Readers - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 315

jX

Z A

X Ls

R Ls R

Z 0 (50 Ω )

Figure 11.20 Representation of Z A in the impedance level (Z plane)

jX

X C2p

Z A

X Ls

C 1 ∼ 1/(X Ls - X C2s )

X C2s

C 2 ~ 1/X C2p

R Ls R

Z 0 (50 Ω )

Figure 11.21 Transformation path with C ls and C 2p

As is clear from the impedance plane in Figure 11.21, C 2p is determined exclusively
by the serial resistance R ls of the antenna coil. For a serial resistance R LS of precisely
50 , C 2p can be dispensed with altogether; however greater values for R ls are not per-
missible, otherwise a different matching circuit should be selected (Fricke et al. , 1979).
We further find for C ls :

1

C 1s = ( 11 . 6 )

Z 0 R LS − R LS
ω

L s −

ω 2

·