Exercise 2-1 Beamwidth Measurement EXERCISE OBJECTIVE When you have completed this exercise, you will be able to evaluate the -3 db beamwidth of the Phased Array Antenna. You will use a reference cylindrical target that you will move on the target table in order to evaluate the beamwidth. DISCUSSION Beamwidth is usually defined by the angles at which the antenna power pattern falls 3 db (half power) below the main beam peak. Sometimes, the term beamwidth is used to describe the angle between any two points on the pattern, such as the angle between the -10 db points. However, in this case, the term beamwidth will be reserved to describe the -3 db beamwidth or the Half Power Beamwidth (HPBW). In a phased array with a finite number of beams, the antenna beams are usually separated such that a beam pattern crosses the adjacent beam at the -3 db point. This is called the crossover point. The crossover point can be varied by increasing the number of beams within a given scan range or by "widening" each beam (reducing the number of elements in the array or reducing the spacing between the elements). The beamwidth is also used to describe the antenna's resolution capabilities, i.e. the ability to distinguish between two adjacent radiating sources or targets. If the angular separation between the two targets is smaller than the HPBW, then the two targets will be recognized as a single target. In a Rotman lens phased array, resolution depends on the number of beams within the scan range. In any phased array, the beamwidth depends on the number of elements and on the spacing between these elements. The beamwidth also depends on the scan angle. As the beam is scanned toward the endfire direction, the beam broadens due to the fact that the effective aperture presented by the array is progressively smaller than its physical aperture. The beam broadening phenomenon increases the half power beamwidth and hence reduces the directivity. For a large array, the half power beamwidth (HPBW) increases approximately as 1/cos( ) with being the scan angle. It is possible to predict the HPBW at any angle (not close to endfire) from the following relation: HPBW = HPBW Broadside / cos( ) Procedure Summary In the section H-Plane Beamwidth Measurement, you will place a target on the target table and adjust the distance between the target and the radar. Then, you will find the maximum echo on the oscilloscope screen as well as the location of the -3 db points on each side of the maximum. Finally, using the measured values, you will calculate the H-Plane beamwidth. 2-3
In the section E-Plane Beamwidth Measurement, you will turn the antenna on its side and repeat the same procedure. PROCEDURE Set-up and calibration G 1. Before beginning this exercise, the main elements of the Radar Training System (the antenna, the target table and the training modules) must be set up as shown in Appendix A. Turn on all modules and make sure the POWER ON LEDs are lit. G 2. Make sure that the LVRTS software has been started and that the Radar Training System has been connected, adjusted and calibrated according to the instructions in Appendix B. Then set the RF POWER switch on the Radar Transmitter to the STANDBY position. Note: DO NOT connect the power cable to the MOTOR POWER INPUT of the Rotating-Antenna Pedestal. H-Plane Beamwidth Measurement G 3. Place the cylinder target on the target table. Move the target table so that when the target is at the center of the target table (at X = 45 cm, Y = 45 cm), the target range is 200 cm (measured from the closest edge of the cylinder). G 4. On the Radar Transmitter, turn the RF POWER on. G 5. On the Phased Array Antenna Controller, set the SCAN MODE to MANUAL, the BEAM SEQUENCE to INCREMENTAL and the DISPLAY MODE to BEAM NUMBER. Use the POSITION/SPEED buttons to select beam 0. G 6. Connect probe E to TP3 (PRF) in the Display Processor tab and connect probe 1 to TP14 (VIDEO OUTPUT) in the MTI Processor tab. Show the Oscilloscope and adjust it as follows: Channel 1.............. 0.1 V/div (DC coupling) Channel 2.............................. Off Time Base....................... 0.35 ms/div Trigger Source........................... E Trigger Level........................... 2 V Trigger Slope............................ + Trigger Coupling......................... DC 2-4
Set the oscilloscope to Continuous Refresh (in the View menu, select Continuous Refresh or click in the oscilloscope toolbar.) You should see the VIDEO OUTPUT signal on the oscilloscope screen. G 7. In the System Settings, set the Gain to 4 and set Video Integrator to On in order to reduce the high frequency noise. Set the Video Integrator Pulses as desired. G 8. Move the target to the center of the Target Table (you can do this by pressing the MODE button on the Target Controller one or more times until the POSITION MODE is selected). G 9. With the desired beam selected, turn the PAA from side to side in order to maximize the echo amplitude. Note the maximum amplitude value and the X position of the target in the Max. column of Table 2-1. Enter one-half the maximum amplitude under L-HP and R-HP. Note: When the target travels perpendicular to the radar line of sight, the amplitude variation is symmetrical on both sides of the maximum. BEAM NUMBER PARAMETER L-HP MAX. R-HP HPBW ( ) 0 15 7 8 Table 2-1. H-Plane Beamwidth Measurement Table. G 10. With the Target Controller in POSITION MODE, use the X-axis POSITION button to move the target to the left until the echo amplitude is half its maximum value. This reduces the power by 6 db (20 log 0.5 = -6 db). Note the X position displayed on the Target Controller in the L-HP column of Table 2-1. Although the echo power has dropped by 6 db below the maximum level at this position, this represents the -3 db point of the antenna main lobe because the radar signal goes through the antenna twice, once at transmission and once at reception (2 x 3 db = 6 db). 2-5
G 11. Use the X-axis + POSITION button to move the target to the right until the echo amplitude is half its maximum value. Note the X position displayed on the Target Controller in the R-HP column of Table 2-1. G 12. Referring to Figure 2-1, use the following equation to calculate the Half Power Beamwidth HPBW and note your results in Table 2-1. D x is the difference between the L-HP and R-HP X positions. The target table was positioned so that D y = 200 cm. Figure 2-1. HPBW equation. 2-6
G 13. Repeat steps 8 through 12 with beams 15, 7 and 8 selected and note your results in Table 2-1. Is there a difference between the beamwidth of the center beams (0 and 15) and that of the end beams (7 and 8) of the Phased Array Antenna? E-Plane Beamwidth Measurement G 14. Unplug all cables from the Phased Array Antenna. Turn the Phased Array Antenna 90 on its left side. Carefully place it back on the pedestal ensuring that the rubber feet fit into the mounting holes on the support and reconnect all the cables. Then, repeat steps 8 through 12 for the center beams (0 and 15). Note your results in Table 2-2. BEAM NUMBER PARAMETER L-HP MAX. R-HP HPBW ( ) 0 15 Table 2-2. E-Plane Beamwidth Measurement Table. Compare the half-power beamwidth in the H-plane with that in the E-plane. G 15. On the Radar Transmitter, make sure that the RF POWER switch is in the STANDBY position. The RF POWER STANDBY LED should be lit. If no one else will be using the system, turn off all equipment. CONCLUSION In this exercise, you measured the H-plane beamwidth by measuring the maximum as well as the -6 db points for two center beams and two end beams. Then, you turned the antenna on its side in order to measure the E-plane beamwidth. In theory, the half power beamwidth is greater in the H-plane than it is in the E-pane. This is due to the fact that the array has a smaller physical aperture in the H-plane (24 cm) than in the E-plane (39 cm). 2-7
REVIEW QUESTIONS 1. Define the term beamwidth. 2. In a Rotman lens based phased array, what does the angular resolution depend on? 3. In phased array antennas in general, what does the beamwidth depend on? 2-8