Fundamental Operating Principles - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 47

Table 3.7 Free space path loss a F at different frequen-
cies and distances. The gain of the transponder's antenna
was assumed to be 1.64 (dipole), the gain of the reader's
antenna was assumed to be 1 (isotropic emitter)

Distance r 868MHz 915MHz 2.45GHz

0.3 m 18.6 dB 19.0 dB 27.6 dB

1 m 29.0 dB 29.5 dB 38.0 dB

3 m 38.6 dB 39.0 dB 47.6 dB

10 m 49.0 dB 49.5 dB 58.0 dB

Figure 3.20 Active transponder for the frequency range 2.45 GHz. The data carrier is supplied
with power by two lithium batteries . The transponder's microwave antenna is visible on the
printed circuit board in the form of a u-shaped area (reproduced by permission of Pepperl &
Fuchs, Mannheim)

a power saving 'power down' or 'stand-by' mode. If the transponder moves out of
range of a reader, then the chip automatically switches over to the power saving
'power down' mode. In this state the power consumption is a few µ Aatmost.The
chip is not reactivated until a sufficiently strong signal is received in the read range
of a reader, whereupon it switches back to normal operation. However, the battery
of an active transponder never provides power for the transmission of data between
transponder and reader, but serves exclusively for the supply of the microchip. Data
transmission between transponder and reader relies exclusively upon the power of the
electromagnetic field emitted by the reader.