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“Radio and TV systems” LECTURE № 1 -2 1. GENERAL INFORMATION ABOUT MODERN RADIO AND TV BROADCASTING SYSTEMS 2. GENERAL MODEL FOR RADIO AND TV BROADCASTING SYSTEM 3. FEATURES OF PROPAGTION RADIO WAVES DIFFERENT BROADCASTING BANDS 5. FEATURES OF THE ANTENNA AND FEEDER SYSTEMS
Educational - methodical support of discipline “Radio and TV systems” Main literature 1. Radio and TV systems. Lectures /V. Loshakov. 2011. 2. Телекоммуникационные системы и сети : Уч. пособие в 3 томах. Том 2 -Радиосвязь, радиовещание, телевидение/ Катунин Г. П. идр. под ред. проф. В. П. Шувалова. -Изд. 2 -е, испр и доп. -М. : Горячая линия – Телеком. 2005. -672 с. 3. 4. Телевидение: Учебник для вузов. 5 -е изд. , перераб. и доп. Под ред. П. В. Шмакова -М. : Связь, 1999. -432 с. 3. Выходец А. В. Звуковое радиовещание. Учебное пособие -Одесса: Феникс, 2005. -246 с. 4. Локшин Б. А. Цифровое вещание. - М. : САЙРУС СИСТЕМ, 2001. -448 с. 5. Шахнович И. В. Современные технологии беспроводной связи. Глава 4. –М. : Техносфера, 2006, с. 67 -102. 6. Digital television : satellite, cable, terrestrial, iptv, mobile tvin the dvb framework/Herve Benoit. – 3 rd ed. . ISBN 978 -0 -240 -52081 -0. 2008. 7. IP multicast with applications to IPTV and mobile DVB-H / Daniel Minoli. ISBN 978 -0 -470 -25815 -6 (cloth) 2008. 8. IPTV and Internet Video: New Markets in Television Broadcasting. Linacre House, Jordan Hill, Oxford OX 28 DP, UK. 2007 Additional literature Methodical instructions on laboratory works and a practical training on discipline 1. “Radio and TV systems”.
2. General model Radio and TV broadcasting system The correct operation and of management of radio and TV broadcasting systems is impossible without deep understanding of principles of construction and features radio transmitters, receivers, antenna systems, specificity propagation radio waves of different broadcasting bands, standard and formats broadcasting baseband signals and feature of its conversion for transmission through digital – analog channel. Assume information source is digital: generates a string of bits that must be transmitted using electromagnetic waves (no wires): - modulates a carrier - sinusoidal signals – suitable carriers, characterized by: - amplitude: amplitude modulation - frequency: frequency modulation - phase: phase modulation Example: BPSK: 5
Frequency bands and wavelengths • Different frequency for the carrier different physical properties: – – – propagation beyond the horizon energy absorption by the air propagation through rain, walls, etc. attenuation with distance sources of noise • These properties can be better understood in terms of the wavelengths of the radiation. • Next slides: frequency bands allocation and corresponding wavelengths 6
Frequency Band Allocations RADIO IR VISIBLE UV X-RAYS GAMMA RAYS RADIO VLF 3 k LF 30 k MF 300 k HF 3 M VLF: Very Low Frequency MF: Medium Frequency VHF: Very High Frequency SHF: Super High Frequency VHF 30 M UHF SHF 300 M 3 G EHF 30 G 300 GHz LF: Low Frequency HF: High Frequency UHF: Ultra High Frequency EHF: Extremely High Frequency 7
Wavelengths of Frequency Bands • • VLF, LF long waves (VLW, LW) MF medium waves (MW) HF, VHF short waves(SW, VSW) UHF, SHF microwaves Propagate well beyond line of sight The distance the signal travels decreases as the frequency increases • EHF millimeter waves • Above microwave region, only certain windows of frequencies propagate freely through air, rain, etc. • Infrared and visible light will not penetrate walls • X-rays and gamma rays interact with matter 8
Some Frequency Allocations AM Radio: 120 – 300 k. Hz (LF) 540 – 1, 600 k. Hz (MF) AM + Digital Radio (SF): 120 m 90 m 75 m 60 m 49 m 41 m 31 m 25 m 21 m 19 m 16 m 13 m 11 m 2300 3200 3900 4750 5950 7200 9500 11650 13600 15100 17550 21450 25670 2495 3400 4000 5060 6200 7400 9900 12050 13800 15600 17900 21850 26 FM Radio: 88 – 108 MHz (VHF) TV: 49 – 220 MHz (VHF) & 460 – 820 MHz [not contiguous] (UHF) Satellite Broadcast: 10. 7 GHz – 12. 75 GHz ↓ down 13. 0 GHz – 15. 05 Ghz ↑ up (SHF) ISM = Industry, Science & Medicine - transmit power of 1 watt or less. ISM Bands: 902 - 928 MHz 2. 4 - 2. 4835 GHz 5. 725 - 5. 850 GHz 9
Ground atmosphere effect on the radio wave propagation The gaseous mantle of the earth, which is stretched to the height more than 1000 km, is called the atmosphere. The atmosphere is subdivided into three basic spheres (layer): the troposphere - atmospheric boundary layer, whose upper layer lies on height 10… 14 km; the stratosphere - layer of the atmosphere to heights 60… 80 km; the ionosphere - ionized air layer of low density above the stratosphere, which passes then into the Earth radiation belts Ionosphere • Three layers – D: low frequencies can be refracted but the high frequencies tend to pass on through – E: signals as high as 20 MHz can be refracted while higher ones pass through – F: during the day light hours there are two layers: F 1 and F 2
Propagation Effects • Large scale propagation • Several phenomena occur when a wave propagates close to the earth surface – Reflection – a wave encounters objects larger than its wavelength – Diffraction – when a radio path is obstructed by an irregular surface. Secondary waves are generated, resulting in bending of waves around and behind the obstacle. – Scattering – when radio wave travels through a medium containing lots of small (compared to the wavelength) objects 11
Ionosphare • The layers that form the ionosphere vary greatly in altitude, density, and thickness with the varying degrees of solar activity. • The upper portion of the F layer is most affected by sunspots or solar disturbances • There is a greater concentration of solar radiation during peak sunspot activity. • The greater radiation activity the more dense the F layer and the higher the F layer becomes and the greater the skip distance
• F: during the night hours the ionization layer is relatively constant and the higher frequencies can be refracted • During the night hours, the D and E layers virtually disappear and signals that would be refracted at lower levels now are refracted at higher levels. • This results in greater skip (jamp) distances and better reception at greater distances than in the daytime hours.
Propagation of VLF and LF waves of broadcasting bands Distinctive feature of VLF and LF waves is their ability well to bend around the earth. Therefore intensity of a field of a terrestrial wave is considerable on distances 1500. . . 2000 km from a source of electromagnetic waves. Propagation of MF waves can be as surface (ground) and the sky ( ionospheric ) waves. Waves of this band have limited range of propagation in the day time , which it is increasing (extending) at night time. In this band waves has appeared and there was the basic way of Radio broadcasting - the synchronous broadcasting allowing considerably to improve the technical and economic characteristic of a broadcasting network and simultaneous improvement of radio reception quality. The MF band in many countries (USA, GB ) become basic for the Radio broadcasting.
Propagation of short waves (SF) At propagation SF waves energy of a ground (surface) wave is strongly absorbed by a terrestrial surface, especially over a cross-country terrain. The diffraction phenomenon does not play an appreciable role as these waves are absorbed usually earlier short waves, than there is notable a curvature of the earth Quiet Zone or Skip Zone: The space between the point where the ground wave is completely dissipated and the point where the first sky wave is received
To a choice of working frequencies SF Broadcasting band Critical Frequency: The highest frequency that will be returned to the earth when transmitted vertically under given ionospharic conditions. Critical Angle: The highest angle with respect to a vertical line at which a radio wave of a specified frequency can be propagated and still be returned to the earth from. Maximum usable frequency (MUF) The highest frequency that is returned to the earth from the ionosphere between two specific points on earth. Optimum Working frequency: The frequency that provides for the most consistent communication path via sky waves.
Fading: Variations in signal strength that may occur at the receiver over a period of time. • Solar Cycle Every 11 years the sun undergoes a period of activity called the "solar maximum", followed by a period of quiet called the "solar minimum". During the solar maximum there are many sunspots, solar flares, and coronal mass ejections, all of which can affect communications and weather here on Earth.
The Sun goes through a periodic rise and fall in activity which affects HF communications; solar cycles vary in length from 9 to 14 years. At solar minimum, only the lower frequencies of the HF band will be supported by the ionosphere, while at solar maximum the higher frequencies will successfully propagate, figure 1. 4. This is because there is more radiation being emitted from the Sun at solar maximum, producing more electrons in the ionosphere which allows the use of higher frequencies. One way we track solar activity is by observing sunspots. Sunspots are relatively cool areas that appear as dark blemishes on the face of the sun. They are formed when magnetic field lines just below the sun's surface are twisted and poke though the solar photosphere. The twisted magnetic field above sunspots are sites where solar flares are observed to occur, and we are now beginning to understand the connection between solar flares and sunspots.
Propagation waves with wavelength shorter 10 m Radio Waves shorter 10 m as shown above are broken into four bands, which often named as ultra-short waves (VHF 30 -300 MHz, UHF 3003000 MHz, SHF 3 -30 GHz, USHF 30 -300 GHz). For many years VHF use was limited to the requirement of direct visibility between antennas of the transmitter and the receiver which follows from line of sight propagation (direct visibility) of these waves. Really, diffraction practically is not inherent for these bands, and they cannot bend around camber ( obstacles) of a terrestrial surface. Degree ionization of an ionosphere is insufficient for reflection radiowaves of these bands. Propagation range on distance of direct visibility (taking into account a refraction) makes Taking into account a refraction makes (at f<1000 MHz)
• Propagation in free space: ignores any interactions – Antenna radiates a sine wave with the carrier frequency speed of light – Friis free space equation: Pt = transmit power Pr = received power Gt, Gr = transmit/receive antenna gains r = distance between the antennas The received powers can also be expressed in d. B or d. Bm: Lp- path loss coefficient Lp, д. Б = 92. 4 + 20 lg r, км + 20 lg f, ГГц 10 r=100 km, f=100 MHz 10 Lp = 92. 4 + 20 lg 100 + 20 lg 0. 1 =92. 4+40 -20=112. 4 d. B
Propagation close to the earth surface, when antenna heights are small compared with the distance between antennas: Propagation along the earth’s surface: 2 -ray model: flat earth assumption; ground wave reflected; delay; phase shift; attenuation. path loss coefficient in compliance with 2 -ray model In practice: measurements Pr (d. B) Free space 2 -ray model Distance r (meters) Fit a a straight line from measurements -the slope gives the propagation exponent
4. Features of the antenna and feeder systems General principles of the construction of antennas. Antenna is the necessary element of any radiotransmitter and radio receiving equipment. The antenna of radio transmitter (transmitting antenna) is intended for the transformation of high-frequency current into the energy of the electromagnetic waves radiated by it. The antenna of radio receiver is intended for the transformation of the electromagnetic waves into the electric current energy of high frequency accepted by it. The nature of the processes, proceeding in the transmitting and receiving antennas, determines the reversibility of their use (reciprocity principle). l 4 / I l 1 / l 1 a a/ I/ l/4 I Elementary electric dipole (vibrator) a l 1 / l 1 a/ I/ l 4 / Current distribution at the opened LL and transformation LL into the antenna of type the elementary dipole The quarter-wave antenna Current distribution at the end with the locked line and transformation LL into the antenna of type the loop vibrator
Power in antenna (Ра) - the input power from transmitter (through feed) Efficiency - ratio radiated power to input power of antenna Input resistance of the antenna. It has active resistance and reactance components. At adjustment antenna in resonance it is purely active loading for the feed and it is used most effectively. xa , Ra Ra xa » 75 Ом 0. 25 0. 75 1 l/ l
CHARACTERISTIC IMPEDANCE The two conductors comprising a transmission line have capacitance between them as well as inductance due to their length. This combination of series inductance and shunt capacitance gives a transmission line a property known as characteristic impedance. Distributed capacitance, inductance and resistance in a two wire transmission line If the series inductance per unit length of line LS and the parallel capacitance per unit length CP are known, and the loss resistances can be neglected, one can calculate the characteristic impedance of a transmission line from the following equation:
Antenna feeds is a transmission lines are generally characterized by the following properties: balance-to-ground characteristic impedance attenuation per unit length velocity factor electrical length BALANCE TO GROUND Balance-to-ground is a measure of the electrical symmetry of a transmission line with respect to ground potential. A transmission line may be unbalanced or balanced. An unbalanced line has one of its two conductors at ground potential. A balanced transmission line has neither conductor at ground potential. An example of an unbalanced transmission line is coax. The outer shield of coax is grounded. An example of a balanced transmission line is two-wire line. Neither conductor is grounded and if the instantaneous RF voltage on one conductor is +V, it will be –V on the other conductor. Problems can result if an unbalanced transmission line is connected directly to a balanced line. A special transformer, known as a balun (balanced-to-unbalanced transformer) must be used. The schematic diagram of one type of balun is shown below.
Examples: RG-62 coaxial cable has a series inductance of 117 n. H per foot and a parallel capacitance of 13. 5 p. F per foot. What is the characteristic impedance of this cabale? A two-wire line has a series inductance of 315 n. H per foot and a parallel capacitance of 3. 5 p. F per foot. What is the characteristic impedance of this cable? Quite often, the values of LS and CP are not known, but the physical dimensions (conductor diameter, spacing, dielectric properties, etc. ) of the line are known. The following formulas can be used to determine the characteristic impedance of transmission line. For parallel conductor line: where: K is the relative dielectric constant of the material between the two conductors S is the center-to-center separation of the two conductors d is the diameter of the wires. For a coaxial cable: where: K is the relative dielectric constant of the material between the two conductors D is the inside diameter of the outer conductor d is the outside diameter of the inner conductor D and d must be measured in the same units.
REFLECTIONS AND STANDING WAVE RATIO (SWR) RF power goes into the input of the transmission line Z 0, travels to the load and is absorbed by the load resistor ZL. . If ZL ≠ Z 0 arises reflected wave and in transmission line will being standing wave. To express what we know about reflection more concisely, we will define a new quantity, called the reflection coefficient, Г, defined as follows: Г ranges in value from -1 to 1. The table below shows the relationship between the impedance of the termination (load) and the reflection coefficient Impedance of transmission line termination (ZL) Reflection Coefficient, Г ZL = 0 (short circuit) Г = -1 0 < | ZL | < Z 0 -1 < Re(Г) < 0 ZL = Z 0 Г = 0 Z 0 < | ZL | < ∞ 0 < Re(Г) < 1 ZL = ∞ (open circuit) Г= +1 These observations can be summed up in a mathematical formula:
The amplitude of the standing wave varies between a minimum and maximum. The ratio of the maximum to minimum value is known as the voltage standing wave ratio (VSWR) or standing wave ratio (SWR). The SWR measures the degree of mismatch between the load and the transmission line's characteristic impedance. The SWR can take on any value from 1 to infinity. SWR is much easier to measure than Г and thus it is more widely used. Unlike Г , the SWR is always a real positive number greater than or equal to 1. 0. The limiting value of 1. 0 occurs on transmission lines where there are no reflections, that is lines that are terminated in their characteristic impedance. As the terminating impedance becomes either greater or smaller than the characteristic impedance, the SWR increases. The SWR can be computed from the load and characteristic impedances by using the following formula It is also possible to measure the forward and reflected RF power on a transmission line and determine the SWR: Where: PF is the forward RF power; PR is the reflected RF power This formula is very useful because it is relative simple to measure RF power.
Example 1: A resonant transmission line carries 81 watts in the forward direction and 9 watts in the reverse direction. What is the SWR on the line? Example 2: A transmission line whose characteristic impedance is 300 ohms is terminated in a load resistance of 100 ohms. What is the SWR? SWR is encountered frequently in communications work. It is relatively easy to measure and can be used to estimate other quantities such as the magnitude of Г and ZL. Knowledge of the SWR can be important because the RF losses in a transmission line increase as the SWR increases. This is especially true in coaxial transmission lines, which are generally not used when the SWR will be above 3. 0. Parallel conductor ladder line, whose dielectric is primarily air, can be used at SWR's of as high as 12. 0 without problems. SWR can be measured directly with an SWR meter, or computed from values of forward and reflected power. A particularly useful instrument, shown below, is the crossedneedle SWR meter. The crossed-needle SWR meter
Antenna directional characteristics Antenna direktivity - ability to radiate electromagnetic waves in certain directions (radiation directivity). This antenna property is represented by radiation pattern which graphically shows dependence of intensity of a field or radiated power on a direction. Е-плоскость ДН Н-plane КНД E-plane
The effective height of the antenna (hд). Quantity of the energy radiated by each element of antenna is proportionally to current passing on it. As current distribution in the antenna non-constant, radiation by various elements different: it most intensively in a current antinode (wave antinode – пучность) and is equal to zero in current knot (узел) Antennas of long and medium waves: a - grounded vibrator with extending coil; b – inverted-L antenna; c- current distribution in the antenna with extending coil; d - current distribution in the inverted-L antenna with extended coil; e - T-shaped antenna; е – umbrella-tipe antenna.
Tower-antenna (а) and antenna-mast (b): 1 – base insulator; 2 – capacitive cap; 3 – light barrier of mast; 4 - insulators Nadenenko dipole
A vertical mast radiator, Chapel Hill, North Carolina A mast radiator (or 'radiating tower') is a radio mast or tower in which the whole structure itself functions as an antenna. This design is commonly used for transmitting antennas operating at low frequencies, in the VLF, LF and MF ranges, in particular those used for AM broadcasting. The metal mast is electrically connected to the transmitter. Its base is usually mounted on a nonconductive support to insulate it from the ground. A mast radiator is a form of monopole antenna. "Super Turnstile" type transmitting antenna for VHF low band television broadcasting station, Germany.
In phase horizontal antenna Influence of number vibrators on the radiation pattern of the in phase horizontal antenna in a vertical plane
The rhombic antenna System from two vibrators: а - vibrator with active reflector; б - the vibrator with a passive reflector; в - the vibrator with the passive director
Antennas of VHF, UHF and SHF In these bands the antennas possessing directed properties at least in one plane are used mainly. At small length of a wave such aerials are compact enough, that allow to do their rotating. Thanks to it there is a possibility to receive the big prize in power and reducing mutual interference of radio stations to carry out communication in any desirable directions. Dipol (а) and Loop (б) vibrators and there radiation pattern (в) Antenna Yagy (wave channel) (а) and its radiation pattern (б)
d dipole antenna Television antennas. These six antennas are of a common type called a Yagi-Uda antenna, widely used at VHF and UHF frequencies Antenna Yagy (wave channel)
"Super Turnstile" type transmitting antenna for VHF low band television broadcasting station, Germany. Transmitting TV antenna Horn antenna Periscope antenna mirror antenna - direct focus dish antenna
Dish Antennas reflector рефлектор х Contr reflektor optical exis feedhorn Optical exis z фідер с облучателем Focus F The flat front of wave Two-mirror dish antena of Kasegrena feed The multi beam antenna Parabolic antenna for communicating with spacecraft, Canberra, Australia
With taken out feed horn (offset antenna) Features of a design and orientation in vertical plane of the offset antenna Offset antenna Horn-dish antenna