Presentation on theme: "1. 2 The aim of this Course is to Give a) Basic notions in Radio Propagation at microwave frequencies, b) application to Radio Link Design in the frequency."— Presentation transcript:
2 The aim of this Course is to Give a) Basic notions in Radio Propagation at microwave frequencies, b) application to Radio Link Design in the frequency range from about 450 MHz up to 60 GHz. Means : - Course notes s - Lab simulators
3 Prerequisites: - basic notions in: - Modulation techniques, - Radio equipment and systems - Elementary electromagnetic physics. Conclusion : Course objective : actively involving the reader in navigating through the text and in practicing with exercises in the field of microwave link design.
4 In telecommunications, information can be analog or digital. replaceddigital since the 1970’s, MW Analog systems have been almost completely replaced by digital systems. Now even long distance transmission Now even analog traffic, such as voice calls, are converted to digital signals ( sampling), to facilitate long distance transmission and switching.
5 Terrestrial MW systems have been used since the 1950’s( wartime radar technology). Today, modern digital microwave radio is world widely deployed to transport information over distances of up to 60 kilometers ( sometimes farther). transparent information carried voicedata video Microwave radio is totally transparent to the information carried : which can be voice, data, video, or a combination of all three.
6 Transport can be in a variety of formats :Transport can be in a variety of formats : circuit-switched Time Division Multiplex TDM (TDM) packet packet-based data protocols such as ATM, IP, Ethernet Frame Relay or IP, Ethernet. overlaid In some cases, packetized data can be overlaid on a TDM frame structure such as: - PDH, - SDH or SONET.
7 Microwave radio advantages over cable/fiber- based transmissionMicrowave radio advantages over cable/fiber- based transmission: Rapid Deployment No right-of-way issues – avoid all obstacles Any requirement to seek permissions :cost & time delays. Flexibility Flexibility: simple redeployment & capacity adjustment.
8 Losing customers ≠ Losing assets as in Cables & fibers Easily crosses city terrainEasily crosses city terrain (extremely restricted,& very expensive, to install fiber in city terrains and street crosses). Operator-owned infrastructure - no reliance on competitors. Low start-up capital costsLow start-up capital costs : independent of the link distance.
9 Minimal operational costs. Radio infrastructure already exists Radio infrastructure already exists (rooftops, masts and towers). not susceptible to catastrophic failure Microwave radio is not susceptible to catastrophic failure ( cable cuts,) and can be repaired in minutes instead of hours or days. flood, earthquakes). Better resistance to natural disasters (flood, earthquakes). where the fiber was not always available (the radio is only choice)
10 Fiber is very cost effective where extremely high bandwidths are required. STM-4,radio has an obvious advantage However, in the access portion of the network, where the maximum capacity requirements are less than STM-4, radio has an obvious advantage. Note STM1 = 155 Mbits ; STMn = n*155Mbits
11 The 3 basic components of the radio terminal Two radio terminals are required to establish a MW “hop”. digital terminal equipment 1- digital modem interfaces with digital terminal equipment, converting customer traffic to a modulated radio signal; 2- a radio frequency (RF) unit : Frequency converter + RF amplifier up to around 1 watt. 3- a passive parabolic antenna to transmit and receive the signal.
12 2-Basic configurations for MW terminals Non-protected, ( 1+0) : 1-Non-protected, ( 1+0) : - Any major failure component will result in loss of customer traffic a loss of customer traffic. - cost-effective when traffic is non-critical, or where alternate traffic routing is available. 2-protected (1+1) : Main + hot standby (Monitored Hot Standby (MHSB)) - twice expensive, but No loss of customer traffic.
15 3Space Diversity 3- Space Diversity 4- Frequency Diversity 5- Polarization diversity 6-Angle diversity In addition, Some radios are fitted to use ODU attached directly to the back of the antenna, eliminating antenna feed lines and attendant feed line losses).
16 Used to reinforce the radio dispersive fade margin. The new technology of Mw radio don’t need this type of diversity
18 important characteristicTwo very important characteristic of digital MW transmission is: A- immunity to noise B- the ability of the radio to operate in the presence of adverse environmental conditions. B- the ability of the radio to operate in the presence of adverse environmental conditions.
19 A- immunity to noise NoiseNoise refers to the effects caused by unwanted electromagnetic signals that interfere and corrupt the received signal.
20 “licensed” frequency bands tightlyMicrowave systems operate in so-called “licensed” frequency bands between 2 and 38 GHz (tightly regulation the use of these frequencies ensure that each operator will not cause interference to other links operating in the same area). tightlyworldwide basisITU.The frequency band characteristics are also tightly specified on a worldwide basis by ITU.
21 stringentEquipment are controlled, to meet stringent specifications ( ITU standards, National as FCC and ETSI). “unlicensed” 2.4 and 5.8 GHzThis is in contrast to the “unlicensed” frequency bands of 2.4 and 5.8 GHz : countermeasures to avoid noise and interferencespread-spectrum transmission No control, Unlicensed systems themselves incorporate countermeasures to avoid noise and interference, such as spread-spectrum transmission
22 B- the ability of the radio to operate in the presence of adverse environmental conditions. a perception that microwave is still unreliable due to “fading”. This is largely a remnant of the analog days. However, digital radio systems today are able to counteract fading effects in a number of ways
23 Fading is known to occur as a result of primarily two phenomena. 1- Firstly, multipath interference affects mainly lower frequencies below 18 GHz. 1- Firstly, multipath interference affects mainly lower frequencies below 18 GHz. This happens when the reflected signal arrive slightly later than the direct signal path.it reduces the ability of the receiver to correctly distinguish the data carried on the signal. This happens when the reflected signal arrive slightly later than the direct signal path.it reduces the ability of the receiver to correctly distinguish the data carried on the signal.
24 Fortunately, modern radio systems can compensate for this form of interference through countermeasures such as:Fortunately, modern radio systems can compensate for this form of interference through countermeasures such as: signal equalization [using DSP-filtering to cancel the echoes (pre-echoes & post- echoes) due to Multipath]. Forward Error Correction, diversity receiver configurations. reliability Multipath fade measurement parameter is often called the reliability of the link ) ( (ذا ثقة)-
25 2- precipitation the higher frequenciesabove 18 GHz Secondly, precipitation, mostly in the form of rain, can severely affect microwave radio systems in the higher frequencies above 18 GHz. Microwaves cannot penetrate rain Microwaves cannot penetrate rain, so : the heavier the downpour, and the higher the frequency, the greater the signal attenuation. Rain fade measurement parameter is often called the availability of the link
26 counteract rain fade Although there is noway to counteract rain fade other than higher transmission power. The mechanisms of rain fade are very well understood: models extremely accurate tolerances models have been developed by the ITU to enable links to be planned within extremely accurate tolerances based upon particular rainfall profiles.
27 Conclusion : 99.999%, seconds annually “error-free As a result, modern microwave systems can be designed for extremely high link total availabilities in excess of 99.999%, translating to link downtimes of literally seconds annually, which is easily comparable to that provided by supposed “error-free” optical fiber systems. Note : total availability concerns the 2 types of the fade
28 Microwave applications Mobile Cellular Networks service immediate revenue connectto provide service for customers and to generate immediate revenue, cellular carriers need to connect their cell sites to switching stations, and have chosen microwave due to: its reliabilityits reliability speed of deploymentspeed of deployment cost benefits over fiber or leased-line alternativescost benefits over fiber or leased-line alternatives.
29 heavily deployed 3GMicrowave radio will be heavily deployed in the emerging 2.5 and 3G mobile infrastructures: More data usage greater numbers of cell sites.
30 Last Mile Access Last Mile Access broadband connectivity the last mile bottleneck A significant proportion of business premises lack broadband connectivity : Wireless provides the perfect medium for connecting new customers to overcome the last mile bottleneck. multi-point high capacity microwave provides the ideal solution for backhaul of customer traffic access hubs to the nearest fiber Even if an operator chooses to use unlicensed or multi-point wireless technologies to connect customers, high capacity microwave provides the ideal solution for backhaul of customer traffic from access hubs to the nearest fiber
31 Private Networks:Private Networks: high speed LAN / WAN campuscity or country. Companies now have high speed LAN / WAN network requirements and need to connect parts of their business in the same campus, city or country. rapid, high capacity connections Microwave radio is able to provide rapid, high capacity connections that are compatible with Fast and Gigabit Ethernet data networks, enabling LANs to be extended without reliance on fiber build-out.
32 Disaster RecoveryDisaster Recovery wreak havoc Natural (earthquakes, floods, hurricanes ) and man-made (terrorist attack and wars) disasters can wreak havoc on a communications network: Microwave is often used to restore communications when transmission equipment has been damaged by or other natural disasters, or man-made conflicts such as ( Kuwait, Serbia and Kosovo)
33 The Digital Divide Microwave radio plays a key role in bridging the digital divide : quickly establish have to wait. quickly establish a network of access hubs and high-speed backhaul network to bring advanced communications services to areas that would normally have to wait.
34 Developing Nations Microwave has traditionally allowed developing nations the means of establishing state-of-the- art telecommunications quickly over often undeveloped and impractical terrain ( deserts, jungle or frozen terrain where laying cable would be all but impossible.
35 Control and Monitoring Public transport organizations, railroads, and other public utilities are major users of microwave. carry control These companies use microwave to carry control and monitoring information to and from power substations, pumping stations, and switching stations.
37 A- Understanding db & db units : A- db : The ratio of 2 signals may be expressed in db by : in case of voltages : V 1 /V 2 ( V 2 /V 1 ) db = 20 log 10 ( V 2 /V 1 ) in case of Powers : P 1 /P 2 ( P 2 /P 1 ) db = 10 log 10 ( P 2 /P 1 )
38 ExampleExample : a signal if 10 w is applied to long transmission line. The power measured at the load end is 7 W. What is the loss in db Solution :Solution :
39 Table of some common ratios Voltage ratio (db) 20 log 10 R Power ratio (db) 10 log 10 R FactorRatio ® 0.00 11:1 63.0122:1 2010.001010:1 4020.00100100:1 6030.0010001000:1 -20-100.11/10 -40-200.011/100 -60-300.0011/1000
40 30 dBis an increase of 1000X in power 20 dBis an increase of 100X in power 10 dBis an increase of 10X in power 6 dBis an increase of 4X in power 3 dBis an increase of 2X in power 2 dBis an increase of 1.6X in power 1 dBis an increase of 1.25X in power 0 dBis no increase or decrease in power -1 dBis a decrease of 20% in power -2 dBis a decrease of 37% in power (roughly a decrease of 1/3) -3 dBis a decrease of 50% in power -6 dBis a decrease of 75% in power -10 dBis a decrease of 90% in power -20 dBis a decrease of 99% in power -30 dBis a decrease of 99.9% in power
41 B) : - db-power units - dbw : is the unit of power expressed relatively to 1W P (dbw) = 10 log 10 P (w) - dbm : is the unit of power expressed relatively to 1mW P (dbm) = 10 log 10 P (mw) Attention : 0dbw = 30dbm= 1W 0 dbm = -30dbw = 1mw
42 P (dbm) = P(dbW) + 30 P(dbW) = P (dbm) -30
43 Example Problem If the two antennas in the drawing are "welded" together, how much power in dbm will be measured at point A? (Line loss L1 = L2 = 0.5) –suppose no ideal antenna coupling Multiple choice: a. 16 dBm b. 28 dBm c. 4 dBm d. 10 dBm e. < 4 dBm
44 Answer: The antennas do not act as they normally would since the antennas are operating in the near field. They act as inefficient coupling devices resulting in some loss of signal. In addition, since there are no active components, you cannot end up with more power than you started with. The correct answer is "e. < 4 dBm." 10 dBm - 3 dB - small loss -3 dB = 4 dBm - small loss
45 Example : Convert 10dbm in dbw ; -2dbw in dbm Solution : given P(dbW) = P (dbm) -30 = 10-30 = -20 dbW P (dbm) = P(dbW) + 30 = -2 + 30 = 28 dbm Example : consider the 2 following configurations 10dbm ? Gain 3dbGain 10db
46 C) - db-voltage units - dbmv : is used in RF receiver in which the system impedance is 50 Ω. It is the unit of voltage expressed relatively to 1mv v (dbmv) = 20 log 10 v (mv)
47 -dbµv : is used in RF receiver in which the system impedance is 50 Ω. It is the unit of voltage expressed relatively to 1µv : v (dbµv) = 20 log 10 v (µv) Example : The received RF effective voltage at the input of radio receiver is 0.5mv. Find the input voltage in dbµv & the input power in dbm Solution
48 -Field db units : Electromagnetic field a- Electric field E in dbµv /m E (dbµv/m) = 20 log 10 E (µv/ m) b- Magnetic field H = E/377 where H in A/m and E in v/ m
50 Power and Field Db-units
51 c) received power in dBm at the RX-antenna where G r is the RX-antenna gain. P dbm = E dbµv/m + G r - 20 log (F MHZ ) – 77.2 In case of isotropic Rx-antenna P dbm = E dbµv/m - 20 log (F MHZ ) – 77.2 Received voltage into 50 input receiver: P (dBm) = U (dBµV) - 107
52 Example:Example: The received RF effective voltage at the input of radio receiver is 0.5mv a) Find the input voltage in dbµv & the input power in dbm. b) knowing that the receiver’s antenna has 20db Gain and the transmitted frequency is 10GHZ, Find the Electric field at the antenna location In dbµv/m and V/m. Deduce the Magnetic field value.
54 Exercises on Db A cable has 6 dB signal loss. Find the signal at the output of this cable,knowing that the input signal is 1mW. an amplifier has 15 dB of gain. Find the signal at the output of this amplifier,knowing that the input signal is 1mW. Complete the following sentences: –a)Every time you double (or halve) the power level, you add (or subtract) ……. dB to the power level. This corresponds to a ……. percent gain or reduction. –b) ……dB gain/loss corresponds to a tenfold increase/decrease in signal level. A 20 dB gain/loss corresponds to a ……….-fold increase/decrease in signal level.
55 Exercises on Dbm (dB milliWatt) A signal strength or power level 0 dBm is defined as …. mW (milliWatt) of power into a terminating load such as an antenna or power meter. Small signals are negative numbers. For example, typical 802.11b WLAN cards have +15 dBm (….mW) of output power. They also specify a -83 dBm (………pW.) RX sensitivity (minimum RX signal level required for 11Mbps reception). Additionally, a) 125 mW is ….. dBm, and b) ….mW is 24 dBm.
56 CH2- Antenna and space propagation propagation Recommended Software Andrew Recommended Software Andrew
57 Antenna Basic questions accept reject which cause some antennas to accept one wave and reject others?: frequency The physical size of an antenna : defines the efficiently radiated or received frequency The shape of the antenna determine the directivity of an antenna angular pointing The property of polarization describes the angular pointing of the EM field vector
58 Antenna Electromagnetic field radiation : General discussion two basic functions An antenna serves two basic functions: matches 1- it matches the characteristic impedance of the transmission line to the intrinsic impedance of free space (To avoid any reflections back to the source or load) direct 2 - Second, the antenna is designed to direct the electromagnetic radiation in the desired direction.
59 Isotropic Point Radiator fictitiouspoint radiator real It is a fictitious ideal isotropic point radiator. it would radiate power equally well in all directions in a volume sense. it would have an omni directional pattern in all planes. All real antennas have some directivity. Isotropic antenna practically doesn’t exist Omni-directional fictive Hertz Isotropic point antenna G=1 fold or G = 0 Dbi
60 Radiation Pattern power density orientationin The radiation pattern is a plot of the relative strength (more often power density in db )of the antenna radiation as a function of the orientation in a given plane. Example :radiation pattern of ANTMAN :ANDREW CORPORATION MODNUM:FP10-34 LOWFRQ:3400; HGHFRQ:3900
64 Antenna Gain ideal isotropic the same powerRatio of the power density at a particular location from an antenna with directivity to the power density from an ideal isotropic antenna radiating the same power: the result is : The power is taken away from some directions and added to the power in other directions, and the result is :
65 The antenna referenceThe antenna reference - most often used is : the hypothetical (Gain units dbi). "real-life" - Sometimes a "real-life" antenna such as the (Gain units dbd).
67 ReciprocityReciprocity be same Basically, it states that the properties of the antenna used for transmission will be same as when used for reception. the transmitting antenna larger power In realistic terms, the transmitting antenna must be constructed to handle a much larger power level than at the receiver similar field patterns and impedance propertiesThe best interpretation is to assume similar field patterns and impedance properties of a given antenna used in the TX or RX.
68 Antenna ReciprocityAntenna Reciprocity
69 Power density of electromagnetic c- Power density of electromagnetic energy energy in W/m 2 : An ideal isotropic point radiator transmitting power P T. The power density p d upon the surface of the sphere of radius r will be equal at all points and will be In free space propagation ( far field ) E in v/m, Pd = E 2 / 377 Sometimes we define the radiation intensity as steradian. the unit of U is watts / steradian. Know that
70 Power density of electromagneticPower density of electromagnetic energy energy in W/m2
71 the radiation intensity in watts / steradian
72 Solid AnglesSolid Angles solid angle spanned by a cone is measured by the area of intersection of the cone with a sphere: differential solid angle can be assigned a direction. Unit: steradian (full sphere = 4 )
73 Example : 1-The power density 10 km from a transmitting antenna is 0.06 ìW/m2. Determine the radiation intensity. 2-The radiation intensity from a transmitting antenna is 50 W/sr. Determine the power density of a receiving antenna located 25 km from the transmitting station. Solution 1- 2-
74 2- E.M Radiation From an Antenna Time-varying radially awayTime-varying voltages and currents in an antenna produce time-varying electric and magnetic fields that travel radially away from the antenna at a velocity determined by the medium in which the electromagnetic fields are propagating. near field and far field.There are two distinct regions of electric and magnetic fields surrounding an antenna: near field and far field.
75 They are defined by the distance from the antenna as a function of the wavelength of the electromagnetic radiation and size of the antenna D. The fields in the far field are transverse fields; i. e., the electric and magnetic field intensities are transverse to the direction of propagation: This condition is referred to as plane wave propagation. –Both transverse and radial electric and magnetic field intensities exist in the near field region.
76 patterns for the far field.The radiation patterns that describe the radiation intensity of the antenna as a function of angle are usually patterns for the far field. Example 1: Determine the distance from a 100- MHz half-wavelength dipole to the boundary between the near field and the far field. Solution: = 3 m. Therefore, the length of the dipole is 1.5m. The distance to the far field can then be determined
77 Example 2 A parabolic reflector antenna with a diameter D operates at 2.3 GHz. Determine the far field distance for this antenna. Case1 : D= 1m = 0.13 m. R ff = 2D 2 / = 15m Case2 : D= 20m = 0.13 m. R ff = 2D 2 / = 6Km far-field distance for a high-gain antenna can be very large D=20m then R ff = 6Km :The preceding analysis shows that the far-field distance for a high-gain antenna can be very large. The measurement of the far field radiation pattern for a large antenna operating at a high frequency can be a very difficult task.
78 15-4 Radiation Patterns three-dimensional plotIn general, the radiation pattern of an antenna is a three-dimensional plot of the relative strength or radiation intensity of an antenna as a function of the coordinate systems. 3-D pair of two-dimensional plots:Since it is difficult to present 3-D information, typically the radiation patterns are shown as a pair of two-dimensional plots:
83 - Polar-coordinate graph for anisotropic (directive) radiator.
84 The first plot shows the radiation intensity as a function of the angle in the plane of the electric field intensity vector - E plane pattern and the second plot shows the radiation intensity as a function of the angle in the plane of the magnetic field - H plane pattern. The E plane and H plane are orthogonal to each other and are referred to as the principal plane patterns.
85 Typical E and H plane plots: half-wave dipole antennaConsider a simple half-wave dipole antenna aligned with the y-axis with the center at the origin. A typical E plane radiation pattern for a half-wave dipole antenna is shown in Figure (a). This plane is the –y z plane in this case. A typical H plane radiation pattern for a half-wave dipole is shown in Figure (b). This plane is the –x z plane for the orientation given. Note that the pattern is omnidirectional in the H plane.
87 Normalized Gain FunctionsNormalized Gain Functions Radiation patterns are often normalized to the maximum gain by dividing the gain as a function of the two angles by the maximum gain to obtain the normalized gain. The normalized gain will be represented as This means that the normalized maximum gain is 1, and the gain at other angles is less than 1.
88 significant dynamic range 1 down to 10 -4 or lessSince normalized antenna patterns cover a significant dynamic range, typically from 1 down to 10 -4 or less, antenna radiation patterns are normally plotted in decibels on a linear scale, usually on a polar plot. G (db) = 10 log G (folds) G (db) = 10 log G (folds)
89 Antenna Beamwidths and Sidelobes 1 over the desired angular beamwidth0 at all other angles An ideal antenna would have a radiation pattern whose normalized gain is 1 over the desired angular beamwidth and 0 at all other angles. Beamwidth -3 dB The antenna beamwidth is defined as the included angle between the -3 dB (Half power gain) points on the normalized gain pattern.
91 Lobes main lobebeamThe main lobe is the antenna beam defined between the first null on either side of the maximum gain angle. Typically for high-gain antennas, the null-to-null beamwidth is 2.5 times the 3-dB beamwidth.
92 sidelobesAn antenna will usually radiate some power in undesired directions. The radiation pattern of the Figure has several sidelobes. The levels of the sidelobes determine how much power is radiated in these undesired directions. If the antenna is a receiving antenna, the sidelobes will determine the levels of undesired signals that could be received.If the antenna is a receiving antenna, the sidelobes will determine the levels of undesired signals that could be received.
93 BacklobeBacklobe undesiredAnother undesired part of the radiation pattern when single direction transmission is desired is the backlobe. front-to-backA quality factor called the front-to-back ratio is important in these cases. As shown in the Figure, the absolute value of the front-to-back ratio of a dipole is 1, which in decibel form would be 0dB. A dipole cannot tell if the signal is coming from the front or back of the antenna.
94 Directivity and Antenna Gain15-6 Directivity and Antenna Gain There are two commonly employed terms used to describe the radiation characteristics of an antenna: directivity and antenna gain. Directivity lossless antennaantenna gain includes the ohmic losses Directivity is a characteristic of the radiation pattern of an ideal lossless antenna while the antenna gain includes the ohmic losses of the antenna physical structure. Directivity : The directivity D of an antenna is defined from the radiation pattern as
96 Antenna Gain Antenna gain is defined as the ratio of the maximum radiation intensity U max to the maximum radiation intensity U ref from a reference antenna with same power input to the antenna. The difference between directivity and gain is that directivity is referenced to the power radiated by the antenna, while gain is referenced to the power delivered by the transmission line to the antenna. Therefore, gain is always less than or equal to directivity, the difference being the power dissipated in the antenna ohmic losses. Normally, antenna gain is expressed as a power ratio and is usually specified in decibels as
97 The value of gain depends on the gain of the reference antenna. It is important to know what reference has been used for the antenna gain. Two of the common references are as follows: 1. a lossless isotropic antenna, in which the radiation intensity is uniform over the sphere surrounding the antenna, i. e., all 4 steradians 2. a reference dipole. The lossless isotropic antenna is a theoretical concept and has never been realized in practice, while the dipole is readily available.
98 Antenna measurements of gain are usually referenced to a standard dipole for low-gain antennas or to a standard-gain horn for higher-gain antennas. Accurate theoretical calculations of the gain referenced to a lossless isotropic antenna are possible for both the standard dipole and the standard-gain horn. The absolute gain of a standard half-wave dipole with respect to an isotropic radiator is 1.643 or 2.16 dB Dbi = Dbd +2,16
100 Exercise in dbd &dbi dBd (dB dipole)dBd (dB dipole) The gain an antenna has over a dipole antenna at the same frequency. A dipole antenna is the …..., least gain practical antenna that can be made. The term dBd generally is used to describe antenna gain for antennas that operate under 1GHz (1000Mhz), ( manufacturers calibrate their equipment using a simple dipole antenna as the standard. Then they replace it with the antenna they are testing. The difference in gain (in dB) is reference to the signal from the dipole).
101 dBi (dB isotropic)dBi (dB isotropic) Unfortunately, an isotropic antenna cannot be made in the real world, but it is useful for calculating theoretical fade and System Operating Margins. The gain of Microwave antennas (above 1 GHz) is generally given in dBi. A dipole antenna has 2.14 dB gain over a 0 dBi isotropic antenna. So if an antenna gain is given in dBd, not dBi, …… 2.15 to it to get the dBi rating. For example, if an antenna has 5 dBd gain, it would have ………. dBi gain.
102 Antenna EfficiencyAntenna Efficiency It depends on the ohmic losses of the antenna. It is the ratio of the total power radiated from the antenna / the power delivered to the antenna from the transmission line. It is also equal where D and G are the absolute values of directivity and gain.
103 1- An antenna is transmitting 200 W of power. The maximum power density at a distance of 10 km is 3.184 mW/m2. Determine the directivity of the antenna.
104 2- An antenna with a directivity of 16 dB is transmitting a power of 1 kW. Determine the maximum power density at a distance of 50 km from the antenna.
105 3-An antenna has an efficiency of 95% and the directivity is 33 dB. Determine the antenna gain in dB.
106 Effective Area of an RX Antenna15-7 Effective Area of an RX Antenna (capture area) (capture area ). It is the area by which the power density in watts per unit of area is multiplied to obtain power in watts delivered to the load. It is close to the physical area of the antenna. The effective antenna area A e of the parabolic reflector is given by
108 15-8 Polarization orientation of the electric field intensity vector relative to the surface of the earth.By definition, the polarization of an electromagnetic wave propagating in free space is the orientation of the electric field intensity vector relative to the surface of the earth. linear and elliptical.There are two basic types of polarization: linear and elliptical.
109 lineardoes not change orientationIn linear polarization, the electric vector does not change orientation as it travels away from an observer : 2 Types : H& V elliptical rotates as In elliptical polarization, the electric vector rotates as it travels away from an observer, and the tip of the electric vector traces an ellipse : sense here mean Clockwise and anticlockwise.
110 antenna design and orientationAn antenna transmits vertical, horizontal, right-hand (RH) circular, or left-hand (LH) circular polarization depending on the antenna design and orientation. 30 dBThere is a significant cross-polarization loss of approximately 30 dB. This loss also occurs in case of cross sens rotation in circularly polarized antennas. polarization diversity in communication systems.This characteristic is used to provide polarization diversity in communication systems.
111 Polarization Requirements for Various Frequencies wave typeGround wave Sky waves Direct waves (including satellite) frequency BandLow & Medium Short wavesVHF, UHF,SHF Polarization possible to be usedVertical Vertical or Horizontal Polarization practically to be usedVertical Horizontal Vertical orHorizontal Why The earth is fairly good conductor,short out Eh a) Less auto and electro ignition, b) Less building absorption c) More simple support antenna structure No ionosphere entry or reflexion
112 15-9 Antenna Impedance and Radiation Resistance complex impedancewhen it is excited by an appropriate AC source The antenna acts like an a complex impedance to the source providing power to it Z = R + jX RF bridgeThis impedance can be measured by an appropriate RF bridge.
113 Antenna ImpedanceAntenna Impedance Ideally, it should be purely resistive R at the frequency of operation and equal to the characteristic impedance of the line connected to it.Ideally, it should be purely resistive R at the frequency of operation and equal to the characteristic impedance of the line connected to it. Self Impedance of the isolated antenna self-impedance If an antenna is isolated from ground and any other surrounding objects, this impedance is the self-impedance of the antenna & at the resonance it is purely resistive ( Z= R +j0)
114 Mutual impedance of the antenna. other antennas, objects, or ground is near When other antennas, objects, or ground is near the antenna, the currents flowing in these objects have an influence on the antenna impedance. mutual impedancenearby The antenna impedance is then determined both by the self-impedance of antenna and by a mutual impedance between the antenna and the nearby objects.
115 Radiation Resistance losslessThe radiation resistance is the real part of the complex antenna impedance of a lossless antenna. It is equal to Where : - P rad is the amount of this energy leaving a sphere surrounding the antenna per unit of time is the power radiated by the antenna. - I rms is the rms value of the antenna current magnitude at the input terminals of the antenna.
116 Example : An antenna has an rms current of 3 A flowing into the antenna, and it is transmitting 1 kW of power. Determine the radiation resistance of the antenna. Solution The radiation resistance is determined from (15-29).
119 Dipoles One of the simplest and most commonly used antennas is the half-wave dipole, formed from a two-wire parallel transmission line as shown in The following figure. Starting with an open-ended line, which has a voltage maximum at the open end and a voltage minimum back from the open end, the two conductors are bent 90 o from the transmission line as illustrated in the Figure.
120 The theoretical length of the antenna is the Diameter d of the wires is assumed to be much smaller than the length, and the spacing D at the feed point must be small compared with the length.
122 Practically Mounted dipole
123 Input Impedance of Dipole A voltage minimum and a current maximum occur at the feed point, which means that the impedance is a minimum at that point. The actual value of the impedance of the half- wave dipole is 73+ 42.5 j .
124 eliminated tuning the antenna shortening the length The reactive component can be eliminated by tuning the antenna, which is accomplished by shortening the length by about 5% from 0.5 to 0.475, corresponding to approximately 95% of the theoretical length. 73 resistiveWhen properly tuned, the half-wave dipole has an impedance of 73 resistive, which for a lossless dipole is the radiation resistance of the antenna.
125 balanced antennaThe dipole is a balanced antenna and must, therefore, be fed by a balanced transmission line. balunSince the most common transmission line providing the best impedance match is coaxial cable, a balun must be used to properly connect a coaxial cable to a dipole.
126 Radiation PatternsRadiation Patterns examples of patterns.The E plane and H plane radiation patterns of the half-wave dipole were shown in the following Figure as examples of patterns. The E plane pattern is like a doughnut with two maxima broadside to the dipole and a null at both ends of the dipole. The 3-dB beamwidth is 78 o. The isotropic power gain or directivity for a lossless /2 dipole is 1.643 folds or 2.16 dB.
128 parallelAs shown in the Figure, the polarization of the dipole is parallel to the dipole. uniform circularThe H plane pattern is illustrated in the Figure and is a uniform circular pattern with a constant gain for an angle of 360 o about the dipole.
129 Effective AreaEffective Area The effective area of a dipole can be determined from the isotropic gain and is for f= 98MHz( =3m) ; A e = 1.218m 2
130 Folded DipoleFolded Dipole A folded dipole is constructed from a /2 length of 300- twin-lead transmission line (see figure). 73- 280 . The combination of the 73- self-impedance of the dipole and the mutual impedance from the parallel conductor connected at both ends increases the antenna impedance of the folded dipole to 280 . folded dipoleTherefore, the folded dipole is a balanced 280- antenna, which closely matches the 300- twin-lead balanced transmission line.
132 Practically mounted Folded antenna
133 The folded dipole is the perfect antenna for stations that require a truly professional antenna for their broadcasting. One powerful advantage of a folded dipole antenna is that is has a wide bandwidth, in fact a one octave bandwidth. This is the reason it was often used as a TV antenna for multi channel use. Folded dipole antennas were mainly used in conjunction with Yagi antennas. No tuning or adjustment is needed for any frequency on the band which makes this antenna the only one to use if you plan to move your transmitters frequency often.
134 Specifications for the folded professional antenna Max Power Input300 Watts Impedance50 Ohms Gain0dBd VSWRBetter than 1:1.5 (88 - 108 MHz) Frequency Range88 - 108 MHz (No Tune) ConnectorN-Type Female Dimension1600mm(height) x 150mm x 35mm Weight2kg
136 15-11 Antennas Above a Ground Plane A ground plane is a uniform good ground plane surface beneath an antenna- constructed from good conductors, or at some frequencies, the earth acts as a good ground plane. Electromagnetic fields cannot exist in a perfect conductor, and any wave incident upon a perfect conductor will be reflected.
137 Figure illustrates this situation. To satisfy the boundary condition that no tangential component of electric field can exist there, the reflected wave will be shifted in phase by 180 o. A concept known as image theory is used to determine the characteristics of an antenna above a ground plane.
138 is like a direct wave from an identical antenna located within the ground planeThe reflected wave is like a direct wave from an identical antenna located within the ground plane the same distance from the boundary as the real antenna is above the ground plane. This situation is illustrated in the following Figure. The image antenna is similar to an image formed in a mirror at optical frequencies.
141 Monopole AntennaMonopole Antenna /4 monopole antennaAn important and commonly used antenna is the /4 monopole antenna on a ground plane as shown in the Figure. unbalanced antenna vertical polarizationThis is an unbalanced antenna since the feed point is between the monopole and ground and it has vertical polarization. 21 j The radiation resistance is 36.5 , and the antenna impedance has a reactive component of 21 j . When the monopole is located close to the ground plane, the image antenna forms a dipole as shown in the previous Figure.
142 only one-halfThe E plane radiation pattern is that of a /2 dipole with only one-half of the pattern above the ground plane. The ground plane can be achieved either by a grounded metal disc or by radial wires as shown in the following Figure. roof of a vehicleThe roof of a vehicle such as a car or truck can form a ground plane for a /4 monopole.