June 15, 2014
June 15, 2014
June 18, 2014
24.88.1 - 24.88.12
A Portable and Low-cost RF Measurement System for Instructional Use Abstract In this work, we develop a RF measurement system that consists of portable transmitter and receiver modules operating at the 2.4GHz band using Wi-Fi technologies. Originally designed as a hands-on lab tool for antenna radiation pattern measurements, this system can be a viable tool for multiple instructional tasks such as provide demos in lectures and serve as a measurement tool in labs for various Electrical Engineering (EE) and Electronic Engineering Technology (EET) courses. Relevant courses include those that cover antenna theory, RF signal propagation modeling, and communication systems which are fundamentals in upper-division EE/EET curriculum. The system renders several promising features: portable, low-cost, simple, and compact in size, to name a few. In this paper, we ﬁrst introduce the components and the mechanism of the RF measurement system. We then present a laboratory exercise that designed for an antenna radiation pattern study using the developed system. Detailed lab procedures and results collected from a recent upper-division communication systems course in an EET program are provided. The lab results demonstrate the effectiveness of the developed system. Additional assessment data from students’ feedback further shows that the lab experience using the measurement system has been engaging. The proposed system provides a feasible solution for programs which are not equipped with complex and expensive lab facilities and resources for RF and antenna measurements instructional needs. I. I NTRODUCTION Antenna theory together with RF signal propagation modeling, as fundamentals of electronic/RFcommunication curriculum in Electrical Engineering (EE) and Electronic Engineering Technology (EET)programs, are often perceived as abstract and difﬁcult topics. An effective and commonly used teachingtechnique to facilitate students’ understanding of these topics is the use of hands-on lab exercises tocomplement lectures. For instance, antenna measurements can be taken to obtain antenna radiationpatterns for different types of antennas through laboratory measurements , , . Conventionally,antenna and RF signal propagation measurements require complex lab facilities and instruments such asAntenna Test Chambers, network analyzer, or ﬁeld strength meters (e.g., , ). The cost of equippingsuch facilities and instruments might be prohibitive and impractical for smaller EET/EE programs that donot have a special focus in these areas. Moreover, some instruments are not closely related to students’every day experience and real-life examples. Another common practice is to use wireless transceiversfor antenna lab measurements to send a RF signal through a transmitter unit and capture the receivedsignal strength at the receiver. For instance, in , the authors described an antenna radiation pattern labmeasurement system utilizing the Ritron DTX-450 transceiver and an Icom communications receiverthat both operate at 450M Hz. Their system provides a more hands-on approach with the selectedpractical wireless transceivers modules. The cost of the entire lab system, however, is still relativelyhigh (over $600 including the expenses on major hardware components such as a transceiver, a receiver,and a rotator). At present, our EET program offers introductory courses that emphasize fundamentals in broadsubject areas such as communications, controls, DSP, and power systems. However, we do not have aconcentration on some speciﬁc ﬁelds such as antenna or EM theory which usually demands expensiveand complex research and lab facilities and test instruments. In this work, to best cope with our availablebudget and resources and to meet our pedagogical needs, we have developed a low-cost, simple, andportable RF measurement system. It can be a viable instructional tool for topics such as antenna theory,antenna measurements, and RF signal propagation modeling. Besides the cost savings (below $150 persystem), compared to other existing systems, its transceiver modules adopt the popular 2.4G Hz Wi-Fitechnologies and consist of off-the-shelf wireless routers and wireless network cards as the transmitterand receiver units. The Wi-Fi enabled devices are more accessible and familiar to students and areexcellent real-life examples of RF communication systems. As such, the developed system rendersstudents a great learning opportunity that connects abstract theories learned in classrooms to real-worldexamples. In this paper, we will also present a successful laboratory exercise designed for antenna radiationpattern measurements using this system. The experiment data collected from a recent communicationsystem class in an EET program and students feedback demonstrate that the developed lab setup andthe measurement system are efﬁcient and effective to achieve the teaching outcomes. In summary, the developed RF measurement system can serve a variety of lab activities and instruc-tional tasks for antenna, EM wave propagation, and communication systems related topics. Both themeasurement system and the laboratory exercise can be readily adapted to courses that cover similartopics in other institutions. The paper is organized as follows. In the next section, we present system components and features.In Section III, we describe a hands-on laboratory designed for antenna radiation pattern measurements.Assessment results are provided in Section IV. We conclude in Section V. 3 II. RF M EASUREMENT S YSTEM In this section, we present the components and features of the proposed system. As depicted in Fig.1, the major components of the transmitting module include a transmitting antenna, a modiﬁed wirelessWi-Fi router, cable, and an antenna rotator. The receiving module consists of a computer, a wirelessnetwork card, and a received signal strength acquisition software. A photo of the actual transmitter unitis provided in Fig. 2. antenna cable computer with wireless router RF signal acquisition software wireless network card rotatorTransmitter Module Receiver ModuleFig. 1. RF measurement system diagram Detailed descriptions and cost of each component are summarized in Table I and Table II. We highlightseveral features as follows: • Depending on the use of the measurement system, the antenna and the router set up might be different. For antenna radiation pattern measurements, the router can be modiﬁed such that its pre- packaged antenna might be removed and replaced with one that is designed and built by students. We will discuss a similar case in the next section. • For the current version, the antenna rotator is manually operated. A user controls the tuning knob to rotate the antenna for a certain angle. • The entire receiver unit is a computer equipped with a wireless network card and a free software (the version we have adopted is named as “Network stumbler”) to capture the received RF signal strength. Most students own laptops which typically are conﬁgured with a wireless network card. Therefore, the receiver unit is very cost-effective and accessible to students. • Compared to other wireless transmitter and receiver modules, the choice of using 2.4G Hz Wi-Fi devices is more appealing due to the fact that these devices are also more accessible and students are in general familiar with these components (e.g., a Wi-Fi wireless router). • All of the components except the computer are off-the-shelf parts. The antenna is tuned at 2.4G Hz and can be constructed using 16 AWG wire. The support is built by a PVC pipe. • The measurement system in a whole is portable and easy to relocate to different indoor or outdoor locations. • The total cost is about $150 per system and is far less expensive than existing systems. TABLE I C OMPONENTS IN THE TRANSMITTER MODULE Component Function Cost tx antenna radiate a RF signal students design and build < $10 Wi-Fi 2.4G Hz router generates a 2.4G Hz RF signal $40 rotator rotate the antenna between 0 − 365 degrees $80 cable feeds a signal from the router to the antenna $10 total cost: $140 TABLE II C OMPONENTS IN THE RECEIVER MODULE Component Function Cost wireless network card receives the RF signal $10 or free if use laptop computers software “Network stumbler” acquire Wi-Fi signal strength readings free computer hosts the network card and the software free of use in labs or students use own laptop computers total cost: $10 III. A NTENNA R ADIATION PATTERN L AB D ESIGN Hands-on laboratory activities have been well acknowledged as an essential part of engineeringand engineering Technology curricula and made critical impact in improving teaching efﬁciency andenhancing students understanding of abstract topics . In this section, we elaborate a successfullaboratory exercise that is designed for antenna radiation pattern measurement using the proposed RF 5Fig. 2. Transmitter module.measurement system. Antenna radiation pattern, used to characterize an antenna, plays an importantrole in understanding antenna theory. Lab objectives and lab procedures are presented below in brief.A. Lab Objectives and Pedagogical Goals This lab exercise (a two-hour lab session) aims to provide students a hands-on opportunity to • enhance understanding in antenna radiation pattern theory. • accumulate hands-on skills in antenna measurements. • be familiar with and practice constructing commonly used dipole antennas.B. Lab Procedures The lab includes three components: Pre-lab assignment, in-lab measurements, and post-lab analysis. 6 The pre-lab assignment requires students to get familiar with two types of dipole antennas throughtheoretical studies, speciﬁcally: • Calculate the length of a half-wavelength dipole for 2.4G Hz Wi-Fi applications. • Calculate the length of a 1.5-wavelength dipole for 2.4G Hz Wi-Fi applications. • Sketch theoretical 2D radiation patterns of the two selected antennas and observe the features of each radiation pattern. The in-lab measurements consists of a few tasks as speciﬁed brieﬂy as follows: • Step I: build a half-wavelength dipole and a 1.5-wavelength dipole using the 16 AWG wire as tx antennas, respectively. Note that the transmitter is a wireless router. For the half-wavelength dipole antenna, it will be connected to the router through a piece of cable using the center feed method. Therefore, the antenna consists of two identical sections each with a length slightly > L/2 and L represents the wavelength. • Step II: for each antenna, place the transmitter and receiver unit at a certain distance d. Obtain measurements of the received signal ﬁeld intensity through “Network stumbler” readings and sketch the actual radiation pattern plot. For each angle, take multiple readings and use the average value as the ﬁnal result to combat possible noisy data. • Step III: move the receiver unit to a farther distance, repeat step II. The post-lab analysis assignment is summarized as below: • Comment on the actual radiation pattern plots. • Compare the theoretical radiation pattern with the actual ones obtained through lab measurements. Are these patterns consistent? If not, explain why the difference exist. • Specify the difference observed from the radiation pattern plots when the distance between the transmitter and the receiver increases. Explain the difference. • Sketch the voltage and current distributions (i.e., standing waves) of the two antennas. • (optional) Could you suggest any improvements or other methods or tools for obtaining more accurate radiation pattern measurements? IV. A SSESSMENT R ESULTS In this section, we will discuss some assessment results to validate that the proposed measurementtool and the developed laboratory activity are effective. 7A. Lab experiment Data To gauge the effectiveness and the performance of the proposed RF measurement system, we providein the following some lab measurement data collected from students who took a recent communicationsystem course in our EET program in winter 2013. In Fig. 3, the actual radiation pattern based on labmeasurement for a half-wavelength dipole is demonstrated. As a comparison, the theoretical radiationpattern is illustrated in Fig. 4. We note that the actual pattern plot of Fig. 3 assumes the number 8shape in general and the characteristics of the ﬁeld strength of a half-wavelength dipole antenna isclearly present. There are several small regions that are not quite consistent with the theoretical patternplot, however, considering the factors such as the noisy setting of the lab room and the accuracy of thereceived signal strength readings provided by the free software, the overall actual pattern plot appearsto be acceptable and provides a reasonable estimate of the theoretical pattern.Fig. 3. Actual half-wavelength dipole antenna radiation pattern based on lab measurements.B. Student Feedback In addition, students feedback were also collected regarding the developed measurement tool and thelab exercise. Students were unanimously positive about the lab experience and felt that the lab was 8Fig. 4. Theoretical half-wavelength dipole antenna radiation pattern.fun and engaging. All students indicated that they enjoyed the hands-on activities of constructing andtesting dipole antennas, taking measurements, and analyzing the lab results. The measurement system isconvenient to use and requires minimal training or troubleshooting. The impact of having a real hands-on lab on their understanding of antenna-related concepts is far profound than that from lecturing-onlyor lecturing with software simulations. Moreover, a few students also provided insightful and useful suggestions on how the lab might beimproved. For instance, one pointed out that “I’d suggest in the future some other extended tasks couldbe explored such as studying the impact of walls and ﬂoor orientations”. Another commented that themeasurement data looked quite noisy at times and there is a need for more efﬁcient ways of conductingthe lab measurements. V. C ONCLUSIONS In this paper, we present a simple, portable, and low-cost RF measurement system that is suitable forinstructional uses for topics related to antenna theory, RF signal propagation modeling, and communi-cation systems. Moreover, we also introduce a laboratory exercise for antenna radiation measurementusing the proposed system. Lab experiment data from students in a communication system course havevalidated the effectiveness of the system. In addition, students’ feedback show that the designed lab 9tool and lab exercise have been successful. The proposed system might be a viable low-budget optionfor similar courses in EE and EET programs in other institutions to meet their instructional needs. R EFERENCES H. Xie, Y. Liang, and Q. Wang, A laboratory measurement method of antenna radiation pattern, Springer, 2012. V. Bhavsar, N. Blas, H. Nguyen, and A. Balandin, Measurement of antenna radiation patterns, Lab Manual, UC-Riverside, 2000. H. Matzner, S. Levy, and D. Ackerman, RF Laboratory Manual - Antenna, 2008. L.D. Feisel and A.J. Rosa, “The role of the laboratory in undergraduate engineering education,” Journal of Engineering Education, pp. 121–130, Jan 2005.
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