Measurements Corner Brian E. Fischer General DynamicsAdvanced Information Systems Michigan Research and DevelopmentCenter 1200 Joe Hall Drive, Ypsilanti, MI48197, USA Tel: +1 (734) 480-5125 Fax: +1 (734) 480-5328 Email: Brian.Fischer@gd-ais.com Ivan J. lahaie General DynamicsAdvanced InformationSystems Michigan Research and Development Center 1200 Joe Hall Drive, Ypsilanti,M148197, USA Tel: +1 (734) 480-5207 Fax: +1 (734) 480-5328 Email: Ivan.LaHaie@gd-ais.com Antenna-Pattern Measurement Using Spectrum Analyzer for Systems with Frequency Translation Sadegh Farzaneh 1, Alper K. OzturK, Abdel R. SebaK, and Robert Paknys2 1Eco1e Polytechnique de Montreal, Polygram Research Center 2500 ch. De Polytechnique, L5808, Montreal, QC, H3T 1J4, Canada Tel: +1 (514) 340-4711 ext. 2015; E-mail: sadegh.farzaneh@polymtl.ca 2Concordia University, Electrical and Engineering Department 1455 de Maisonneuve Blvd. W., Montreal, Quebec, Canada Tel: +1 (514) 848-2424 ext. 4082, Fax: (514) 848-2802 E-mail: a_ozturk@ece.concordia.ca.abdo@ece.concordia.ca.paknys@ece.concordia.ca Abstract Pattem measurements using a network analyzer are typically based on measuring the transmission coefficient, S21' When the transmitting and receiving frequencies are different, it is not possible to directly measure the antenna pattern using a network analyzer. In this paper, an antenna-pattern measurement system using a spectrum analyzer, designed to measure the radiation pattern of an antenna with a microwave sampling beamformer (MSBF) structure, is presented. A synthesized oscillator was used as the transmitter in the measurement setup. The instruments were controlled through GPIB by a program specifically designed for the system. High-quality pattern measurements were obtained for several antenna types. The measurement results were verified using HFSS simulations. Keywords: Anechoic chambers (electromagnetic); antenna measurements; antenna radiation patterns; spectrum analyzer; beam steering; frequency conversion; microwave sampling beamformer 1. Introduction Vector network analyzers (VNA) have become standard tools in indoor antenna-pattern measurement systems. Antennapattern measurement using a vector network analyzer is based on measuring the transmission coefficient (S21) at a given frequency. The modern vector network analyzers are convenient for antenna- 126 pattern measurements in terms of frequency control and stability, spectral purity, and dynamic range requirements [1]. However, when the transmitting and receiving frequencies are different, it is not possible to directly measure the antenna pattern using a vector network analyzer. State-of-the-art vector network analyzers allow S-parameter measurements when there is a frequency mixing in the receiver. For this type of measurement, a complicated and tedious calibration procedure, which usually requires an extra calibration IEEEAntennasand Propagation Magazine, Vol. 51, No.3, June 2009
kit, is necessary. Furthermore, the accuracy of the measurement is rather intolerant to the imperfections in the calibration process. In this paper, a simple and efficient antenna-pattern measurement system using a spectrum analyzer is presented. The setup was originally developed to measure the radiation pattern of a microwave sampling beamfonner (MSBF) [2, 3], but has been used for measurement of other antenna types. In an MSBF structure, antenna-element signals are switched using fast switches that are driven by pulses with controlled pulse widths and time delays. By adjusting the time delay and pulse width, the phase shift and the attenuation are controlled. In the MSBF structure, the frequency of the received signal is initially the same as the transmitting frequency, but it is shifted to a different frequency after switching [2]. The beamfonner output frequency is therefore different from the transmitting frequency. The measurement setup is based on using a synthesized oscillator (SO) as the transmitter, and a spectrum analyzer (SA) as the receiver. The antenna under test (AUT) is connected to the spectrum analyzer and the pattern is measured in the receiving mode. There are other antenna examples for which the transmitter and receiver frequencies are different. Local beamfonning [4] is one example, in which the phase shifters are located in the local oscillators and the mixer output signals experience the phase shift. Another example is hybrid analog-digital beamfonning [5]. In this case, the frequency of the beamfonner's output is different from the transmitting frequency due to the application of the beamforming weights in the analog intermediate-frequency (IF) domain. A similar situation arises in active antennas [6], particularly the frequency-conversion type. In such applications, active elements, such as mixers, are integrated with the antennas. The output frequency ofthe integrated structure is thus usually different than the RF frequency. There are several instances where spectrum-analyzer measurements are advantageous. For example, an advantage is the flexibility in choosing the type ofrf source that can be used. This is specifically apparent in pattern measurements for mobile terminals. When using a vector network analyzer, an RF feed cable must be used to connect the mobile terminal to the vector network analyzer. The effect of cable radiation on the measured pattern was studied in [7]. It was shown that the disruption caused by the feed cable was avoided when a spectrum analyzer was used as the receiving instrument. A pattern-measurement setup with a spectrum analyzer allows for the study of the spatial response of the possible harmonics in the receiver or transmitter. In receiving subsystems, high dynamic range and sensitivity are important requirements, and noise reduction is necessary for most antenna measurements. In a spectrum analyzer, noise reduction can be accomplished by reducing the resolution bandwidth. The effect of noise on the displayed signal can also be reduced by using the video filter. Video filtering performs an averaging over the received signal, thereby reducing the rapid variations caused by noise. This function is useful for measuring a low-level signal close to the noise level. However, a limitation of pattern measurement with a spectrum analyzer is that only signal amplitude is measured. Therefore, when pattern phase is desired, it cannot be used. The work in this paper is organized as follows. The specifications of the instruments used in the setup and the controller software are described in Section 2. The MSBF is briefly described, and various pattern measurements are presented in Section 3. The measured patterns are verified by comparison with the simulation results. 2. Measurement Setup 2.1 Hardware Block Diagram The pattern measurements were carried out in a rectangular anechoic chamber that measured 4 m in length, 3 m in width, and 2 m in height, as shown in Figure 1. The setup used an HP8672A synthesized oscillator that operated over 2 GHz to 18 GHz as the transmitter source. A double-ridged EMCO 3118 waveguide hom antenna that operated from I GHz to 18 GHz was used as the source antenna. The output of the AUT was connected to the HP8569B spectrum analyzer, which had a frequency range of 10 MHz to 22 GHz. The antenna positioner was controlled by an ARA HD20lE antenna-positioner controller. The spectrum analyzer, synthesized oscillator, and the ARA HD20lE azimuth rotator were interfaced through a GPID card to a PC, as shown in Figure 2. The controller software was developed in LabVIEW for automated pattern measurements. In order to reduce the cable losses, the synthesized oscillator and the spectrum analyzer were placed inside the chamber. Figure 1. The anechoic chamber with transmitting and receiving antennas. Pc with Labview8 HP8569B Spectrum Analyzer Antenna under test (AUT) HD201E Positioner Controller HPIB Connection HP8672A Synthesized Osci llator Figure 2. The measurement setup hardware. Anechoic Chambe IEEEAntennasand Propagation Magazine, Vol. 51, No.3, June 2009 127
2.2 Control Software The antenna positioner, the spectrum analyzer, and the synthesized RF oscillator were controlled through the GPill interface by a program specifically designed for the system. Figure 3 shows the front panel of the control program. The measurement frequency, measurement plane (H or E), start, stop, and rotation step angles were the required input parameters. Once the program was executed, the measurements were acquired based on the measurement parameters and displayed on the screen in several formats. A polar plot ofthe measured pattern on a db scale was displayed as the antenna rotated. In order to obtain a proper pattern, the db range of the polar plot had to be provided prior to execution. This should be chosen based on the maximum received power and minimum measured power. Typical values for the maximum and the minimum were -30dBm and -IOOdBm, respectively. The measurement data was also stored in a user-defined file, and shown in a table on the screen in real time. The spectrum ofthe spectrum analyzer's display, with proper power and frequency scales, as well as the parameters ofthe power and frequency axes of the spectrum analyzer, were displayed. The current rotation angle and measured pattern were also shown as the antenna rotated. A simplified flowchart of the data-acquisition procedure is shown in Figure 4. First, the RF power is turned on. The output power range and level, and the RF frequency, are then adjusted, based on the specified values. In order to monitor the measured data in real time, the frequency and power axes settings on the spectrum analyzer display are acquired. Next, the antenna posi- ~ I U'A r8lswi ~ '72 ~ SW 2 ~ ~ 8 0 ' I. SW I. RPf p, (I) : Figure S. An MSBF receiver structure. Output tioner is rotated to the desired angle. At each angular position, the trace of the spectrum analyzer is read and displayed on the front panel. The parameters of the horizontal and vertical axes of the spectrum analyzer are also acquired to locate the desired frequency in the trace and to find the amplitude at the current angular position. This value is marked on the polar plot and saved in the data file. This procedure is repeated until the antenna positioner reaches the stop angle. Finally, the RF power is turned off, in order to avoid hazardous radiation effects. In pattern measurements, it is essential to reduce the noise effects in the receiving instrument. In the HP8569B spectrum analyzer, the noise can be reduced in three different ways: averaging, reducing the frequency resolution, and reducing the video filter bandwidth. Since averaging slows down the measurement speed, in this measurement setup, frequency resolution and video filter control were used for noise reduction. Since the noise in the spectrum analyzer has a wide bandwidth, the total noise power that passes through can be controlled by adjusting the width of the resolution bandwidth filters. Video filtering was used for smoothing the rapid fluctuations caused by the noise. This was accomplished by reducing the cutoff frequency of the video filter below the bandwidth of the resolution bandwidth filter. In this case, the rapid fluctuations of the signal envelope were filtered out by the video filter. This resulted in smoothing the signal on the display. Read the SA frequency and power axis settings 3. MSBF Structure No 'P. = 'P...+ (n - 1).1'P 3.1 Fundamentals The fundamental idea of the MSBF structure relies on the Nyquist sampling theorem that a signal with bandwidth B can be reconstructed if sampled with a rate Is ~ 2B. Figure 5 shows a block diagram ofthe MSBF structure, where in each branch there is a phase switch after the low-noise amplifier (LNA). The switch in the lth element is controlled using a rectangular control pulse train p/ (t). This pulse train is periodic with frequency Is,and has an adjusted normalized-to-period pulse width, '/' and a time delay t s / ' which control the attenuation and phase shift, respectively. It can be shown that ts/ and '/ are related to the normalized amplitude Ow/In) and phase (Lw/) of the desired weight on the corresponding element by Figure 4. A simplified flowchart of the LabVIEW program. (la) 128 IEEEAntennasand Propagation Magazine, Vol. 51, No.3, June 2009
Ii' WltlD"'l(ARP I) \ to. /'" U.IJ, I:.. - --. ( - --. ~- -... -- -,. --, Figure 3. The front panel of the LabVIEW control program. 90. ~ 60., 0 t ; -; 30 (l) "0 :;: 0 Oil ~ -30 'I,. I 5796 5801 5806 Frequency, (MHz) 5810 5795 5800 5805 Frequency, (MHz) 5810 Figure 7a. The measured power in dbm as a function of frequency and angle, for a frequency resolution of 2 MHz. Figure 7b. The measured power in dbm as a function of frequency and angle, for a frequency resolution of 30 khz. IEEE Antennasand Propagation Magazine, Vol. 51, No.3, June 2009 129
LWf if t f =----. s 2" 2 (Ib) o In Equation (Ia), if is adjusted in the range [0,0.5] for full-range amplitude control and with a fixedif' and t sf is adjusted in the range [0,1] for full-range phase-shift control using Equation (Ib). In Figure 5, in each antenna element a phase switch with a 180 0 phase shifter is used instead ofa simple switch. If a simple switch is used, the received power is wasted during the OFF state of the switch. In other words, through using this switch the transmission - 9 coefficient ofthe switch changes from 0 to -I, which is equivalent to a 6 db power increase at the output of the switch [2]. In addition, through using this switch, the antenna and combiner ports are always matched. Moreover, using this switch attenuates or totally removes some undesired spectral replicas. -60 " o -Measurement... HFSS -15 50 180 Figure 6c. The measured and HFSS-simulated radiation patterns for a scan angle of Os =-10 0 o -.,.- 0-9 o -90 90 180 Figure 6a. The measured and HFSS-simulated radiation patterns for a scan angle of Os =0 0-120 120-30 o 0 180 Figure 6d. The measured and HFSS-simulated radiation patterns for a scan angle of Os =35 0 3.2 Prototype Description -9-12 -15 -Measurement...HFSS 180 Figure 6b. The measured and HFSS-simulated radiation patterns for a scan angle of Os = 20 0 The four-element inset-feed microstrip-patch antenna array presented in [2] was measured using the pattern-measurement setup. Rogers RO-4350B substrate with h=20 mils, E: r =3.66, and tanc5 = 0.0031, was used for the antenna and microwave circuitry. A return loss of around 14 db with a 10 db bandwidth of 50 MHz was measured for the antenna array. The antenna array signals were combined using a corporate feed, which was composed of three two-way Wilkinson combiners. The Wilkinson combiners were designed for Ie - Is, which was equal to 5.7935 GHz with Is =6.49 MHz. For each two-way Wilkinson combiner, a return loss of 16 db, an isolation of 18 db, and an insertion loss of 3.1 db were measured. After each antenna ele- 130 IEEEAntennasand Propagation Magazine, Vol. 51, No.3, June 2009
ment, a phase switch was implemented that had a insertion loss of around 1.2 db and a return loss of better than 15 db over a bandwidth of400 MHz. The control hardware had four branches that generated four signals to control the switches. Each branch had a pulse-width control block, a time-delay block, and a PIN driver IC. In order to have all branches synchronized, the same oscillator drove all four branches. In each branch, the pulse width was controlled, and then the time delay. The outputs of the time-delay les were fed to the PIN-diode drivers to provide the high currents required during the ON-OFF transition of the PIN diodes. For each pulse-width control block, a 12-bit programmable pulse generator with a pulse-width step of 0.25 ns was used. The pulse width was adjusted using 12-bit binary inputs, which could also be controlled directly using a processor. For the time-delay generator, an eight-bit programmable delay line was used, where the time delay varied in 1 ns steps. The high time-delay and pulse-width resolutions made it possible to achieve high-resolution phase shift and attenuation in the MSBF structure. 3.3 Measurement Results In this part, the measurement setup was used to verify the beam-steering capability of the MSBF structure. The synthesized oscillator and the center frequency of the spectrum analyzer were adjusted to 5.8 GHz. The switching frequency of the control circuit was chosen to be 6.49 MHz. Figures 6a-6d shows the steered uniform normalized patterns measured when the control-pulse time delays and pulse widths were adjusted for scanning at Os = 0, 20, -10, and 35 with respect to the broadside of the array, respectively. In each case, the pattern obtained using HFSS simulation is also shown for comparison. In the HFSS simulations, only the antenna array with proper excitations was simulated, and the MSBF structure was not included in the simulations. It was observed that the pattern was scanned to the corresponding angles in all cases. In addition, the measurements agreed well with the HFSS simulation results. The received power pattern for a fourelement inset-feed microstrip-patch antenna array as a function of scan angle and frequency is shown in Figure 7. The radiation pattern of the array was measured over a wide frequency range. Three different harmonic frequencies were observed. It was noted that the scan angles were different at each frequency. Based on the theory of the MSBF structure and the prototype, the three replicas occurred at Ie - Is = 5793.51 MHz, Ie = 5.8 GHz, and I; + Is = 5806.49 MHz. In Figure 7a, the corresponding maximum power angles with respect to broadside should have occurred at -35, 0, and 55, respectively. In Figure 7b, they should have occurred at 50, 0, and -40 0, respectively. In order to demonstrate the effect of resolution bandwidth, different measurements were performed using different resolutions. The frequency resolution was adjusted to 2 MHz in Figure 7a, and to 30 khz in Figure 7b. It was observed that the noise level decreased as the frequency resolution decreased. The video-filter frequency was the same for the two cases. It was noted that video filtering did not affect the average noise level on the display. It was used for smoothing the rapid noise fluctuations to measure low-level signals that were close to the noise level. On the other hand, the noise floor and sensitivity could be improved by using a smaller frequency resolution. 4. Conclusion A simple and efficient antenna-pattern measurement system was designed to measure the radiation pattern of a microwave sampling beamformer (MSBF) structure. When the transmitting and receiving frequencies are different, it is not possible to directly measure the antenna pattern using a network analyzer. In the measurement setup, we used a synthesized oscillator as the transmitter. The antenna under test was connected to the spectrum analyzer, and the pattern was measured in the receiving mode. A program was developed in Lab VIEW for controlling the instruments and monitoring the measurements in real time. Illustrative examples were presented to demonstrate the accuracy of the measurement setup. 5. References 1. IEEE Standard Board, IEEE Standard Test Procedures lor Antennas, Piscataway, NJ, IEEE, 1990. 2. S. Farzaneh and A. Sebak, "A Novel Amplitude-Phase Weighting for Analog Microwave Beamforming," IEEE Transactions on Antennas and Propagation, AP-54, 7, July 2006, pp. 1997-2008. 3. S. Farzaneh and A. Sebak, "Microwave Sampling Beamformer: Prototype Verification and Switch Design," IEEE Transactions on Microwave Theory and Techniques, 57, 1, January 2009, pp. 36 44. 4. Y. Ji, K. Inagaki, O. Shibata, and Y. Karasawa, "Receive Mode of Optical Signal Processing Multibeam Array Antennas," IEEE Microwave and Guided Wave Letters, 8, 7, July 1998, pp. 251-253. 5. T. Nishio, Y. Wang, H. P. Tsai, and T. Itoh., "A High-Speed Adaptive Antenna Array with Simultaneous Multiple-Beamforming Capability," IEEE Transactions on Microwave Theory and Techniques, 51, 12, December 2003, pp. 2483-2494. 6. J. Lim and T. Itoh, "Active Integrated Antennas," IEEE Transactions on Microwave Theory and Techniques, 42, 12, December 1994,pp.2186-2194. 7. S. Saario, D. V. Thiel, J. W. Lu, and S. G. O'Keefe, "An Assessment of Cable Radiation Effects on Mobile Communications Antenna Measurements," 1997 IEEE International Symposium on Antennas and Propagation, Montreal, Canada, July 1997. EID IEEE Antennasand Propagation MagaZine, Vol. 51, No.3, June 2009 131
易迪拓培训 专注于微波 射频 天线设计人才的培养网址 :http://www.edatop.com 射频和天线设计培训课程推荐 易迪拓培训 (www.edatop.com) 由数名来自于研发第一线的资深工程师发起成立, 致力并专注于微波 射频 天线设计研发人才的培养 ; 我们于 2006 年整合合并微波 EDA 网 (www.mweda.com), 现已发展成为国内最大的微波射频和天线设计人才培养基地, 成功推出多套微波射频以及天线设计经典培训课程和 ADS HFSS 等专业软件使用培训课程, 广受客户好评 ; 并先后与人民邮电出版社 电子工业出版社合作出版了多本专业图书, 帮助数万名工程师提升了专业技术能力 客户遍布中兴通讯 研通高频 埃威航电 国人通信等多家国内知名公司, 以及台湾工业技术研究院 永业科技 全一电子等多家台湾地区企业 易迪拓培训课程列表 :http://www.edatop.com/peixun/rfe/129.html 射频工程师养成培训课程套装该套装精选了射频专业基础培训课程 射频仿真设计培训课程和射频电路测量培训课程三个类别共 30 门视频培训课程和 3 本图书教材 ; 旨在引领学员全面学习一个射频工程师需要熟悉 理解和掌握的专业知识和研发设计能力 通过套装的学习, 能够让学员完全达到和胜任一个合格的射频工程师的要求 课程网址 :http://www.edatop.com/peixun/rfe/110.html ADS 学习培训课程套装该套装是迄今国内最全面 最权威的 ADS 培训教程, 共包含 10 门 ADS 学习培训课程 课程是由具有多年 ADS 使用经验的微波射频与通信系统设计领域资深专家讲解, 并多结合设计实例, 由浅入深 详细而又全面地讲解了 ADS 在微波射频电路设计 通信系统设计和电磁仿真设计方面的内容 能让您在最短的时间内学会使用 ADS, 迅速提升个人技术能力, 把 ADS 真正应用到实际研发工作中去, 成为 ADS 设计专家... 课程网址 : http://www.edatop.com/peixun/ads/13.html HFSS 学习培训课程套装该套课程套装包含了本站全部 HFSS 培训课程, 是迄今国内最全面 最专业的 HFSS 培训教程套装, 可以帮助您从零开始, 全面深入学习 HFSS 的各项功能和在多个方面的工程应用 购买套装, 更可超值赠送 3 个月免费学习答疑, 随时解答您学习过程中遇到的棘手问题, 让您的 HFSS 学习更加轻松顺畅 课程网址 :http://www.edatop.com/peixun/hfss/11.html `
易迪拓培训 专注于微波 射频 天线设计人才的培养网址 :http://www.edatop.com CST 学习培训课程套装该培训套装由易迪拓培训联合微波 EDA 网共同推出, 是最全面 系统 专业的 CST 微波工作室培训课程套装, 所有课程都由经验丰富的专家授课, 视频教学, 可以帮助您从零开始, 全面系统地学习 CST 微波工作的各项功能及其在微波射频 天线设计等领域的设计应用 且购买该套装, 还可超值赠送 3 个月免费学习答疑 课程网址 :http://www.edatop.com/peixun/cst/24.html HFSS 天线设计培训课程套装套装包含 6 门视频课程和 1 本图书, 课程从基础讲起, 内容由浅入深, 理论介绍和实际操作讲解相结合, 全面系统的讲解了 HFSS 天线设计的全过程 是国内最全面 最专业的 HFSS 天线设计课程, 可以帮助您快速学习掌握如何使用 HFSS 设计天线, 让天线设计不再难 课程网址 :http://www.edatop.com/peixun/hfss/122.html 13.56MHz NFC/RFID 线圈天线设计培训课程套装套装包含 4 门视频培训课程, 培训将 13.56MHz 线圈天线设计原理和仿真设计实践相结合, 全面系统地讲解了 13.56MHz 线圈天线的工作原理 设计方法 设计考量以及使用 HFSS 和 CST 仿真分析线圈天线的具体操作, 同时还介绍了 13.56MHz 线圈天线匹配电路的设计和调试 通过该套课程的学习, 可以帮助您快速学习掌握 13.56MHz 线圈天线及其匹配电路的原理 设计和调试 详情浏览 :http://www.edatop.com/peixun/antenna/116.html 我们的课程优势 : 成立于 2004 年,10 多年丰富的行业经验, 一直致力并专注于微波射频和天线设计工程师的培养, 更了解该行业对人才的要求 经验丰富的一线资深工程师讲授, 结合实际工程案例, 直观 实用 易学 联系我们 : 易迪拓培训官网 :http://www.edatop.com 微波 EDA 网 :http://www.mweda.com 官方淘宝店 :http://shop36920890.taobao.com 专注于微波 射频 天线设计人才的培养易迪拓培训官方网址 :http://www.edatop.com 淘宝网店 :http://shop36920890.taobao.com