Standardisation - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 234

To this end the card incorporates a large area antenna coil typically with 3-6 windings
of wire (see Figures 2.11 and 2.12).
The magnetic field generated by the reader must be within the range 1 . 5A/m ≤
H ≤ 7 . 5 A/m. Thus the interrogation field strength H min of a proximity coupling smart
card is automatically H min ≤ 1 . 5 A/m. This is the only way to ensure that a smart
card with an interrogation field strength H min = 1 . 5 A/m can be read by a reader that
generates a field strength of just 1.5A/m (e.g. a portable, battery operated reader
with a correspondingly lower transmission power), at least at distance x = 0 from the
transmission antenna (smart card in contact) (Berger, 1998).
If the field strength curve of a reader and the interrogation field strength of a prox-
imity coupling smart card are known, then the range of the system can be calculated.
The field strength curve of a typical reader in accordance with ISO 14443 is shown in
Figure 9.11 (see Section 4.1.1.1). In this case, a smart card interrogation field strength
of 1.5 A/m results in a range of 10 cm.
Unfortunately it was not possible to agree to a common communication interface in
the development of this standard. For this reason, two completely different procedures
for the data transfer between reader and proximity coupling smart card have found a
place in ISO 14443 — Type A and Type B. A smart card only has to support one of
the two communication procedures. A reader conforming to the standard, on the other
hand, must be able to communicate equally well by both procedures, and thus support
all smart cards. This means that the reader must switch between the two communication
procedures (polling) periodically during 'idle' mode ('wait for smart card').
However, the reader may not switch between the two procedures during an existing
communication relationship between reader and card.

8

H max = 7.5 A/m

6

H (x)

4

2

H min = 1.5 A/m

0 0

0.05 0.1 0.15 0.2

x

Figure 9.11 Typical field strength curve of a reader for proximity coupling smart cards (antenna
current i 1 = 1A, antenna diameter D = 15 cm, number of windings N = 1)