Physical Principles of RFID Systems - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 130

G 1

r, E(r) G 2

P EIRP = P 1 G 1 P S

A e , A s ( s )

P 1
P 3

P e

r, E s (r)

Reader Transponder

Figure 4.74 Model of a microwave RFID system when a transponder is located in the inter-
rogation zone of a reader. The figure shows the flow of HF power throughout the entire system

It is relatively simple to make the conversion between the components of the two
equivalent circuits. For example, the transponder impedance Z T can be determined
from the functional or the simplified equivalent circuit, as desired (Figure 4.75).

1

Z T = jX T + R T =

( 4 . 96 )

1
R L +

1
Z mod

jωC 2 +

The individual components R T and X T of the simplified equivalent circuit can also
be simply determined from the components of the functional equivalent circuit. The
following is true:

1

R T = Re (4.97)

1
R L +

1
Z mod

jωC 2 +

Dipole

X T

C 2 R L Z mod

R T

Transponder chip Equivalent circuit

Figure 4.75 Functional equivalent circuit of the main circuit components of a microwave
transponder (left) and the simplified equivalent circuit (right)