Chapter-9 CONCLUSIONS AND FUTURE SCOPE 9.1 CONCLUSIONS This thesis presented a detailed study about six different new antenna designs developed to demonstrate the reconfigurable concept employing electrical reconfiguration technique using PIN diodes. The new designs presented are the Reconfigurable rectangular patch antenna (RRPA), Reconfigurable wheel antenna (RWA), Reconfigurable meandered line antenna, Reconfigurable cavity backed square spiral antenna, and Reconfigurable substrate integrated waveguide cavity backed slot antenna and Reconfigurable diamond shape patch antenna. A vast literature survey was conducted on the available reconfigurable antennas starting from reconfigurable antennas implemented with mechanically movable parts and arrays to microstrip reconfigurable antennas implemented with mechanical and semiconductor switches. The survey revealed that conventional mechanical switches are not practical for reconfigurable antenna applications due to their large size and are not compatible with the printed circuit board. They are preferred only for lower frequency and high power handling situations. Solid state switches such as PIN diode and FET s are most widely used to implement reconfigurable antennas electrically, among these PIN diode switches can offer promising characteristics for reconfigurable antennas. Therefore, in this work, all reconfigurable antennas are designed and fabricated using PIN diodes. Reconfigurable rectangular patch antenna single element for frequency reconfiguration and 1X8 linear array for both frequency & pattern reconfiguration using PIN diodes have been discussed in Chaper3. It has been shown that the pattern can be steered by controlling the supply of PIN diodes in each iteration. Therefore, the requirement of phase shifters in the phased array radar can be eliminated thus reducing the system cost and complexity. Experimental data have demonstrated the concepts of reconfigurable antenna by switching of PIN diodes 174
for multiple radar frequencies. The simulated results are in good agreement with the measured results. Fabrication accuracy can further improve the results of the designed antenna array. Simulated and experimental data presented in Chapter 4 demonstrated the concepts of single element reconfigurable wheel antenna and its array by switching OFF and ON of PIN diodes for multiple bands of frequencies. The performance of RWA can be further improved by proper designing of driver circuit in the antenna structure. The technique has taken the advantage of different number of radiating lengths with the use of PIN diode switches, each configuration resonating at different frequency, In array radiation pattern there is a grating lobe within 35 deg for X-band, therefore the main beam can be steered only within ±15 deg. For S-band there is no grating lobe as the inter element spacing is less than a wavelength. Multiband meander line antenna design for four states are presented in Chapter 5.A total of 4 PIN diode switches were incorporated in to the antenna geometry to achieve frequency reconfiguration, for experimental verification. Fourth iteration has been fabricated and return loss, pattern measurements have been carried out for the same. The simulated and experimental data have demonstrated the concepts of multiband reconfigurable antenna by switching OFF and ON of PIN diodes for multiple band frequencies. The technique has taken the advantage of different number of radiating lengths with the use of PIN diode switches, each configuration resonating at multiband frequencies. 175
The development, design, simulation and measurement of the cavity backed reconfigurable spiral antenna were presented in Chapter 6. The measured results are in good agreement with the simulated one. In Band-II, the ripples are little high because of the biasing circuit effect. This can be avoided by proper isolation between RF and DC bias. The controlling of PIN diodes in real application can be implemented using FPGA control to achieve the switching speed. The operational frequency can be still further increased by multilayer spiral and proper broad band matching. Design and development of a SIW antenna with dual state and dual band for C- band applications is discussed in Chapter 7.Two high performance PIN diode switches were incorporated in to the new design to give dual band in both the states, The corresponding biasing network of the diodes are also integrated in the antenna geometry. The measured antenna performance was similar to the predicted simulation performance and suggested that by using reconfigurable multiband approach we can eliminate the bulky and expensive filters in modern multi-band systems to improve the out-of-band noise rejection performance. 176
A new microstrip antenna with triple-polarization diversity for C-band applications is demonstrated with six discrete antenna states in Chapter 8. To achieve the polarization reconfigurability, one SP3T switch to select the feed location and 4 PIN diodes have been used to connect the truncated patches to the main patch and the biasing network of the diodes are also integrated in the antenna geometry. The types of achieved polarization are linear, circular and elliptical. The purity of polarization has been estimated by measuring the axial ratio of the developed proto type antenna, and it is found that it is less than 4dB for CP and more than 30dB for linear. Table 9.1 provides a summary of the important performance characteristics for the six antennas developed in this work. 177
178
Achieving the pattern reconfigurability without significant changes in the operating frequency is somewhat difficult because of the relationship between the source currents and the antenna structure. Here, antenna structure has been modified to achieve the pattern thus resulting small changes in the operating frequency. Pattern reconfigurability demonstrated with switches in this work is similar to that achieved with traditional phased arrays but without the inherent costs of phase shifters. 9.2 FUTURE SCOPE The reconfigurable antenna designs using PIN diodes reported in this dissertation may be extended by using the RF micro electro mechanical systems (MEMS) switches which give the superior performance than the PIN diodes with respect to bandwidth, linearity, power consumption, insertion loss and isolation. The specific disadvantage of this PIN diode is that, it is unsuitable for reconfigurable antenna design where a large number of switches may be employed and individual device losses have a cumulative impact on overall antenna performance. Additionally, the non-linear nature of solid-state semiconductor switches always has the potential to introduce undesirable inter-modulation products into the RF signal path. The controlling of PIN diodes/mems can be made programmable. In order to control more number of switches, a switch matrix can be preloaded into a memory as a look-up table and that memory will be recalled by a simple embedded program. 179
AGILENT ADS MOMENTUM Momentum is a part of Advanced Design System and gives the simulation tools need to evaluate and design modern communications systems products. The key features of momentum as follows. An electromagnetic simulator based on the Method of Moments Adaptive frequency sampling for fast, accurate, simulation results Optimization tools that alter geometric dimensions of a design to achieve performance specifications Comprehensive data display tools for viewing results Equation and expression capability for performing calculations on simulated data Full integration in the ADS circuit simulation environment allowing EM/Circuit Co-simulation Momentum is an electromagnetic simulator that computes S-parameters for general planar circuits, including microstrip, slot line, stripline, coplanar waveguide, and other topologies. Vias and air bridges connect topologies between layers, so we can simulate multilayer RF/microwave printed circuit boards, hybrids, multichip modules, and integrated circuits. Momentum gives a complete tool set to predict the performance of high-frequency circuit boards, antennas, and ICs. Momentum optimization extends momentum capability to a true design automation tool. The momentum optimization process varies geometry parameters automatically to help us achieve the optimal structure that meets the circuit or device performance goals. Momentum visualization is an option that gives users a 3-dimensional perspective of simulation results, enabling us to view and animate current flow in conductors and slots, and view both 2D and 3D representations of far-field radiation patterns. The following section describes the overview of the momentum. 180
MOMENTUM OVERVIEW Momentum commands are available from the Layout window. The following steps describe a typical process for creating and simulating a design with Momentum: 1. Create a physical design. You start with the physical dimensions of a planar design, such as a patch antenna or the traces on a multilayer printed circuit board. There are three ways to enter a design into Advanced Design System: Convert a schematic into a physical layout Draw the design using Layout Import a layout from another simulator or design system. Advanced Design System can import files in a variety of formats. 2. Choose Momentum or Momentum RF mode. Momentum can operate in two simulation modes: microwave or RF. You can select the mode based on your design goals. Use Momentum (microwave) mode for designs requiring full-wave electromagnetic simulations that include microwave radiation effects. Use Momentum RF mode for designs that are geometrically complex, electrically small, and do not radiate. You might also choose Momentum RF mode for quick simulations on new microwave models that can ignore radiation effects, and to conserve computer resources. 3. Define the substrate characteristics. A substrate is the media upon which the circuit resides. For example, a multilayer PC board consists of various layers of metal, insulating or dielectric material, and ground planes. Other designs may include covers, or they may be open and radiate into air. A complete substrate definition is required in order to simulate a design. The substrate definition includes the number of layers in the substrate and the composition of each layer. This is also where you position the layers of your physical design within the substrate, and specify the metal characteristics of these layers. 181
4. Solve the substrate. Momentum calculates the Green s functions that characterize the substrate for a specified frequency range. These calculations are stored in a database, and used later on in the simulation process. 5. Assign port properties. Ports enable you to inject energy into a circuit, which is necessary in order to analyze the behavior of your circuit. You apply ports to a circuit when you create the circuit, and then assign port properties in Momentum. There are several different types of ports that you can use in your circuit, depending on your application. 6. Add a box or a waveguide. These elements enable you to specify boundaries on substrates along the horizontal plane. Without a box or waveguide, the substrate is treated as being infinitely long in the horizontal direction. This treatment is acceptable for many designs, but there may be instances where a boundaries need to be taken into account during the simulation process. A box specifies the boundaries as four perpendicular, vertical walls that make a box around the substrate. A waveguide specifies two vertical walls that cut two sides of the substrate. 7. Create Momentum components. Momentum components can be used in the schematic design environment in combination with all the standard ADS active and passive components to build and simulate circuits including the parasitic layout effects. The Momentum engine is automatically invoked to generate an S- parameter model for the Momentum component during the circuit simulation. 8. Set up and generate a circuit mesh. A mesh is a pattern of rectangles and triangles that are applied to a design in order to break down (discretize) the design into small cells. A mesh is required in order to simulate the design effectively. You can specify a variety of mesh parameters to customize the mesh to your design, or use default values and let Momentum generate an optimal mesh automatically. 9. Simulate the circuit. You set up a simulation by specifying the parameters of a frequency plan, such as the frequency range of the simulation and the sweep type. 182
When the setup is complete, you run the simulation. The simulation process uses the Green s functions computed for the substrate, plus the mesh pattern, and the currents in the design are calculated. S-parameters are then computed based on the currents. If the Adaptive Frequency Sample sweep type is chosen, a fast, accurate simulation is generated, based on a rational fit model. 10. View the results. The data from Momentum simulation is saved as S- parameters or as fields. Use the Data Display or Visualization to view S- parameters and far-field radiation patterns. 183
ANTENNA MEASUREMENTS General Requirements of Antenna Measurement Procedures The ideal condition for measuring the far field characteristics of an antenna is its illumination by a uniform plane wave. This is a wave, in which has a plane wave front with the field vectors being constant across it. If D max is the maximum dimension of the antenna under test (AUT), a distance R min from the source of a spherical wave is given by R min = 2D 2 /λ This will ensure that the maximum phase difference between a plane wave and the spherical wave at the aperture of the AUT is 22.5 often many antennas, because of their complex structural configuration and excitation method cannot be investigated analytically. Experimental results are needed soften to validate theoretical data. Experimental investigations suffer from a number of drawbacks such as: 1. For pattern measurements, the distance to the far field region ( ) is too long even for outside ranges. It also becomes difficult to keep unwanted reflections from the ground and the surrounding objects below acceptable levels. 2. In many cases, it may be impractical to move the antenna from the operating environment to the measuring site. 3. For some antennas, such as phased arrays, the time required to measure the necessary characteristics might be enormous. 4. Outside measuring systems provide an uncontrolled environment, and they do not possess an all- weather capability. 5. Enclosed measuring systems usually cannot accommodate large antenna systems (such as ships, aircrafts and large spacecrafts). 184
6. Measurements techniques in general are expensive. Some of the above shortcomings can be overcome by using special techniques such as the far-field pattern prediction from near-field measurements scale model measurements, and automated commercial equipment specifically designed for antenna measurements and utilizing computer assisted techniques. Because of the accelerated progress made in aerospace / defense related systems (with increasingly small design margins), more accurate measurement methods were necessary. To accommodate these requirements improved instrument and measuring techniques were developed which include tapered anechoic chambers, compact ranges, near field probing techniques and swept frequency measurements, indirect measurements of antenna characteristics and automated test system s performance are the pattern (amplitude and phase), gain, efficiency, impedance, etc. Antenna Test Ranges The testing and evaluation of antennas are performed in antenna ranges. Typically there exist indoor and outdoor ranges and limitations are associated with both of them. Outdoor ranges are not protected from environmental conditions whereas indoor facilities are limited by space restrictions. Because some of the antenna characteristics are measured in the receiving mode and require far field criteria, the ideal field incident upon the test antenna should be a uniform plane wave. To meet this specification a larger space is usually required and it limits the value of indoor facilities. The classification of the test ranges is shown in Fig.1. 185
Figure.1 Different Types of Antenna Test Ranges Advantages of outdoor ranges: 1. Large antennas can be tested. 2. Very low frequency antennas can be tested. 3. No absorbers are required. 4. No need to do complicated near field to far field conversion. Limitations: 1. Interference from external environment. 2. High power transmitters due to long distances. Advantages of Indoor ranges: 1. No interference from external environment. 2. Accurate results by implementation of near field transformation. 3. Transmitting power is limited. 4. Availability of quit zone in indoor ranges. Limitations: 1. Large antennas cannot be tested, far field is very large. 186
2. For more accurate results proper grounding and shielding of chamber is necessary. Outdoor Test Ranges Elevated Ranges Elevated ranges are usually designed to operate mostly over smooth terrains. The antennas are mounted on towers or roofs of adjacent buildings. These ranges are used to test physically large antennas. A geometrical configuration is shown in the Fig.2. The contributions from the surrounding are usually reduced or eliminated by 1. Carefully selecting the directivity and side lobe level of the antenna. 2. Clearing the line of sight between the antennas. 3. Redirecting or absorbing any obstacles from the range surface and/or from any obstacles that cannot be removed. 4. Utilizing special signal processing techniques such as modulation tagging of the desired signal by using short pulses. Tx Antenna Direct Ray Rx Antenna Reflected Ray Figure.2 Elevated range 187
Ground Reflection Ranges In general, there are two basic types of antenna ranges, the reflections and the free-space. The reflection ranges can create a constructive interference in the region of the test antenna, which is referred to as the quite zone. This is accomplished by designing the ranges so that secular reflections from the ground, as shown in the Fig.3 combine constructively with direct rays. Tx Antenna Rx Antenna Direct Ray Reflected Ray Figure.3 Reflection range Usually it is desired for the illuminating field to have small and symmetric amplitude taper. This can be achieved by adjusting the transmitting antenna height while maintaining constant that of the receiving surface and they are usually employed in the UHF region for measurements of patterns of moderately broad antenna. They are also used for operating in the UHF. Slant Ranges Reflecting surface Slant ranges are designed so that the test antenna, along with its positioner, is modulated at a fixed height on a non conducting tower while the source (transmitting) antenna is placed near the ground, as shown in the Fig.4. The source antenna is positioned so that the pattern maximum, of its free space radiation is 188
oriented towards the center of the test antenna. The first null directed toward the ground spectral reflection point to suppress reflected signals. Slant ranges in general are more compact than elevated ranges as they require less land Test Antenna Source Antenna Figure.4 Slant Range Indoor Test Ranges Anechoic Chamber The Anechoic Chambers are the most popular antenna measurement sites especially in microwave frequency range. They provide convenience and controlled EM environment. However, they are very complex and expensive facilities. An Anechoic Chamber is typically a large room whose walls, floor, ceiling are first EM isolated by steel sheet. Besides, all inner surfaces of the chamber are lined with RF/Microwave absorbers. Absorbing materials are with much improved characteristics proving reflection coefficients as low as -50 db at normal incidence for a thickness of about four wave lengths are used in the chamber. Reflections increases with increase in angle of incidence. A typical absorbing element has the form of pyramid or a wedge shape. Pyramids are designed to absorb best the waves in normal incidence, while they do not perform well at large angles of incidence. Their resistance gradually decreases as the pyramid s cross section increases. 189
The pyramidal and wedge shaped absorbers are shown in Fig.5 and Fig.6 these absorbers are used in the anechoic chamber for the better results. Figure.5 Pyramidal shaped absorbers Figure.6 Wedge shaped absorbers Wedges, on other hand, perform much better than pyramids for waves, which travel nearly parallel to their ridges. There are two types of anechoic chamber designs: 190
1. Rectangular Chambers It is usually designed to stimulate free space conditions. High quality absorbing material such as carbon impregnated poly eurithrene pyramidal absorbers are used, on surfaces that reflect energy directly towards the test region in order to reduce the reflected energy level as shown in Fig.7. Even though the sidewalls, floor and ceiling are covered with absorbing material, significant specular reflections can occur from these surfaces, especially for the case of large angles of the incidence. One precaution that can be taken is to limit the angles of incidence to those for which the reflected energy is for below the level consistent with the accuracy required for the measurements to be made in the chamber. Often, for the high quality absorbers, this limit is taken to be a range of incidence angles of 0 0 to 70 0 (as measured from the normal to the wall). For the rectangular chamber this leads to a restriction of the overall width or height of the chamber. The actual width and height chosen shall depend upon the magnitude of the errors that can be tolerated and upon the measured characteristics of the absorbing material used to line the walls. Additionally the room width and the size of the source antenna should be chosen such that no part of the main lobe of the source antenna is incident upon the side walls, floor and ceiling. 191
Figure.7 Rectangular anechoic chamber 2. Tapered Chambers The design of both chambers is based on geometrical optics considerations, whose goal is to minimize the amplitude and phase ripples in the test zone, which are due to the imperfect absorption by the wall lining. The Tapered chamber has the advantage of turning by moving the source antenna closer to (at higher frequencies) or further closer (at lower frequencies) the apex of the taper. Thus, the reflected rays are adjusted to produce nearly constructive interference with the directed rays at the test location. Simple anechoic chambers are limited by distance requirements of the far-field measurements of large antennas or scatterers. There are two type basic approaches to overcome this limitation. One is presented by the compact Antenna Test Ranges (CATRs), which produce a nearly uniform plane wave in a very short distance via a system of reflectors. Another 192
approach is presented by techniques based on near-field zone or in Fresnel zone of AUT. The tapered anechoic chamber is shown in the Fig.8. Figure.8 Tapered anechoic chamber Compact Antenna Test Ranges Microwave antenna test measurements often require that the radiator under test be illuminated by a uniform plane wave. This is usually achieved only in the far field region, which in many cases dictates very large distances. Feed Figure.9 Compact ranges 193
The compact ranges are shown in this Fig.9. The requirement of the plane wave illumination can be achieved by techniques that require smaller distances and the use of a reflector. To accomplish this source antenna is used as an offset feed that illuminates a paraboloidal reflector. The illuminated reflector converts he impinging spherical waves into plane waves. The geometrical arrangement is shown in figure. This techniques lead to far field pattern simulation. It requires smaller distances than conventional methods and it is referred to as a compact range. Usually the linear dimensions of the reflector are three to four times greater than those of the test antenna. Network Analyzer RF or microwave energy can be viewed as a light wave. The energy is either reflected from or transmitted through the test device. By measuring the amplitude ratios and phase differences between the incident and the two (reflected and transmitted) new waves we can determine the reflection (impedance) and transmission characteristics of the device. There may be many names for these measurements, some use magnitude information only (scalar), others include both magnitude and phase information (vector). All names can be classified under the general headings of transmission and reflection. Figure.10 Internal architecture of network analyzer 194
Here a HP-8722D vector network analyzer has been used for VSWR measurements all developed antennas A network analyzer measurement system can be divided into four major parts. 1. A signal source providing the incident signal. 2. Signal separation devices to separate the incident, reflected and transmitted signals. 3. A receiver to convert the microwave signals to a lower intermediate (IF) signal. 4. Signal processor/display sections to process the IF signals and display the information on CRT. The signal source (RF or microwave) produces the incident signal used to simulate the test device. The test device responds by the reflecting part of the incident and transmits the remaining part. By sweeping the frequency of the source the frequency response of the test device can be determined. The next step in the measurement process is to separate the incident, the reflected and the transmitted signals. Once separated, their individual magnitude and phase differences can be measured. This can be accomplished through the use of directional couplers, bridges, power splitters or even high impedance probes. Reflection measurements require a directional device. Separation of the incident and reflected signals can be accomplished using either a dual directional coupler or a power splitter with a single directional coupler or bridge. The receiver provides the means of converting the RF or microwave voltages to a lower IF or DC signal to allow for a more accurate measurement. Lastly, the IF signals must be measured and processed before the relevant information can be displayed in an appropriate format on the CRT. Source Antennas for Antenna Ranges With the reception of a few highly specialized installations, antennas test ranges are designed to operate over wide band of frequencies. This means that 195
they shall be equipped with a family of source antennas and signal sources covering the entire band. The antennas shall, of course, have the beam widths and polarization properties consistent with the measurements to be performed on the range. For frequencies above 400 MHz families of parabolas with broadband feeds are most often used and for frequencies above 1 GHz horn antennas are used. A pyramidal horn antenna is being used as a source antenna for measurements. Signal Sources The selection of the transmitter depends upon several system considerations. There are a number of types of signal sources available such as triode cavity oscillators, klystrons, magnetrons, backward wave oscillators and various solid-state oscillators. Whatever type of signal is chosen, the following performance requirements apply: Frequency control: A means shall be available to tune the signal source to the desired frequency. For the case of oscillators that can be electrically tuned, an adjustable, regulated power supply is required. Frequency stability: Since the antennas and their associated radio-frequency circuitry are highly frequency sensitive, it is necessary that the signal-source frequency remain constant over the measured period, which may be in excess of 30 minutes. Spectral purity: Some types of oscillations are rich in harmonics, which if transmitted, would contaminate the desired signal. In some cases spurious or nonharmonically related signals are generated. Hence the source selected must have degree of spectral purity. Power level: The required power output of the signal source for a particular measurement is dependent upon the source and the test antenna gains, the receiver sensitivity, the transmission loss between the two antennas, and the dynamic range required for the measurement. Accordingly power level must be chosen. 196
Modulation: For some systems amplitude modulation is required, hence the signal sources should have that capability. There are cases where special pulse shaping is required to reduce the distortion of the pulse spectrum. Receiving Systems The receiving subsystem used in the antenna s amplitude/pattern measurement system may be simply a crystal detector (usually mounted directly on the test antenna or in the case of a scale model, inside the model) and its associated amplifier, the output of which supplies the signal to the recorder. With this system the transmitter is usually modulated. For high sensitivity even the mixers can be used in conjunction with receivers. Antenna Pattern Recorder The Antenna-pattern recorder provides a means of obtaining a visual display of the antenna pattern. It is used to plot the relative signal strength received by the test antenna as a function of the angular position of the antenna. The signal to be plotted is obtained from the output of a receiver or directly from a microwave detector, depending upon the type of receiving system used. The position information is normally obtained from synchro transmitters or digital encoders geared to the positioned axes. Typical antenna-pattern recorders are electro-mechanical devices employing servo systems to drive the recorder axes. A chart servo system usually positions the recording paper as a function of the angular position of the antenna. A pen servo system positions a recording pen in response to the amplitude of the input signal. Ink-writing systems are mostly used in preference to electric, thermal, pressure-sensitive or photographic systems because of the high quality, high writing speed, reproducibility, economy and simplicity of an ink system. The antenna pattern may be recorded in either polar or rectangular format. The polar form is often preferred for plotting patterns of antennas that are not 197
highly directional. The polar format is particularly useful for visualizing the power distribution in space. In the rectangular format the signal amplitude is the y-axis (ordinate) and the position angle is the x-axis (abscissa). The rectangular format permits narrow beam patterns to be recorded in finer detail because the pattern does not become crowded in regions of relatively low gain as it does in a polar graph. To provide adequate resolution in a rectangular display of patterns of different beam widths, selectable chart scales are required. Data Processing and Control Computers An on-line instrumentation minicomputer provides a natural solution to the data gathering, control and data-processing requirements of an automatic antennameasurement system. Instrumentation computers can be equipped with a variety of input-output devices, depending upon the requirements of the particular measurement program. Computer plotters can be employed to provide a variety of visual displays of antenna patterns such as contour plots and three-dimensional plots. For lengthy measurement programs or for programming convenience, a larger central computer at the user s facility can process the recorded data. Measurement of the Antenna Testing of the antenna includes the measurement of return loss and radiation pattern. From the obtained radiation patterns gain and beam widths are calculated. 1. RETURN LOSS MEASUREMENT In the measurement of return loss HP-8722D vector network analyzer has been used. Measurement Procedure 1. Adjust the sweep oscillator RF power level so that the reference channel level is in operate position of the scale. This ensures that there is enough power for phase locking. 198
2. Select sweep frequency range by selecting start and stop frequency. 3. Select one port s 11 for calibration measurement. 4. Select log amplitude mode on display. 5. Calibrate the network analyzer by connecting the standard short circuit, open circuit and matched loads at the test port. Observe the trace on the display to get a solid reference line. 6. Remove the standards and connect the antenna and observe the shift in the trace of the display. The display can be changed for obtaining the return loss, reflection coefficient, and impedance over the selected frequency band. Return loss in db=20log (ρ), where ρ is the reflection coefficient and 1+ ρ VSWR = 1 ρ Calibration of the network analyzer is done by using the standard loads supplied by the manufacturer. All the measurements are carried out carefully by not disturbing the cable setup, which is necessary for accurate measurement. 2. RADIATION PATTERN MEASUREMENT IN AN ANECHOIC CHAMBER Measurement Procedure 1. Mount the antenna under test on the antenna positioned as shown in Fig.11. 2. Mount the transmitting antenna, which is connected to a signal source. 3. Transmit the signal of the desired frequency from the transmitting antenna. 4. Receive the signal from the crystal detector that in turn is applied to the spectrum analyzer. 5. Adjust the attenuation of the spectrum analyzer to ensure that the signal is within the range of the spectrum analyzer. 199
6. To obtain the pattern in orthogonal plane, rotate the test antenna by 90 and repeat step 5. Figure.11 Setup for antenna radiation pattern measurement in an anechoic chamber For the test antenna the radiation pattern measurements were carried out for both horizontal and vertical polarizations. Beam width Beam width is calculated from the radiation pattern measured on the calibrated chart. The half power beam width is equal to the angular width between directions where the gain decreases by 3dB (the radiated field reduces to 1/ 2 if the maximum value.). 200
Gain The power gain of an antenna is 4π times the ratio of the power radiated per unit solid angle in the direction of maximum radiation to the net power accepted by the antenna from its generator. Two general categories of gain measurement methods exist. These are the absolute-gain measurements and the gain transfer measurements. The first method is used when extremely high accuracies are necessary and is usually employed in laboratories that specialize in the calibration of standards. Here we used the second method in which the gain of the antenna under test is measured by comparing it to that of the standard gain antenna. Gain of the antenna is measured by comparing gain pattern of the antenna under test to that of the standard linear isotropic antenna. Radiation pattern of the test antenna and standard gain antenna are measured with the same transmitting antenna. The difference between the measured power levels of the standard gain antenna and test antenna gives the gain of the test antenna. The gain measurements require essentially the same environment as the pattern measurements, although they are not so much sensitive to reflections and EM interference. To measure the gain of the antennas operating above 1 GHz, usually, free-space ranges are used. Between 0.1 GHz and 1 GHz, ground reflection ranges are used. Procedure 1. Fix vertical polarization of the transmit antenna 2. Transmit signal for known frequency 3. Mount the test antenna in azimuth plane rotate the antenna through 360 o and record the power received on the recorder. Note down the power output of the transmitter at each frequency. 4. Replace the test antenna with standard gain horn antenna and record the power received in the spectrum analyzer without any change at transmit or receive end. Make sure that the test power output of the transmitter is same as that at step3. 201
5. For gain at various frequencies repeat steps 1 to 4 Gain Calculations 1. Calculate the gain of the antenna under test using the following procedure 2. From the Gain vs. frequency plot of the standard gain horn, calculate the Gain of the standard Gain horn (say X db) 3. For the same frequency find the difference in db between the amplitude of the test antenna and standard Gain horn (say Y db) 4. The gain of the antenna under test for the frequency is given by G=(Y- X+A) db 3. RADIATION PATTERN MEASUREMENT USING OUTDOOR ANTENNA TEST RANGE This facility is used at Astra Microwave Products Limited, Hyderabad There are three outdoor antenna test ranges installed in Astra Microwave products Limited. These are 22m, 120m, and 1km. with the following salient features. The range that was selected to perform the testing of the various antennas discussed in this thesis was the 22m outdoor elevated range. Ranges: 3 outdoor ranges (22m,120m, and 1km) Frequencies: 100 MHz to 18 GHz Measurement type: amplitude Dynamic ranges: 80 db Sensitivity: -124 dbm Maximum size of the antenna 6m Positioner: azimuth over elevation Measurement of Directional Pattern Measurement of the directional pattern of the antenna reveals a lot about the functioning of the antenna and gives an overview about its performance. The pattern is plotted in both the horizontal as well as the vertical plane of the antenna 202
by using a transmitting antenna operating in the same frequency band of the AUT. There are three outdoor antenna test ranges installed in Astra Microwave products Limited. These are 22m, 120m, and 1km. The range that was selected to perform the testing of the various antennas in this thesis was the 22m outdoor elevated range. Figure.12 Antenna test set up The transmitting signal is generated by a sweep oscillator at the transmit antenna. The transmitted signal is approximately amplified with a suitable gain to overcome the path losses that are especially prominent in the microwave frequencies. The received signal is fed to a network analyzer and later to the digital pattern recorder that plots the received pattern at various points. 203
Gain Measurement Procedure The method of comparison was employed to measure the gain of the AUT. In this method, a standard antenna of known gain is connected to the receiver and the transmit antenna is pointed in the direction of maximum signal intensity. The input to the transmitting antenna is adjusted to a convenient level, and the readings are noted. The difference in the readings of the two antennas is calculated. This value is either subtracted from or added to the gain of the standard gain antenna depending on whether the AUT signal is lower than or higher than the standard gain antenna signal respectively. This final value gives us the gain of the AUT. Test procedure for measurement of Antenna Beam width Set the center frequency of the antenna in the signal source. Align the direction of both the transmitting and the receiving antennas on the same angle of elevation. Rotate the antenna under test through 360º in the azimuth with the help of positioner. Plot the radiation pattern by using the digital pattern recorder. repeat the steps for the entire band of frequencies for the antenna under test. Test Set Up for the Radiation Pattern and Gain measurements Mount the standard gain antenna of known frequency on the positioner. Keep it on axis direction. Set the center frequency band in the transmitter. Record the plot of the standard gain antenna with the help of the spectrum analyzer for on the axis of the standard gain antenna on the center frequency. Dismount the standard gain antenna and place the antenna under test in its place. 204
Now rotate the antenna in 360 degrees in azimuth with the help of the positioner and record the antenna gain with the help of the DPR. Find the difference of the gains of the standard gain antenna and antenna under test and add the known gain of the standard gain antenna to the difference. The final result so obtained gives the gain at the center frequency. Repeat the above steps in the entire band of frequencies of the frequency band. Axial Ratio Measurement Axial ratio is the ratio of major axis to the minor axis of the polarization ellipse. The axial ratio is determined as a function direction by using the rotating source method. The method consists of continuously rotating a linearly polarized source antenna (a pyramidal horn antenna is being used for the measurements) as the direction of observation of the test antenna is changed. This method is of greatest value for testing nearly circularly polarized antennas. The rotating source antenna causes the tilt angle t w of the incident field to rotate at the same rate. Care shall be taken to ensure that the time response of the recording system can adequately follow the excursions in t w. 205
Figure.13 Test set up for radiation pattern and gain measurements SNO EQUIPMENT QUANTITY 1 Synthesized micro sweeper 1 2 Spectrum Analyzer 1 3 Azimuth over elevation positioner 1 4 Flam & Russell Positioner controller 1 5 Positioner cables 1 6 ACORN Digital pattern recorder 1 Flam & Russell Inc-944 (version 2) a) CPU b) Color monitor c) Laser printer 7 Rotary joint 1 TABLE.1 List of Equipments used in Test set up 206
PHOTOGRAPHS OF THE ACTUAL MEASUREMENT SET UP USED Figure.14 Network analyzer kit Figure.15 Anechoic chamber 207
Figure.16 Test set up for radiation pattern, gain measurement Figure.17 Transmitting antenna used 208