antenna design of rfid applications

8/3/2019 Antenna Design of RFID Applications

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GANPAT UNIVERSITY JOURNAL OF ENGINEERING & TECHNOLOGY, VOLUME 1, ISSUE 1, MARCH 2011 1

Antenna Design of RFID ApplicationsP.R. Patel, Ashish Raval, Hitesh D. Patel

Abstract Passive RFID tags utilize an induced antenna coil voltage for operation. This induced AC voltage is rectified to provide a

voltage source for the device. As the DC voltage reaches a certain level, the device starts operating. By providing an energizing RF

signal, a reader can communicate with a remotely located device that has no external power source such as a battery. Since the

energizing and communication between the reader and tag is accomplished through antenna coils, it is important that the device mustbe equipped with a proper antenna circuit for successful RFID applications.

KeywordsRFID, Antenna Circuit, RFID, Tag, Micro ID

1.INTRODUCTION

An RF signal can be radiated effectively if the lineardimension of the antenna is comparable with thewavelength of the operating frequency. However, thewavelength at 13.56 MHz is 22.12 meters. Therefore, it isdifficult to form a true antenna for most RFID applications

[1]. Alternatively, a small loop antenna circuit that isresonating at the frequency is used. A current flowing intothe coil radiates a near-field magnetic field that falls offwith r-3. This type of antenna is called a magnetic dipoleantenna.

For 13.56 MHz passive tag applications, a few microhenries of inductance and a few hundred pF of resonantcapacitor are typically used [3]. The voltage transferbetween the reader and tag coils is accomplished throughinductive coupling between the two coils. As in a typicaltransformer, where a voltage in the primary coil transfers tothe secondary coil, the voltage in the reader antenna coil is

transferred to the tag antenna coil and vice versa. Theefficiency of the voltage transfer can be increasedsignificantly with high Q circuits.

This section is written for RF coil designers and RFIDsystem engineers. It reviews basic electromagnetic theorieson antenna coils, a procedure for coil design, calculationand measurement of inductance, an antenna tuningmethod, and read range in RFID applications.

2.REVIEWOFABASICTHEORYFORRFIDANTENNADESIGN

2.1 Current and Magnetic Fields

Amperes law states that current flowing in a conductorproduces a magnetic field around the conductor. Themagnetic field produced by a current element, as shown inFigure 1, on a round conductor (wire) with a finite length isgiven by:

Equation 1:

Where:I = currentr = distance from the center of wire0 = permeability of free space and given as 4 Kx 10

(Henry/meter)

In a special case with an infinitely long wire where:1 = -1802 = 0

Equation 1 can be rewritten as:

Equation 2:

Fig. 1: calculation of magnetic field b at location p due to current i on astraight conducting wire

The magnetic field produced by a circular loop antennais given by:

Equation 3:

WhereI = currenta = radius of loopr = distance from the center of loop0 = permeability of free space and given as

4 x 10-7 (Henry/meter)

The above equation indicates that the magnetic fieldstrength decays with 1/r3. A graphical demonstration isshown in Figure 3. It has maximum amplitude in the planeof the loop and directly proportional to both the currentand the number of turns, N.[8]

Equation 3 is often used to calculate the ampere-turnrequirement for read range. A few examples that calculatethe ampere-turns and the field intensity necessary to powerthe tag will be given in the following sections


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Fig 2: calculation of magnetic field b at location p due to current i onthe loop

Fig 3: decaying of the magnetic field b vs. Distance r

3.INDUCEDVOLTAGEINANANTENNACOIL

Faradays law states that a time-varying magnetic fieldthrough a surface bounded by a closed path induces avoltage around the loop.

Figure 4 shows a simple geometry of an RFID applica-tion. When the tag and reader antennas are in closeproximity, the time-varying magnetic field B that isproduced by a reader antenna coil induces a voltage (calledelectromotive force or simply EMF)[4] in the closed tagantenna coil. The induced voltage in the coil causes a flowof current on the coil. This is called Faradays law. Theinduced voltage on the tag antenna coil is equal to the timerate of change of the magnetic flux [5]

Equation 4:

where:N = number of turns in the antenna coil = magnetic flux through each turn

The negative sign shows that the induced voltage acts insuch a way as to oppose the magnetic flux producing it.

This is known as Lenzs law and it emphasizes the fact thatthe direction of current flow in the circuit is such that theinduced magnetic field produced by the induced currentwill oppose the original magnetic field.

The magnetic flux in Equation 4 is the total magneticfield B that is passing through the entire surface of the

antenna coil, and found by:

Equation 5:

Where:B = magnetic field given in Equation 2S = surface area of the coil = inner product (cosine angle between two vectors)

of vectors B and surface area SNote: Both magnetic field B and surface S are vector

quantities.

The presentation of inner product of two vectors inEquation 5 suggests that the total magnetic flux that ispassing through the antenna coil is affected by anorientation of the antenna coils. The inner product of twovectors becomes minimized when the cosine angle betweenthe two are 90 degrees, or the two (B field and the surface ofcoil) are perpendicular to each other and maximized whenthe cosine angle is 0 degrees.

The maximum magnetic flux that is passing through thetag coil is obtained when the two coils (reader coil and tagcoil) are placed in parallel with respect to each other. Thiscondition results in maximum induced voltage in the tagcoil and also maximum read range. The inner productexpression in Equation 5 also can be expressed in terms of amutual coupling between the reader and tag coils. Themutual coupling between the two coils is maximized in theabove condition.

Fig 4: a basic configuration of reader and tag antennas in rfidapplications


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Using Equations 3 and 5, Equation 4 can be rewritten as:

Equation 6:

WhereV= voltage in the tag coili1 = current on the reader coila = radius of the reader coilb = radius of tag coilr = distance between the two coilsM = mutual inductance between the tag and readercoils, and given by:

Equation 7:

The above equation is equivalent to a voltage transfor-mation in typical transformer applications. The currentflow in the primary coil produces a magnetic flux thatcauses a voltage induction at the secondary coil.

As shown in Equation 6, the tag coil voltage is largelydependent on the mutual inductance between the two coils.The mutual inductance is a function of coil geometry and

the spacing between them. The induced voltage in the tagcoil decreases with r-3. Therefore, the read range alsodecreases in the same way.

From Equations 4 and 5, a generalized expression forinduced voltage Vo in a tuned loop coil is given by:

Equation 8:

Where:f = frequency of the arrival signalN = number of turns of coil in the loopS = area of the loop in square meters (m)Q = quality factor of circuit

Bo = strength of the arrival signala = angle of arrival of the signal

In the above equation, the quality factor Q is a measureof the selectivity of the frequency of the interest.

Fig 5. Orientation dependency of the tag antenna

The induced voltage developed across the loop antennacoil is a function of the angle of the arrival signal. Theinduced voltage is maximized when the antenna coil isplaced in parallel with the incoming signal where a = 0.

Example 1: calculation of b-field in A tag coil

The MCRF355 device turns on when the antenna coil

develops 4 VPP across it. This voltage is rectified and the devicestarts to operate when it reaches 2.4 VDC. The B-field to induce

a 4 VPP coil voltage with an ISO standard 7810 card size (85.6 x

54 x 0.76 mm) is calculated from the coil voltage equation

VO= 2fNSQBocos = 4

Where the following parameters are used in the abovecalculation:

Tag coil size = (85.6 x 54) mm2 (ISO card size) =0.0046224 m2 Frequency = 13.56 MHz Number of turns = 4

Q of tag antenna = 40 coilAC coil voltage to = 4 VPP turn on the tagCos = 1 (normal direction, = 0).

Example 2: number of turns and Current (ampere-turns)

Assuming that the reader should provide read range of15 inches (38.1 cm) for the tag given in the previous


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example, the current and number of turns of a readerantenna coil is calculated from Equation 3:

= 0.43 ampere-turn

Equation 12:

Wherea = radius of coilr= read range

The result indicate that optimum loop radius a is 1.414times the demanded read range r

4.WIRETYPESANDOHMICLOSSES

4.1 DC Resistance of Conductor and Wire Types

The diameter of electrical wire is expressed as theAmerican Wire Gauge (AWG) number. The gauge numberis inversely proportional to diameter, and the diameter isroughly doubled every six wire gauges. The wire with asmaller diameter has a higher DC resistance. The DCresistance for a conductor with a uniform cross-sectionalarea is found by:

Equation 13:

Where:l = total length of the wirea = conductivity of the wire (mho/m)S = cross-sectional area = Kr2a = radius of wire

For the resistance must be kept small as possible forhigher Q of antenna circuit. For this reason, a largerdiameter coil as possible must be chosen for the RFID

circuit. Table 5 shows the diameter for bare and enamel-coated wires, and DC resistance.

4.2 AC Resistance of Conductor

At DC, charge carriers are evenly distributed throughthe entire cross section of a wire. As the frequencyincreases, the magnetic field is increased at the centre of theinductor. Therefore, the reactance near the centre of the

wire increases. This results in higher impedance to thecurrent density in the region. Therefore, the charge moves

away from the centre of the wire and towards the edge ofthe wire.

Equation 13:

Where:f =frequency = permeability (F/m) = 0 r0 = Permeability of air = 4 re x 10-7 (h/m)r = 1 for Copper, Aluminum, Gold, etc

= 4000 for pure Iron = Conductivity of the material (mho/m)= 5.8 x 107 (mho/m) for Copper= 3.82 x 107 (mho/m) for Aluminium= 4.1 x 107 (mho/m) for Gold

= 6.1 x 107 (mho/m) for Silver= 1.5 x 107 (mho/m) for Brass

8.CONCLUSION

Passive RFID tags utilize an induced antenna coilvoltage for operation. Review of basic theory of antennadesign by ampere and farads law. This induced ACvoltage is rectified to provide a voltage source for thedevice. As the DC voltage reaches a certain level, thedevice starts operating. By providing an energizing RFsignal, a reader can communicate with a remotelylocated device that has no external power source such asa battery. Since the energizing and communication between the reader and tag is accomplished throughantenna coils, it is important that the device must beequipped with a proper antenna circuit for successfulRFID applications. Calculation of b-field in a tag coil and

number of turns, Current (ampere-turns) and wire typesand ohmic losses.

REFERENCES

[1] Hailong Zhu, Shengli Lai, Hongyue Dai Solutions of MetalSurface Effect for HF RFID, Pervasive Computing, ieee, volume5, issue 1, Jan.-March 2006, pp. 25-33.

[2] V. G. Welsby, The Theory and Design of Inductance Coils, JohnWiley and Sons, Inc., 1960.


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[3] Frederick W. Grover, Inductance Calculations WorkingFormulas and Tables, Dover Publications, Inc., New York, NY.,1946.

[4] Keith Henry, Editor, Radio Engineering Handbook, McGraw-Hill Book Company, New York, NY., 1963.

[5] H.M. Greenhouse, IEEE Transaction on Parts, Hybrid, andPackaging, Vol. PHP-10, No. 2, June 1974.

[6] K. Fujimoto, A. Henderson, K. Hirasawa, and J.R. James, SmallAntennas, John Wiley & Sons Inc., ISBN 0471 914134, 1987

[7] James K. Hardy, High Frequency Circuit Design, RestonPublishing Company, Inc.Reston, Virginia, 1975.

[8] Simon Ramo, Fields and Waves in Communication Electronics,John Wiley, 1984.

First A. Author Mr. P.R.Patel is presently working as a Head in the E. C.engineering department at the B. S. Patel Polytechnic College, GanpatUniversity, Kherva, Mehsana, and Gujarat, India. He has master degree inelectronics and communication engineering system. His research interest iswireless communication and telecommunication. He has 13 year academicand industrial experience. He is member of different technical bodies inIndia.

Second B. Author Jr. Mr. Hitesh D. Patel is presently working as asenior lecturer in the E. C. engineering department at the B. S. PatelPolytechnic College affiliated to Ganpat University, Kherva, Mehsana,Gujarat, India. He has master degree in electronics and communicationengineering system. His research interest is optical communication andindustrial electronics system. He is member of different technical bodiesin India.

Third C. Author Mr. Ashish R Raval is presently working as a lecturer inE.C. engineering department at the B. S. Patel Polytechnic Collegeaffiliated to Ganpat University, Kherva, Mehsana, Gujarat. India. Hisresearch interest is microcontroller and wireless communication system. Heis member of different technical bodies in India.

antenna design of rfid applications

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