Physical Principles of RFID Systems - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 139

Figure 4.85 By suitable design of the transponder antenna the impedance of the antenna can be
designed to be the complex conjugate of the input impedance of the transponder chip (reproduced
by permission of Rafsec, Palomar-Konsortium, PALOMAR-Transponder)
always high enough to operate the circuit. The voltage supplied by the antenna is used
to activate the transponder by means of a detection circuit. In the absence of external
activation the transponder is switched into a power saving mode in order to save the
battery from unnecessary discharge.
Depending upon the type of evaluation circuit a much lower received power P e is
needed to activate the transponder than is the case for a comparable passive transponder.
Thus the read range is greater compared to a passive transponder. In practice, ranges
of over 10 m are normal.
4.2.6.4 Reflection and cancellation
The electromagnetic field emitted by the reader is not only reflected by a transponder,
but also by all objects in the vicinity, the spatial dimension of which is greater than
the wavelength λ o of the field (see also Section 4.2.4.1). The reflected fields are super-
imposed upon the primary field emitted by the reader. This leads alternately to a local
damping or even so-called cancellation (antiphase superposition) and an amplification
(in-phase superposition) of the field at intervals of λ 0 / 2 between the individual min-
ima. The simultaneous occurrence of many individual reflections of varying intensity
at different distances from the reader leads to a very erratic path of field strength E
around the reader, with many local zones of cancellation of the field. Such effects
should be expected particularly in an environment containing large metal objects, e.g.
in an industrial operation (machines, metal pipes etc.).
We are all familiar with the effect of reflection and cancellation in our daily lives. In
built-up areas it is not unusual to find that when you stop your car at traffic lights you
are in a 'radio gap' (i.e. a local cancellation) and instead of your favourite radio station