Antennas and Propagation

1. Antennas and Propagation Chapter 5

2. Introduction • An antenna is an electrical conductor or system of conductors • Transmission - radiates electromagnetic energy into space (involves both E and H fields) • Reception - collects electromagnetic energy from space • In two-way communication, the same antenna can be used for transmission and reception (simplex or with duplexers to isolate the different transmit and receive frequencies) • Reference Data for Radio Engineers and similar handbooks provide good reference sources for antennas and propagation topics

3. Radiation Patterns • Radiation pattern • A graphical representation of the radiation properties of an antenna (far-field) • Idealized (perfect ground), impacts by the surrounding environment normally neglected • Depicted as two-dimensional cross section (elevation & azimuth) • Beam width (or half-power beam width) • Measure of directivity of antenna • Reception pattern • Receiving antenna’s equivalent to radiation pattern • Antenna modeling software very common tool (computer based and very accurate, e.g., NEC, MININEC)

4. Types of Antennas • Isotropic antenna (idealized, free space environment) • Radiates power equally in all directions • Dipole antennas • Half-wave dipole antenna (or Hertz antenna) • Quarter-wave vertical antenna (or Marconi antenna, normally vertically polarized) • Parabolic Reflective Antenna • Focus

5. Antenna Gain • Antenna Gain (Directivity) • Power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna [usual reference is an isotropic antenna (dBi) but sometimes a simple ½  antenna is a more practical reference; good sales trick to use an isotropic reference when a dipole is inferred resulting in a 1.64 power gain] • Antenna gain doesn’t increase power; only concentrates effective radiation pattern • Effective area (related to antenna aperture) • Physical size and shape of antenna as related to the operational wavelength of the antenna

6. Some Simple Antenna Patterns Feed Point – one connection to vertical (monopole) and one connection to the ground plane Feed Point at (balanced feed)

7. Antenna Gain • Relationship between antenna gain and effective area • G = antenna gain • Ae= effective area (see Table 5.2 for Area to Ae relationships) • f = carrier frequency • c = speed of light (~ 3 x 108 m/s in a vacuum) •  = carrier wavelength

8. Propagation Modes • Ground-wave propagation • Sky-wave propagation • Line-of-sight propagation • See Table 5-3 for all aspects of RF transmission characteristics

9. Ground Wave Propagation (LF/MF)

10. Ground Wave Propagation • Follows contour of the earth • Can propagate considerable distances • Frequencies up to 2 MHz (all frequencies will have some ground wave/near field) • Examples • AM radio (generally) • LF and MF • Low frequencies which can be effected by daytime/nighttime

11. Sky Wave Propagation (HF) Normally only one hop

12. Sky Wave Propagation • Signal reflected from ionized layer of atmosphere back down to earth (dependent on sun’s radiation) • Signal can travel a number of hops, back and forth between ionosphere and earth’s surface; both a short path and a long path (opposite direction around earth) can occur • Reflection effect caused by refraction • Examples (3 – 30 MHz) • Amateur radio • Short-wave radio • Good propagation models based on sun observations are readily available (MUF)

13. Line-of-Sight Propagation(VHF and above)

14. Line-of-Sight Propagation • Transmitting and receiving antennas must be within line of sight • Satellite communication – signal above 30 MHz not reflected by ionosphere • Ground communication – antennas within effective line of site due to 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

15. 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

16. Line-of-Sight Equations • Maximum distance between two antennas for LOS propagation: • h1 = height of antenna one in meters • h2 = height of antenna two in meters • Note that d is in kilometers (km)

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

18. Attenuation • Strength of signal falls off with distance over transmission medium (exponential) • Attenuation factors for unguided media: • Received signal must have sufficient strength so that circuitry in the receiver can interpret the signal (without overloading the front-end of the receiver) – receiver sensitivity related to internally generated noise • Signal must maintain a level sufficiently higher than noise to be received without error • Attenuation is greater at higher frequencies and any attenuation results in signal distortion

19. Free Space Loss • Free space loss for an ideal isotropic antenna • Pt = signal power at transmitting antenna • Pr = signal power at receiving antenna •  = carrier wavelength [ c =  f ] • d = propagation distance between antennas • c = speed of light ( ~ 3 x 108 m/s in a vacuum ) where d and  are in the same units (e.g., meters)

20. Free Space Loss • Free space loss equation can be recast: or using f

21. Free Space Loss (dB)

22. Free Space Loss • Free space loss accounting for gain of other antennas • Gt = gain of transmitting antenna • Gr = gain of receiving antenna • At = effective area of transmitting antenna (aperture) • Ar = effective area of receiving antenna • d and λ in meters

23. Free Space Loss • Free space loss accounting for gain of other antennas can be recast as

24. Categories of Noise • Thermal Noise • Intermodulation noise • Crosstalk • Impulse Noise

25. Thermal Noise • Thermal noise due to agitation of electrons • Present in all electronic devices and transmission media (white noise) • Function of temperature • Cannot be eliminated (except at temperatures of absolute 0oK) • Particularly significant for satellite communication (where the satellite frequencies don’t have many other noise sources)

26. Thermal Noise • Amount of thermal noise to be found in a bandwidth of 1Hz for any device or conductor is: • N0 = noise power density in watts per 1 Hz of bandwidth • k = Boltzmann's constant = 1.3803 x 10-23 J/K • T = temperature, in Kelvins (absolute temperature)

27. Thermal Noise • Noise is assumed to be independent of frequency • Thermal noise present in a bandwidth of B (Hertz) for N in watts, k = Boltzmann’s Constant = 1.38 X 10-23 Joules/KelvinT = temperature in degrees Kelvin (absolute temperature) or for N in decibel-watts (dBW)

28. Noise Terminology • Intermodulation noise – occurs if signals with different frequencies share the same medium in association with some nonlinear device • Interference caused by a signal produced at a frequency that can be multiples of the sum or difference of original frequencies; result of nonlinear devices (a mixer - just about all electronic devices are nonlinear) • Crosstalk – unwanted coupling between signal paths (excessive signal strength, no isolation, etc.) • Impulse noise – irregular pulses or noise spikes • Short duration and of relatively high amplitude • Caused by external electromagnetic disturbances (lightning), or faults and flaws in the communications system • Not a big problem for analog data but the primary error source for digital transmission

29. Expression Eb/N0 a commonly used ratio in digital communications • Ratio of signal energy per bit to noise power density per Hertz • The bit error rate for digital data is a function of Eb/N0 • Given a value for Eb/N0 to achieve a desired error rate, parameters of this formula can be selected • As bit rate R increases, transmitted signal power (S) must increase to maintain required Eb/N0 • Ratio doesn’t depend on bandwidth as does SNR (Shannon’s maximum channel capacity which has a bandwidth term) (Signal Power) (Time for 1 bit) Noise Power

30. Spectral Efficiency for digital signals Rewriting Shannon’s Channel Capacity C wrt SNR (noise) C = B log2(1 + S/N) S/N = 2 C/B – 1 thus Eb/No = B/C(2 C/B – 1) Which allows us to the find the required noise ratio Eb/No for a given spectral efficiency C/B (see Example 5.6 on page 113)

31. Other Impairments • Atmospheric absorption – water vapor and oxygen contribute to attenuation (microwave) • Multipath – obstacles reflect signals so that multiple copies with varying delays are received (shadow fading – obstruction of signal by objects in the straight-line path) • Refraction – bending of radio waves as they propagate through the atmosphere

32. Multipath Propagation • Reflection - occurs when signal encounters a surface that is large relative to the wavelength of the signal • Diffraction - occurs at the edge of an impenetrable body that is large compared to the wavelength of the radio wave (signals received without a direct line-of-sight) • Scattering – occurs when incoming signal hits an object whose size in the order of the wavelength of the signal or less (difficult to predict) • If there isn’t a clear LOS, multipath can be the primary means of signal reception

33. Multipath Propagation Figure 5.10 Sketch of Reflection (R), Scattering (S) and Diffraction (D) Propagation Mechanisms in a non-LOS case

34. Wireless Propagation Prediction Tool(Wireless Insite Site Specific Simulation Tool from Remcom) • Originally developed for outdoor urban radio propagation predictions. Just one of many models and tools available. • Extended to irregular terrain, foliage, indoor and indoor-outdoor predictions. • Physics-based models using ray methods with GTD/UTD diffraction (geometric/uniform theory of diffraction) • Uses computer graphics techniques for fast ray shooting. • Includes 2D, full vector 3D, and fast 3D algorithms. • Evaluate E-fields using UTD and material dependent reflection and transmission coefficients, combine E-fields with antenna patterns to find path loss, time-of-arrival, angle-of-arrival, etc.

35. Propagation ModelsWireless Insite Continued • Two-Dimensional “Urban Canyon” Model Assumes very tall buildings, a flat ground and low antenna heights Omits paths over rooftops and shadowing by terrain • Full Three-Dimensional Model Executes a full 3-D ray tracing Full implementation of UTD formulation • Fast Three-Dimensional Model Primarily for urban environments Based on a 2-D ray tracing including transmitted rays Analytically determines over-rooftop propagation paths • Irregular Terrain Model Propagation paths in the vertical plane

36. GUI - Features Wireless Insite Continued • Import, edit and view geometrical features (buildings, terrain, foliage, etc.) • Specify electromagnetic properties of materials • Specify location and properties of antennas • Specify location of field points • Set frequency or waveform characteristics • Set calculation parameters • Select desired output • Plot and view output • Extensive website details available at www.remcom.com

37. Wireless RF Environment (slow)

38. Types of Fading • Fast fading (usually movement over very short distances) • Slow fading (movement in excess of wavelengths; environment) • Flat fading (or non-selective fading, constant fading over entire signal frequencies, e.g., path loss) • Selective fading ( e.g., dependent on frequency, unequal over the frequencies associated the signal) • Rayleigh fading (multiple indirect paths, e.g., no LOS, thus multipath components dominate, worst-case scenario, can be the dominant factor in an outdoor environment, special case of Rician distribution.) • Rician fading (direct LOS path and a number of weaker indirect paths/small-scale fading – multipaths such as found in an indoor environment. As the dominant LOS becomes weaker, e.g. fades away, the composite signal degenerates from a Rician distribution to a Rayleigh distribution.)

39. 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) (especially digital) • One or more delayed copies of a pulse may arrive at the same time as the primary pulse for a subsequent bit, e.g., modulation pulses are spread in time into the adjacent symbols. The modulation bandwidth exceeds the coherence bandwidth of the channel. • This is the major obstacle to high speed data transmission over wireless channels.

40. Intersymbol Interference (ISI) Original Transmission Received

41. Error Compensation Mechanisms • Forward error correction (coding) • Adaptive equalization • Diversity techniques • All three categories are used to combat error rates in a wireless communications system • Good technical reference: Chapter 7 in Rappaport’s Wireless Communications textbook

42. Forward Error Correction • Transmitter adds error-correcting code to data • 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 • Very high levels of redundancy in wireless (2  3 times) • Backward Error Correction would be ACK/NAK, request to retransmit. Not practical (time delays).

43. Adaptive Equalization • Can be applied to transmissions that carry analog or digital information in a channel with time varying characteristcs. • Analog voice or video • Digital data, digitized voice or video • Used to combat intersymbol interference (ISI), a major obstacle, created by multipath within time dispersive channels • Involves ‘restoring’ dispersed symbol energy back into its original time interval • Techniques • Lumped analog circuits • Sophisticated digital signal processing algorithms (usually adaptive, processor based techniques) • Linear Equalization Circuit Figure 5-14 (DSP)

44. Block Diagram of a Simplified Communications System Using a Receiver Adaptive Equalizer

45. Diversity Techniques • Diversity is based on the fact that individual channels experience independent fading events • Space diversity – techniques involving physical transmission path (multiple antennas) • Frequency diversity – techniques where the signal is spread out over a larger frequency bandwidth or carried on multiple frequency carriers (spread spectrum) • Time diversity – techniques aimed at spreading the data out over time (effective on fast fading in conjunction with FEC techniques)

46. Interleaving Data to Spread the Effects of Error Bursts

47. Useful Web Sites from Stallings Chapter 5 - Antennas and Propagation • About Antennas: Good source of information and links. • U.S. Frequency Allocation Chart: Chart plus background paper

48. Suggested Chapter 5 Problems • Review Questions • Problems 5.3, 5.5, 5.11, 5.14 and 5.17 • Discussion question – Why is the move of WLANs to 5 Ghz (from 2.5 GHz) impacting device battery life? (Hint: radio physics)

49. A Hobby Gone Amok 7-30 MHz Log Periodic in 1985 3 element 40 M Beam & UHF/VHF Log Periodic Erection of 40 Meter Beam