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 as a function of space coordinates Depicted as two-dimensional cross section to characterize the performance of an antenna Reception pattern Receiving antenna’s equivalent to radiation pattern

The distance from the antenna to each point on the radiation pattern is proportional to the power radiated from the antenna in that direction. The isotropic antenna produces an omnidirectional radiation pattern of equal strength in all directions, so the A and B vectors are of equal length. For the antenna pattern of Figure b, the B vector is longer than the A vector, indicating that more power is radiated in the B direction than in the A direction

Radiation Pattern Beam width (or half-power beam width) Measure of directivity of antenna is the angle within which the power radiated by the antenna is at least half of what it is in the most preferred direction.

Time vs. Distance When the horizontal axis is time, as in Figure 2.3, graphs display the value of a signal at a given point in space as a function of time With the horizontal axis in space, graphs display the value of a signal at a given point in time as a function of distance At a particular instant of time, the intensity of the signal varies as a function of distance from the source Wavelength () - distance occupied by a single cycle of the signal Or, the distance between two points of corresponding phase of two consecutive cycles the intensity of the signal varies in a sinusoidal way as a function of distance from the source.

Types of Antennas Isotropic antenna (idealized) Dipole antennas A point in space that radiates power equally in all directions Radiation pattern is a sphere with the antenna at the center Dipole antennas Half-wave dipole antenna (or Hertz antenna) Quarter-wave vertical antenna (or Marconi antenna) Parabolic Reflective Antenna

Types of Antennas λ = c/f There is an inverse relationship between frequency and wavelength: λ = c/f the lower the frequency, the longer the wavelength (the longer the antenna); the higher the frequency, the shorter the wavelength (The shorter the antenna).

Types of antennas Example for Quarter wave antenna For transmitting a signal of wavelength λ the antenna height must be λ/4. So if we want to send a 1 Hz (λ=3*10^8 m) signal ( very very low frequency) using an antenna , its height must be 75,000 Km ( impossible to build such a huge antenna ). If the same signal is modulated to some high frequency say 88 MHZ ( λ = 3.4 m ) , antenna height needed is 0.8522 m (quite easy to construct !!! )

Parabolic reflective antenna used in terrestrial microwave and satellite applications. A parabola is the locus of all points equidistant from a fixed line and a fixed point not on the line. The fixed point is called the focus and the fixed line is called the directrix (Figure 5.4a). A cross section through the paraboloid parallel to its axis forms a parabola and a cross section perpendicular to the axis forms a circle.

Parabolic reflective antenna If a source of electromagnetic energy (or sound) is placed at the focus of the paraboloid, and if the paraboloid is a reflecting surface, then the wave will bounce back in lines parallel to the axis of the paraboloid; Figure 5.4b shows this effect in cross section. In theory, this effect creates a parallel beam without dispersion. In practice, there will be some dispersion, because the source of energy must occupy more than one point. If incoming waves are parallel to the axis of the reflecting paraboloid, the resulting signal will be concentrated at the focus.

Antenna Gain Antenna gain Power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna) with the same input power. The increased power radiated in a given direction is at the expense of other directions. In effect, increased power is radiated in one direction by reducing the power radiated in other directions.

Antenna Gain The term Antenna Gain describes how much power is transmitted in the direction of peak radiation to that of an isotropic source.  An antenna with a gain of 3 dB means that the power received from the antenna will be 3 dB higher (twice as much) than what would be received from a lossless isotropic antenna with the same input power. Antenna Gain is sometimes discussed as a function of angle, but when a single number is quoted the gain is the 'peak gain' over all directions. The gain of a real antenna can be as high as 40-50 dB for very large dish antennas. Directivity can be as low as 1.76 dB for a real antenna (example: short dipole antenna), but can never theoretically be less than 0 dB.

Antenna Gain Effective area Related to physical size and shape of antenna Relationship between antenna gain and effective area G = antenna gain Ae = effective area f = carrier frequency c = speed of light ( 3 ×108 m/s)  = carrier wavelength

Ground-wave propagation Sky-wave propagation Line-of-sight propagation 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

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 In a vacuum, an electromagnetic wave (such as light or a radio wave) travels at approximately 3 × 10^8 m/s. This is the constant, c, commonly referred to as the speed of light, but actually referring to the speed of light in a vacuum. In air, water, glass, and other transparent or partially transparent media, electromagnetic waves travel at speeds less than c.

This phenomenon is easily observed by partially immersing a stick in water. The result will look much like Figure 5.6, with the stick appearing shorter and bent.

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 sight due to refraction by the atmosphere

Line-of-Sight Propagation The radio horizon is the locus of points at which direct rays from an antenna are tangential to the surface of the Earth.

Line-of-Sight Propagation

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

Line-of-Sight Equations Maximum distance between two antennas for LOS propagation: h1 = height of antenna one h2 = height of antenna two

Line-of-Sight Propagation Max distance between two antenna One is 100m high One is at the ground level ? Receiving antenna is 10m high, to achieve the same distance, how high must be the transmitting antenna?

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

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

Attenuation The first and second factors are dealt with by attention to signal strength and the use of amplifiers or repeaters to boost the signal at regular intervals. The third factor is known as attenuation distortion. Because the attenuation varies as a function of frequency, the received signal is distorted, reducing intelligibility. Frequency components of the received signal have different relative strengths than the frequency components of the transmitted signal. To overcome this problem, techniques are available for equalizing attenuation across a band of frequencies. One approach is to use amplifiers that amplify high frequencies more than lower frequencies.

Free Space Loss A transmitted signal attenuates over distance because the signal is being spread over a larger and larger area. => Free Space Loss attenuation

Free Space Loss Free space loss, ideal isotropic antenna Pt = signal power at transmitting antenna Pr = 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)

Free Space Loss Free space loss equation can be recast: As the frequency increases, the free space loss also increases, which would suggest that at higher frequencies, losses become more burdensome.

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 Ar = effective area of receiving antenna

Free Space Loss Free space loss accounting for gain of other antennas can be recast as for the same antenna dimensions, the longer the carrier wavelength (lower the carrier frequency f), the higher is the free space path loss.

Free Space Loss Isotropic antenna Antenna with gain at a fixed distance an increa.se in frequency results in an increased loss measured by 20 log (f). Antenna with gain At a fixed antenna area, then the change in loss is measured by -20 log(f); there is actually a decrease in loss at higher frequencies.

Categories of Noise Additional unwanted signals that are inserted somewhere between transmission and reception. 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 uniformly distributed across the frequency spectrum and hence is often referred to as white noise.

Thermal Noise Amount of thermal noise to be found in a bandwidth of 1Hz in any device or conductor is: N0 = 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

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 the mixing of signals at frequencies f1and f2 might produce energy at the frequency f1+f2. This derived signal could interfere with an intended signal at the frequency f1+f2. Crosstalk – unwanted coupling between signal paths experienced by anyone who, while using the telephone, has been able to hear another conversation

Noise Terminology 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 Impulse noise is generally only a minor annoyance for analog data. For example, voice transmission may be corrupted by short clicks and crackles with no loss of intelligibility. However, impulse noise is the primary source of error in digital data transmission. For example, a sharp spike of energy of 0.01 s duration would not destroy any voice data but would wash out about 560 bits of data being transmitted at 56 kbps.

Expression Eb/N0 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 BER is a decreasing 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 must increase to maintain required Eb/N0

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

Fading The term fading refers to the time variation of received signal power caused by changes in the transmission medium or path(s). In a fixed environment, fading is affected by changes in atmospheric conditions, such as rainfall. But in a mobile environment, where one of the two antennas is moving relative to the other, the relative location of various obstacles changes over time, creating complex transmission effects.

Types of Fading Fast fading Slow fading Flat fading Selective fading Rayleigh fading Rician fading

Fast and Slow fading

Flat and Selective fading Flat fading, or nonselective fading, is that type of fading in which all frequency components of the received signal fluctuate in the same proportions simultaneously. Selective fading affects unequally the different spectral components of a radio signal.

Rayleigh and Rician Rayleigh fading occurs when there are multiple indirect paths between transmitter and receiver and no distinct dominant path, such as an LOS path. This represents a worst case scenario. Rician fading best characterizes a situation where there is a direct LOS path in addition to a number of indirect multipath signals. The Rician model is often applicable in an indoor environment whereas the Rayleigh model characterizes outdoor settings.

Error Compensation Mechanisms Forward error correction Adaptive equalization Diversity techniques

Forward Error Correction 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 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 Multiple nearby antennas may be used to receive the message, with the signals combined in some fashion to reconstruct the most likely transmitted signal. 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 (TDM)