Physical Principles of RFID Systems - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 142

l /2-Dipole

P = P e

S

R r

S

R T = R r

P e

U 0

P S = P e P e

(a) Equivalent circuit of the antenna (b) Partial absorption in power matching

Z mod

S S

R T = 0

P S 4P e P S-mod

j Pt-mod

= R T R r

=

Data

(c) Reflection of 4 P, where R T = 0 (d) Technical application:

modulated backscatter

R mod

C mod

S S

P S-mod
j Pt-mod

P S-mod
j Pt-mod

R T = R r R T = R r

Data Data

(e) Ohmic (real) modulation (f) Capacitive modulation

Figure 4.87 Generation of the modulated backscatter by the modulation of the transponder
impedance Z T ( = R T )

as real quantities in RFID systems, but as complex quantities. The relative change in
value and phase of the scatter aperture σ can be expressed using the following equation
(PALOMAR, 18000):

λ 0 · G · Z mod
4 · π · R r

σ = ( 4 . 108 )