Principles of Electronic Communication Systems

1. Principles of ElectronicCommunication Systems Third Edition Louis E. Frenzel, Jr.

2. Chapter 14 Antennas and Wave Propagation

3. Topics Covered in Chapter 14 • 14-1: Antenna Fundamentals • 14-2: Common Antenna Types • 14-3: Radio-Wave Propagation

4. 14-1: Antenna Fundamentals • The interface between the transmitter and free space and between free space and the receiver is the antenna. • At the transmitting end the antenna converts the transmitter RF power into electromagnetic signals; at the receiving end the antenna picks up the electromagnetic signals and converts them into signals for the receiver.

5. 14-1: Antenna Fundamentals Radio Waves • A radio signal is called an electromagnetic wave because it is made up of both electric and magnetic fields. • Whenever voltage is applied to the antenna, an electric field is set up. • This voltage causes current to flow in the antenna, producing a magnetic field. • These fields are emitted from the antenna and propagate through space at the speed of light.

6. 14-1: Antenna Fundamentals Radio Waves: Magnetic Fields • A magnetic field is an invisible force field created by a magnet. • An antenna is a type of electromagnet. • A magnetic field is generated around a conductor when current flows through it. • The strength and direction of the magnetic field depend upon the magnitude and direction of the current flow. • The SI unit for magnetic field strength is ampere-turns per meter.

7. 14-1: Antenna Fundamentals Figure 14-1: Magnetic field around a current-carrying conductor. Magnetic field strength H in ampere-turns per meter = H = II(2 πd).

8. 14-1: Antenna Fundamentals Radio Waves: Electric Field • An electric field is an invisible force field produced by the presence of a potential difference between two conductors. • For example, an electric field is produced between the plates of a charged capacitor. • An electric field exists between any two points across which a potential difference exists. • The SI unit for electric field strength is volts per meter. • Permittivityis the dielectric constant of the material between the two conductors.

9. 14-1: Antenna Fundamentals Figure 14-2: Electric field across the plates of a capacitor.

10. 14-1: Antenna Fundamentals Radio Waves: Magnetic and Electric Fields in a Transmission Line • At any given time in a two-wire transmission line, the wires have opposite polarities. • During one-half cycle of the ac input, one wire is positive and the other is negative. • During the negative half-cycle, the polarity reverses. • The direction of the electric field between the wires reverses once per cycle. • The direction of current flow in one wire is always opposite that in the other wire. Therefore, the magnetic fields combine.

11. 14-1: Antenna Fundamentals Radio Waves: Magnetic and Electric Fields in a Transmission Line • A transmission line is made up of a conductor or conductors. • Transmission lines do not radiate signals efficiently. • The closeness of the conductors keeps the electric field concentrated in the transmission line dielectric. • The magnetic fields mostly cancel one another. • The electric and magnetic fields do extend outward from the transmission line, but the small amount of radiation that does occur is extremely inefficient.

12. 14-1: Antenna Fundamentals Figure 14-3: (a) Magnetic and electric fields around a transmission line. (b) Electric field. (c) Magnetic fields.

13. 14-1: Antenna Fundamentals Antenna Operation: The Nature of an Antenna • If a parallel-wire transmission line is left open, the electric and magnetic fields escape from the end of the line and radiate into space. • This radiation is inefficient and unsuitable for reliable transmission or reception. • The radiation from a transmission line can be greatly improved by bending the transmission-line conductors so they are at a right angle to the transmission line.

14. 14-1: Antenna Fundamentals Antenna Operation: The Nature of an Antenna • The magnetic fields no longer cancel; they now aid one another. • The electric field spreads out from conductor to conductor. • Optimum radiation occurs if the segment of transmission wire converted into an antenna is one quarter wavelength long at the operating frequency. • This makes an antenna that is one-half wavelength long.

15. 14-1: Antenna Fundamentals Figure 14-5: Converting a transmission line into an antenna. (a) An open transmission line radiates a little. (b) Bending the open transmission line at right angles creates an efficient radiation pattern.

16. 14-1: Antenna Fundamentals Antenna Operation • The ratio of the electric field strength of a radiated wave to the magnetic field strength is a constant and is called the impedance of space, or the wave impedance. • The electric and magnetic fields produced by the antenna are at right angles to one another, and are both perpendicular to the direction of propagation of the wave.

17. 14-1: Antenna Fundamentals Antenna Operation • Antennas produce two sets of fields, the near field and the far field. • The near fielddescribes the region directly around the antenna where the electric and magnetic fields are distinct. • The far fieldis approximately 10 wavelengths from the antenna. It is the radio wave with the composite electric and magnetic fields. • Polarization refers to the orientation of magnetic and electric fields with respect to the earth.

18. 14-1: Antenna Fundamentals Antenna Reciprocity • Antenna reciprocity means that the characteristics and performance of an antenna are the same whether the antenna is radiating or intercepting an electromagnetic signal. • A transmitting antenna takes a voltage from the transmitter and converts it into an electromagnetic signal. • A receiving antenna has a voltage induced into it by the electromagnetic signal that passes across it.

19. 14-1: Antenna Fundamentals The Basic Antenna • An antenna can be a length of wire, a metal rod, or a piece of tubing. • Antennas radiate most effectively when their length is directly related to the wavelength of the transmitted signal. • Most antennas have a length that is some fraction of a wavelength. • One-half and one-quarter wavelengths are most common.

20. 14-2: Common Antenna Types The Dipole Antenna • One of the most widely used antenna types is the half-wave dipole. • The half-wave dipole, also called a doublet, is formally known as the Hertz antenna. • A dipole antenna is two pieces of wire, rod, or tubing that are one-quarter wavelength long at the operating resonant frequency. • Wire dipoles are supported with glass, ceramic, or plastic insulators at the ends and middle.

21. 14-2: Common Antenna Types Figure 14-10: The dipole antenna.

22. 14-2: Common Antenna Types The Dipole Antenna • The dipole has an impedance of 73 Ωat its center, which is the radiation resistance. • An antenna is a frequency-sensitive device. • To get the dipole to resonate at the frequency of operation, the physical length must be shorter than the one-half wavelength computed by λ = 492/f. • Actual length is related to the ratio of length to diameter, conductor shape, Q, the dielectric (when the material is other than air), and a condition known as end effect.

23. 14-2: Common Antenna Types The Dipole Antenna • End effect is a phenomenon caused by any support insulators used at the ends of the wire antenna and has the effect of adding capacitance to the end of each wire. • The actual antenna length is only about 95 percent of the computed length. • If a dipole is used at a frequency different from its design frequency, the SWR rises and power is lost.

24. 14-2: Common Antenna Types The Dipole Antenna: Antenna Q and Bandwidth • The bandwidth of an antennais determined by the frequency of operation and the Q of the antenna according to the relationship BW = fr/Q. • The higher the Q, the narrower the bandwidth. • For an antenna, low Q and wider bandwidth are desirable so that the antenna can operate over a wider range of frequencies with reasonable SWR. • In general, any SWR below 2:1 is considered good in practical antenna work.

25. 14-2: Common Antenna Types The Dipole Antenna: Antenna Q and Bandwidth • The Q and thus the bandwidth of an antenna are determined by the ratio of the length of the conductor to the diameter of the conductor. • Bandwidth is sometimes expressed as a percentage of the resonant frequency of the antenna. • A small percentage means a higher Q, and a narrower bandwidth means a lower percentage.

26. 14-2: Common Antenna Types The Dipole Antenna: Conical Antennas • A common way to increase bandwidth is to use a version of the dipole antenna known as the conical antenna. • The center radiation resistance of a conical antenna is much higher than the 73 Ωusually found when straight-wire or tubing conductors are used. • The primary advantage of conical antennas is their tremendous bandwidth. • They can maintain a constant impedance and gain over a 4:1 frequency range.

27. 14-2: Common Antenna Types Figure 14-14: The conical dipole and its variation. (a) Conical antenna. (b) Broadside view of conical dipole antenna (bow tie antenna) showing dimensions. (c) Open-grill bow tie antenna.

28. 14-2: Common Antenna Types The Dipole Antenna: Dipole Polarization • Most half-wave dipole antennas are mounted horizontally to the earth. • This makes the electric field horizontal to the earth and the antenna is horizontally polarized. • Horizontal mounting is preferred at the lower frequencies because the physical construction, mounting, and support are easier. • This mounting makes it easier to attach the transmission line and route it to the transmitter or receiver.

29. 14-2: Common Antenna Types The Dipole Antenna: Radiation Pattern and Directivity • The radiation pattern of any antenna is the shape of the electromagnetic energy radiated from or received by that antenna. • Most antennas have directional characteristics that cause them to radiate or receive energy in a specific direction. • The radiation is concentrated in a pattern that has a recognizable geometric shape. • The measure of an antenna’s directivity is beam width, the angle of the radiation pattern over which a transmitter’s energy is directed or received.

30. 14-2: Common Antenna Types Figure 14-15: Three-dimensional pattern of a half-wave dipole.

31. 14-2: Common Antenna Types The Dipole Antenna: Antenna Gain • A directional antenna can radiate more power in a given directionthan a nondirectional antenna. In this “favored” direction, it acts as if it had gain. • Antenna gain of this type is expressed as the ratio of the effective radiatedoutput power Pout to the input power Pin.

32. 14-2: Common Antenna Types The Dipole Antenna: Antenna Gain • Effective radiated power is the actual power that would have to be radiated by a reference antenna (usually a nondirectional or dipole antenna) to produce the same signal strength at the receiver as the actual antenna produces. • The power radiated by an antenna with directivity and therefore gain is called the effective radiated power (ERP). ERP = ApPt

33. 14-2: Common Antenna Types The Dipole Antenna: Folded Dipole • A popular variation of the half-wave dipole is the folded dipole. • The folded dipole is also one-half wavelength long. • It consists of two parallel conductors connected at the ends with one side open at the center for connection to the transmission line. • The impedance of this antenna is 300 Ω. • Folded dipoles usually offer greater bandwidth than standard dipoles. • The folded dipole is an effective, low-cost antenna that can be used for transmitting and receiving.

34. 14-2: Common Antenna Types Figure 14-18: Folded dipole. (a) Basic configuration. (b) Construction with twin lead.

35. 14-2: Common Antenna Types Marconi or Ground-Plane Vertical Antenna • The one-quarter wavelength vertical antenna, also called a Marconi antenna, is widely used. • It is similar in operation to a vertically mounted dipole antenna. • The Marconi antenna offers major advantages because it is half the length of a dipole antenna.

36. 14-2: Common Antenna Types Marconi or Ground-Plane Vertical Antenna: Radiation Pattern • Vertical polarization and omnidirectional characteristics can be achieved using a one-quarter wavelength vertical radiator. This antenna is called a Marconi or ground-plane antenna. • It is usually fed with coaxial cable; the center conductor is connected to the vertical radiator and the shield is connected to earth ground. • The earth then acts as a type of electrical “mirror,” providing the other one-quarter wavelength making it equivalent to a vertical dipole.

37. 14-2: Common Antenna Types Figure 14-20: Ground-plane antenna. (a) One-quarter wavelength vertical antenna. (b) Using radials as a ground plane.

38. 14-2: Common Antenna Types Marconi or Ground-Plane Vertical Antenna: Ground Plane, Radials, and Counterpoise • When a good electrical connection to the earth has been made, the earth becomes what is known as a ground plane. • If a ground plane cannot be made to earth, an artificial ground can be constructed of several one-quarter wavelength wires laid horizontally on the ground or buried in the earth. • These horizontal wires at the base of the antenna are called radials,and the collection of radials is called a counterpoise.

39. 14-2: Common Antenna Types Marconi or Ground-Plane Vertical Antenna: Antenna Length • For many applications, e.g., with portable or mobile equipment, it is not possible to make the antenna a full one-quarter wavelength long. • To overcome this problem, shorter antennas are used, and lumped electrical components are added to compensate for the shortening.

40. 14-2: Common Antenna Types Marconi or Ground-Plane Vertical Antenna: Antenna Length • The practical effect of this design is a decreased inductance. The antenna no longer resonates at the desired operating frequency, but at a higher frequency. • To compensate for this, a series inductor, called a loading coil,is connected in series with the antenna coil. • The loading coil brings the antenna back into resonance at the desired frequency.

41. 14-2: Common Antenna Types Figure 14-22: Using a base leading coil to increase effective antenna length.

42. 14-2: Common Antenna Types Directivity • Directivity refers to an antenna’s ability to send or receive signals over a narrow horizontal directional range. • The physical orientation of the antenna gives it a highly directional response or directivity curve. • A directional antenna eliminates interference from other signals being received from all directions other than the desired signal.

43. 14-2: Common Antenna Types Directivity • A highly directional antenna acts as a type of filter to provide selectivity. • Directional antennas provide greater efficiency of power transmission. • Directivity, because it focuses the power, causes the antenna to exhibit gain, which is one form of amplification.

44. 14-2: Common Antenna Types Figure 14-25: Radiation pattern of a highly directional antenna with gain. (a) Horizontal radiation pattern. (b) Three-dimensional radiation pattern.

45. 14-2: Common Antenna Types Directivity • To create an antenna with directivity and gain, two or more antenna elements are combined to form an array. • Two basic types of antenna arrays are used to achieve gain and directivity: • Parasitic arrays. • Driven arrays.

46. 14-2: Common Antenna Types Parasitic Arrays • A parasitic array consists of a basic antenna connected to a transmission line plus one or more additional conductors that are not connected to the transmission line. • These extra conductors are referred to as parasitic elements and the antenna is called a driven element. • A Yagi antenna is made up of a driven element and one or more parasitic elements.

47. 14-2: Common Antenna Types Figure 14-26: A parasitic array known as a Yagi antenna.

48. 14-2: Common Antenna Types Driven Arrays • A driven array is an antenna that has two or more driven elements. • Each element receives RF energy from the transmission line. • Different arrangements of the elements produce different degrees of directivity and gain. • The three basic types of driven arrays are the collinear, the broadside, and the end-fire. • A fourth type is the wide-bandwidth log-periodic antenna.

49. 14-2: Common Antenna Types Driven Arrays: Collinear Antenna • Collinear antennas usually consist of two or more half-wave dipoles mounted end to end. • Collinear antennas typically use half-wave sections separated by shorted quarter-wave matching stubs which ensure that the signals radiated by each half-wave section are in phase. • Collinear antennas are generally used only on VHF and UHF bands because their length becomes prohibited at the lower frequencies.

50. 14-2: Common Antenna Types Figure 14-29: Radiation pattern of a four-element collinear antenna.