Antenna Theory and Design Antenna Theory and Design Associate Professor: WANG Junjun 王珺珺 School of Electronic and Information Engineering, Beihang University F1025, New Main Building wangjunjun@buaa.edu.cn 13426405497
Chapter 7 Antenna Measurements
Introduction Accurate measurements are necessary to establish the actual performance of antennas: their gain, pattern, polarization, bandwidth, efficiency, etc. Antennas having strict specifications are needed in many applications as in mobile and personal communications, satellite communications, remote sensing, and radar. In many cases, antenna properties can be calculated theoretically very accurately. However, for complex antennas this might not be possible-too many idealizations or simplifications have to be made. Often the modeling of the usage environment is difficult, e.g., if the antenna is close to the human head or is installed on an airplane. Even if the properties of ideal antennas can be calculated, performance of real world antennas has to be checked by measurements, because due to fabrication tolerances, and in some cases due to fabrication errors, they may not work as well as predicted. The measurement results give valuable information for troubleshooting.
Chapter 7 Antenna Measurements Basic Concepts Radiation Pattern Gain Measurements Typical Sources of Error In Antenna Measurements Measurement Ranges
7.1 Basic Concepts The most common antenna measurement is to measure its radiation properties like directional pattern, gain or phase pattern in the far field. Typical configuration of the measurement of radiation properties is shown in Fig.7-1.The basic procedure is to place a transmitting or receiving source antenna at different locations with respect to the Antenna Under Test(AUT) and thus get a number of samples of the pattern. Figure 7-1 Typical configuration for the measurement of the radiation properties of an antenna
7.1 Basic Concepts 7.1.1 Reciprocity in Antenna Measurements It was indicated already above that the AUT can act as either a receiving antenna or a transmitting antenna. This is of course due to the reciprocity principle. Two important consequences of the principle from the antenna measurement point of view were given: The transmitting and receiving patterns are the same. Power flow is the same either way. Thus it is clear that all radiation parameters of the AUT can be measured in either transmission or reception mode. This is especially useful in cases, where, for example, the AUT is an integral part of a larger device acting as either a receiver or a transmitter thus defining the direction of the signal.
7.1 Basic Concepts 7.1.1 Reciprocity in Antenna Measurements However, in practical antenna measurements one has to be careful in applying the reciprocity principle: 1 The emfs in the terminals of the interchanged antennas are of the same frequency. 2 The media are linear, passive and isotropic. 3 The power flow is equal for matched impedances only. In standard careful measurements the 1st condition should be met without problems. The 2nd condition should be considered always, when the AUT includes any active or ferrite components as an integral part. The 3rd condition is perhaps the most difficult and may lead into measurement results, which seem to violate the reciprocity principle.
7.1.1 Reciprocity in Antenna Measurements Fig.7-2 shows a typical configuration with the instrumentation for the measurement of the radiation properties of an AUT. Fig.7-2 V e e V e e 11 22 R t (1 ) 2 FS AUT 2 T 1 T SA 1 AUT R Where V = voltage detected by the receiver from transmission line 2, V 1 1 2 2 V = voltage supplied by the transmitter into transmission line 1,V = complex propagation factor of transmission line 1 between the transmitter and source antenna, = complex propagation factor of transmission line 2 between the AUT and receiver antenna, = length of transmission line 1,m = length of transmission line 2,m = voltage reflection coefficient of the transmitter output = voltage reflection coefficient of the source antenna = voltage reflection coefficient of the AUT = voltage reflection coefficient of the receiver input = voltage transmission coefficient between the antenna termimals (7-1)
7.1.1 Reciprocity in Antenna Measurements V e e V e e 11 22 R t (1 ) 2 FS AUT 2 T 1 T SA 1 AUT R 1 1 2 2 Above it has been assumed that the voltage transmission coefficients of the transmitter output, source antenna input and receiver input are included in the calibration procedures into, and respectively. Therefore, only the voltage transmission coefficient 1- of the AUT output is taken into account. The first and last term on the right side of(1)are the voltage transmission coefficients of the transmission lines including multiple reflections. Now, according to the reciprocity principle we can exchange the input and the output( and ) if we fulfill the conditions above.
7.1.1 Reciprocity in Antenna Measurements However, this is not possible normally, because we have to exchange at least the transmitter (generator) and receiver and sometimes also parts of the cabling. In this case the exchange does not change the result significantly, if the numerators of the first and last term on the right side of (1) do not change much, which means that The reflection coefficients and and the cables (length, attenuation) are identical, which is seldom the case in practice, The reflections are small, i.e.. This depends on the measurement system and can usually be approximated to be true only for a measurement where a calibrated vector network analyzer is used. Problems occur especially when the matching of the antennas or the measurement devices is poor. In this case the exchange of the generator and receiver will change the result and it would appear that the reciprocity does not hold.
7.1 Basic Concepts 7.1.2 Near-Field and Far-Field The measuring practices and constrains depend largely on the distance of this surface from the AUT. It has been defined already that one can find several regions of radiated field in the vicinity of the antenna. There are the reactive near-field region, the radiative near-field or Fresnel region, and the far-field or Fraunhofer region(fig 7-3). Figure 7-3 Radiation pattern in near- field and far-field regions.
7.1 Basic Concepts 7.1.2 Near-Field and Far-Field As we are almost always interested in the radiation properties in the far field, it is obvious that the measurement usually also takes place in the far field. There are several advantages of the far-field measurement: The measured field pattern is valid for any distance in the far-field region; only simple transformation of the field strength according to l/r is required. If a power pattern is required, only power (amplitude) measurement is needed. The result is not very sensitive to the changes in the location of the phase center of the antennas and thus the rotation of the AUT does not cause significant measurement errors. Coupling and multiple reflections between the antennas are not significant.
7.1.2 Near-Field and Far-Field The main disadvantage of the far-field measurements is the required large distance between the antennas leading to large antenna ranges. The distance can be too large for a measurement in an anechoic chamber or it can result in atmospheric attenuation. In these cases one needs to consider measurements in the radiating near-field region, where so called near-field measurements take place. The reactive or evanescent near-field region is seldom used for antenna measurements, because it normally is located too close to the AUT and because mutual impedance caused by reactive coupling between the antennas makes the measurement complicated.
7.1.2 Near-Field and Far-Field The border of the regions around the AUT are defined based on dominances of certain properties of the field. The outer boundary of the reactive near-field region is typically defined to be at the distance beyond which the amplitude of the far-field component is larger than those of reactive near-field components. For small elementary dipoles, it is at the distance determined by the radius of the radian sphere, that is, where r rnf 2 = distance to small AUT at the outer boundary of the reactive near field, m (7-2) = wavelength, m The inner boundary of the far-field region is most often defined as the distance, where the curvature of the spherical wave front at the edge of the AUT causes a phase difference of radians compared to the center of the antenna, which is equal to a curvature of. This lead to the well-known Rayleigh distance giving the border between the Fresnel and Fraunhofer regions: 2 2D (7-3) rff where = distance to AUT at the inner boundary of the far-field region, m = wavelength, m D = largest dimension of the physical aperture of the antenna, m
7.1.2 Near-Field and Far-Field Phase error at the edges of a test antenna in the far-field when illuminated by a spherical wave.
7.1.2 Near-Field and Far-Field In some cases the allowed error due to the curvature is smaller and thus large farfield distances like are sometimes applied. For small antennas, the curvature criterion may not be adequate to define the far-field condition. Actually, when, is smaller than! Thus in these cases additional criteria for the proper distance for far-field measurement must be used. If we assume that the near fields add in random phase to the far field and set the peak-to-peak ripple caused by this to, we get an additional condition for the measurement distance: r ' rnf rff L/40 10 1 where = distance to AUT with certain level of the reactive near fields, m = peak-to-peak error caused by the reactive near fields, db (7-4)
7.1.2 Near-Field and Far-Field Another far-field condition is based on the requirement that the change in the measurement distance r due to rotation of the AUT should not have a large effect on the result obtained. Setting the peak-to-peak limit of this effect to, we get the condition '' 2Dm (7-5) rff L/40 10 1 where = distance to AUT with certain effect of the rotation, m = largest mechanical dimension of the antenna, m = peak-to-peak error caused by the rotation of AUT, db In 7-5 we have estimated that the peak-to-peak change of the distance between the phase center of the AUT and the source antenna is.
7.1.2 Near-Field and Far-Field Example 7.1.2 Near-and Far-Field Distances for a Normal-Mode Helix on a Handheld Cellular Phone. A normal-mode helical antenna is located on top of a handset at 900MHz. The length of the antenna is 25 mm and the length of the handset chas sis 110 mm. Find the distances where different near-and far-field criteria are met, when required peak-to-peak measurement uncertainty due to proximity effects is 0.5 db. Solution Now the maximum dimension of the antenna should be estimated to be the combined length of the phone chassis and the antenna element, so = = 135.This is due to the fact that for a device smaller than half wavelength typically the whole metallic body is occupied by the currents inducing the radiation. (a) The outer boundary of the reactive near field is at a distance given by Eq. 7-2 : = 0.333/2 = 53 (b) The Fraunhofer region starts at a distance given by Eq.7-3 = 2 0.135 /0.333 m=110mm (c) Now = 0.5dB and we get from Eq.7-4 the minimum distance, where the effect of the near fields is small enough: = 0.333/2 34.2 = 1.8 (d) Eq 7-5 gives the minimum distance, where the effect of the rotation of the AUT is small enough: = (2 0.135) 8.20 = 2.2
7.1.3 Coordinate System The IEEE standard spherical coordinate system is shown in Fig.7-4. Moving the source antenna along lines of constant or constant results in conical cuts or cuts, when is constant, and great-circle cuts or cuts, when is constant. In measuring the polarization of the antenna a standard practice is to establish a local coordinate system for each measurement direction (, ). Figure 7-4 Standard coordinate system for antenna measurements showing conical, great circle and principal plane cuts
Chapter 5 Antenna Measurements Basic Concepts Radiation Pattern Gain Measurements Typical Sources of Error In Antenna Measurements Measurement Ranges
7-2 Radiation Pattern 7-2-1 Instrumentation The instrumentation required to accomplish a measuring task depends largely on the functional requirements of the design. An antenna-range instrumentation must be designed to operate over a wide range of frequencies, and it usually can be classified into five categories : 1. source antenna and transmitting system 2. receiving system 3. positioning system 4. recording system 5. data-processing system
7-2-1 instrumentation A block diagram of a system that possesses these capabilities is shown in Fig 7-5 Figure 7-5 Instrumentation for typical antennarange measuring system
7-2-1 instrumentation The source antennas are usually log-periodic antennas for frequencies below 1 GHz, families of parabolas with broadband feeds for frequencies above 400 MHz, and even large horn antennas. The system must be capable of controlling the polarization. Continuous rotation of the polarization can be accomplished by mounting a linearly polarized source antenna on a polarization positioner. Antennas with circular polarization can also be designed, such as crossed log-periodic arrays, which are often used in measurements. The transmitting RF source must be selected so that it has frequency control, frequency stability, spectral purity, power level, and modulation. The receiving system could be as simple as a bolometer detector, followed possibly by an amplifier, and a recorder. More elaborate and expensive receiving systems that provide greater sensitivity, precision, and dynamic range can be designed. One such system is a heterodyne receiving system, which uses double conversion and phase locking, which can be used for amplitude measurements. A dual-channel heterodyne system design is also available, and it can be used for phase measurements.
7-2-1 instrumentation To achieve the desired plane cuts, the mounting structures of the system must have the capability to rotate in various planes. This can be accomplished by utilizing rotational mounts (pedestals), two of which are shown in Figure 7-6. Figure 7-6 Azimuth-over-elevation and elevation-over-azimuth rotational mounts.
7-2-1 instrumentation There are primarily two types of recorders; one that provides a linear (rectangular) plot and the other a polar plot. The polar plots are most popular because they provide a better visualization of the radiation distribution in space. The recording instrumentation is usually calibrated to record relative field or power patterns. Power pattern calibrations are in decibels with dynamic ranges of 0 60 db. For most applications, a 40-dB dynamic range is usually adequate and it provides sufficient resolution to examine the pattern structure of the main lobe and the minor lobes.
7-2-1 instrumentation In an indoor antenna range, the recording equipment is usually placed in a room that adjoins the anechoic chamber. To provide an interference free environment, the chamber is closed during measurements. To monitor the procedures, windows or closed circuit TVs are utilized. In addition, the recording equipment is connected, through synchronous servo-amplifier systems, to the rotational mounts (pedestals) using the raditional system shown in Figure 7-7(a). Figure 7-7a Block diagrams of typical instrumentations for measuring rectangular and polar antenna and RCS patterns. Traditional system
7-2-1 instrumentation The system can record rectangular or polar plots. Position references are recorded simultaneously with measurements, and they are used for angular positional identification. As the rotational mount moves, the pattern is graphed simultaneously by the recorder on a moving chart. One of the axes of the chart is used to record the amplitude of the pattern while the other identifies the relative position of the radiator. A modern configuration to measure antenna and RCS patterns, using a network analyzer and being computer automated, is shown in Figure 7-7(b). Figure 7-7(b) Block diagrams of typical instrumentations for measuring rectangular and polar antenna and RCS patterns. Computer automated system
7-2-1 instrumentation Figure 7-7c Indoor range with automatic control.
7-2-1 instrumentation Figure 7-7d Anechoic chamber room
7-2-1 instrumentation RCS & far-field Antenna Measurement System RCS and far-field antenna measurement system is based on intelligent instruments, and personal computers (IPC), the automatic monitoring system, with data acquisition, analysis and processing capabilities. Testing System to control computers at the core command and control functions with the corresponding vector network analyzer, the receiving table. Polarization turntable and other peripheral equipment, and far-field antenna measurement and automatic completion of the measurement tasks RCS.
7-2-1 instrumentation
7-2 Radiation Pattern 7-2-2 Amplitude Pattern The total amplitude pattern of an antenna is described by the vector sum of the two orthogonally polarized radiated field components. In many applications, antenna pattern measurements must be made in situ to preserve the environmental performance characteristics. A typical system arrangement that can be used to accomplish this is shown in Figure 7-8. The source is mounted on an airborne vehicle, which is maneuvered through space around the test antenna and in its far-field, to produce a plane wave and to provide the desired pattern cuts. The tracking device provides to the recording equipment the angular position data of the source relative to a reference direction. Figure 7-8 System arrangement for in situ antenna pattern measurements.
7-2 Radiation Pattern 7-2-3 Phase Measurements Two basic system techniques that can be used to measure phase patterns at short and long distances from the antenna are shown respectively, in Figures 7-9(a) and 7-9(b). For the design of Figure (a), a reference signal is coupled from the transmission line, and it is used to compare, in an appropriate network, the phase of the received signal. For large distances, this method does not permit a direct comparison between the reference and the received signal. In these cases, the arrangement of Figure (b) can be used in which the signal from the source antenna is received simultaneously by a fixed antenna and the antenna under test. Figure 7-9 Near-field and far-field phase pattern measuring systems.
Chapter 5 Antenna Measurements Basic Concepts Radiation Pattern Gain Measurements Typical Sources of Error In Antenna Measurements Measurement Ranges
7-3 Gain Measurements The most important figure of merit that describes the performance of a radiator is the gain. Usually there are two basic methods that can be used to measure the gain of an electromagnetic radiator: absolute-gain and gain-transfer (or gaincomparison) measurements. The absolute-gain method is used to calibrate antennas that can then be used as standards for gain measurements, and it requires no a priori knowledge of the gains of the antennas. Gain-transfer methods must be used in conjunction with standard gain antennas to determine the absolute gain of the antenna under test. The two antennas that are most widely used and universally accepted as gain standards are the resonant λ/2 dipole (with a gainof about 2.1 db) and the pyramidal horn antenna (with a gain ranging from 12 25 db). Both antennas possess linear polarizations. The dipole, in free-space, exhibits a high degree of polarization purity. However, because of its broad pattern, its polarization may be suspect in other than reflection-free environments. Pyramidal horns usually possess, in free-space, slightly elliptical polarization (axial ratio of about 40 to infinite db). However, because of their very directive patterns, they are less affected by the surrounding environment.
7-3 Gain Measurements 7-3-1 Absolute-Gain Measurements There are a number of techniques that can be employed to make absolute-g ain measurements. Where A. Two-Antenna Method 4 R P r G0t G0 r 20log10 10log db db 10 Pt = gain of the transmitting antenna (db) = gain of the receiving antenna (db) = received power (W) = transmitted power (W) R= antenna separation (m) = operating wavelength (m) If the transmitting and receiving antennas are identical (7-6) reduces to (7-6) 1 4 R P r G0t G0r 20log10 10log db db 10 2 Pt By measuring R, λ, and the ratio of Pr/Pt, the gain of the antenna can be found. (7-7)
7-3-1 Absolute-Gain Measurements B. Three-Antenna Method If the two antennas in the measuring system are not identical, three antennas (a, b, c) must be employed and three measurements must be made (using all combinations of the three) to determine the gain of each of the three. Three equations (one for each combination) can be written, and each takes the form of (7-6). Thus (a-b Combination) (a-c Combination) (b-c Combination) 4 R P rb 20log 10 log db Pta Ga Gb 10 10 db 4 R P rc 20log 10log db Pta Ga Gc 10 10 db (7-8a) (7-8b) 4 R P rc Gb Gc 20log10 10log db db 10 Ptb (7-8c) From these three equations, the gains (Ga)dB, (Gb)dB, and (Gc)dB can be determined provided R, λ, and the ratios of Prb/Pta, Prc/Pta, and Prc/Ptb are measured.
7-3-1 Absolute-Gain Measurements C. Extrapolation Method The extrapolation method is an absolute-gain method, which can be used with the three-antenna method, and it was developed to rigorously account for possible errors due to proximity, multipath, and nonidentical antennas.
7-3-1 Absolute-Gain Measurements D. Ground-Reflection Range Method A method that can be used to measure the gain of moderately broad-beam antennas, usually for frequencies below 1 GHz, has been reported. The method takes into account the specular reflections from the ground (using the system geometry of Fig 7-10), and it can be used with some restrictions and modifications with the two or three-antenna methods. Figure 7-10 Geometrical arrangement for reflection range.
7-3-1 Absolute-Gain Measurements D. Ground-Reflection Range Method To make measurements using this technique, the system geometry of Figure 7-10 is utilized. Usually it is desirable that the height of the receiving antenna be much smaller than the range (. Also the height of the transmitting antenna is adjusted so that the field of the receiving antenna occurs at the first maximum nearest to the ground. Doing this, each of the gain equations of the two- or threeantenna methods take the form of 4 RD P r rr D Ga G db b D db ADB Pt RR 20log10 10log10 20log10 (7-9) The only quantity that needs to be determined is the factor r which is a function of the radiation patterns of the antennas, the frequency of operation, and the electrical and geometrical properties of the antenna range.
7-3 Gain Measurements 7-3-2 Gain-Transfer (Gain-Comparison) Measurements The method most commonly used to measure the gain of an antenna is the gaintransfer method. This technique utilizes a gain standard (with a known gain) to determine absolute gains. Initially relative gain measurements are performed, which when compared with the known gain of the standard antenna, yield absolute values. The method can be used with free-space and reflection ranges, and for in situ measurements. The procedure requires two sets of measurements. In one set, using the test antenna as the receiving antenna, the received power into a matched load is recorded. In the other set, the test antenna is replaced by the standard gain antenna and the received power into a matched load is recorded. Writing two equations of the form of (7-6) or (7-9), for free-space or reflection ranges, it can be shown that they reduce to P T GT GS 10log db db 10 P ( 7-10) S where and are the gains (in db) of the test and standard gain antennas.
7-3-2 Gain-Transfer (Gain-Comparison) Measurements If the test antenna is circularly or elliptically polarized, gain measurements using the gain-transfer method can be accomplished by at least two different methods. One way would be to design a standard gain antenna that possesses circular or elliptical polarization. The other approach would be to measure the gain with two orthogonal linearly polarized standard gain antennas. The total gain of the circularly or elliptically polarized test antenna can be written as G G G 10log 10 T db TV TH (7-11) and are, respectively, the partial power gains with respect to vertical-linear and horizontal-linear polarizations. G TV is obtained, using (5-10), by performing a gain-transfer measurement with the standard gain antenna possessing vertical polarization. The measurements are repeated with the standard gain antenna oriented for horizontal polarization. This allows the determination of G TH. Usually a single linearly polarized standard gain antenna (a linear λ/2 resonant dipole or a pyramidal horn) can be used, by rotating it by 90, to provide both vertical and horizontal polarizations. This approach is very convenient.
Chapter 5 Antenna Measurements Basic Concepts Radiation Pattern Gain Measurements Typical Sources of Error In Antenna Measurements Measurement Ranges
7-4 Typical Sources of Error In Antenna Measurements Any measured quantity has a margin of error. Thus, the complete value for the gain of an antenna might be indicating a half decibel uncertainty. To reduce the measurement uncertainty to an acceptable level, the critical sources of error have to be recognized. A pure plane wave (uniform phase and amplitude) is an ideal test field for the measurement of far-field pattern. However, there are inevitably deviations from the plane wave. The test field imperfections are virtually indistinguishable from antenna errors and therefore, it is worth to spend a lot of expense and labor to reduce them. Coupling to the reactive near field, alignment errors, interfering signals, effects of the atmosphere, leaking and radiating cables, instrumentation errors, etc., are further sources of error in antenna measurements.
7-4 Typical Sources of Error In Antenna Measurements 7-4-1 Phase Error and Amplitude Taper Due to Finite Measurement Distance Let us assume that the AUT is a planar antenna, which is receiving a wave coming from the direction of the main beam axis. If the measurement distance is too small, the fields received by different parts of the AUT will not be in phase and there will be a quadratic phase error(fig). At the far-field limit between the aperture edge and center is distance halves this phase error., the phase difference. Doubling the measurement Figure 7-11 Phase error and amplitude taper across the aperture of an AUT
7-4-1 Phase Error and Amplitude Taper Due to Finite Measurement Distance Due to the phase error, the measured gain is smaller and the side lobes are higher than in the ideal plane wave case. Also the nulls of the radiation pattern become filled. For measuring antennas having moderate side-lobe levels(down to about -30dB), the distance is usually adequate. At this distance, the measured gain is about 0.06dB smaller than the real far-field gain. If the measurement distance gets shorter, the measurement errors increase rapidly and the near-in side lobes merge with the main beam and either appear as shoulders or disappear altogether.
7-4 Typical Sources of Error In Antenna Measurements 7-4-2 Reflections Reflections from surroundings produce field variations(amplitude and phase ripples)in the test zone as the direct wave and reflected waves interfere. Even small reflected waves may cause large measurement errors, because the fields of the waves are added, not the powers. Reflections are especially harmful in the measurement of low side lobes. A small reflection coupled to the AUT through the main lobe may completely mask the direct wave coupled through the side lobe. Different ways to reduce the reflections or their influence are studied.
7-4 Typical Sources of Error In Antenna Measurements 7-4-3 Other Sources of Error In addition to these common measurement range imperfections, there are some other possible sources of error: Coupling to the reactive near field may be significant at low frequencies. Antenna measurements are three-dimensional vector field measurements. Therefore, many kind of alignment errors are possible. Man-made interfering signals may couple to the sensitive receiver especially on outdoor ranges. At large measurement distances, the effects of the atmosphere may be considerable...
Chapter 5 Antenna Measurements Basic Concepts Radiation Pattern Gain Measurements Typical Sources of Error In Antenna Measurements Measurement Ranges
7-5 Measurement Ranges Generally, one wants to know the far-field performance of an antenna, since the majority of antennas operate in this mode. In a basic far-field measurement, the AUT is separated from the source antenna by a distance that is at least. Small antennas can be measured indoors in an anechoic chamber. Often,however, far-field ranges have to be physically large outdoor ranges. For the measurement of large millimeter and submillimeter wave antennas having far-field distances of several kilometers or more, a compact antenna test range or a near-field range is a necessity. Which range type suits best for the measurement of a certain antenna depends mainly on the physical size and frequency of the antenna. Sometimes, the best measurement site is the environment where the antenna is used.
7-5 Measurement Ranges 7-5-1 Elevated Ranges The basic far-field range is an elevated range (Fig.7-12). Antennas are placed high on towers, buildings or hills to reduce the effects of environment. In most cases, the AUT is operated as a receiving antenna. Figure 7-12 Elevated range to minimize reflected waves
7-5 Measurement Ranges 7-5-1 Elevated Ranges The range length is usually determined by the far-field criterion. For the measurement of low-side-lobe antennas even longer ranges than may be needed. The range width should be sufficient to keep the main beam of the source antenna within it. The antenna heights and the source antenna pattern should be selected so that the main beam of the source antenna does not illuminate the ground between the antennas. The lower limit for the source antenna diameter,, follows from the requirement that the first null of the pattern should fall above the base of the test tower: 1.5R DT H (7-12) R From the maximum accepted amplitude taper of 0.25dB follows the upper limit for the source antenna diameter: R DT 3D (7-13) Combining the two above-mentioned requirements for the source antenna diameter gives that the AUT should be mounted at least five times as high as its diameter D.
7-5-1 Elevated Ranges Example 7-5-1 Amplitude Taper on Test Range The source antenna and the AUT are both parabolic reflector antennas, which have a diameter of 1 meter. The operating frequency is 10 GHz and the separation of the antennas is. Estimate the amplitude taper. Solution The half-power beamwidth of a parabolic reflector is about. At the measurement distance, This angle corresponds to a transverse distance of. can be assumed that the shape of the main beam (measured in decibels) is nearly parabolic. Therefore, the pattern level of the source antenna is at the edge of the AUT. It can be verified also from Eq.(7-13) that the source antenna diameter (1m) is too large if the maximum allowed edge taper is 0.25 db.. It
7-5 Measurement Ranges 7-5-2 Ground-Reflection Ranges At VHF and lower frequencies, ground reflection are difficult to avoid because a directional source antenna is very large and a ground-reflection range(fig.7-13) can be used. Figure 7-13 Ground-reflection range with direct and reflected waves
7-5-2 Ground-Reflection Ranges Antennas are placed above a flat reflecting surface with(or without)buried round screen. The specular reflection appears to come from the image source. The real and image source from together an interference pattern. The AUT is placed in the first lobe of the interference pattern. This lobe has the flattest amplitude distribution. Assuming that the image is negative, i.e., the reflection coefficient of the surface is -1, the peak of the first lobe is at the height where R H R 4HT = wavelength, m = Separation between the antennas, m ; = Height of the source antenna, m This equation indicates that the ideal height is a function of wavelength. Therefore, the height of the source antenna or AUT may need to be adjusted for each measurement frequency. The height of the AUT should be at least 3.3D to make sure that the amplitude taper is less than 0.25dB. (7-14) The surface of a ground-reflection range should be smooth, which requires that the rms height variation of the surface,, is small enough: Where h M sin = wavelength, m ; M= smoothness factor ; = grazing (7-15)
7-5 Measurement Ranges 7-5-3 Anechoic Chambers and Absorbing Materials The walls, ceiling and floor of an anechoic chamber are covered completely with absorbing material. An anechoic chamber simulates a reflectionless free space and allows all-weather antenna measurements in a controlled laboratory environment. In an anechoic chamber, the test area is isolated from interfering signals much better than at outdoors. Anechoic chambers can be used for the far-field measurements of small antennas. Large antenna can be measured on compact antenna test ranges and near-field ranges, which are usually installed in anechoic chambers.
7-5-3 Anechoic Chambers and Absorbing Materials Absorbing Materials. An ideal absorber provides an impedance match for incoming waves at all frequencies and angles of incidence. A tapered impedance transition from free space to the back of the absorber can be obtained by changing the resistivity of the medium gradually or by geometric shaping. Pyramids and wedges are widely used shapes. Pyramids work best at normal incidence, and they scatter as a random rough surface if they are large compared to the wavelength. On the other hand, wedges work well at large incident angles with the wedge direction along the plane of incidence but not as well as the pyramids at normal incidence. Figure 7-14 Absorbers: (a) pyramids. (b) wedges.
7-5-3 Anechoic Chambers and Absorbing Materials Anechoic Chamber Types. Anechoic Chambers are usually rectangular or tapered. Fig 7-15 (a) shows a rectangular chamber. The end walls and the center parts of the sidewalls, floor and ceiling are covered with pyramids. Other parts are covered with wedges. The antennas are placed on the middle line of the chamber; the source antenna close to one end wall, the AUT a little further away from the other end wall. The test zone where the reflections are minimized is called the quiet zone The dimensions of the chamber should be such that the angle of incidence on sidewalls is less than 60". At larger angles the reflections would be large. Typically, the length to width (or height) ratio is 2 : 1. The source antenna should be chosen so that its main beam does not illuminate the sidewalls, ceiling, and floor. Figure 7-15 (a) rectangular Anechoic chambers. At frequencies below about 1GHz, the rectangular chamber having absorbers of reasonable size has a high level of reflection, then a tapered chamber works better.
7-5-3 Anechoic Chambers and Absorbing Materials In a tapered chamber(fig 7-15 (b)), the source antenna is closed to the apex of the tapered section and the specular reflections occur close to the source. The phase difference of the direct wave and the specular reflections changes slowly in the quiet zone which results in a more planar wavefront than in the case of a rectangular chamber. At higher frequencies, the source is moved from the apex closer to the rectangular section and the chamber is used as a normal rectangular chamber. Figure 7-15. (b) tapered Anechoic chambers