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

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]

174 Radio Frequency Identification from System to Applications

*Pl*<sup>=</sup>

nature and the impedance of the chip is capacitive.

complex conjugate match and equaled with that of chip.

transferred.

is complex conjugate of *Zs*.

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

*Ric*

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

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

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*

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

<sup>2</sup> (*Rant* <sup>+</sup> *Ric*) ² + (*Xant* <sup>+</sup> *Xic*) <sup>²</sup> *<sup>V</sup>* <sup>²</sup> *ant* (1)

In this section the passive UHF RFID tag design is discussed. This RFID tag is textile made and involving the human body as the object to be tagged.

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]

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

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 radiation.

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 antenna and microchip to obtain the maximum reading distance.
