Antenna Circuit Design for RFID Applications
Author: Youbok Lee, Ph.D. Microchip Technology Inc.
REVIEW OF A BASIC THEORY FOR RFID ANTENNA DESIGN
Current and Magnetic Fields
Ampere’s law states that current flowing in a conductor produces a magnetic field around the conductor. The magnetic field produced by a current element, as shown in Figure 1, on a round conductor (wire) with afinite length is given by:
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 sourcesuch as a battery. Since the energizing and communication between the reader and tag is accomplished through antenna coils, it is important that the device must be equipped with a proper antenna circuit for successful RFID applications. An RF signal can be radiated effectively if the linear dimension of the antenna is comparable with the wavelength of the operating frequency. However, the wavelengthat 13.56 MHz is 22.12 meters. Therefore, it is difficult to form a true antenna for most RFID applications. Alternatively, a small loop antenna circuit that is resonating at the frequency is used. A current flowing into the coil radiates a near-field magnetic field that falls off with r-3. This type of antenna is called a magnetic dipole antenna. For 13.56 MHz passive tag applications, a fewmicrohenries of inductance and a few hundred pF of resonant capacitor are typically used. The voltage transfer between the reader and tag coils is accomplished through inductive coupling between the two coils. As in a typical transformer, where a voltage in the primary coil transfers to the 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 increased significantly with high Q circuits. This section is written for RF coil designers and RFID system engineers. It reviews basic electromagnetic theories on antenna coils, a procedure for coil design, calculation and measurement of inductance, an antenna tuning method, and read range in RFID applications.
µo I B φ = -------- ( cos α2 – cos α 1 ) 4πr where: I = current r = distance from the center of wire µ0 = permeability of free space and given as 4 π x 10-7 (Henry/meter) In a special case with an infinitely long wire where: α1 = -180° α2 = 0° Equation 1 can be rewritten as: ( Weber ⁄ m )
µo I B φ = -------2πr ( Weber ⁄ m )
FIGURE 1: CALCULATION OF MAGNETIC FIELD B AT LOCATION P DUE TO CURRENT I ONA STRAIGHT CONDUCTING WIRE
Ζ Wire α2 α α1 0 r R P X
B (into the page)
2003 Microchip Technology Inc.
The magnetic field produced by a circular loop antenna is given by:
µ o INa B z = --------------------------------2 2 3⁄2 2(a + r ) µ o INa 1 ------------------ --- 3 2 r
2 2 2 2
FIGURE 2: CALCULATION OF MAGNETIC FIELD B ATLOCATION P DUE TO CURRENT I ON THE LOOP
for r >>a
α R r
where I = current a = radius of loop r = distance from the center of loop µ0 = permeability of free space and given as 4 π x 10-7 (Henry/meter) The above equation indicates that the magnetic field strength decays with 1/r3. A graphical demonstration is shown in Figure 3. It has maximum amplitude in the planeof the loop and directly proportional to both the current and the number of turns, N. Equation 3 is often used to calculate the ampere-turn requirement for read range. A few examples that calculate the ampere-turns and the field intensity necessary to power the tag will be given in the following sections.
y V = V o sin ωt
P Bz z
FIGURE 3: DECAYING OF THE MAGNETIC FIELD B VS. DISTANCE r...