6 Chapter One 40 80-150-120-90-60-300306090120150180 Figure1.5 Rectangular plot of radiation pattern half-power beamwidth is sometimes referred to as the 3-dB beamwidth. Both horizontal and vertical beamwidths are usually considered. 1.1.3.2 Sidelobes and Nulls No antenna is able to radiate all the energy in one preferred direction.Some energy is inevitably radiated in other directions with lower levels than the main beam.These smaller peaks are referred to as sidelobes,commonly specified in dB down from the main lobe Inan antea radiation in which the effective radiated power is at a minimum.A null often has a narrow directivity angle compared to that of the main beam.Thus,the null is useful for several purposes,such as suppressing interfering signals in a given direction Com ing the fron-atio of directional antenas is ofte he ratio of the e maximum directivity of an antenna to its directivity in the opposite direction.For example,when the radiation pattern is plotted on a relative dB scale,the front-to-back ratio is the difference in dB between the level of the maximum radiation in the forward direction and the level of radiation at 180.This number is meaningless for an anidirectional antenna,but it gives one an idea of the amount of power directed forward on a very directional antenna. 1.1.4 Polarization of the Antenna a is the orientation of the electric field
6 Chapter One half-power beamwidth is sometimes referred to as the 3-dB beamwidth. Both horizontal and vertical beamwidths are usually considered. 1.1.3.2 Sidelobes and Nulls No antenna is able to radiate all the energy in one preferred direction. Some energy is inevitably radiated in other directions with lower levels than the main beam. These smaller peaks are referred to as sidelobes, commonly specified in dB down from the main lobe. In an antenna radiation pattern, a null is a zone in which the effective radiated power is at a minimum. A null often has a narrow directivity angle compared to that of the main beam. Thus, the null is useful for several purposes, such as suppressing interfering signals in a given direction. Comparing the front-to-back ratio of directional antennas is often useful. This is the ratio of the maximum directivity of an antenna to its directivity in the opposite direction. For example, when the radiation pattern is plotted on a relative dB scale, the front-to-back ratio is the difference in dB between the level of the maximum radiation in the forward direction and the level of radiation at 180°. This number is meaningless for an omnidirectional antenna, but it gives one an idea of the amount of power directed forward on a very directional antenna. 1.1.4 Polarization of the Antenna The polarization of an antenna is the orientation of the electric field (E-plane) of the radio wave with respect to the Earth’s surface and is Figure 1.5 Rectangular plot of radiation pattern 0 -10 -20 -30 -40 -180 -150 -120 -90 -60 -30 0 30 60 90 120 150 180
Fundamentals of Antennas 7 determined by the physical structure and orientation of the antenna.It has nothing in common with the antenna directionality terms:horizon- tal,vertical,and circular Thus,a simple straight wire antenna will have one polarization when mounted vertically and a different polariz ation when mounted horizontally.Electromagnetic wave polarization filters are structures that can be employed to act directly on the electromag- netic wave to filter out wave energy of an undesired polarization and to pass wave energy of a desired polarization. Reflectio ge rally affect polarization.For radio waves,the most important reflector is the ionosphere he polarization f signals reflected from it will change unpredictably.For signals reflected by the ionosphere,polarization cannot be relied upon.For line-of-sight communications,for which polarization can be relied upon,having the transmitter and receiver use the same polarization can make a huge signal qu lity;many ens nce is commonly seen,and I this is more than enough to make up the difference between reasonable communication and a broken link. Polarization is largely predictable from antenna construction,but especially in directional antennas,the polarization of sidelobes can be quite that of the main propagation lobe For radioante nas,polarization corresponds to th of the ment in an antenna.A vertical omnidirectional WiFi antenna will have vertical polarization (the most common type).One exception is a class of elongated waveguide antennas in which a vertically placed antenna is horizontally polarized.Many commercial antennas are marked as to the of their emitted s e orientations over tim e projected onto an imaginary plane perpendicular to the direction of motion of the radio wave.In the most general case,polarization is elliptical(the projection is oblong),meaning that the polarization of the radio waves emitting from the antenna is varying over time.Two special cases are linear p rization(the ellipse colapses intoaline) ation(in which the ellipse var mally).In lir ar polarization. the antenna compels the electric field of the emitted radio wave to a par ticular orientation.Depending on the orientation of the antenna mount- ing.the usual linear cases are horizontal and vertical polarization.In circular polarization,the antenna continuously varies the electric field of the radio e through all p ssible values of its regard to the Ea rth's surface.Circul ar polarizatio ones,are classified as right-hand polarized or left-hand polarized using a"thumb in the direction of the propagation"rule.Optical researchers use the same rule of thumb,but point it in the direction of the emitter. not in the direction of propagation,and so their use is opposite to that of radio engineers. Some ar ennas,such the helical antenna,produce
Fundamentals of Antennas 7 determined by the physical structure and orientation of the antenna. It has nothing in common with the antenna directionality terms: horizontal, vertical, and circular. Thus, a simple straight wire antenna will have one polarization when mounted vertically and a different polarization when mounted horizontally. Electromagnetic wave polarization filters are structures that can be employed to act directly on the electromagnetic wave to filter out wave energy of an undesired polarization and to pass wave energy of a desired polarization. Reflections generally affect polarization. For radio waves, the most important reflector is the ionosphere—the polarization of signals reflected from it will change unpredictably. For signals reflected by the ionosphere, polarization cannot be relied upon. For line-of-sight communications, for which polarization can be relied upon, having the transmitter and receiver use the same polarization can make a huge difference in signal quality; many tens of dB difference is commonly seen, and this is more than enough to make up the difference between reasonable communication and a broken link. Polarization is largely predictable from antenna construction, but especially in directional antennas, the polarization of sidelobes can be quite different from that of the main propagation lobe. For radio antennas, polarization corresponds to the orientation of the radiating element in an antenna. A vertical omnidirectional WiFi antenna will have vertical polarization (the most common type). One exception is a class of elongated waveguide antennas in which a vertically placed antenna is horizontally polarized. Many commercial antennas are marked as to the polarization of their emitted signals. Polarization is the sum of the E-plane orientations over time projected onto an imaginary plane perpendicular to the direction of motion of the radio wave. In the most general case, polarization is elliptical (the projection is oblong), meaning that the polarization of the radio waves emitting from the antenna is varying over time. Two special cases are linear polarization (the ellipse collapses into a line) and circular polarization (in which the ellipse varies maximally). In linear polarization, the antenna compels the electric field of the emitted radio wave to a particular orientation. Depending on the orientation of the antenna mounting, the usual linear cases are horizontal and vertical polarization. In circular polarization, the antenna continuously varies the electric field of the radio wave through all possible values of its orientation with regard to the Earth’s surface. Circular polarizations (CP), like elliptical ones, are classified as right-hand polarized or left-hand polarized using a “thumb in the direction of the propagation” rule. Optical researchers use the same rule of thumb, but point it in the direction of the emitter, not in the direction of propagation, and so their use is opposite to that of radio engineers. Some antennas, such the helical antenna, produce
8 Chapter One circular polarizations.However,circular polarization can be generated from n a lin early polarized a r feedi ing the antenn a by two ports with equal magnitude and with a 90 phase difference between them. From the linear field components in the far zone,the circular polariza- tion can be presented as E.(8=E,8)+E,8,) (17) 2 E,(8,1=E,8)-E8,p) (1.8) 2 where E is the copolar of the circular polarization,in this case,the left- hand CP and E.is the on,or the e right-hand CP. practice,regardle sing termin loy,matching linearly polarized antennas is important,or the received signal strength is greatly reduced.So,horizontal polarization should be used with hori- zontal antennas and vertical with vertical.Intermediate matching will cause the loss of some signal strength,but not as much as a complete mismatch.Tra on ve hicles with large e moti al free dom commonly use circularly polarized antennas so there will never be a complete mismatch with signals from other sources.In the case of radar,these sources are often reflections from rain drops. In order to transfer maximum power between a transmit and receive antenna,both antennas must have the same spatial orientation,the ame polari 1Z8 nd the same axial r a tio wh en the antenr are not aligned or do not have the same polarization,power transfer between the two antennas will be reduced.This reduction in power transfer will reduce the overall system efficiency and performance as well.When transmit and receive antennas are both linearly polarized, physical antenna misalig ent will result in a polarization mismatch loss,which can be determined using the following formula: Loss (dB)=20 log (coso) (1.9 where ois the difference in alignment angle between the two antennas. For15°，the loss is approximately0.3dB;for30°，the loss is1.25dB;for 45°，the loss is3dB;and for90°，the loss is infinite. In short,the greater the mismatch in polarization between a transmit- ting and rece ntenna the great er the apparent loss will be.You e6。m调e mg a int-to-point link and rotate one antenna until you see the lowest received signal.Then bring your link online and orient the other end to match polarization
8 Chapter One circular polarizations. However, circular polarization can be generated from a linearly polarized antenna by feeding the antenna by two ports with equal magnitude and with a 90° phase difference between them. From the linear field components in the far zone, the circular polarization can be presented as E E jE c ( , ) ( , ) ( , ) θ φ θ φ θ φ φ θ = + 2 (1.7) E E jE x ( , ) ( , ) ( , ) θ φ θ φ θ φ φ θ = − 2 (1.8) where Ec is the copolar of the circular polarization, in this case, the lefthand CP, and Ex is the cross-polarization, or the right-hand CP. In practice, regardless of confusing terminology, matching linearly polarized antennas is important, or the received signal strength is greatly reduced. So, horizontal polarization should be used with horizontal antennas and vertical with vertical. Intermediate matching will cause the loss of some signal strength, but not as much as a complete mismatch. Transmitters mounted on vehicles with large motional freedom commonly use circularly polarized antennas so there will never be a complete mismatch with signals from other sources. In the case of radar, these sources are often reflections from rain drops. In order to transfer maximum power between a transmit and receive antenna, both antennas must have the same spatial orientation, the same polarization sense, and the same axial ratio. When the antennas are not aligned or do not have the same polarization, power transfer between the two antennas will be reduced. This reduction in power transfer will reduce the overall system efficiency and performance as well. When transmit and receive antennas are both linearly polarized, physical antenna misalignment will result in a polarization mismatch loss, which can be determined using the following formula: Loss (dB) = 20 log (cosf) (1.9) where f is the difference in alignment angle between the two antennas. For 15°, the loss is approximately 0.3 dB; for 30°, the loss is 1.25 dB; for 45°, the loss is 3 dB; and for 90°, the loss is infinite. In short, the greater the mismatch in polarization between a transmitting and receiving antenna, the greater the apparent loss will be. You can use the polarization effect to your advantage on a point-to-point link. Use a monitoring tool to observe interference from adjacent networks, and rotate one antenna until you see the lowest received signal. Then bring your link online and orient the other end to match polarization
Fundamentals of Antennas 9 This technique can sometimes be used to build stable links,even in noisy radio environments. 1.1.5 Antenna Efficiency in ratio between e= 1.10) P Different types of efficiencies contribute to the total antenna effi- ciency.The total antenna efficiency is the multiplication of all these efficiencies.Effici ncy is affected by the losse within the antennaitself and the reflectic ue tothe mismatch at the ina on the equivalent circuit on Figure 1.1,we can compute the radiation efficiency of the antenna as the ratio between the radiated powers to the input power,which is only related to the conduction losses and the dielectric losses of the antenna structure as P R ,==RR+ (1.11) Due to the mismatch at the antenna terminal,the reflection efficiency et=(1-rP) (1.12) Then the total efficiency is defined as e=ereref (1.13) In this formula,antenna radiation efficiency only includes conduction efficiency and dielectric efficiency and does not include reflection effi- cie t of the total effici factor.More ver,the IEEE stan ate that gain does not ind oses arisingfrom impedance mismatches and polarization mismatches.' Efficiency is the ratio of power actually radiated to the power input into the antenna terminals.A dummy load may have an SWR of 1:1 but an efficiency of o.as it absorbs all r wer and radiates heat but not RF showin th t SWR alone is n e me e of ar anten ion resistance. which can only be measured as part of total resistance,including loss resistance.Loss resistance usually results in heat generation rather than
Fundamentals of Antennas 9 This technique can sometimes be used to build stable links, even in noisy radio environments. 1.1.5 Antenna Efficiency Antenna efficiency is the measure of the antenna’s ability to transmit the input power into radiation.1–4 Antenna efficiency is the ratio between the radiated powers to the input power: e P P r = in (1.10) Different types of efficiencies contribute to the total antenna efficiency. The total antenna efficiency is the multiplication of all these efficiencies. Efficiency is affected by the losses within the antenna itself and the reflection due to the mismatch at the antenna terminal. Based on the equivalent circuit on Figure 1.1, we can compute the radiation efficiency of the antenna as the ratio between the radiated powers to the input power, which is only related to the conduction losses and the dielectric losses of the antenna structure as e P P R R R r R R r r r r l = = = + in in (1.11) Due to the mismatch at the antenna terminal, the reflection efficiency can be defined as eref = − ( | | )1 2 Γ (1.12) Then the total efficiency is defined as e = er eref (1.13) In this formula, antenna radiation efficiency only includes conduction efficiency and dielectric efficiency and does not include reflection efficiency as part of the total efficiency factor. Moreover, the IEEE standards state that “gain does not include losses arising from impedance mismatches and polarization mismatches.”5 Efficiency is the ratio of power actually radiated to the power input into the antenna terminals. A dummy load may have an SWR of 1:1 but an efficiency of 0, as it absorbs all power and radiates heat but not RF energy, showing that SWR alone is not an effective measure of an antenna’s efficiency. Radiation in an antenna is caused by radiation resistance, which can only be measured as part of total resistance, including loss resistance. Loss resistance usually results in heat generation rather than
10 Chapter One radiation and reduces efficiency.Mathematically,efficiency is calculated as radia tion resistance divided by total resistance 1.1.6 Directivity and Gain intensity averaged over all directions."In other words,the directivity of a nonisotropic source is equal to the ratio of its radiation intensity in a given direction,over that of an isotropic source I4πI D-U P. (1.14) where D is the directivity of the antenna;U is the radiation intensity of the antenna;U:is the radiation intensity of an isotropic source;and P,is the total powe radiated. Sometimes,the direction of the directivity is not specified.In this case,the direction of the maximum radiation intensity is implied and the maximum directivity is given as U 4πJ =U (1.15) where D.is the maximum directivity and Uis the maximum radia- tion intensity Amore general radia- tion patterns as functions of spherical coordinate angles and 4元 D= A 1.16) where A is the beam solid angle and is defined as the solid angle in which,if the antenna radiation intensity is constant (and maximum value).all power would flow through it.Directivity is a dimensionless use it is the ratio of two radiation inte ensities.Ther dBi.The directiv ity of an antenna can be easily estimated from the radiation pattern of the antenna.An antenna that has a narrow main lobe would have better directivity than the one that has a broad main lobe:hence,this antenna is more directive.In the case of antennas with one narrow major lobe and very negligible minor lobes,the beam solid angle can be app mated as the product of the half-power beamwidths in two perpen cular planes 2A=02 (1.17)
10 Chapter One radiation and reduces efficiency. Mathematically, efficiency is calculated as radiation resistance divided by total resistance. 1.1.6 Directivity and Gain The directivity of an antenna has been defined as “the ratio of the radiation intensity in a given direction from the antenna to the radiation intensity averaged over all directions.” In other words, the directivity of a nonisotropic source is equal to the ratio of its radiation intensity in a given direction, over that of an isotropic source1–4 : D U U U i r P = = 4π (1.14) where D is the directivity of the antenna; U is the radiation intensity of the antenna; Ui is the radiation intensity of an isotropic source; and Pr is the total power radiated. Sometimes, the direction of the directivity is not specified. In this case, the direction of the maximum radiation intensity is implied and the maximum directivity is given as D U U U i Pr max max max = = 4π (1.15) where Dmax is the maximum directivity and Umax is the maximum radiation intensity. A more general expression of directivity includes sources with radiation patterns as functions of spherical coordinate angles q and f: D A = 4π Ω (1.16) where ΩA is the beam solid angle and is defined as the solid angle in which, if the antenna radiation intensity is constant (and maximum value), all power would flow through it. Directivity is a dimensionless quantity because it is the ratio of two radiation intensities. Therefore, it is generally expressed in dBi. The directivity of an antenna can be easily estimated from the radiation pattern of the antenna. An antenna that has a narrow main lobe would have better directivity than the one that has a broad main lobe; hence, this antenna is more directive. In the case of antennas with one narrow major lobe and very negligible minor lobes, the beam solid angle can be approximated as the product of the half-power beamwidths in two perpendicular planes: Ω Θ Θ A r r = 1 2 (1.17)