Lesson Title: Electromagnetics and Antenna Overview

Transcription

1 Page 1 of 5 Lesson Title: Electromagnetics and Antenna Overview 6/26/09 Copyright 2008, 2009 by Dale R. Thompson {d.r.thompson@ieee.org} Rationale Why is this lesson important? Why does the student need this lesson? How does this lesson fit in the larger module? RFID uses a radio frequency channel to communicate, which is governed by electromagnetic theory and requires both transmitting and receiving antennas. The student needs to understand the basic concepts of electromagnetic radiation and antennas. This lesson gives an overview of the theory required to understand the media interface layer. Objective(s) What will the student know, be able to do, and value at the end of this lesson? This is smaller amounts of information than the module objectives. The student will be able to evaluate the period, frequency, wavelength, and phase of a signal. He will be able to differentiate the near and far field. Finally, he will be able to evaluate link budgets of a given system. Exploration Explicit concepts related to the Module goal are explored. It is at this point that the student will be provided basic information about the topic and the chance to explore some basic concepts about the topic. This is where the instructor imparts information. Electromagnetic (EM) radiation is caused by charged particles that are accelerated. Charged particles have an electric field. Moving charged particles create a magnetic field, which in turn creates electromagnetic radiation sometimes called an electromagnetic wave or electromagnetic field. Therefore, changing currents are required to create electromagnetic radiation. Electromagnetic radiation has both a magnetic and electric field. Common periodic signals o Sine o Cosine Characteristics of periodic signals o Period o Frequency o Phase (time delay) Lead vs lag Could measure absolute time like seconds More common to use a radians or degrees Antennas with a periodic signal create electromagnetic radiation o Near-field coupling or inductive coupling Area from the antenna to the point where the electromagnetic field forms Field starts at the antenna as purely magnetic If transmitting and receiving antenna are close (less than a wavelength), currents are induced on the receiving antenna like a transformer

2 Page 2 of 5 Inductive (transformer) or capacitive coupling can only happen in the near field Distance where EM field needs to be considered spherical instead of planar Angular field distribution dependent on distance Amount of current determined by differences in distance between two antennas Magnetic field decreases by a factor of 1/(r^3) in free space, where r is distance between the tag and reader antenna When the distance is small there is enough energy to run an IC with complex calculations such as required for cryptography o Far-field coupling or radiation Area some distance from the transmitting antenna at which the electromagnetic wave has fully formed and separated from the antenna. The electric and magnetic fields propagate as an electromagnetic wave. In the far field, inductive coupling is not possible Distance from antenna at which the EM wave can be considered planar Further away from antenna waves considered planar instead of spherical Angular field distribution independent of distance from antenna If transmitting and receiving antenna are further apart (more than a wavelength), radiation caused by differences in propagation time between parts of the receiving antenna. EM field decrease by a factor of 1/r, where r is distance between the tag and reader antenna Capture EM waves and rectify them for DC to run circuits Uses backscattering for modulation because load modulation is not possible. When tag antenna is tuned to a frequency it absorbs energy. When tag is purposely mistuned, it reflects energy and this can be detected by the reader. o Rules of thumb for approximating boundary between near and far field Two cases If antenna size is comparable to the wavelength (like UHF), o r = (2)(d^2)f/c o d = maximum antenna dimension o f = frequency o c = speed of light If antenna size much smaller than the wavelength (like HF), o r = c/(2*pi*f) Table of typical near-field/far-field boundary for LF, HF, and UHF Periodic signal voltage and power o Voltage o Power Decibels o Signal power range is large o Introduce decibels o Power ratios o Absolute power in db dbm dbw Antenna overview o Isotropic antenna o Dipole

3 Page 3 of 5 o Gain EIRP ERP o Polarization Linear Circular RHP LHP RFID antennas o Reader antennas Bistatic configuration: One reader antenna is used for transmitting and a different antenna is used for receiving. Monostatic configuration: The same reader antenna is used for both transmitting and receiving. Patch antenna Circular antenna Near-field antenna o Tag antennas Dipoles and bent dipoles Meandered antenna Folded dipole Polarization Near-field antennas Reflection Several questions are posed to the student to answer and then often discuss as a class. This is an attempt to determine whether the student "gets" the basic concepts delivered above. If they do get it, move on to engagement. If they do not get it, go back to exploration above. It could be as simple as asking a few probing questions or as complex as asking the student to write a paper. What are the two components of electromagnetic radiation? What is the period and frequency of a particular sine wave? Given the shown two signals, which signal lags? What is the time delay? What is the phase difference? What is the difference between near-field and far-field coupling? Given the peak voltage of a sinusoidal signal, what is the average power? Given the RMS voltage of a sinusoidal signal, what is the average power? What is the absolute power in dbm of a signal with power X? What is the difference between EIRP and ERP? What is an advantage and disadvantage of using circular polarization on a reader antenna? What is the difference between a bistatic and monostatic configuration? Given the following antennas, identify its name and if it would be used for a reader or tag. Engagement Concepts learned in the Exploration are further developed by conducting experiments, designing and building solutions, and solving problems. This is an attempt to cause the student to apply the new knowledge. By applying the new knowledge, the student is much more likely to retain this information.

4 Page 4 of 5 This engagement could be accomplished through a debate, an experiment, a problem solving activity, or anything else that would cause the student to demonstrate understanding and competence. Homework assignment o Calculate the period, frequency, and phase of a signal o Do calculations in decibels o Circular polarization thought problem o Calculate near-field/far-field boundary for a system o Calculate EIRP and ERP of an antenna Expansion Provide opportunities for students to expand the concepts to more general or global situations including connection to the Module goal. Expand back to the big ideas of the module and prepare for the next lesson. Can you speculate a design that uses the bistatic configuration AND circular polarization? Why do tags often use bent and folded dipoles? On a tag, what can be done to help receive more power despite its orientation on a box? Lesson Assessment Assess student understanding of the lesson content. This does not have to be a full-blown examination. It could be a graded homework assignment, a quiz, a performance examination, a graded problem solving activity, or something similar. Homework assignment Equipment None Software None References Daniel M. Dobkin, The RF in RFID: passive UHF RFID in practice, Oxford, UK: Elsevier, ISBN: K. Finkenzeller, RFID Handbook: Fundamentals and Applications in Contactless Smart Cards and Identification, R. Waddington, Trans., 2nd ed., Hoboken, New Jersey: John Wiley & Sons, P. V. Nikitin, K. V. S. Rao, and S. Lazar, An Overview of Near Field UHF RFID, in Proc. IEEE Int l Conf. RFID, Grapevine, TX, Mar , 2007, pp R. Want, An introduction to RFID technology, IEEE Pervasive Computing, vol. 5, no. 1, pp , Jan.-Mar

5 Page 5 of 5 Copyright Notice This material is Copyright 2008, 2009 by Dale R. Thompson. It may be freely redistributed in its entirety provided that this copyright notice is not removed. It may not be sold for profit or incorporated in commercial documents without the written permission of the copyright holder. Acknowledgment These materials were developed through a grant from the National Science Foundation at the University of Arkansas. Any opinions, findings, and recommendations or conclusions expressed in these materials are those of the author(s) and do not necessarily reflect those of the National Science Foundation or the University of Arkansas. Liability Release The curriculum activities and lessons have been designed to be safe and engaging learning experiences and have been field-tested with university students. However, due to the numerous variables that exist, the author(s) does not assume any liability for the use of this product. These curriculum activities and lessons are provided as is without any express or implied warranty. The user is responsible and liable for following all stated and generally accepted safety guidelines and practices.