TV
• Conical Antenna
Made up of various elements of different lengths, arranged in a way very similar to the rods of an umbrella
• Internal Antenna V
Consists of two conductors arranged in a vertical plane in the form of the letter V
• Antenna rhomboid
Located in a horizontal plane and occupies a large space
Is diamond-shaped
• most important parameters of an antenna:
• Characteristic impedance
Impedance presented by the antenna at a given frequency
Also called radiation resistance
For this impedance dipole antenna is placed on the 73 ohm value is quadrupled for a folded dipole.
An antenna can be modeled by a voltage from which we extract the maximum power

The maximum power transfer is obtained when: Zl = Z * a
The maximum power transfer between a generator device and a receiving device there is when:
Rsalida generator = receptor reentry

     Since the characteristic impedance of the coaxial cables used in the distribution of TV signals is 75 ohms is desirable to incorporate an impedance adapter device between a Yagi antenna or a folded dipole and the coaxial cable
Generally, this device also allows the transition between a symmetric structure as the dipole and an asymmetric structure as the coaxial cable - balun 


The basic function of an antenna is to extract part of the power associated with an electromagnetic wave propagates in space and transfer it to a system of guided waves.
The following parameters define the behavior of an antenna in relation to the different directions of space:
• Directivity:
An antenna ability to concentrate the radiated power in a given address space or, conversely, the ability to absorb the incident power in this direction.
Ability of an antenna to receive the signal from a given direction with an intensity higher than that obtained in other directions.
It should be noted conveniently the antenna in the direction in which the directivity is maximum uptake  maximum mean power associated with the traveling electromagnetic wave.
With a high directivity of the antenna gives the advantage of attenuating reflections, interference or unwanted signals from other directions.
• Gain
The antenna gain quantifies the relationship between energy radiated or emitted by a directional antenna in the direction of maximum energy transfer and radiate or receive a reference antenna oriented in the same way at that point.
• Radiation pattern: