**2. RFID basics**

The RFID uses wireless technology to identify the objects. It consists of RFID tag and a reader. The bi directional communication between the tag and the reader is accomplished by the Radio Frequency (RF) part of the electromagnetic spectrum, to carry information between an RFID tag and reader. There are two types of RFID tag. Passive RFID tags are the ones that does not require any external power supply and works by receiving the signal from reader and

© 2013 Vojtech et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Vojtech et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

retransmit the signal back to reader. Active RFID tag consists of external source in them. These are more complex than passive RFID tags and also give long range communication between tag and reader, when compared with passive tag.

The basic block diagram describes the bi directional communication between the tag and the reader, see Figure 1. The tag antenna in the block diagram receives the RF signal from the reader. This signal is received by the tag antenna, rectified and supplied to the chip to power it up. After the chip is powered up, it now acts as a source and retransmits the signal back to the reader. The reader after receiving the signal sends further to the computer to process the data. The method used to send the signal back to the reader from the tag is called back scattering.

The on chip oscillator in the transponder is used to provide the clock pulse to the logic and also defines the data rate. On chip oscillator present in the transponder oscillates at a frequency

If the supply voltage is less than the threshold voltage, the oscillator and the logic cannot function properly and thus the transponder cannot be activate. At this condition, the logic is in reset position. This ensures that the transistor Q2 is off during power up and do not let any

Among the two transistors, Q1 is turned on during power up. Q2 is the modulation transistor which when turned on, loads the antenna with the information from the tag. Q2 is active when

In order to have a maximum power transferred from antenna to the chip, the antenna should be designed such that the impedance of the antenna is conjugated matched with that of chip for the given frequency. Generally the chip has capacitive impedance so to have a perfect match

**Parameter Symbol Test conditions Min Type Max Units**


869 MHz ; -10dBm 915 MHz ; -10dBm 2.45 GHz ; -10dBm

The tag antenna receives RF energy from the reader. The tag antenna works for a definite resonant frequency. So when the reader transmit RF signal with the desired frequency, the tag receives the signal and supplies to the chip which is attached to it in the transponder. The chip

**Table 1.** Electrical characteristics of IC EM4222, VM-VA=2V, TA=25°C, unless otherwise specified [2]

400 1.0

512 1.4 1 128-j577 132-j553 80-j232

600 1.8 5

RFID Textile Antenna and Its Development http://dx.doi.org/10.5772/53521 173

KHz V μA Ω Ω Ω

the data is to be transmitted from transponder to reader.

**Figure 2.** Block diagram UHF transponder EM4222 [2]

the antenna impedance should be inductive in nature.

Fosc Vwu I STAT Zin Zin Zin

of 512 kHz.

false operation to act.

Oscillator frequency Wake up voltage

Static Current Consumption Input Series Impedance Input Series Impedance Input Series Impedance

**2.2. RFID matching**

**Figure 1.** Block diagram of RFID system

#### **2.1. RFID transponder**

RFID Transponder is basically a radio transmitter and receiver. It mainly consists of two parts, antenna and the integrated circuit (IC). The main function of an antenna is to capture the radiated electromagnetic field by the reader at a definite frequency. The received electromag‐ netic energy is converted to electrical power and supplied to integrated circuit. The IC chip in the transponder has the capability to store the information to be transmitted to the reader, execute the series of command and also sometimes stores new information sent by the reader [1]. The IC chip mainly consists of a rectifier which rectifies the alternating voltage (AC) received by antenna to the continuous voltage (DC) and supplies to the rest of the circuit in the IC chip.

The IC used for the research is EM4222. This is a read only UHF identification device. The EM4222 is used as a passive chip for UHF transponder. It does not have any internal power supply source. The RF beam is transmitted by the reader. The antenna in the transponder receives the signal, rectifies it and supply the rectified voltage to the chip. The basic block diagram is shown in Figure 2.

From the block diagram, it can be seen that the radio beam is received at the terminal *A* in the chip. This signal is rectified to a DC voltage. The shunt regulator is used to limit the input voltage to the logic circuit. It also protects the Schottky diode which is used as a rectifier.

**Figure 2.** Block diagram UHF transponder EM4222 [2]

retransmit the signal back to reader. Active RFID tag consists of external source in them. These are more complex than passive RFID tags and also give long range communication between

The basic block diagram describes the bi directional communication between the tag and the reader, see Figure 1. The tag antenna in the block diagram receives the RF signal from the reader. This signal is received by the tag antenna, rectified and supplied to the chip to power it up. After the chip is powered up, it now acts as a source and retransmits the signal back to the reader. The reader after receiving the signal sends further to the computer to process the data. The method used to send the signal back to the reader from the tag is called back

RFID Transponder is basically a radio transmitter and receiver. It mainly consists of two parts, antenna and the integrated circuit (IC). The main function of an antenna is to capture the radiated electromagnetic field by the reader at a definite frequency. The received electromag‐ netic energy is converted to electrical power and supplied to integrated circuit. The IC chip in the transponder has the capability to store the information to be transmitted to the reader, execute the series of command and also sometimes stores new information sent by the reader [1]. The IC chip mainly consists of a rectifier which rectifies the alternating voltage (AC) received by antenna to the continuous voltage (DC) and supplies to the rest of the circuit in

The IC used for the research is EM4222. This is a read only UHF identification device. The EM4222 is used as a passive chip for UHF transponder. It does not have any internal power supply source. The RF beam is transmitted by the reader. The antenna in the transponder receives the signal, rectifies it and supply the rectified voltage to the chip. The basic block

From the block diagram, it can be seen that the radio beam is received at the terminal *A* in the chip. This signal is rectified to a DC voltage. The shunt regulator is used to limit the input voltage to the logic circuit. It also protects the Schottky diode which is used as a rectifier.

tag and reader, when compared with passive tag.

172 Radio Frequency Identification from System to Applications

scattering.

**Figure 1.** Block diagram of RFID system

diagram is shown in Figure 2.

**2.1. RFID transponder**

the IC chip.

The on chip oscillator in the transponder is used to provide the clock pulse to the logic and also defines the data rate. On chip oscillator present in the transponder oscillates at a frequency of 512 kHz.

If the supply voltage is less than the threshold voltage, the oscillator and the logic cannot function properly and thus the transponder cannot be activate. At this condition, the logic is in reset position. This ensures that the transistor Q2 is off during power up and do not let any false operation to act.

Among the two transistors, Q1 is turned on during power up. Q2 is the modulation transistor which when turned on, loads the antenna with the information from the tag. Q2 is active when the data is to be transmitted from transponder to reader.

In order to have a maximum power transferred from antenna to the chip, the antenna should be designed such that the impedance of the antenna is conjugated matched with that of chip for the given frequency. Generally the chip has capacitive impedance so to have a perfect match the antenna impedance should be inductive in nature.


**Table 1.** Electrical characteristics of IC EM4222, VM-VA=2V, TA=25°C, unless otherwise specified [2]

#### **2.2. RFID matching**

The tag antenna receives RF energy from the reader. The tag antenna works for a definite resonant frequency. So when the reader transmit RF signal with the desired frequency, the tag receives the signal and supplies to the chip which is attached to it in the transponder. The chip after getting sufficient voltage is able to wake up and hence retransmit the signal at the same frequency to the reader. Thus the purpose of matching an antenna with its load is to insure that maximum power transferred from antenna to chip. To do this, it is needed to have a perfect match between the antenna and the chip. Perfect antenna matching can be achieved by changing the dimension of an antenna, by adding a reactive component or implementing both of them. A mathematical expression can be overviewed as depicted in Figure 3.

**Figure 3.** Series model for transponder chip and antenna [3]

The power delivered form antenna to the load or chip is given as [3]:

$$P\_{lw} \frac{R\_{ic}}{2\left[ (R\_{ant} + R\_{ic})^2 + (X\_{ant} + X\_{ic})^2 \right]} V \,^2{}\_{ant} \tag{1}$$

**3. H-slot microstrip patch antenna for UHF RFID**

involving the human body as the object to be tagged.

In this section the passive UHF RFID tag design is discussed. This RFID tag is textile made and

RFID Textile Antenna and Its Development http://dx.doi.org/10.5772/53521 175

The designed antenna layout is an H-shape slot place onto a patch, Figure 4.

**Figure 4.** Geometry of nested - slot. The microchip is placed in the central gap of the slot [4]

also the impedance matching is done by tuning the internal slot size.

antenna and microchip to obtain the maximum reading distance.

radiation.

The patch with H-slot is placed on a substrate and grounded by a conductive material to decouple from the human body. H-slot is a tuning slot for the required conjugate impedance matching between the microchip and the antenna tag. The maximum size of the antenna is 150 mm x 180 mm and the gain is rather poor around -7 dB due to the bidirectional radiation of the slot. But the maximum gain can be increased by increasing width of the tag antenna. And

Dielectric material of this patch antenna has a thickness of *h* and it has a longer face of it in the lower part which is placed on the human body through the conductive ground plane. It is an advantage to have longer ground plane because it will avoid the effect of human body

The radiation is produced by the patch open edge and by the slot. To achieve better radiation performance, width of the antenna can be increased depending of available place for tag. The dimension of the central gap is kept fix by the microchip packing but for tuning the other dimensions of the slot are optimized. The perfect conjugate matching should be done between

In the above equation it can be seen that the maximum power can be delivered from the antenna to the IC only if *Ric= Rant* and *Xic= - Xant*. Thus it can be observed that the maximum power can be delivered from antenna to load only if they are conjugate matched. This gives one of the favorable conditions for antenna designer as generally the antenna impedance is inductive in nature and the impedance of the chip is capacitive.

In this research the antenna is designed to work at 869 MHz. At this frequency the input series impedance of the chip is 128-j577 Ω. Thus the requirement is to have antenna impedance of 128+j577 Ω such that it is complex conjugate matched with the load and maximum power is transferred.

Conjugate Match Factor (CMF) is the factor which tells how good matching is done between the chip impedance and the antenna impedance. It can be described as the ratio between antenna input power with given chip impedance *Zs* and antenna impedance *Za* assuming *Za* is complex conjugate of *Zs*.

The value of CMF changes between 0 and 1 in linear. To receive maximum power from the reader and retransmit the maximum power to the reader, the antenna impedance should be complex conjugate match and equaled with that of chip.
