Exercise 1-3. Radar Antennas EXERCISE OBJECTIVE DISCUSSION OUTLINE DISCUSSION OF FUNDAMENTALS. Antenna types
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1 Exercise 1-3 Radar Antennas EXERCISE OBJECTIVE When you have completed this exercise, you will be familiar with the role of the antenna in a radar system. You will also be familiar with the intrinsic characteristics of a radar antenna. DISCUSSION OUTLINE The Discussion of this exercise covers the following points: Antenna types Antenna characteristics Antenna Fields. Radiation pattern. Directivity. Power gain. Aperture. Angular resolution. DISCUSSION OF FUNDAMENTALS The role of any antenna is to radiate the transmitted signal into space and/or capture the received signal. The antenna is the transducer between waveguides or transmission lines and free space. In a pulsed radar system, the antenna serves to concentrate the transmitted signal into a narrow beam pointing in the desired direction, and to capture echo signals from only the desired direction. Radar antennas therefore usually have high directivity. Antenna types An isotropic antenna is a hypothetical antenna which radiates equally in all directions. The theoretical characteristics of an isotropic antenna are useful when describing those of practical antennas. The most common type of antenna in radar is the parabolic reflector. The parabola is illuminated by a source of energy called the feed (usually a waveguide horn), which is situated at the focus of the parabola and directed towards the reflector. This type of antenna is illustrated in Figure 1-36a. Because of the characteristics of a parabola, any ray from the feed is reflected by the reflector in a direction parallel to the axis of the parabola. Furthermore, the distance traveled by any ray from the feed to the reflector and then to a plane perpendicular to the axis of the parabola is independent of its path. This means that the signal originating at the feed is converted to a plane wavefront of uniform phase. One disadvantage of the basic parabolic antenna shown in Figure 1-36a is that the feed blocks some of the signal. This can be remedied by using an offset feed, as shown in Figure 1-36b. Many other types of directional antennas exist, such as the Cassegrain and phased-array antennas, but their description is beyond the scope of this manual. Festo Didactic
2 Ex. 1-3 Radar Antennas Discussion of Fundamentals Figure Parabolic antennas. 48 Festo Didactic
3 Ex. 1-3 Radar Antennas Discussion of Fundamentals Antenna characteristics Antenna Fields The area in front of a directional antenna can be divided into three regions, or fields. Immediately in front of the antenna is the near field. A little further away is the Fresnel region. In this region, the rays from the antenna are not parallel, and the radiation characteristics of the antenna vary with the distance from the antenna at which they are observed. The farthest region from the antenna is the Fraunhofer, or far-field region. In this region, the antenna and the observation point are sufficiently far from each other that the rays from the antenna can be considered to be parallel, and the radiation characteristics of the antenna are independent of distance. Radar antennas usually operate in this region. Radiation pattern A very useful concept in describing antenna characteristics is the radiation pattern. The radiation pattern is a three-dimensional graph of the field distribution of an antenna, in the far-field region. A radiation pattern therefore represents the distribution of energy transmitted by the antenna, as a function of direction. Although the term radiation pattern is used, it applies just as well to receiving antennas, as it also indicates the relative signal level of received power as a function of direction. Although the complete radiation pattern of an antenna is a three-dimensional function, a radiation pattern in one plane only often provides a sufficient indication of the antenna characteristics. For this reason, the radiation pattern is usually given as a two-dimensional graph representing the power or amplitude of the radiation in one plane (horizontal or vertical) versus the angle from the antenna beam axis. To characterize an antenna more completely, two radiation patterns, in planes at right angle to each other, may be used. The radiation pattern of an antenna is usually made by rotating the antenna while measuring the level of power received from another source as a function of the antenna orientation. To obtain a valid pattern, the environment must be free from all obstacles, such as walls and buildings which can act as reflectors and cause measurement errors. For this reason, radiation patterns are often traced by placing the antenna in an anechoic chamber. Figure 1-37 shows examples of two radiation patterns. The pattern marked O is that of an isotropic antenna. Note that the radiation is uniform in all directions. The other radiation pattern, marked A, is that of a directional antenna. The radiation pattern of the directional antenna in Figure 1-37 is composed of several lobes. The main lobe is the largest. This shows that most of the power from the antenna is radiated in the direction of the main lobe, which is the direction of the antenna axis. Festo Didactic
4 Ex. 1-3 Radar Antennas Discussion of Fundamentals Figure Radiation patterns (in db) of a directional antenna A and an isotropic antenna O. Besides the main lobe, there are usually several smaller sidelobes. These are lobes other than the main one, and represent radiation in directions other than that of the antenna axis. Sidelobes extend in all directions, even backward from the antenna. They contain a considerable amount of the transmitted power. 50 Festo Didactic
5 Ex. 1-3 Radar Antennas Discussion of Fundamentals Sidelobe radiation is generally undesired, as it is wasted. Since the radiation pattern also applies to reception, the sidelobes represent directions from which the antenna may pick up unwanted signals, such as echoes from the ground or buildings, or interference from jamming stations or other radars. Although sidelobes cannot be eliminated, antennas are usually designed to reduce them to a minimum. Radar antennas are usually designed to be highly directional. The beamwidth is a measure of the directivity of an antenna. The beamwidth may be considered to be the angle in degrees between opposite sides of the main lobe. This is the null-to-null beamwidth. A more common definition of beamwidth is the angle between points where the power has dropped by one-half (3 db) from maximum power. This is the 3-dB beamwidth or half-power beamwidth, and is shown in Figure The 3-dB beamwidth is usually approximately equal to one-half the null-to-null beamwidth. Directivity Another important antenna characteristic is how much the antenna concentrates energy in one particular direction. This can be expressed in terms of radiation intensity. The radiation intensity in a given direction is the power radiated per unit solid angle in that direction. The unit of solid angle is the square radian, or steradian, and is the angle subtended by an area on the surface of a sphere equal to the square of the radius. Since the area of a sphere is times the radius squared, a sphere contains steradians. An isotropic antenna radiates its energy uniformly in all directions, that is, over a solid angle of steradians. The radiation intensity is therefore independent of direction. This means that the radiation intensity in any direction is equal to the total power transmitted divided by : For a non-isotropic antenna, the radiation intensity varies with direction. The directive gain for a particular direction is defined as follows: (1-4) The average radiation intensity is equal to the radiation intensity of an isotropic antenna transmitting the same total power. The directivity is defined as the maximum value of the directive gain: (1-5) For pulsed radar systems using a scanning or rotating antenna, it is essential that the antenna have a high directivity. Festo Didactic
6 Ex. 1-3 Radar Antennas Discussion of Fundamentals Power gain The directivity of an antenna is determined solely by the radiation pattern. To express the how efficiently the antenna transforms the available power into radiated power concentrated in one direction, we use the term power gain (often called gain). The gain is equal to the directivity multiplied by a radiation efficiency factor. This factor takes into account antenna dissipative losses, but not losses due to impedance mismatch etc. Note that the gain is always less than the directivity, except for an ideal, lossless antenna where they would be equal. Gain can also be expressed in terms of radiation intensity. It is equal to the ratio of the radiation intensity in the direction of interest, to the radiation intensity that would be obtained if all of the power accepted by the antenna were radiated isotropically. Aperture The aperture of a unidirectional antenna is the size of the antenna's frontal area expressed in terms of the wavelength. It is defined as that portion of the plane surface which is perpendicular to the direction of maximum radiation and through which the major part of the radiation passes. The effective aperture is less than the aperture as it takes into account an efficiency factor. The effective aperture is directly related to gain by the relation: (1-6) where is the gain. is the effective aperture. is the signal wavelength. Angular resolution The angular resolution of an antenna is the ability of an antenna to distinguish between targets at different angles. Theoretically, the angular resolution could be as small as the 3-dB beamwidth. In practice, however, when there is noise, when the echo strengths are not equal, and when the target detection threshold in the receiver section is not ideal, the angular resolution is not as good. Usually, it is somewhere between 1 to 1.5 times the antenna's 3-dB beamwidth. The effect of beamwidth on angular resolution is illustrated in Figure In Figure 1-38a, the beam is so wide that it illuminates the two targets simultaneously. As the antenna turns, scanning the targets, the two targets shown will produce a single blip in the display. In Figure 1-38b, the beam is much narrower. As the antenna turns, each target will be illuminated successively, producing two distinct blips on the display. 52 Festo Didactic
7 Outline Figure Effect of beamwidth on angular resolution. PROCEDURE OUTLINE The Procedure is divided into the following sections: Setting up the system Radiation pattern of the Radar Antenna Setting up the basic pulsed radar Angular resolution of the basic pulsed radar PROCEDURE Setting up the system In this section, you will set up the measurement circuit that will be used to determine the radiation pattern of the Radar Antenna. The block diagram of this system is shown in Figure a a The Radar Transmitter provides a frequency-modulated continuous-wave (FM- CW) RF signal. It is convenient to use an FM-CW signal, rather than a pulsed or CW signal, as the frequency, and therefore the phase, of an FM-CW signal is constantly changing. Because of this, slight changes in range do not cause large changes in the amplitude of the demodulated signal, as is the case with pulsed radar. FM-CW radar will be studied in Unit 3 of this manual. The FM-CW signal is radiated in the direction of the Radar Antenna using a horn antenna. The Radar Receiver is connected to the RF OUTPUT of the Rotating-Antenna Pedestal in order to demodulate the signal received by the Radar Antenna. The demodulated signal is provided by the FM-CW OUTPUT of the Radar Receiver. A 50 load is connected to the RF INPUT of the Rotating-Antenna Pedestal since the Radar Antenna is used in reception only. Festo Didactic
8 1. The main elements of the Radar Training System, that is the antenna and its pedestal, the target table and the training modules, must be set up properly before beginning this exercise. Refer to Appendix B of this manual for setting up the Radar Training System, if this is not done yet. Set up the modules on the Power Supply / Antenna Motor Driver as shown in Figure Figure Module Arrangement. On the Radar Transmitter, make sure that the RF POWER switch is in the STANDBY position. On the Antenna Controller, make sure that the MANual ANTENNA ROTATION MODE is selected and that the SPEED control is in the 0 position. Set the POWER switch of the Power Supply to the I (on) position, and then those of the other modules. 2. Figure 1-40 shows the block diagram of the system you will use to determine the radiation pattern of the Radar Antenna. Connect the modules according to this block diagram, except for the horn which will be connected later in this exercise. a You should use a medium-length (approximately 75 cm) SMA cable to connect the RF OUTPUT of the Rotating-Antenna Pedestal to the RF INPUT of the Radar Receiver. 54 Festo Didactic
9 Figure Block diagram of the system used to determine the radiation pattern of the Radar Antenna. 3. On the Antenna Controller, select the POSITION MODE. In this mode, the Antenna Controller DISPLAY indicates the angle in the horizontal plane between the antenna beam axis and a reference axis. This angle is expressed in degrees. Festo Didactic
10 Using the SPEED control, set a very low speed and let the antenna make at least one turn while observing the Antenna Controller DISPLAY, then stop the antenna rotation. Using the SPEED control, position the antenna so that the Antenna Controller DISPLAY indicates 0. The antenna is now aligned with the reference axis. 4. Connect the cable of the target table to the multi-pin connector located on the rear panel of the Target Controller. Make sure that the surface of the target table is free of any objects and then set the POWER switch of the Target Positioning System to the I (on) position. Place the Rotating-Antenna Pedestal so that the antenna beam axis is aligned with the metal rail of the target table. Move the target table so that the grid is located approximately 1.1 m from the antenna horn. Make sure that the metal rail of the target table remains aligned with the antenna beam axis while moving the target table. An example for positioning the Rotating-Antenna Pedestal and the target table is shown in Figure Figure Position of the Rotating-Antenna Pedestal and target table. 5. Assemble the horn, the waveguide-to-sma coaxial adaptor and the support pin of the horn as shown in Figure Fix the horn assembly on the mast of the target table so that its aperture is directed towards the parabolic reflector of the Radar Antenna. 56 Festo Didactic
11 Figure Horn assembly. On the Target Controller, select the POSITION MODE of the DISPLAY, then use the Y-axis POSITION controls to set the Y-axis coordinate to 75 cm. Connect the CW/FM-CW RF OUTPUT of the Radar Transmitter to the horn assembly using an extra-long SMA cable (approximately 4 m). 6. Make the following adjustments: On the oscilloscope Channel V/DIV (AC coupled) Channel V/DIV (AC coupled) Vertical Mode... ALT. Time Base ms/div Trigger... Channel 2 On the Radar Transmitter RF OSCILLATOR FREQUENCY... MOD. This setting on the Radar Transmitter allows the frequency of the RF OSCILLATOR to be modulated by the FREQUENCY MODULATION section. On the Radar Transmitter, set the DEVIATION control so that a 1.2-V p-p triangular wave signal is displayed on the oscilloscope screen. Set the FREQUENCY control of the FREQUENCY MODULATION section so that the frequency of the triangular wave signal is approximately equal to 600 Hz. This signal modulates the frequency of the RF OSCILLATOR signal. Festo Didactic
12 7. On the Radar Transmitter, depress the RF POWER push button. The RF POWER ON LED should start to flash on and off. This indicates that RF power is being radiated by the horn. a Since the radiation levels of the Radar Training System are very low, there is no danger to anyone standing near or in front of the antenna. Remember however, that with a full-scale radar, you must always make sure that no one could be exposed to dangerous radiation levels before turning on the RF power. Slightly vary the orientation of the horn while observing the oscilloscope screen in order to maximize the amplitude of the FM-CW OUTPUT signal. Manually rotate the Rotating-Antenna Pedestal slightly while observing the oscilloscope screen in order to maximize the amplitude of the FM-CW OUTPUT signal. Do not alter the antenna orientation using the Antenna Controller. The antenna beam axis should be now perfectly aligned with the horn beam axis. Figure 1-43 shows an example of what you might observe on the oscilloscope screen. Figure Signal modulating the frequency of the RF OSCILLATOR and signal demodulated by the Radar Receiver. a In this exercise, you are often asked to vary the position of the Rotating- Antenna Pedestal or the orientation of the target while the RF power is on. This requires standing near or in front of the antenna. This practice could be very dangerous with a full-scale radar and should normally be avoided. However, the low radiation levels of the Radar Training System allow these manipulations to be carried out safely. 58 Festo Didactic
13 Radiation pattern of the Radar Antenna In this section, you will determine the radiation pattern of the Radar Antenna. To do this, you will measure the level of the demodulated signal for various orientations of the Radar Antenna. For each orientation of the Radar Antenna, you will calculate the ratio between the demodulated signal level and the maximum demodulated signal level. You will then plot the results on a polar graph. From this plot, you will determine the 3-dB beamwidth of the Radar Antenna. 8. On the Radar Transmitter, slightly vary the setting of the DEVIATION control in order to minimize the amplitude variations of the FM-CW OUTPUT signal that occur at the peak of the triangular wave signal. Adjust the oscilloscope as follows: Vertical Mode... Channel 1 Time Base s/div Trigger... EXT. The oscilloscope display should resemble Figure Figure Non-synchronized oscilloscope display of the FM-CW OUTPUT signal. 9. Use the oscilloscope to measure the peak-to-peak voltage of the FM-CW OUTPUT signal and note the result in the first row of the RECEIVED SIGNAL VOLTAGE ( ) column of Table 1-2. This voltage corresponds to (0 ) and will be used to calculate the relative powers using the formula indicated at the top of the RELATIVE POWER columns. In the same row of Table 1-2, but in the ANTENNA ORIENTATION ( ) column, note the angle indicated by the Antenna Controller DISPLAY. Festo Didactic
14 ANTENNA ORIENTATION Table 1-2. Determining relative power levels versus antenna orientation. RECEIVED SIGNAL VOLTAGE RELATIVE POWER 20 LOG ANTENNA ORIENTATION RECEIVED SIGNAL VOLTAGE RELATIVE POWER 20 LOG degrees V p-p db degrees V p-p db 10. Using the SPEED control of the Antenna Controller, vary the antenna orientation by steps of approximately 2 for the angles between 0 and 10. For each antenna orientation, note the angle indicated by the Antenna Controller DISPLAY and the peak-to-peak voltage of the FM-CW OUTPUT signal in the appropriate columns of Table 1-2. a It may be difficult using the SPEED control to obtain the exact antenna orientation desired. However, the backlash of the antenna driving mechanism allows the Radar Antenna to be turned one or two degrees by hand in order to obtain the desired orientation. 60 Festo Didactic
15 For the angles between 10 and 350, use the SPEED control of the Antenna Controller to set the antenna orientation where the amplitude of the FM-CW OUTPUT signal is minimum or maximum. For each of these minima or maxima, note the angle indicated by the Antenna Controller DISPLAY and the peak-to-peak voltage of the FM-CW OUTPUT signal in the appropriate columns of Table 1-2. Since the amplitude of the FM-CW OUTPUT signal can be quite stable over fairly long intervals of antenna rotation, make sure, however, that you take a reading at least every ten degrees. Using the SPEED control of the Antenna Controller, vary the antenna orientation by steps of approximately 2 for the angles between 350 and 0. For each antenna orientation, note the angle indicated by the Antenna Controller DISPLAY and the peak-to-peak voltage of the FM-CW OUTPUT signal in the appropriate columns of Table For each antenna orientation, calculate the relative power using the formula indicated at the top of the RELATIVE POWER columns. From the results contained in the ANTENNA ORIENTATION and RELATIVE POWER columns of Table 1-2, plot the radiation pattern of the Radar Antenna in Figure Festo Didactic
16 Figure Radiation pattern of the Radar Antenna. From this radiation pattern, determine the 3-dB beamwidth of the Radar Antenna. 12. On the Radar Transmitter, place the RF POWER switch in the STANDBY position. Remove the horn assembly from the mast of the target table. Disconnect all cables except that of the target table and those which are necessary for the antenna driving system. 62 Festo Didactic
17 Remove the 50 load from the RF INPUT of the Rotating-Antenna Pedestal. Setting up the basic pulsed radar In this section, you will set up a basic pulsed radar and calibrate the A-scope display. The block diagram of the system is shown in Figure Figure 1-46 shows the block diagram of the basic pulsed radar that can be obtained using the Radar Training System. Connect the modules according to this block diagram. Festo Didactic
18 Figure Block diagram of the basic pulsed radar. 64 Festo Didactic
19 14. Make the following adjustments: On the Radar Transmitter RF OSCILLATOR FREQUENCY... CAL PULSE GENERATOR PULSE WIDTH... 1 ns On the Radar Synchronizer PRF MODE... SINGLE PRF Hz 15. Refer to Appendix C of this manual to calibrate the A-scope display so that its origin is located at 1.1 m from the antenna horn and its range span is equal to 3.6 m. Once you have finished the calibration, the display on the oscilloscope should resemble Figure Figure Calibrated A-scope display of a fixed target located at the origin. Angular resolution of the basic pulsed radar In this section, you will experiment on the angular resolution of the basic pulsed radar. To do this, you will scan two targets with the Radar Antenna and observe the resulting blips on the A-scope display. You will also make an approximation of the angular resolution of the basic pulsed radar. 16. On the Target Controller, use the Y-axis POSITION control to place the target at the far end of the target table, then vary the target range by a few millimeters so that the peak voltage of the target blip on the A-scope display is positive and maximum. Festo Didactic
20 On the Antenna Controller, use the SPEED control to slowly rotate the Radar Antenna in one direction then in the other, so that it scans the target. Repeat this a few times while observing the A-scope display, then realign the antenna beam axis with the target. Describe what happens on the A-scope display as the Radar Antenna scans the target. Explain. 17. Place the fixed mast provided with the target table at the following coordinates: X = 0 cm and Y = 90 cm. Install the other small metal plate target on the fixed mast and set the target so that it faces the Radar Antenna. a For the rest of this exercise, the target installed on the mast mounted on the movable carriage of the target table will be called the movable target, whereas the target installed on the fixed mast will be called the fixed target. On the Antenna Controller, use the SPEED control to align the antenna beam axis with the fixed target. Slightly vary the orientation and range of the fixed target so that the blip on the A-scope display is positive and maximum. On the Antenna Controller, use the SPEED control to slowly rotate the Radar Antenna in one direction then in the other, so that it scans the two targets. Repeat this a few times while observing the A-scope display, then realign the antenna beam axis with the fixed target. Describes what happens on the A-scope display as the Radar Antenna scans the two targets. 18. On the Target Controller, use the X-axis POSITION control to set the X-axis coordinate of the movable target to approximately 35 cm. This approaches the movable target towards the fixed target. On the Antenna Controller, use the SPEED control to align the antenna beam axis with the movable target. On the Target Controller, use the Y-axis POSITION control to vary the movable target range by a few millimeters so that the peak voltage of the blip on the A-scope display is positive and maximum. Slightly vary the orientation of the movable target in order to maximize the peak voltage of the blip. On the Antenna Controller, use the SPEED control to slowly rotate the Radar Antenna in one direction then in the other, so that it scans the two targets. Repeat this a few times while observing the A-scope display. 66 Festo Didactic
21 Ex. 1-3 Radar Antennas Conclusion Describe what happens on the A-scope display as the Radar Antenna scans the two targets. Explain. 19. Continue to approach the movable target towards the fixed target, about 5 cm at a time, until the two targets are virtually unresolved as the Radar Antenna scans them. Each time you move the movable target, adjust the range and orientation so that the peak voltage of its blip is positive and maximum. On the Antenna Controller, use the SPEED control to successively align the antenna beam axis with the movable target and the fixed target. The antenna is properly aligned when the amplitude of the target blip is maximal on the A- scope display. For each target, note the angle indicated by the Antenna Controller DISPLAY. Calculate the difference between the two angles. This result is the angle, with respect to the Radar Antenna, which separates the two targets. What does this angle correspond to? Compare this to the angular resolution that one would expect according to the antenna radiation pattern plotted earlier in the exercise. 20. On the Radar Transmitter, make sure that the RF POWER switch is in the STANDBY position. The RF POWER STANDBY LED should be lit. Place all POWER switches in the O (off) position and disconnect all cables. CONCLUSION In this exercise, you plotted the radiation pattern of the Radar Antenna. From this radiation pattern, you determined the 3-dB beamwidth of the Radar Antenna. You then scanned two targets with the Radar Antenna and found that the pulsed radar has a finite angular resolution. Finally, you approximated the angular resolution of the Radar Antenna. Festo Didactic
22 Ex. 1-3 Radar Antennas Review Questions REVIEW QUESTIONS 1. Why does the antenna in a pulsed radar system usually have a high directivity? 2. What does a radiation pattern represent? 3. Is sidelobe radiation desirable? Explain. 4. Give two definitions of beamwidth and the approximate relationship between them. 5. Define angular resolution and give the approximate relationship between angular resolution and beamwidth. 68 Festo Didactic
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