Fast Solution for Main Beam Pattern Measurement Using Digital Beam Forming Technique Le Quang Thao, Dam Trung Thong Nguyen Thi Ngoc Minh Radio Physics Department, Physics Faculty Hanoi University of Science, VNU Hanoi, Vietnam thaolq@vnu.edu.vn Mobile: 84.983.71.29.41 Scientific and Technological Institutes of Military Radar Institute 17 Hoang Sam, Hanoi, Vietnam minh_viet08447@yahoo.com Abstract—In this paper, we suggest a fast method to measure the main beam pattern of phased array antenna This method is done by electrical steering which using Digital Beam Forming (DBF) technique to steer the main beam to designed direction with little change to the main beam and the sidelobe instead of using mechanical steering The result tested on our 4x8 array antenna supports that this method has acceptable accuracy for the main beam pattern Keywords: digital beam-forming; electrical steering; phase array antenna; far-field measurement; main beam; side lobe I INTRODUCTION Phased array antenna which is known as smart antenna has been used more and more in wireless communication, medical, radar, etc… with the developing of DBF technique Because using DBF, we can easily change the phased shifting or amplitude scaling of each element in phased array antenna in order to change its main beam direction as its pattern to designed direction Far-field pattern is one of the most important properties of an antenna Therefore, as the developed of antenna technique, we had many methods to measure it [1], [2] But for most of methods, it took a long time [3] to measure because we had to sample data from every direction In order to reduce time of measuring the antenna pattern while testing quality of antenna, we should care much more about the main beam and apart of sidelobe near the main beam than other directions Furthermore, to phased array antenna, we suggest that we could use DBF technique to steer the main beam instead of sampling data by used mechanical steering method Normally, steering beam will change the pattern of antenna, but theory and simulation has shown that there is algorithm [4] could steer the beam with least change in main beam pattern Therefore this algorithm could be used for a quick main beam pattern measurement Our method has been tested by measuring the main beam pattern from -30 to 30 degree in azimuth plane of our channels, 4x8 elements antenna (Figure 1) It operates at Xband, and is used in radar systems Figure channels, 4x8 elements antenna II DIGITAL BEAM FORMING TECHNIQUE Phased array antenna [5] is known as active antenna or smart antenna It means we can easily change it pattern by changed some antenna’s parameter such as element’s distance, permittivity ε, etc… to change phased shifting and amplitude scaling for all elements in array In DBF, the phased shifting and amplitude scaling for each antenna element, and summation for receiving or transmitting are done digitally (figure 2) Therefore it make phased array antenna more flexible, now we could change antenna pattern without change to antenna system Everything can be done by general - purpose DSP or dedicated chip For simple, suppose that we have a linear array antenna with n elements and the data received from each channel is: s = [s1 , s2 , , sn ] To steer the beam to designed angle, we use a correlation matrix w with each element is phased shifting and amplitude scaling for each antenna channel: w T = [ w1 , w2 , , wn ] (Transpose is convenient for matrix product) 978-1-4577-0536-6/11/$26.00 ©2011 IEEE (1) 6655 (2) Now we consider the case when we add more phased shifting Δφk like in (3), (5) to kth element in compare with 1st elements Suppose that at initial the highest direction or main beam is at degree From (5) we have: § Δφλ0 ã â d = arcsin ă (6) It means that if we shift the phase of kth element to almost antenna pattern is steered idea of beam forming wT = [1, e j Vφ , , e j ( n−1)Vφ ] Forget s, w and DBF for instant, we come back to normal beam forming method for linear array antenna For the signal comes from ϑ in the azimuth plane (figure 3) We have: With Δφ (3) elements, and Δφk is the different in phase of kth from 1st d 2π λ0 sin ϑ (4) Therefore: Δφk = kd 2π λ0 sin ϑ (5) From (5) we see that signal come from different angle have different phased shifting Therefore pattern of array antenna is not the same in all direction, there will be some directions with higher amplitude than the others Figure Signal come from (7) At here, we equalized amplitude scaling because if we change this parameter it’ll change radiated energy Apply this to the signal, the pattern will be steered: § Δφλ0 · = arcsin ă â 2d is the different phase between consecutive Δφ = βΔr sin ϑ = k Δφ , degrees This is the basic As above, if we use DBF technique, we need build matrix w with each element of w is the phased shifting like in (6) and equalized amplitude scaling for each antenna element Because ϑ is related to arcsin function, we choose w in this form: Figure Digital beam forming system for receiver Δφk = k Δφ ϑ Base on this equation, by changing pattern to any arbitrary angle (8) Δφ we can steer the We have simulated this algorithm in matlab (figure and 5) by steering main beam from to ± 15 degrees From the simulation, although there is a little changed in the pattern but the ratio of S/N and the main beam didn’t change so much This result shows that our idea in using electrical steering in measuring main beam pattern is possible, but we still need experimental result to support it ϑ Figure Simulation result of steering beam 6656 Therefore, using this method could reduce a lot of time for measuring Figure Simulation result of steering beam in polar III MAIN BEAM PATTERN MEASUREMENT Antenna far-field pattern refer to the performance of antenna Normally, people want their antenna has narrowed main beam with low sidelobe level But the matter of fact is that we couldn’t reach both of them [5], we have to choose either main beam or sidelobe Improving measurement method is as important as improving antenna performance In present, there are main methods [2] to measure far-field pattern of antenna: directly sample data in far-field, indirectly by sample data in near-field with data was transmitted from a standard antenna For both methods, it’s needed to steer whole antenna to every direction to sample data Although we have automated system done it, it’s stilled cost a long time to sample all data In industrial environment where there are large a mount of antennas were produce, it’s difficult to examine the quality of product To reduce time cost, we could sample less data, for example we could measure only the main beam because if the antenna has the same design and main beam pattern normally has the same quality But it’s still cost a lot of time Figure Far-field measurement’s apparatus V MEASUREMENT RESULT With our schedule measurement, we have finished measuring for our channels, 4x8 elements array antenna The result reached less than one minute since begun sample data is shown in figure From the result we can see the main beam pattern from -30 to 30 degrees in the bottom left corner At any time, in the bottom-right corner is phase different of channels and in the top is the phase of each channel Our measurement method need sample only once, in comparison with other methods [3] which need sample at a lot of direction Therefore, it’s obviously that our method could reduce a lot of time in measuring As mention above, if we use DBF technique with only changing phased difference, the pattern’s appearance near the main beam is almost the same Base on it, we suggest that we could use DBF technique to steer the antenna’s pattern about ± 30 degrees IV MEASUREMENT SCHEDULE In order to perform our measurement, we need a standard antenna which can operate at X-band, a display device to show the result in a chamber room We also need a DSP board to perform DBF technique and send result to display device Our measurement contains steps: Firstly, we need to fix transmitted antenna, then tested antenna was put far enough from transmitted antenna and looked directly to transmitted antenna (figure 6) for calibration Secondly, we sample data and using DSP to calculate the main beam pattern of tested antenna Because using DBF to steer received data can perform simultaneously for many directions, we need sample only once 6657 Figure Experimental result VI REFERENCES CONCLUSION This paper discusses about an antenna measurement method using DBF technique which can reduce a lot of time in measuring antenna main beam pattern Our experimental result supports that this method is possible and could be used widely, especially in testing quality of antenna ACKNOWLEDGMENT We would like to thank Vietnamese Scientific and Technological Institutes of Military for their supported equipments in measuring experimental result [1] [2] [3] [4] [5] 6658 Myron D Fanton “Array antenna pattern measurement techniques” ERI Technical Series, Vol 6, April 2006 “Antenna measurement theory” reprinted with the permission of ORBIT/FR Inc Mathew D Valerio and Robert R Romanofsky “Implementing an automated antenna measurement system” NASA/TM-2003-212337 Németh Z., Imre S., Balázs F., "Adaptív antennarendszerek", Híradástechnika, LVII évfolyam, május, 2002 Hubregt J Visser “Array and phased array antenna basics” - John Wiley & Sons, Ltd (2005) ... (figure 6) for calibration Secondly, we sample data and using DSP to calculate the main beam pattern of tested antenna Because using DBF to steer received data can perform simultaneously for many... beam 6656 Therefore, using this method could reduce a lot of time for measuring Figure Simulation result of steering beam in polar III MAIN BEAM PATTERN MEASUREMENT Antenna far-field pattern refer... pattern is steered idea of beam forming wT = [1, e j Vφ , , e j ( n−1)Vφ ] Forget s, w and DBF for instant, we come back to normal beam forming method for linear array antenna For the signal comes