Physical Principles of RFID Systems - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 111

(a) t = 0 (b) t = T/ 8 (c) t = T/4

(d) t = 3T/ 8 (e) t = T/2

(f) Current and voltage
distribution on a
dipole

U

l /2 l /2

f G

I

Figure 4.56 The creation of an electromagnetic wave at a dipole antenna. The electric field E
is shown. The magnetic field H forms as a ring around the antenna and thus lies at right angles
to the electric field

at a distance of λ/ 2 π the electromagnetic field begins to separate from the antenna
and wanders into space in the form of an electromagnetic wave. The area from the
antenna to the point where the electromagnetic field forms is called the near field of the
antenna. The area after the point at which the electromagnetic wave has fully formed
and separated from the antenna is called the far field .
A separated electromagnetic wave can no longer retroact upon the antenna that
generated it by inductive or capacitive coupling. For inductively coupled RFID systems
this means that once the far field has begun a transformer (inductive) coupling is no

Table 4.5 Frequency and wavelengths of
different VHF - UHF frequencies

Frequency Wavelength (cm)

433 MHz 69 (70 cm band)

868 MHz 34

915 MHz 33

2.45 GHz 12

5.8 GHz 5.2