OBJECTIVES: PROPAGATION INTRO RADIO WAVES POLARIZATION LINE OF SIGHT, GROUND WAVE, SKY WAVE IONOSPHERE REGIONS PROPAGATION, HOPS, SKIPS ZONES THE
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1 WAVE PROPAGATION
2 OBJECTIVES: PROPAGATION INTRO RADIO WAVES POLARIZATION LINE OF SIGHT, GROUND WAVE, SKY WAVE IONOSPHERE REGIONS PROPAGATION, HOPS, SKIPS ZONES THE IONOSPHERIC LAYERS ABSORPTION AND FADING SOLAR ACTIVITY AND SUN SPOTS MF, HF CRITICAL FREQUENCIES UHF, VHF, SPORADICE, AURORAS, DUCTING SCATTER, HF, VHF,UHF
3 Propagation: how radio waves get from point A to point B. The events occurring in the transmission path between two stations that affect the communications between the stations.
4 Electromagnetic radiation comprises both an Electric and a MagneticField. The two fields are at right-angles to each other and the direction of propagation is at right-angles to both fields.
5 Two types of waves: Transverse waves and Longitudinal Transverse waves:vibration is from side to side; that is, at right angles to the direction in which they travel.
6 Longitudinal waves: Vibration is parallel to the direction of propagation. Sound waves, Pressure waves are longitudinal.oscillate back and forth, vibrations along or parallel to their direction of travel.
7 POLARIZATION: The polarization of an antenna is the orientation of the electric field with respect to the Earth's surface and is determined by the physical structure of the antenna and by its orientation. Radio waves from a vertical antenna will usually be vertically polarized. Radio waves from a horizontal antenna are usually horizontally polarized.
8 Ground Wave is a surface wave that propagates close to the surface of the Earth. Line of Sight (Ground wave or Direct Wave) is propagation of waves travelling in a straight line. The rays or waves are deviated or reflected by obstructions and cannot travel over the horizon or behind obstacles. Most common of the radio propagation modes at VHF and higher frequencies.
9 Space Waves: travel directly from an antenna to another without reflection on the ground. Occurs when both antennas are within line of sight of each another, distance is longer that line of sight because most space waves bend near the ground and follow practically a curved path. Sky Wave (Skip/ Hop/ Ionospheric Wave) is the propagation of radio waves bent (refracted) back to the Earth's surface by the ionosphere. HF radio communication (between 3 and 30 MHz) is a result of skywave propagation.
10 Attenuation: when the distance doubles, the signal becomes half less strong. obstacles placed between emitter, receiver, and travelling around the earth; radio waves lose their energy as they forced to bend to follow the earth curvature. Reflection:similar to its optical counterpart as wave enters in contact with a surface. Refraction: the bending of waves that occurs when they pass through a medium (air or ionosphere) produce variation in the velocity of waves.
11 Diffraction: due to its high frequency bends around the edge of the object and tends to make the borders of it lighter. That means that some light reaches well some places that we considered as plunged into darkness. Skip Zone: It is defined as the region between the furthest transmission points and the nearest point refracted waves can be received. Within this region, no signal can be received as there are no radio waves to receive. Skip Distance: It is defined as the least distance between point of transmission and the point of reception.
12 The portion of the received energy at a distant receiving point may travel over any of the possible modes of propagations. Ground or surface wave propagation Sky wave or Ionospheric wave propagation Space wave propagation (Tropospheric propagation) The modes of propagation depends upon Frequency of operation Distance between transmitting and receiving antennas.
13 The ground wave is a wave that is guided along the surface of the earth and is vertically polarized. This wave permits the propagation around the curvature of the earth. This mode of propagation exists when the transmitting and receiving antennas are close to the surface of the earth and is supported at its lower edge by the presence of the ground.
14 The earth attenuation increases as frequency increases. So this mode of propagation is suitable for low and medium frequency i.e. upto 2 MHz only. It is called as medium wave propagation and is used in local broadcasting. At high frequency, wave attenuation by ground is much more than at low frequency over the same ground. All the signals received during day time is due to ground wave propagation.
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16 Electric field strength E at a distance from TX antenna due to ground wave, E = 120 π h t h r I s (volt/meter) λ d where, 120 π Intrinsic impedance of free space h t, h r Effective heights of transmitting and receiving antennas I s Antenna currents d Distance between TX and RX antennas λ Wavelength If d is large, the reduction in the field strength due to ground attenuation and atmospheric absorption increases and thus actual voltage received at receiving point decreases.
17 According to sommerfield, the field strength for ground wave propagation for a flat earth is given by, Eg = E 0 A d where, Eg Ground wave field strength at the surface of earth at unit distance form the TX antenna E 0 Ground wave field strength A Attenuation factor (Earth losses) D Distance from TX antenna
18 Unit distance field strength E 0 depends on Power radiation of TX antenna Directivity in vertical and horizontal planes If the antenna is non directional in the horizontal plane, producing a radiated field which is proportional to the cosine of the angle of elevation, then the field at unit distance for a radiated power is given by where E 0 = 300 ( p)/d v/m, P is the radiated power in KW and D is the distance in metres.
19 The attenuation factor A depends on Frequency Dielectric constant Conductivity of earth The attenuation factor is expresses in terms of two auxiliary variables, the numerical distance p and phase constant b. These two constants are determined by Frequency distance and dielectric characteristics of ground.
20 The parameters p and b are given by Where, P = πdcosb/xλ B = tan -1 [(ε r +1)/x] = 2b 2 b 1 X = 1.8 * σ/f(hz) mhos/cm ε r = dielectric constant of the earth relative to air. σ = Conductivity of the earth.
21 The parameters p and b are given by Where, P = πdx/λcosb B = 180 b1 b1 = tan -1 [(ε r -1)/x] b 2 = tan -1 (ε r /x) b2 = power factor angle of the impedance offered by the earth to the flow of current.
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23 Medium and high frequencies of 2 to 30 MHz Reflection from the ionized region in the upper atmosphere called Ionosphere (50Km to 400Km above earth surface). Ionosphere act as a reflected surface More than 30 MHz- not reflected & penetrate into Ionosphere.
24 Refection from Ionosphere called as ionosphere propagation. Suitable for 2 to 30 MHz- called as short wave propagation. Long distance point to point communication- called as point to point propagation. Possible with multiple reflectionextremely long distance communication.
25 It is necessary to study the medium above the earth, through which the radio waves propagate. The atmosphere consists of three regions Troposphere Ionosphere Outer atmosphere
26 Region below E layer Responsible for attenuation of high frequency in day time. Lower most region of Ionosphere. Height range of 50 km to 90 km. Present at day Disappears at night Critical frequency is 100 KHz. Electron density is ranging from to per cm 3 Reflecting VLF Signals. Absorbing HF signals. Ionization increases with solar activity.
27 Layer occurs during day light hours. Electron density Day 10 5 to 4.5x10 5 per cm 3 Night 5x10 3 to 10 4 per cm 3 Max electron density at noon & summer(at 110 km) Useful in long distance communication in day hours Height range from 90 to 140 km. Critical frequency 3 MHz to 5 MHz. Also known as Kennelly Heaviside layer.
28 Height- 140 km to 400 km from earth surface. Top most layer & highly ionized layer. Remains ionized irrespective of hours. Critical frequency is 5MHz to 7MHz. During day, F layer is found to split up into two layers called, F1 Layer F2 Layer
29 F1 Layer Upper most region Height 140 km to 250 km. Critical frequency at noon 5 MHz to 7 MHz Electron Density 2x10 5 to 4.5x 10 5 per cm 3 Formed by ionization of Oxygen atoms. More absorption of HF waves. F2 layer Upper most region(above F1) Height 250km to 400 km. Critical frequency at noon 10 MHz to 12 MHz Electron Density 3x10 5 to 2x 10 6 per cm 3 Formed by ionization of UV & X rays. More important reflecting medium for HF Radio Waves.
30 In ionosphere collisions due to presence of large no of electrons, neutral atoms & heavy ions. Absorption of energy takes place from the radio waves passes through ionosphere. Velocity of electron(v) = ee (v-j ω)/ m(v 2 + ω 2) Conduction current density( J)= NeV Conductivity (σ) = Ne 2 v /m(v 2 + ω 2 ) Effective dielectric constant(k)= k 0 - [Ne 2 v /m(v 2 + ω 2 ) ]
31 Sky waves reflected from ionized layers. Single Hop or Multi Hop reflections. Radio wave -> Electrons Vibration. Vibration creates an a.c current α velocity of vibration. Effect of mag.field electron I is Inductive Total current decreases and dielectric constant also reduced- causes path of radio wave bend towards earth.
32 Fading is the fluctuation in the received signal strength at the receiver or a random variation in the received signal is known as fading undesirable variations in the intensity of the waves received at the receiver. Due to variation in the heights, density of ionization in the different layers of the ionosphere and due to interference between two waves of different path lengths.
33 The most common method to minimize fading is to use a Automatic voltage control or Automatic gain control in the receiver. When signal fades down the noise level, AVC fails to help because no amount of amplification will make the signal usable. So, diversity reception is used to duplicate some part of signal and even if one part experiences a deep fade, the other part may not.
34 Time diversity reception: In this method, the signal is sent through the channel more than once. It has the disadvantage of reducing the traffic capacity of the circuit. Frequency diversity reception: In this system, the same information is transmitted and received on two or more different frequencies. It is used in radiotelegraph circuits. Since it uses additional frequency, it leads to waste of frequency spectrum.
35 The voltages induced in antennas spaced a few meters apart do not fade simultaneously. When one antenna system is in fading zone, one or more of the others may not be. In HF band, minimum separation is 300 to 600m. In UHF band, minimum separation is 10 to 20m. Each antennas are provided with a separate receiver. Only the signal from the strongest receiver is passed to the output. Thus, the reception becomes good.
36 In ionosphere, the earth s magnetic field exerts a force on the vibrating electrons producing twisting effect on their paths. This reacts on the incident radio waves. The earth s magnetic field splits up the incident waves into two components. Both the waves bend different amounts by the ionosphere and hence travel through it along different paths.
37 The rates of energy absorption and velocities also differ. The waves have elliptical polarization and rotate in opposite directions. The critical frequency of extra ordinary wave is greater than critical frequency by an amount half the gyro frequency. The earth s magnetic field also effects the polarization of the incident radio wave. The average strength of the terrestrial magnetic field is 40A/m.
38 Gyro frequency is defined as the frequency whose period is equal to the revolution of an electron in its circular orbit under the influence of the earth s magnetic field of the flux B. ω g = B(e/m), 2πf g = B(e/m), f g = Be/2πm Attenuation is maximum near f g, and so avoided in propagation work. At high frequencies when f > f g, the electron motion follows an ellipitial path. At lower frequencies when f < f g, the electron motion follows an spiral path (loop).
39 Defined as the highest frequency which can be reflected by a particular layer at vertical incidence. Also called as plasma frequency and expresses in MHz It is proportional to the square root of the maximum electron density in the layer. Refractive index µ= sin i/sin r F c = 81N m
40 Defined as the height to which a short pulse of energy sent vertically upward. It will always be greater than the actual height. If it is known, it is easy to calculate the angle of incidence required for the wave to return at a desired point. The measurement of virtual height is normally carried out by means of an instrument known as ionosonde. H = CT/2, C velocity of light, T Round trip time.
41 Transmit a signal that consists pulses of RF energy of short duration. Receiver which is located close to the transmitter picks up both the direct and the reflected signals. The spacing between these signals on the time axis of CRO gives a measurement of the height of the layer. Useful in transmission path calculations.
42 It is the maximum frequency for which reflection takes place for a given distance of propagation. If f >MUF, then wave penetrates the ionized layer and will not be reflected back. Used for sky wave communication. Value varies from 8 to 35 MHz for a distance of 1000 km. Secant law : f muf = f c sec i
43 Is a frequency limit below which the signal to noise ratio fails to reach an acceptable value for the service required. Depends on the transmitted power. Day-time LUF > Night-time LUF. Sky wave absorption increases with decreasing frequency and hence reducing the received field intensity. The atmospheric noise and man made electrical noise increase with decreasing frequency.
44 Normal variations diurnal, seasonal, height and thickness variations. Abnormal variations Sudden Ionospheric Disturbance Ionospheric storms Sporadic E layer reflection Tides and winds Sunspot cycle Fadings Whistlers
45 The layers of air one above the other have different temperatures and water vapour contents. The boundary surfaces between layers of air form a duct or a sort of leaky wave guide which guides the EM wave between its wall. The highest frequency are continuously refracted in the duct and reflected by the ground so that they propagate around the curvature for beyond the line of sight even upto a distance of 1000 Km. Requirement Temperature inversion.
46 It is defined as The minimum distance from the transmitter at which a sky wave of given frequency is returned to earth by the ionosphere. The minimum distance from the transmitter to a point where sky wave of a given frequency is first received. The minimum distance within a sky wave of given frequency fails to be reflected back.
47 The higher the frequency, the higher the skip distance. For a frequency less than the critical frequency of a layer, skip distance is zero. If a wave frequency exceeds the critical frequency, the effect of the ionosphere depends on the angle of incidence at the ionosphere. It is the distance skipped over by the sky wave. When f = f muf, the skip distance is D skip = 2h [(f muf /f c ) 2-1]
48 The coverage of transmission distance between transmitter and receiver in more than one hop is called multi-hop propagation. The longest single hop propagation is obtained when the transmitted ray is tangential at the earth surface. The maximum practical distance covered by a single hop is 2000 km for E layer and 4000 km for F2 layer. Multi-hop propagation paths occur when the semi circumference of the earth is just over 20,000 km.
49 When the wave is horizontally polarized, the electric field is perpendicular to the plane of incidence and parallel to the reflecting surface. The phase of the reflected wave differs from that of the incident wave by nearly 180 degrees for all angle of incidence. For angle of incidence near gazing, the reflected wave is equal in magnitude but 180 degrees out of phase with the incident wave for all frequencies and all ground conductivities.
50 When the wave is vertically polarized, the electric field is parallel to the plane of incidence and magnetic vector H is parallel to the boundary surface. When the angle of incidence increases form zero, the magnitude and phase of the reflected wave decreases rapidly. The magnitude reaches a minimum and the phase goes through -90 degree at an angle known as Pseudo-Brewster angle. At angles of incidence above Brewster angle, the magnitude increases again and phase approaches zero.
Polarization orientation of the electric field vector with respect to the earth s surface (ground).
Free space propagation of electromagnetic waves is often called radio-frequency (rf) propagation or simply radio propagation. The earth s atmosphere, as medium introduces losses and impairments to the
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