1. Antennas and Propagation Chapter 5

2. Introduction An antenna is an electrical conductor or system of conductors Transmission - radiates electromagnetic energy into space Reception - collects electromagnetic energy from space In two-way communication, the same antenna can be used for transmission and reception

3. Radiation Patterns Radiation pattern Graphical representation of radiation properties of an antenna as a function of space coordinates Depicted as two-dimensional cross section to characterize the performance of an antenna Reception pattern Receiving antenna’s equivalent to radiation pattern

4. The isotropic antenna produces an omnidirectional radiation pattern of equal strength in all directions, so the A and B vectors are of equal length. For the antenna pattern of Figure b, the B vector is longer than the A vector, indicating that more power is radiated in the B direction than in the A direction The distance from the antenna to each point on the radiation pattern is proportional to the power radiated from the antenna in that direction.

5. Radiation Pattern Beam width (or half-power beam width) Measure of directivity of antenna is the angle within which the power radiated by the antenna is at least half of what it is in the most preferred direction.

6. Radiation Pattern Beam width (or half-power beam width) measurement of the area over which the antenna receives signal. it is measured to the "half-power points," which is the number of degrees between the points where the gain is 3 dB less than the gain for the antenna's strongest direction.

7. Types of Antennas Isotropic antenna (idealized) A point in space that radiates power equally in all directions Radiation pattern is a sphere with the antenna at the center Dipole antennas Half-wave dipole antenna (or Hertz antenna) Quarter-wave vertical antenna (or Marconi antenna) Parabolic Reflective Antenna

8. The antenna length is proportional to the wavelength

9. Types of Antennas There is an inverse relationship between frequency and wavelength: λ = c/f the lower the frequency, the longer the wavelength (the longer the antenna); the higher the frequency, the shorter the wavelength (The shorter the antenna).

10. Types of antennas Example for Quarter wave antenna For transmitting a signal of wavelength λ the antenna height must be λ/4. So if we want to send a 1 Hz (λ=3*10^8 m) signal ( very very low frequency) using an antenna, its height must be 75,000 Km ( impossible to build such a huge antenna ). If the same signal is modulated to some high frequency say 88 MHZ ( λ = 3.4 m ), antenna height needed is 0.8522 m (quite easy to construct !!! )

11. Parabolic reflective antenna used in terrestrial microwave and satellite applications.

13. Parabolic reflective antenna A parabola is the locus of all points equidistant from a fixed line and a fixed point not on the line. The fixed point is called the focus and the fixed line is called the directrix.

14. Parabolic reflective antenna If a source of electromagnetic energy (or sound) is placed at the focus of the paraboloid, and if the paraboloid is a reflecting surface, then the wave will bounce back in lines parallel to the axis of the paraboloid In theory, this effect creates a parallel beam without dispersion.

15. If incoming waves are parallel to the axis of the reflecting paraboloid, the resulting signal will be concentrated at the focus.

16. Antenna Gain Measures the gain that can be introduced by using a directional antenna compared to the isotropic omni-directional one with the same input power.

17. Antenna Gain Power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna) with the same input power. The increased power radiated in a given direction is at the expense of other directions. In effect, increased power is radiated in one direction by reducing the power radiated in other directions.

18. Antenna Gain Antenna gain The gain of a directional antenna is compared to an omni-directional (or isotropic) antenna. The isotropic antenna radiates equally in all directions and has a gain of 1 (0dB). The directional antenna focuses the energy in one direction The isotropic antenna is a theoretical antenna (like a ball), radiating in a perfect sphere in all directions.

19. Antenna Gain The term Antenna Gain describes how much power is transmitted in the direction of peak radiation to that of an isotropic source. An antenna with a gain of 3 dB means that the power received from the antenna will be 3 dB higher (twice as much) than what would be received from a lossless isotropic antenna with the same input power.

20. Antenna Gain The gain of a real antenna can be as high as 40-50 dB for very large dish antennas. Directivity can be as low as 1.76 dB for a real antenna (example: short dipole antenna), but can never theoretically be less than 0 dB.

21. Antenna Gain vs Beam width Generally, the higher the gain, the smaller the beam width. An antenna with high gain and low beam width receives signals over a smaller area, but receives such signals at a strong strength (longer distance); whereas, an antenna with low gain and high beam width receives signals over a larger area, but at weaker strengths (shorter distance).

22. Antenna Gain Effective area Related to physical size and shape of antenna is a measure of how effective an antenna is at receiving the power of radio waves. Relationship between antenna gain and effective area G = antenna gain A e = effective area f = carrier frequency c = speed of light ( 3 ×10 8 m/s) = carrier wavelength

23. Propagation Modes A signal radiated from an antenna travels along one of three routes: Ground-wave propagation: up to 2 MHz Sky-wave propagation: from 2 MHz to 30 MHz Line-of-sight propagation: from 30 MHz

24. Ground Wave Propagation

25. Follows contour of the earth Can Propagate considerable distances Frequencies up to 2 MHz Example AM radio

26. Sky Wave Propagation

28. Signal reflected from ionized layer of atmosphere back down to earth Frequencies from 2 MHz to 30 MHz Signal can travel a number of hops, back and forth between ionosphere and earth’s surface Reflection effect caused by refraction Examples Amateur radio CB radio

29. Refraction Refraction – bending of microwaves by the atmosphere Velocity of electromagnetic wave is a function of the density of the medium When wave changes medium, speed changes Wave bends at the boundary between mediums In a vacuum, an electromagnetic wave (such as light or a radio wave) travels at approximately 3 × 10^8 m/s. This is the constant, c, commonly referred to as the speed of light, but actually referring to the speed of light in a vacuum. In air, water, glass, and other transparent or partially transparent media, electromagnetic waves travel at speeds less than c.

30. This phenomenon is easily observed by partially immersing a stick in water. The result will look much like Figure 5.6, with the stick appearing shorter and bent.

31. Line-of-Sight Propagation

32. Satellite communication – signal above 30 MHz not reflected by ionosphere Therefore, the signal can be transmitted between an earth station and a satellite Ground communication – Transmitting and receiving antennas must be within line of sight

33. Line-of-Sight Propagation The radio horizon is the locus of points at which direct rays from an antenna are tangential to the surface of the Earth.

34. Line-of-Sight Propagation

35. Line-of-Sight Equations Optical line of sight Effective, or radio, line of sight d = distance between antenna and horizon (km) h = antenna height (m) K = adjustment factor to account for refraction, rule of thumb K = 4/3

36. Line-of-Sight Equations What is the Maximum distance between two antennas for LOS propagation: h 1 = height of antenna one h 2 = height of antenna two

37. Line-of-Sight Equations Maximum distance between two antennas for LOS propagation: h 1 = height of antenna one h 2 = height of antenna two

38. Line-of-Sight Propagation Max distance between two antenna for LOS transmission One is 100m high One is at the ground level ? Receiving antenna is 10m high, to achieve the same distance, how high must be the transmitting antenna?

39. Line-of-Sight Equations

40. This is a saving of over 50m in the height of the transmitting antenna It is preferred to raise the transmitting antenna above the ground in order to reduce the necessary height of the transmitter

41. LOS Wireless Transmission Impairments Attenuation and attenuation distortion Free space loss Noise Atmospheric absorption Multipath Refraction Thermal noise

42. Attenuation Strength of signal falls off with distance over transmission medium

43. Attenuation Atmospheric absorption – water vapor and oxygen contribute to attenuation Multipath – obstacles reflect signals so that multiple copies with varying delays are received Refraction – bending of radio waves as they propagate through the atmosphere

45. Multipath Propagation

46. Reflection - occurs when signal encounters a surface that is large relative to the wavelength of the signal

47. Multipath Propagation Diffraction - occurs at the edge of an impenetrable body that is large compared to wavelength of radio wave. Observed when a wave encounters an obstacle or a slit

48. Multipath Propagation Scattering – occurs when incoming signal hits an object whose size in the order of the wavelength of the signal or less

49. The Effects of Multipath Propagation Multiple copies of a signal may arrive at different phases If phases add destructively, the signal level relative to noise declines, making detection more difficult Intersymbol interference (ISI) One or more delayed copies of a pulse may arrive at the same time as the primary pulse for a subsequent bit

50. Attenuation Attenuation factors for unguided media: Factor 1: Received signal must have sufficient strength so that circuitry in the receiver can interpret the signal Factor 2: Signal must maintain a level sufficiently higher than noise to be received without error =>The first and second factors are dealt with by attention to signal strength and the use of amplifiers or repeaters to boost the signal at regular intervals. Factor 3: attenuation distortion: the attenuation varies as a function of frequency Attenuation is greater at higher frequencies,

51. Attenuation Exp: Sound frequencies in air, a frequency will attenuate because the energy of that frequency is transferred to something else. Waves tend to interact with things that have a size comparable to their wavelengths (or larger.) Higher frequencies have shorter wavelengths, and therefore they are more likely to be absorbed by objects they come in contact with, attenuating the sound. Even in an empty room, the particles that make up the air will interact in such a way that higher frequencies are more likely to be converted to heat energy, while low frequencies remain as sound waves.

52. Attenuation Attenuation distortion: Frequency components of the received signal have different relative strengths than the frequency components of the transmitted signal. To overcome this problem, techniques are available for equalizing attenuation across a band of frequencies. One approach is to use amplifiers that amplify high frequencies more than lower frequencies.

53. Free Space Loss A transmitted signal attenuates over distance because the signal is being spread over a larger and larger area. => Free Space Loss attenuation Loss in signal strength that occurs when an electromagnetic wave travels over a line of sight path in free space. In these circumstances there are no obstacles that might cause the signal to be reflected refracted, or that might cause additional attenuation.

54. Free Space Loss Free space loss, ideal isotropic antenna P t = signal power at transmitting antenna P r = signal power at receiving antenna = carrier wavelength d = propagation distance between antennas c = speed of light (3 ×10 ^8 m/s) where d and are in the same units (e.g., meters)

55. Free Space Loss Free space loss equation can be recast: As the frequency increases, the free space loss also increases, which would suggest that at higher frequencies, losses become more burdensome.

56. Free Space Loss Free space loss accounting for gain of other antennas G t = gain of transmitting antenna G r = gain of receiving antenna A t = effective area of transmitting antenna A r = effective area of receiving antenna

57. Free Space Loss Free space loss accounting for gain of other antennas can be recast as for the same antenna dimensions, the longer the carrier wavelength (lower the carrier frequency f), the higher is the free space path loss.

58. Free Space Loss Isotropic antenna at a fixed distance an increa. se in frequency results in an increased loss measured by 20 log (f). Antenna with gain At a fixed antenna area, then the change in loss is measured by -20 log(f); there is actually a decrease in loss at higher frequencies.

59. Categories of Noise Additional unwanted signals that are inserted somewhere between transmission and reception. This is similar to when one is listening to a person talk immediately adjacent to them and a loud noise suddenly drowns out the voice of the person speaking, such that all that can be heard is that loud noise, until the noise either stops or moves far enough away to again hear the speaking person’s voice. Thermal Noise Intermodulation noise Crosstalk Impulse Noise

60. Thermal Noise Thermal noise due to agitation of electrons Present in all electronic devices and transmission media Cannot be eliminated Function of temperature Particularly significant for satellite communication uniformly distributed across the frequency spectrum and hence is often referred to as white noise.

61. Thermal Noise Particularly significant for satellite communication A sun outage occurs because the earth station cannot distinguish between the energy from the sun and its intended communication signal.

62. Thermal Noise Amount of thermal noise to be found in a bandwidth of 1Hz in any device or conductor is: N 0 = noise power density in watts per 1 Hz of bandwidth k = Boltzmann's constant = 1.3803 ×10 -23 J/K T = temperature, in kelvins (absolute temperature)

63. Thermal Noise Noise is assumed to be independent of frequency Thermal noise present in a bandwidth of B Hertz (in watts): or, in decibel-watts

64. Noise Terminology Intermodulation noise – occurs if signals with different frequencies share the same medium Interference caused by a signal produced at a frequency that is the sum or difference of original frequencies the mixing of signals at frequencies f1and f2 might produce energy at the frequency f1+f2. This derived signal could interfere with an intended signal at the frequency f1+f2. Generally speaking: if the frequency components of your signal are f1, f2 and f3 =>n 1 f1+ n 2 f2 + n 3 f3

66. in blue the position of the fundamental carriers, in red the position of dominant IMPs, in green the position of specific IMPs.

67. Noise Terminology Crosstalk – unwanted coupling between signal paths experienced by anyone who, while using the telephone, has been able to hear another conversation

68. Noise Terminology Impulse noise – irregular pulses or noise spikes Short duration and of relatively high amplitude Caused by external electromagnetic disturbances, or faults and flaws in the communications system Impulse noise is generally only a minor annoyance for analog data. For example, voice transmission may be corrupted by short clicks and crackles with no loss of intelligibility. However, impulse noise is the primary source of error in digital data transmission. For example, a sharp spike of energy of 0.01 s duration would not destroy any voice data but would wash out about 560 bits of data being transmitted at 56 kbps.

69. Expression E b /N 0 Ratio of signal energy per bit to noise power density per Hertz The bit error rate for digital data is a function of E b /N 0 BER is a decreasing function of E b /N 0 Given a value for E b /N 0 to achieve a desired error rate, parameters of this formula can be selected As bit rate R increases, transmitted signal power must increase to maintain required E b /N 0

70. Fading The term fading refers to the time variation of received signal power caused by changes in the transmission medium or path(s). In a fixed environment, fading is affected by changes in atmospheric conditions, such as rainfall. But in a mobile environment, where one of the two antennas is moving relative to the other, the relative location of various obstacles changes over time, creating complex transmission effects.

71. Types of Fading Fast fading Slow fading Flat fading Selective fading Rayleigh fading Rician fading

72. Fast and Slow fading

73. Flat and Selective fading Flat fading, or nonselective fading, is that type of fading in which all frequency components of the received signal fluctuate in the same proportions simultaneously. Selective fading affects unequally the different spectral components of a radio signal.

74. Rayleigh and Rician Rayleigh fading occurs when there are multiple indirect paths between transmitter and receiver and no distinct dominant path, such as an LOS path. This represents a worst case scenario. Rician fading best characterizes a situation where there is a direct LOS path in addition to a number of indirect multipath signals. The Rician model is often applicable in an indoor environment whereas the Rayleigh model characterizes outdoor settings.

75. Error Compensation Mechanisms Forward error correction Adaptive equalization Diversity techniques

76. Forward Error Correction Transmitter adds error-correcting code to data block Code is a function of the data bits Receiver calculates error-correcting code from incoming data bits If calculated code matches incoming code, no error occurred If error-correcting codes don’t match, receiver attempts to determine bits in error and correct

77. Adaptive Equalization Can be applied to transmissions that carry analog or digital information Analog voice or video Digital data, digitized voice or video Used to combat intersymbol interference Mitigating the effect of multipath propagation Techniques Sophisticated digital signal processing algorithms

78. Diversity Techniques Diversity is based on the fact that individual channels experience independent fading events Space diversity – techniques involving physical transmission path Multiple nearby antennas may be used to receive the message, with the signals combined in some fashion to reconstruct the most likely transmitted signal. Frequency diversity – techniques where the signal is spread out over a larger frequency bandwidth or carried on multiple frequency carriers Time diversity – techniques aimed at spreading the data out over time (TDM)

79. Appendix