Rabuck F., Sweeney P.J CompTIA RFID+ Study Guide Exam RF0-101, includes CD-ROM / Exam Essentials

Computers & Technology
Education & Reference
Business & Investing
Science & Math

CompTIA RFID+ Study Guide Exam RF0-101, includes CD-ROM
Authors: Rabuck F., Sweeney P.J
Published year: 2006
Pages: 20-22/136

Buy this book on amazon.com >>

<< Previous page
Table of contents
Next page >>

Exam Essentials

Explain the difference between near field and far field. The near field and the far field are two regions in electromagnetic radiation. The near field is the region near the antenna and extending approximately λ /(2 π ) from the antenna. The near field is an energy storage field primarily characterized by the magnetic field. The far field exists beyond the near field. The far field is an energy propagation field primarily characterized by electromagnetic waves.

Explain nulls and standing waves. Just like nulls, standing waves are caused by the interference of waves, but, unlike nulls, standing waves maintain a measurable amplitude.

Explain the dielectric constant. The dielectric constant is a dimensionless quantity that characterizes the relative electrical permittivity of a material, which is to say the ability of the material to store electrical energy in an electric field. For most materials under most conditions, the dielectric constant has a constant value independent of both the magnitude and direction of the electric field.

Identify RF propagation techniques. Radio frequency energy propagates through a medium in a manner that may be modeled as an electromagnetic wave. An electromagnetic wave consists of two perpendicular components , an electric field and a magnetic field, that are both orthogonal to the direction of travel of the wave. Electromagnetic waves exhibit linearity and may be refracted, deflected, reflected, and attenuated as they propagate through an environment.

Describe antenna field performance as it relates to reflective and absorptive materials. An antenna within an unobstructed free-space environment radiates energy in a pattern specific to that antenna. For antennas commonly used in UHF RFID systems, this pattern is similar in shape to an egg. When RF reflective and RF absorptive materials are present in the environment, the antenna's radiation pattern no longer resembles the simple free-space pattern for the antenna. Reflective material redirects the antenna's radiated energy, thereby distorting the field in a manner specific to the location of the reflective material relative to the antenna. A large sheet of metal, for example, that is placed at a 45-degree angle to the antenna approximately half-way into the antenna's pattern will cause the antenna's radiation pattern to extend 90 degrees to either the left or the right (depending on which of the two 45-degree angles is chosen ). In this way, the radiation pattern can be made to extend around corners. Shielding, such as a cage, can also be used to trap the energy in a confined space. This has the effect of increasing the available energy in that space. RF absorptive materials absorb the energy emitted by the antenna. In this way, they reduce the size of the radiation pattern, limiting its reach in the directions where it must pass through the absorptive materials. Highly absorptive materials can be used to completely absorb energy in certain areas where the amount of RF energy is desired to be reduced.

Key Terms

Before you take the exam, be certain you are familiar with the following terms:

gain
linearity
Maxwell's equations
near field
near-field region
null
permeability
right-hand rule
standing wave

Review Questions

1.	Which type of material might you use to steer electromagnetic waves around a corner? Water Chicken wire Glass Cardboard
2.	Which of the following absorbs electromagnetic wave energy at UHF frequencies? (Select three options.) Water Chicken wire Glass Cardboard
3.	RFID systems that operate in which of the following frequency ranges rely on energy storage fields for communication? (Select two options.) LF HF UHF Microwave
4.	Which of the following properties must be equalized in a resonant coiled antenna? C and L R and C R and L R, C, and L
5.	Which law states that the net magnetic flux flowing out of any closed surface is zero? Faraday's Law Ampere's Law Gauss's Law for electric flux Gauss's Law for magnetic flux
6.	Which type of reader antenna is least likely to communicate with a linearly polarized antenna that is located horizontally? Linear antenna positioned horizontally Linear antenna positioned vertically Circularly polarized antenna positioned horizontally Circularly polarized antenna positioned vertically
7.	A UHF RFID tag may be read through which of the following materials? (Select three options.) Cardboard Water Sand Steel
8.	The magnetic field created by an electric current flowing through a straight wire ___________________. flows from the end of the wire in the direction of the electric current (the north pole), out into space, and back into the other end of the wire (the south pole) flows from the end of the wire in the opposite direction of the electric current (the north pole), out into space, and back into the other end of the wire (the south pole) flows around the wire in concentric circles in a direction following the right-hand rule flows around the wire in concentric circles in a direction following the left-hand rule
9.	The magnetic field of which commonly used RFID frequency range(s) penetrates small amounts of metal to the point that communication between tag and reader may occur? LF HF UHF None of the above
10.	The far-field region begins at roughly what distance from the antenna? λ 2 μ λ / π λ /2 π (2 π )/ λ
11.	The electric field and the magnetic field are ___________________. not interconnected in the dynamic case dependent solely on electric charges not interconnected in the static case parallel in the propagation of an electromagnetic wave
12.	The polarization of an electromagnetic wave is determined by ___________________. the direction of the electric field the direction of the magnetic field the direction of the electric flux the direction of the magnetic flux
13.	The polarization of an electromagnetic wave may be which of the following? (Select all that apply.) Square Linear Elliptical Circular
14.	The magnetic field created by an RFID reader operating in the HF frequency range decreases in proportion to the inverse of what power of the distance from the antenna? 1 2 3 4
15.	The power density available to a tag operating in the UHF far field decreases as the inverse of what power of the distance from the antenna? 1 2 3 4
16.	What are some of the reasons that a reader and a tag may not be able to communicate? The tag is too far from the reader's signal to harvest sufficient operating energy. The tag's multi- path communication signals destructively interfere at the reader's antenna. The tag's multi-path communication signal is incorrectly polarized for the reader's antenna to detect it. The tag's communication signal is significantly weaker than another signal received at the reader's antenna. All of the above.
17.	What common material can be used to mitigate the impact of poorly placed metal in the UHF RFID environment? Sand Water Cardboard Wood
18.	The experienced permeability of what material may change throughout the year? Sand Water Cardboard Metal
19.	Rectangular packages with foil along one side (such as blister packs for drugs) create a sequence of vertical metal bars when placed in a case and viewed from one side. What reader antenna polarization and orientation is best suited to read tags located on the other side of the case? Linear polarized antenna positioned horizontally Linear polarized antenna positioned vertically Circularly polarized antenna positioned horizontally Circularly polarized antenna positioned vertically
20.	Which frequencies are best able to communicate through 12 cm of water? (Select two options.) LF HF UHF Microwave

Answers

1.	B. Chicken wire acts as an efficient reflector of electromagnetic radiation in the UHF frequency spectrum. By placing it at an angle to the radiation source (the RFID antenna), the emitted signal can be directed around corners.
2.	A, C, D. UHF electromagnetic waves do not pass through the metal of the chicken wire but do pass through the other materials.
3.	A, B. LF and HF RFID systems communicate and operate within the near field, which is the energy storage field. UHF and microwave systems operate in the far field, which is the power propagation field.
4.	A. A coiled antenna is resonant when its capacitance (C) and inductance (L) are equalized.
5.	D. Gauss's Law for magnetic flux defines how magnetic fields behave. One of the properties of magnetic fields is that they form closed loops . A magnetic field flows from the north pole of a magnet out into space and back into the south pole of the magnet .
6.	B. A horizontally positioned linearly polarized tag antenna is unlikely to capture any of the energy emitted by a vertically positioned linearly polarized reader antenna. The horizontally positioned linearly polarized reader antenna is most likely to communicate with the tag. The circularly polarized antenna will be able to communicate with the tag regardless of its orientation.
7.	A, B, C. Only steel prevents UHF frequencies from penetrating it. UHF RFID tags may be read through small amounts of water and relatively larger amounts of cardboard and sand.
8.	C. The current flowing through a straight wire does not create a magnetic north pole and south pole. The current creates a magnetic field that forms concentric circles around the wire. The right-hand rule is used to determine the direction of the magnetic field.
9.	A. The magnetic fields generated by the LF frequency ranges do penetrate small thicknesses of metals. This penetration allows readers to communicate with tags located next to and even behind metal surfaces. All other frequency ranges commonly used by RFID systems do not have magnetic fields that penetrate metals to any significant extent.
10.	C. The near field ends at a distance of roughly r = λ /(2 π ) from the antenna, at which point the far field begins.
11.	C. The electric and magnetic fields are disconnected in the static case. Only when the fields are time varying (that is, dynamic) do the fields become interconnected.
12.	A. The convention for describing polarization is to use the direction of the electric field vector at a point in space.
13.	B, C, D. The electric field of an electromagnetic wave may rotate as the wave propagates through space. The electric field may rotate either left or right. If the electric field rotates to the right with constant amplitude, the wave is said to be right circular polarized. Circular polarization is a special case of elliptical polarization, in which the amplitude increases and decreases in a regular fashion as the electric field rotates. If the electromagnetic wave does not rotate as it propagates, it is linearly polarized.
14.	C. In the near field, the magnetic field produced by the antenna diminishes at a rate proportional to 1/ r ³ . The power from the antenna decreases at a rate of 1/ r ⁶ .
15.	B. In the far field, the power density decreases much more slowly than in the near field. The power density in the far field decreases proportional to 1/ r ² .
16.	E. Electromagnetic waves are linear, meaning that the wave experienced at a point in time and location in space is the addition of all waves intersecting then and there. Multi-path delays, absorption by materials in the environment, and reflections and polarizations that result from those reflections are all phenomena that will be experienced by the electromagnetic waves.
17.	B. Water effectively absorbs UHF energy. By placing water in front of metal fixtures that are creating too much reflection, the reflected waves can be eliminated.
18.	C. Cardboard absorbs moisture from the air and its local environment. As the humidity varies throughout the year, so will the moisture content of the cardboard. During times of increased humidity, the cardboard will contain more water, which will decrease its experienced permeability in those conditions.
19.	B. A linear polarized antenna positioned vertically will have all of its energy propagate through the case. The vertically oriented metal foil acts as a filter to electromagnetic radiation that is not vertically oriented. Thus, a linear polarized antenna positioned horizontally will have its energy blocked by the case, and a circularly polarized antenna will have all but the vertically oriented portion of its energy filtered by the case.
20.	A, B. Although UHF and microwave frequencies can communicate through water, the power levels allowed by the local regulations for use by RFID systems prevent them from communicating through large quantities of water. LF and HF frequencies are little affected by water.

<< Previous page
Table of contents
Next page >>