Antennas and Propagation Chapter 5
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
Radiation Patterns Radiation pattern Graphical representation of radiation properties of an antenna Depicted as two-dimensional cross section Beam width (or half-power beam width) Measure of directivity of antenna Reception pattern Receiving antenna s equivalent to radiation pattern
Types of Antennas Isotropic antenna (idealized) Radiates power equally in all directions Dipole antennas Half-wave dipole antenna (or Hertz antenna) Quarter-wave vertical antenna (or Marconi antenna) Parabolic Reflective Antenna Directed concentrated waves from single focus Better single with less dispersion
Types of Antennas Isotropic Dipole Parabolic
Antenna Patterns
Antenna Gain Antenna gain Power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna) Effective area Related to physical size and shape of antenna
Antenna Gain Relationship between antenna gain and effective area G 4A e 2 4f G = antenna gain A e = effective area f = carrier frequency c = speed of light 3 10 8 m/s) = carrier wavelength c 2 2 A e
Example For a parabolic reflective antenna with a 2m Dia. operating at 12 GHz. What is the effective area and antenna gain? A= r 2 = A e = 0.56 (chart on page 100) = c/f = 3 x 10 8 / 12 x 10 9 = 0.025m G 4A e 2 4f G = 4*0.56 2 /0.025m) 2 35,186 10 log 35,186 = G B = 45.46 db c 2 2 A e
Propagation Modes Ground-wave propagation Sky-wave propagation Line-of-sight propagation
Ground Wave Propagation
Ground Wave Propagation Follows contour of the earth Can Propagate considerable distances Frequencies up to 2 MHz Example AM radio
Sky Wave Propagation
Sky Wave Propagation Signal reflected from ionized layer of atmosphere back down to earth 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
Line-of-Sight Propagation
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
Line-of-Sight (LOS) Equations Optical line of sight d 3. 57 Effective, or radio, line of sight h d 3. 57 h d = distance between antenna and horizon (km) h = antenna height (m) K = adjustment factor to account for refraction, rule of thumb K = 4/3
Line-of-Sight (LOS) Equations Maximum distance between two antennas for LOS propagation: ) 3.57 h h h 1 = height of antenna one h 2 = height of antenna two 1 2
LOS Example Maximum distance between two antennas for LOS if one is at 100m and the other is at ground level. d 3. 57 h d = 3.57 * sqrt(4/3 * 100) = 41 km
LOS Wireless Transmission Impairments 1. Free space loss Primary cause of loss in satellite communication 2. Attenuation and attenuation distortion 3. Noise 4. Atmospheric absorption 5. Multipath 6. Refraction 7. Thermal noise
Free Space Loss Free space loss, ideal isotropic antenna P t P r ) 2 4d 4fd ) 2 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) c 2 2
Free Space Loss Free space loss equation can be recast: L db d and are in meters Pt 10log P r 4d 20log ) 20log d) 21.98 db 20log 4fd 20 log 20 log c f ) 20log d ) 147.56 db
Free Space Loss Free space loss accounting for gain of other antennas P t P r ) 2 ) 2 ) 2 4 d d cd ) G G r t A G t = gain of transmitting antenna G r = gain of receiving antenna 2 A t = effective area of transmitting antenna A r = effective area of receiving antenna r A t G f 2 4A e 2 A A r 2 t 4f c 2 2 A e
Free Space Loss Free space loss accounting for gain of other antennas can be recast as L db ) 20log d) 10log A A ) 20log f ) 20log d ) 10log A A t ) 169.54dB 20log t r r
Free Space Loss Determine the free space loss at 4 GHz with a distance of 35,863 km = c/f = 3 x 10 8 / 4 x 10 9 = 0.075m L db ) 20log d) 21.98 db 20log 20log ) 6 0.075 20 log 35.85310 ) 21.98 db195.6db
Attenuation Strength of signal falls off with distance over transmission medium Attenuation factors for unguided media: Received signal must have sufficient strength so that circuitry in the receiver can interpret the signal Signal must maintain a level sufficiently higher than noise to be received without error Attenuation is greater at higher frequencies, causing distortion
Categories of Noise Thermal Noise Intermodulation noise Crosstalk Impulse Noise
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
Thermal Noise Amount of thermal noise to be found in a bandwidth of 1Hz in any device or conductor is: N kt 0 W/Hz ) 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)
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 N ktb N 10logk 10 log T 10log B 228.6 dbw 10 log T 10log B
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 Crosstalk unwanted coupling between signal paths 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
Expression E b /N 0 Ratio of signal energy per bit to noise power density per Hertz E b S / R S N N ktr 0 0 The bit error rate for digital data is a 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
Other Impairments 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
Multipath Propagation
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 wavelength of radio wave Scattering occurs when incoming signal hits an object whose size in the order of the wavelength of the signal or less
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
Types of Fading Fast fading Changes in signal strength between a transmitter and receiver as the distance between the two changes by a small distance of about onehalf a wavelength. Slow fading Changes in signal strength between a transmitter and receiver as the distance between the two changes by a larger distance, well in excess of a wavelength. Flat fading - Nonselective fading Type of fading in which all frequency components of the received signal fluctuate in the same proportions simultaneously
Types of Fading Selective fading Affects unequally the different spectral components of a radio signal. Rayleigh fading Multiple indirect paths of transmission with no dominant path Rician fading Direct LOS path with multiple indirect paths of transmission with no dominant path
Slow and Fast Fading
Error Compensation Mechanisms Forward error correction Adaptive equalization Diversity techniques
Forward Error Correction (FEC) 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
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 Involves gathering dispersed symbol energy back into its original time interval Techniques Lumped analog circuits Sophisticated digital signal processing algorithms
Diversity Techniques Diversity is based on the fact that individual channels experience independent fading events Space diversity techniques involving physical transmission path 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
Transmission Loss for test cables Your average test cable is 3 ft long Your test cable is coax which has an average capacitance of 29.5 pf/ ft and 73.75 nh/ft The propagation delay for a 1 ft cable is: 9 12 t LC 73.7510 29.510 1. 457ns
Transmission Loss for test cables Your velocity of propagation (Vp) is how fast the signal travels down the wire per foot of length. Vp d LC 3 ft 1.457ns 2.06 10 9 ft s 2.06 10 9 ft s 0.3048 ft m 6.2810 8 m s
Wavelength for test cables The wavelength of the test cable is determined by the formula = c/f where c is usually the speed of light (3 x 10 8 m/s) in this case we will use Vp instead of c. What is the wave length of a 3 ft test lead at 1 MHz? Vp f 6.2810 110 6 8 Hz m s 628m
Find odd harmonics of test cables The wavelength of the odd harmonics are 3, 5,7,etc. What are the wavelengths of a 3 ft test lead at 1 MHz at 3 rd, 5 th & 7 th harmonics? 1628m, this is called the fundamental 3 3 628m 1884m 5 5 628m 3140m 7 7 628m 4396m
Find odd harmonic frequencies of a test cable at 1 MHz f 3rd Vp 3 6.2810 8 1884m m s 333.33 khz f 5th Vp 5 6.2810 8 3140m m s 200 khz f 7th Vp 7 6.2810 8 4396m m s 142.86 khz
Homework Page 124 Problems 5.1, 5.2, 5.3, 5.4 & 5.5