Fundamentals of Antennas 11 where,R is the half-power beamwidth in one plane (radians)and 2 is the half-power beamwidth in a plane at a right angle to the other(radi- ans)The same approximation can be used for angles given in degrees as follows: 180)2 元 D=4π 41253 (1.18 01a02a01d02a where is the half-power beamwidth in one plane(degrees)and 2 is the half-power beamwidth in a plane at a right angle to the other (degrees).In planar arrays,a better approximation is 32400 (1.19) 01d02d Gain as a parameter measures the directionality of a given antenna An antenna with low gain emits radiation with about the same power in all directions,whereas a high-gain antenna will preferentially radiate in particular directions.Specifically,the gain,directive gain,or power gain of an antenna is defined as the ratio of the intensity(power per unit urface)radiated by the antenna in a given direction an a rbitrary dis tance divided by th intensity radiated at the sa distance by a hypo thetical isotropic lossless antenna.Since the radiation intensity from a lossless isotropic antenna equals the power into the antenna divided by a solid angle of 4xsteradians,we can write the following equation: G=4rU (1.20) Pn Although the gain of an antenna is directly related to its directivity, antenna gain is a measure that takes into account the efficiency of the ant a as well as its dir ctional capabilities.In cor defned as a measure that takes into account only thed erties of the antenna,and therefore,it is only influenced by the antenna pattern.If,however,we assume an ideal antenna without losses,then antenna gain will equal directivity as the antenna efficiency factor equals 1(100%efficiency).In practice,the gain of an antenna is always less than its directivity. G= 4πU AnU-eaD (1.21) P Equations 1.20 and 1.21 show the relationship between antenna gain and directivity,whereeca is the antenna radiation efficiency factor,D the
Fundamentals of Antennas 11 where, Ω1r is the half-power beamwidth in one plane (radians) and Ω2r is the half-power beamwidth in a plane at a right angle to the other (radians). The same approximation can be used for angles given in degrees as follows: D d d d d ≈ 4 = 180 41253 2 1 2 1 2 π π Θ Θ Θ Θ (1.18) where Ω1d is the half-power beamwidth in one plane (degrees) and Ω2d is the half-power beamwidth in a plane at a right angle to the other (degrees). In planar arrays, a better approximation is6 D d d ≈ 32400 Θ Θ1 2 (1.19) Gain as a parameter measures the directionality of a given antenna. An antenna with low gain emits radiation with about the same power in all directions, whereas a high-gain antenna will preferentially radiate in particular directions. Specifically, the gain, directive gain, or power gain of an antenna is defined as the ratio of the intensity (power per unit surface) radiated by the antenna in a given direction at an arbitrary distance divided by the intensity radiated at the same distance by a hypothetical isotropic lossless antenna. Since the radiation intensity from a lossless isotropic antenna equals the power into the antenna divided by a solid angle of 4p steradians, we can write the following equation: G U P = 4π in (1.20) Although the gain of an antenna is directly related to its directivity, antenna gain is a measure that takes into account the efficiency of the antenna as well as its directional capabilities. In contrast, directivity is defined as a measure that takes into account only the directional properties of the antenna, and therefore, it is only influenced by the antenna pattern. If, however, we assume an ideal antenna without losses, then antenna gain will equal directivity as the antenna efficiency factor equals 1 (100% efficiency). In practice, the gain of an antenna is always less than its directivity. G U P e U P e D r = = = 4 4 π π in cd cd (1.21) Equations 1.20 and 1.21 show the relationship between antenna gain and directivity, where ecd is the antenna radiation efficiency factor, D the
12 Chapter One directivity of the antenna,and G the antenna gain.We usually deal with same direction.The input power must be the same for both antennas while performing this type of measurement.The reference antenna is usually a dipole,horn,or any other type of antenna whose power gain is already calculated or known. P G=Gret P re (1.22) In the case that the direction of radiation is not stated,the power gain is always calculated in the direction of maximum radiation.The maximum directivity of an actual antenna can vary from 1.76 dB for a short dipole to as much as 50 dB for a large dish antenna.The maximum gain of a real antenna has no lower bound and is often-10 db or less for electrically small antenna Antenna absolute gain is another definition for antenna gain.However, absolute gain does include the reflection or mismatch losses: G=eG=eeD (1.23) As defined before,eren is the reflection efficiency,and ecd includes the dielectric and conduction efficiency.The term ee is the total antenna efficiency factor. Takin into。 count polarization effects in the ante nna, we can also define the partial gain of an antenna for a given polarizati on as tha part of the radiation intensity corresponding to a given polarization divided by the total radiation intensity of an isotropic antenna.As a result of this definition for the partial gain in a given direction,we can present the total gain of an antenna as the sum of partial gains for any two orthogonal polarizations Gtotal=Gg+U。 (1.24) &G, (125) Pin Pin The terms Ueand U.represent the radiation intensity in a given direc- tion contained in their respective E-field component. non;power is not added na but sim nply redistributed toprovidemore radiated power in a certain direction than would be transmitted by an isotropic antenna. An antenna designer must take into account the antenna's application
12 Chapter One directivity of the antenna, and G the antenna gain. We usually deal with relative gain, which is defined as the power gain ratio in a specific direction of the antenna to the power gain ratio of a reference antenna in the same direction. The input power must be the same for both antennas while performing this type of measurement. The reference antenna is usually a dipole, horn, or any other type of antenna whose power gain is already calculated or known. G G P P = ref ref max max| (1.22) In the case that the direction of radiation is not stated, the power gain is always calculated in the direction of maximum radiation. The maximum directivity of an actual antenna can vary from 1.76 dB for a short dipole to as much as 50 dB for a large dish antenna. The maximum gain of a real antenna has no lower bound and is often –10 dB or less for electrically small antennas. Antenna absolute gain is another definition for antenna gain. However, absolute gain does include the reflection or mismatch losses: G e G e e D abs eff refl cd = = (1.23) As defined before, erefl is the reflection efficiency, and ecd includes the dielectric and conduction efficiency. The term eeff is the total antenna efficiency factor. Taking into account polarization effects in the antenna, we can also define the partial gain of an antenna for a given polarization as that part of the radiation intensity corresponding to a given polarization divided by the total radiation intensity of an isotropic antenna. As a result of this definition for the partial gain in a given direction, we can present the total gain of an antenna as the sum of partial gains for any two orthogonal polarizations: Gtotal = Gq + Uf (1.24) G U P θ θ π = 4 in & G U P φ φ π = 4 in (1.25) The terms Uq and Uf represent the radiation intensity in a given direction contained in their respective E-field component. The gain of an antenna is a passive phenomenon; power is not added by the antenna but simply redistributed to provide more radiated power in a certain direction than would be transmitted by an isotropic antenna. An antenna designer must take into account the antenna’s application
Fundamentals of Antennas 13 when determining the gain.High-gain antennas have the advantage of longer range and better signal quality but must be aimed carefully in a particular direction.Low-gain antenas have shorter range,but the or entation of the antenna is inconsequential.For example,a dish antenna on a spacecraft is a high-gain device(must be pointed at the planet to be effective)whereas a typical wireless fidelity (WiFi)antenna in a laptop computer is low-gain(as long as the base station is within range,the antenna can be in any orientation in space).Improving horizontal range at the expense of reception above or below the antenna makes sense 1.1.7 Intermodulation Generally,an antenna is passive linear device.However. when such is excited by high enough power,it acts slightly as nonlinear device.The nonlinearity is normally caused by meta l-to-meta joints and nonlinear materials in the antenna structure.Therefore.whep signals with multiple frequencies are fed into nonlinear devices.inter- modulation product terms whose frequencies are different to those of al are gen ated A t -180 to-120 dBe(dB c re An antenna's intermodulation degrades a wireless system's perfor mance if the system has the following features: High transmitted power is adopted. The system is equipped with high receiver sensitivity. One antenna is used for both transmitting and receiving. Signals at more than one frequency are transmitted base stations normally have this entire feature set.base stations om pa odulatio PIM).High an enn na of the ba ponents cause back reflection to the receiver due to the antenna's PIM Since the receiver is highly sensitive and is able to sense very weak signals,the intermodulation signals cause interference.The problem becomes worse if the intermodulation term falls inside the receiving hand beca not be ed by filte ering ple,for th eP.GSM-900 system whose downlink band from 9 35 MHz to 960 MHz and whose uplink band is from 890 MHz to 915 MHz, the 3rd order intermodulation term at the base station side may be 2 x 935-960=910 MHz,which falls inside the uplink band.On the other hand,the PIM problem is not that serious at a client terminal. ach as a cell phor a per onal digital as nt (PDA) with wireless capability,and is normally ignored.On the client side
Fundamentals of Antennas 13 when determining the gain. High-gain antennas have the advantage of longer range and better signal quality but must be aimed carefully in a particular direction. Low-gain antennas have shorter range, but the orientation of the antenna is inconsequential. For example, a dish antenna on a spacecraft is a high-gain device (must be pointed at the planet to be effective) whereas a typical wireless fidelity (WiFi) antenna in a laptop computer is low-gain (as long as the base station is within range, the antenna can be in any orientation in space). Improving horizontal range at the expense of reception above or below the antenna makes sense. 1.1.7 Intermodulation Generally, an antenna is considered a passive linear device. However, when such a device is excited by high enough power, it acts slightly as a nonlinear device. The nonlinearity is normally caused by metal-to-metal joints and nonlinear materials in the antenna structure. Therefore, when signals with multiple frequencies are fed into nonlinear devices, intermodulation product terms whose frequencies are different to those of the input signal are generated. A typical passive intermodulation signal level is from –180 to –120 dBc (dBc relative to carrier power).7–8 An antenna’s intermodulation degrades a wireless system’s performance if the system has the following features: ■ High transmitted power is adopted. ■ The system is equipped with high receiver sensitivity. ■ One antenna is used for both transmitting and receiving. ■ Signals at more than one frequency are transmitted. Base stations normally have this entire feature set. Base stations, therefore, suffer from passive intermodulation (PIM). High-power signals excite the antenna of the base station, and intermodulation components cause back reflection to the receiver due to the antenna’s PIM. Since the receiver is highly sensitive and is able to sense very weak signals, the intermodulation signals cause interference. The problem becomes worse if the intermodulation term falls inside the receiving band because the interference cannot be removed by filtering. For example, for the P-GSM-900 system whose downlink band is from 935 MHz to 960 MHz and whose uplink band is from 890 MHz to 915 MHz, the 3rd order intermodulation term at the base station side may be 2 × 935 − 960 = 910 MHz, which falls inside the uplink band. On the other hand, the PIM problem is not that serious at a client terminal, such as a cell phone, a personal digital assistant (PDA), or a laptop with wireless capability, and is normally ignored. On the client side
14 Chapter One the transmitted power is not that high due to limited battery capacity and forelectrom power transmission does not reduce the quality of uplink as the base station is equipped with a highly sensitive receiver.In addition,the receiver sensitivity is not high at a client terminal,and the reflected PIM level is thus low wer than the noise level.Similarly,the relatively lower eceiver sensitivity does not degrade the downlink t performance as a high-power signal is transmitted from the base station. An antenna's PIM can be measured by a dedicated analyzer.For exam- ple,Summitek Instruments provides such an analyzer.Figure 1.6 shows the block diagram ofa PIM analyzer that measures the PIM ofa two-port device.It has two meas ent modes called reve erse measur ement and forward measurement.As shown in Figure 1.6,a tw e high-power signal is fed into Port 1 of the device under test(DUT).The RF switch is in the "Rev"position for the reverse measurement mode or in the "Fwd"position for the forward measurement.For the measurement of an antenna's PIM.the reverse measurement is used not only because an antenna is a one- -portdevice the e erse e measurement corresponds to the operation condition of a base station antenna. RF switch Receiver <图 2 Figure 1.6 Block diagram of a PIM analyzer
14 Chapter One the transmitted power is not that high due to limited battery capacity and for electromagnetic safety reasons, and thus the PIM reflected to the receiver is weaker than that at the base station. The relatively lowpower transmission does not reduce the quality of uplink as the base station is equipped with a highly sensitive receiver. In addition, the receiver sensitivity is not high at a client terminal, and the reflected PIM level is thus lower than the noise level. Similarly, the relatively lower receiver sensitivity does not degrade the downlink performance as a high-power signal is transmitted from the base station. An antenna’s PIM can be measured by a dedicated analyzer. For example, Summitek Instruments provides such an analyzer. Figure 1.6 shows the block diagram of a PIM analyzer that measures the PIM of a two-port device. It has two measurement modes called reverse measurement and forward measurement. As shown in Figure 1.6, a two-tone high-power signal is fed into Port 1 of the device under test (DUT). The RF switch is in the “Rev” position for the reverse measurement mode or in the “Fwd” position for the forward measurement. For the measurement of an antenna’s PIM, the reverse measurement is used not only because an antenna is a one-port device but also because the reverse measurement corresponds to the operation condition of a base station antenna. Figure 1.6 Block diagram of a PIM analyzer PA PA RF switch Rev Tx Rx Rx Tx Port 1 Port 2 DUT Receiver Fwd
Fundamentals of Antennas 15 1.2 Important Antennas in This Book Here we introduce several antennas that are recently developed and could be relatively new. microstrip antennas as narrow band planar printed antennas.sus pended planar antennas as wideband antennas,and planar monopole as an ultra-wideband antenna(UWB). 1.2.1 Patch Antennas The microstrip patch antenna is a popular printed resonant antenna for narrow-band microwave wireless links that require semihemispherical heavily and is often used as an el ment for an array. Common microstrip antenna shapes are square,rectangular,cir- cular,ring,equilateral triangular,and elliptical,but any continuous shape is possible.Figure 1.7 shows the parameters of circular and rectangular patches.Some patch antennas eschew a dielectric sub- patch in air above a ground planeu rs;the resulting structure is less robust but provides better bandwidth. Microstrip Microstrip antenna Feed poin Feed point Ground plane Ground plane Dielectric Ground plane a (, Ground plane covered by dielectric Figure 1.7 (a)Circular patch,(b)rectangular patch,and(e)side view
Fundamentals of Antennas 15 1.2 Important Antennas in This Book Here we introduce several antennas that are recently developed and could be considered as relatively new. These antennas are conventional microstrip antennas as narrow band planar printed antennas, suspended planar antennas as wideband antennas, and planar monopole as an ultra-wideband antenna (UWB). 1.2.1 Patch Antennas The microstrip patch antenna is a popular printed resonant antenna for narrow-band microwave wireless links that require semihemispherical coverage. Due to its planar configuration and ease of integration with microstrip technology, the microstrip patch antenna has been studied heavily and is often used as an element for an array. Common microstrip antenna shapes are square, rectangular, circular, ring, equilateral triangular, and elliptical, but any continuous shape is possible.9 Figure 1.7 shows the parameters of circular and rectangular patches. Some patch antennas eschew a dielectric substrate and suspend a metal patch in air above a ground plane using dielectric spacers; the resulting structure is less robust but provides better bandwidth. Ground plane Microstrip antenna Feed point Microstrip antenna Feed point Ground plane Rp y x L W y x *Ground plane covered by dielectric z y (a) (b) (c) Ground plane Dielectric substrate Figure 1.7 (a) Circular patch, (b) rectangular patch, and (c) side view