Antennas and Propagation Review/Recap

1. Antennas and PropagationReview/Recap Lecture 17

2. Overview • Antenna Functions • Isotropic Antenna • Radiation Pattern • Parabolic Reflective Antenna • Antenna Gain • Signal Loss in Satellite Communication • Noise Types • Refraction • Fading • Diffraction and Scattering • Fast and Slow Fading • Flat and Selective Fading • Diversity Techniques ……

3. Review Question: Antenna Functionality Q:- What two functions are performed by an antenna?

4. Antenna Definition • An antenna is defined as usually a metallic device (as a rod or wire) for radiating or receiving radio waves. • The IEEE Standard Definitions of Antenna defines the antenna or aerial as “a means for radiating or receiving radio waves.” In other words the antenna is the transitional structure between free-space and a guiding device, as shown in Figure

5. Why Antennas of Different Shapes • In addition to receiving or transmitting energy, an antenna in an advanced wireless system is usually required to optimize or accentuate the radiation energy in some directions and suppress it in others. • Thus the antenna must also serve as a directional device in addition to a probing device. • It must then take various forms to meet the particular need at hand, and it may be a piece of conducting wire, an aperture, a patch, an assembly of elements (array), a reflector, a lens, and so forth. • For wireless communication systems, the antenna is one of the most critical components. A good design of the antenna can relax system requirements and improve overall system performance. • The antenna serves to a communication system the same purpose that eyes and eyeglasses serve to a human

6. Basic Antenna Functions • As Antenna resides between cable/waveguide and the medium air, the main function of antenna is to match impedance of the medium with the cable/waveguide impedance. Hence antenna is impedance transforming device. • The second and most important function of antenna is to radiate the energy in the desired direction and suppress in the unwanted direction. This basically is the radiation pattern of the antenna. This radiation pattern is different for different types of antennas.

8. The Role of Antennas • Antennas serve four primary functions • Spatial filter • directionally-dependent sensitivity • Polarization filter • polarization-dependent sensitivity • Impedance transformer • transition between free space and transmission line • Propagation mode adapter • from free-space fields to guided waves • (e.g., transmission line, waveguide)

9. Spatial filter • Antennas have the property of being more sensitive in one direction than in another which provides the ability to spatially filter signals from its environment. Radiation pattern of directive antenna. Directive antenna.

10. Polarization filter Antennas have the property of being more sensitive to one polarization than another which provides the ability to filter signals based on its polarization. In this example, h is the antenna’s effective height whose units are expressed in meters.

11. Impedance transformer • Intrinsic impedance of free-space, E/H • Characteristic impedance of transmission line, V/I • A typical value for Z0 is 50 . • Clearly there is an impedance mismatch that must be addressed by the antenna.

12. Propagation Mode Adapter • In free space the waves spherically expand following Huygens principle: each point of an advancing wave front is in fact the center of a fresh disturbance and the source of a new train of waves. • Within the sensor, the waves are guided within a transmission line or waveguide that restricts propagation to one axis.

13. Propagation Mode Adapter • During both transmission and receive operations the antenna must provide the transition between these two propagation modes.

14. Transformation of a guided EM wave in transmission line (waveguide) into a freely propagating EM wave in space (or vice versa) with specified directional characteristics Transformation from time-function in one-dimensional space into time-function in three dimensional space The specific form of the radiated wave is defined by the antenna structure and the environment Antenna purpose Space wave Guided wave

15. Antenna functions • Transmission line • Power transport medium - must avoid power reflections, otherwise use matching devices • Radiator • Must radiate efficiently – must be of a size comparable with the half-wavelength • Resonator • Unavoidable - for broadband applications resonances must be attenuated

16. Review Question: Antenna Functionality Q:- What two functions are performed by an antenna? Ans:- Two functions of an antenna are: • For transmission of a signal, radio frequency electrical energy from the transmitter is converted into electromagnetic energy by the antenna and radiated into the surrounding environment (atmosphere, space, water); • For reception of a signal, electromagnetic energy impinging on the antenna is converted into radio-frequency electrical energy and fed into the receiver.

18. Isotropic Antenna Q:- What is an isotropic antenna?

19. Isotropic Antenna • Isotropic antenna or isotropic radiator is a hypothetical (not physically realizable) concept, used as a useful reference to describe real antennas. • Isotropic antenna radiates equally in all directions. • Its radiation pattern is represented by a sphere whose center coincides with the location of the isotropic radiator.

20. Reference Antenna for Gain • Gain is Measured Specific to a Reference Antenna • isotropic antenna often used - gain over isotropic • Isotropic antenna – radiates power ideally in all directions • Gain measured in dBi • Test antenna’s field strength relative to reference isotropic antenna at same power, distance, and angle • -Isotropic antenna cannot be practically realized e.g. A lamp is similar to an isotropic antenna

21. Isotropic

22. Isotropic sphere An Isotropic Source: Gain • Every real antenna radiates more energy in some directions than in others (i.e. has directional properties) • Idealized example of directional antenna: the radiated energy is concentrated in the yellow region (cone). • Directive antenna gain: the power flux density is increased by (roughly) the inverse ratio of the yellow area and the total surface of the isotropic sphere • Gain in the field intensity may also be considered - it is equal to the square root of the power gain.

23. Reference antenna Measuring equipment Actual antenna Measuring equipment Po = Power delivered to the reference antenna S0 = Power received (the same in both steps) P = Power delivered to the actual antenna S = Power received (the same in both steps) Step 1: reference Step 2: substitution Antenna Gain Measurement Antenna Gain = (P/Po) S=S0

24. Isotropic sphere Isotropic Antenna Q:- What is an isotropic antenna? Ans:- An isotropic antenna is a point in space that radiates power in all directions equally.

26. Review: Radiation Pattern Q:- What information is available from a radiation pattern?

27. Radiation Pattern • In the field of antenna design the term radiation pattern (or antenna pattern or far-field pattern) refers to the directional (angular) dependence of the strength of the radio waves from the antenna or other source. • Particularly in the fields of fiber optics, lasers, and integrated optics, the term radiation pattern may also be used as a synonym for the near-field pattern or Fresnel pattern. This refers to the positional dependence of the electromagnetic field in the near-field, or Fresnel region of the source. The near-field pattern is most commonly defined over a plane placed in front of the source, or over a cylindrical or spherical surface enclosing it. • The far-field pattern of an antenna may be determined experimentally at an antenna range, or alternatively, the near-field pattern may be found using a near-field scanner, and the radiation pattern deduced from it by computation. The far-field radiation pattern can also be calculated from the antenna shape by computer programs such as NEC. Other software, like HFSS can also compute the near field.

28. Antenna Radiation Pattern • Radiation pattern • Graphical representation of radiation properties of an antenna • Depicted as two-dimensional cross section • The radiation pattern of an antenna is a plot of the far-field radiation from the antenna. More specifically, it is a plot of the power radiated from an antenna per unit solid angle, or its radiation intensity U [watts per unit solid angle]. This is arrived at by simply multiplying the power density at a given distance by the square of the distance r, where the power density S [watts per square metre] is given by the magnitude of the time-averaged Poynting vector: U=r²S

29. Radiation pattern • The radiation pattern of antenna is a representation (pictorial or mathematical) of the distribution of the power out-flowing (radiated) from the antenna (in the case of transmitting antenna), or inflowing (received) to the antenna (in the case of receiving antenna) as a function of direction angles from the antenna • Antenna radiation pattern (antenna pattern): • is defined for large distances from the antenna, where the spatial (angular) distribution of the radiated power does not depend on the distance from the radiation source • is independent on the power flow direction: it is the same when the antenna is used to transmit and when it is used to receive radio waves • is usually different for different frequencies and different polarizations of radio wave radiated/ received

30. Auxiliaryantenna Antenna under test Large distance Power or field-strength meter Generator Turntable Power Pattern Vs. Field pattern • The power pattern is the measured (calculated) and plotted received power: |P(θ, ϕ)| at a constant (large) distance from the antenna • The amplitude field pattern is the measured (calculated) and plotted electric (magnetic) field intensity, |E(θ, ϕ)| or |H(θ, ϕ)| at a constant (large) distance from the antenna • The power pattern and the field patterns are inter-related:P(θ, ϕ) = (1/)*|E(θ, ϕ)|2 = *|H(θ, ϕ)|2 • P = power • E = electrical field component vector • H = magnetic field component vector •  = 377 ohm (free-space, plane wave impedance)

31. Normalized pattern • Usually, the pattern describes the normalized field (power) values with respect to the maximum value. • Note: The power pattern and the amplitude field pattern are the same when computed and when plotted in dB.

32. 3-D pattern • Antenna radiation pattern is 3-dimensional • The 3-D plot of antenna pattern assumes both angles θ and ϕ varying, which is difficult to produce and to interpret 3-D pattern

33. 2-D pattern • Usually the antenna pattern is presented as a 2-D plot, with only one of the direction angles, θ or ϕ varies • It is an intersection of the 3-D one with a given plane • usually it is a θ = const plane or a ϕ= const plane that contains the pattern’s maximum Two 2-D patterns

34. Example: a short dipole on z-axis

35. Principal Patterns • Principal patterns are the 2-D patterns of linearly polarized antennas, measured in 2 planes • the E-plane: a plane parallel to the E vector and containing the direction of maximum radiation, and • the H-plane: a plane parallel to the H vector, orthogonal to the E-plane, and containing the direction of maximum radiation

36. Example

37. Typical relative directivity- mask of receiving antenna (Yagi ant., TV dcm waves) Antenna Mask (Example 1)

38. Antenna Mask (Example 2) Reference pattern for co-polar and cross-polar components for satellite transmitting antennas in Regions 1 and 3 (Broadcasting ~12 GHz) 0dB Phi -3dB

39. Review: Radiation Pattern Q:- What information is available from a radiation pattern? Radiation Patterns in Polar and Cartesian Coordinates Showing Various Types of Lobes • Ans:- A radiation pattern is a graphical representation of the radiation properties of an antenna as a function of space coordinates.

41. Parabolic Reflective Antenna Q:- What is the advantage of a parabolic reflective antenna?

42. Two Main Purposes of Antenna • Impedance matching: matches impedance of transmission line to the intrinsic impedance of free space to prevent wanted reflection back to source. • Antenna must be designed to direct the radiation in the desired direction. So a parabolic antenna • is a high gain reflector antenna. It is used for television, radio and data communications. It may also be used for radar on the UHF and SHF sections of the electromagnetic spectrum.

43. Reflector Antenna • Reflector antenna such as parabolic antenna are composed of primary radiator and a reflective mirror.

44. Parabolic Reflector Antenna • Any electromagnetic wave incident upon the paraboloid surface will be directed to the focal point. • Primary antenna is used at the focal point of the parabolic reflector antenna instead of isotropic antenna. The isotropic antenna would radiate and receive radiation from all directions resulting in spillover. • Primary antenna should be designed to “illuminate” just the reflector uniformly.

45. Loss

46. Characteristics • Aperture: • r= radius of the diameter • Larger dish has more gain than smaller • Clear line of sight is important

47. Calculations • Physical area: • D= Diameter • Effective area: • = illumination efficiency • Wavelength: • Gain: • 3db beamwidth:

48. Half Power Beamwidth The half power graph showing the angle between the half power point on either side of maximum

49. Radiation Pattern for Parabolic Antenna

50. Advantage of a Parabolic Antenna • The advantage of a parabolic antenna is that it can be used as primary mirror for all the frequencies in the project, provided the surface is within the tolerance limit; only the feed antenna and the receiver need to be changed when the observing frequency is changed. • An advantage of such a design is the small irradiation loss, which allows for an optimum antenna gain. • It is an advantage of such an arrangement that the exciter system and/or the exciter 3 are/is protectively located within the parabola or the parabolic reflector. • Parabolic antenna is the most efficient type of a directional antenna - large front/back ratio, sharp radiation angle and small side lobes. It fits well for noisy locations where other antennas factually do not work. • The antenna's Gain is adequate to the area of the reflector. The reflector can be central-focused(the focus is in the center of the dish) or offset (the focus is off the axis of the dish). • In general, they serve for connection of end users (so-called last mile) to a wireless network. However, in areas with lower intensity of Wi-Fi networks, they can be successfully used also for back-bone links. In fact, this frequency is used for connections up to maximum 10 km