Radio & Telecommunications Systems (1.0)

1. Radio & Telecommunications Systems (1.0) Lecturer: P.M. Cheung (room 326) email:pmcheung@vtc.edu.hk Contact Hours: Lecture: 30 hours (room 310) Tutorial: 15 hours (room324) Lab. :4 experiments(room 324)

2. Content: 1. EM wave & Antenna • 2. Transmitters & Receivers • 3. Telephone systems • 4. TV Systems • Assessment: Course work – 50% • (assignment:10%, lab:20%, test: 20%) • Exam. - 50% • Textbook: Electronic Communications Systems, 3rd ed.,Dungan, Delmar • No lecture notes will be delivered. • Download from intranet: http://172.26.126.61/student

3. Aims • establish an understanding of the elementary principles employed in radio transmitter and receiver systems • introduce the basic system knowledge of various kinds of local telecommunications systems • Co-requisites • Telecommunications Principles 1

4. Learning Strategies • emphasis on the general aspects and appreciation of radio and telecommunications systems • practical examples of various telecommunications systems will be used to promote learning • Assessment • Continuous assessment - 50% • Examination - 50%

5. Content Area • Electromagnetic wave and antenna systems • radiation of electromagnetic wave • modes of propagation • parameters of aerial • practical aerials • Radio transmitters and receivers • block diagrams of : AM transmitters, FM transmitters, superheterodyne receiver • diode detector

6. Telephone systems • fixed network technology, space and time switching • local loop, signalling in call establishment • overview of mobile communications; cellular communications, multiple access • Television systems • scanning, composite video, PAL/NTSC systems, TV transmission/reception • overview of satellite boardcast; orbit, earth station, satellite TV

7. Radio Wave Propagation Radio wave characteristics Radiation from an antenna Propagation characteristics

8. Radio Wave Characteristics • the radiation concept of radio waves • dropping a pebble into a pool of water • water to move up and down • disturbance transmitted as expanding circles of waves • transverse wave or traveling wave • occurring perpendicular to the direction of propagation. • e.g. electromagnetic waves radiated by antennas

9. Frequency • the number of cycles of a sine wave completed in one second expressed in Hz (Figure 1) • Radio Frequencies (RF) • frequencies between 3 kHz and 300 GHz • commonly used in radio communication. • Wavelength (l) • the space occupied by one full cycle of a radio wave at any given instant (Figure 2) l = c / f c = velocity of radio wave = 3x108m/s

10. Figure 1 - Sine wave characteristic

11. Figure 2 - Concept of a wavelength

12. Electromagnetic Radiation • complex form of energy containing both electric and magnetic fields • moving electric field always creates a magnetic field • moving magnetic field always creates an electric field • lines of force of these fields are perpendicular to each other (Figure 3)

13. Figure 3 - Electromagnetic lines of force

14. Wave Polarization • determined by the direction of the electric field of the wave with respect to earth • vertically polarized • electric field of the wave is vertical to the earth (Figure 4A) • horizontally polarized • electric field is horizontal to the earth (Figure 4B) • position of the transmitting antenna determines whether the wave will be vertically or horizontally polarized

15. Figure 4A - Vertically polarized wave

16. Figure 4B - Horizontally polarized wave

17. Radio & Telecommunications Systems Induction/Radiation Field Free Space Impedance Modes of Propagation

18. Radiated field • energy radiated from the conductor or aerial • in the form of an electromagnetic wave • electric and magnetic fields are at right angles to each other • mutually at right angles to the direction of propagation (Figure 1) • magnitude proportional to the frequency of the wave and inversely proportional to the distance from the aerial

19. Figure 1 - Electromagnetic wave

20. Figure 5 - Radiation from an aerial

21. Induction field • near the aerial • energy that is not radiated away from the aerial • magnitude diminishes inversely as the square of the distance from the aerial • the induction field larger than the radiation field • at distances greater than l/2p • radiation field is the larger

22. Free Space Impedance • amplitudes of electric field E & magnetic field H constant relationship to each other. Impedance of free space =E (volts/meter) / H (ampere-turns/meter) =120pW =377 W

23. Propagation Characteristics • electromagnetic wave sent out from an antenna • ground wave • part of the radiated energy travels along or near the surface of the earth • sky wave • another part travels from the antenna upward into space • space wave • energy that travels directly from the transmitting antenna to the receiving antenna

24. Ground Waves • primary mode of propagation in • LF band (30 - 300 KHz) • MF band (300 KHz - 3MHz) • follow the curvature of the earth and actually travel beyond the horizon (Figure 2) • as the frequency increases • more effectively absorbed by the irregularities on the earth's surface • hills, mountains, trees, and buildings

25. Figure 2 - Ground wave propagation

26. Space Waves transmitted signal above 4 or 5 MHz usable ground wave signal is limited to a few miles. signals can be transmitted farther using the space or direct wave (Figure 3) used primarily in VHF band (30 - 300 MHz) UHF band (300 MHz - 3 GHz) limited to line-of-sight distances energy in radio waves at frequencies above 30 MHz moves through space in straight lines like light waves

27. Figure 3 - Space wave propagation

28. Radio Horizon • about one third greater than that of the optical horizon • caused by refraction in the earth's lower atmosphere • density of the earth's atmosphere decreases linearly as height increases • effectively bending the wave slightly downward • follows the curvature of the earth beyond the optical horizon

29. radio horizon for both transmitting and receiving antennas: Dt = 4ÖHt or Dr = 4ÖHr where Dt and Dr = radio horizon distance in kilometers Ht and Hr= height of transmitting (receiving) antenna in meters • maximum space wave communications distance is the sum of the numbers obtained by for both antennas. Dmax= 4ÖHt + 4ÖHr or Dmax =Dt + Dr

30. Sky Waves • ionized layers of the atmosphere between 50 - 400km above the surface of the earth • at certain frequencies and radiation angles • the ionosphere reflects radio waves • radio waves at other frequencies and angles are refracted and return to earth (Figure 4) • amount of refraction depends on • frequency of the wave • density of the ionized layer • angle at which the wave enters the ionosphere.

31. Figure 4 - Sky wave propagation

32. long distance communications • carrier frequencies in the MF and HF bands (3 - 30 MHz) • waves radiated at these frequencies can be refracted back to earth • waves at frequencies above 30 MHz • penetrate the ionosphere and continue moving out into space

33. The Ionosphere: atmospheric conditions continuously change hourly, daily, monthly, seasonally, yearly….. undesirable results are signal absorption, dispersion and fading atmospheric conditions have their greatest effect on the ionosphere graphic illustration of the designations of the ionospheric layers and their approximate altitudes is shown in Figure 1.

34. Figure 1 - Layers in the ionosphere

35. D layer, 50-90 km above the earth lowest layer exists only in the daytime ionization is relatively weak does not affect the travel direction of radio waves absorb energy from the electromagnetic wave attenuates the sky wave

36. MF band signals are completely absorbed by the D layer at night D layer disappears long distance MF transmissions via sky wave E layer, 90-150 km above earth maximum density at noon ionization is so weak at night the layer may disappear

37. F layer, 200-400 km above earth splits into F1 and F2 in the daytime F2 varying from summer to winter F1 layer, 200-220 km above the surface of the earth. F2 layer, 250-350km in winter, 300-500km in summer

38. Frequency bands and major services

39. Refraction of an Electromagnetic Wave • electromagnetic wave travelling in one medium passes into a different medium • direction of travel will probably be altered • wave is said to be refracted. • The ratio is a constant for a given pair of media and is known as the refractive index for the media.

40. wave is transmitted through a number of thin strips (Figure 2) • each strip having an absolute refractive index lower than that of the strip immediately below it • wave will pass from higher to lower absolute refractive • progressively bent awayfrom the normal • wave will becontinuously refracted

41. Figure 2 - Refraction of an electromagnetic wave

42. refractive index n of a layer is related to both the frequency f of the wave and the electron densityNaccording to:

43. Figure 3 - Effect on ionospheric refraction

44. Critical Frequency • the max frequency that can be radiated vertically upwards by a radio transmitter and be returned to earth • wave that travels to the top of the layer • electron density is at its maximum value • angle of refraction becomes 90o • angle of incidence is 0o • therefore

45. Maximum Usable Frequency (MUF) • highest frequency that can be used to establish communication • using the sky wave • between two points • determined by both the angle of incidence of the radio wave and the critical frequency of the layer; thus

46. Optimum Working Frequency (OWF) • ionospheric fluctuations often take place • operation of a link at the m.u.f. would not be reliable • frequency of about 85% of the m.u.f. • used to operate a sky-wave link • known as the optimum working frequency or o.w.f • since the m.u.f. will vary over the working day • necessary to change the transmitter frequency as propagation condition varies

47. Reference • Dungan F.R., “Electronic Communications Systems,” 3rd ed., ITP • Green D.C., “Radio Systems for Technicians,” Longman