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Verfügbare Sprachen
Verfügbare Sprachen
BIS U Basic Manual
8
Reflection, Dispersion and Adsorption of Electromagnetic Waves
Electromagnetic waves radiated from the antenna propagate at nearly the speed of light and
meet objects with different consistencies. The wave can be absorbed and reflected or scattered
in all directions at different intensities.
Apart from the consistency of the material, which may be similar to metal or polar liquids, the
size of the obstacles has a decisive influence on the backscattering characteristics:
Parts of the primary wave that overlap with stray partial waves generated by reflections,
scattering or diffraction on metallic structures in the actual surrounding area can result in local
magnification or reduction in the electric field intensity. If the field intensity decreases so much
that it falls below the response field intensity value for the data carrier, communication between
the processor unit and data carrier is interrupted. However, if interaction in the surrounding area
at a point situated further in front of the antenna causes the field intensity to increase,
communication between the data carrier and processor unit remains stable. Field magnification
can result in super-refraction as a result.
For this reason, it is not possible to specify a reading range for a specific UHF
identification system consisting of a data carrier and antenna/processor unit that is
valid for all applications or boundary conditions.
Figure 12:
As already highlighted, interaction between electromagnetic waves and objects in the actual
surrounding area as well as between the waves themselves results in changes in the anticipated
electric field distribution or free space loss. Selected examples should be used to demonstrate
the effect of this interaction on the performance of UHF technology.
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Rayleigh
The reflections are negligible if their dimensions are much smaller than the
range
wave length.
Resonance
The object size is comparable with the wave length. Resonant absorption and
range
radiation from sharp objects, slots and points are observed and may cause
changes in the polarization direction or result in the magnification or cancella-
tion of fields.
Optical
The object dimensions are large compared to the wave length. The geometry
range
and position of the object (incidence angle of the wave) have an influence on
the backscattering result. Experiences gained from the field of geometrical
optics can be used with approximate equivalence.
8,00
7,00
6,00
5,00
4,00
3,00
Data carrier response
field intensity
2,00
1,00
0,00
0
50
100
Appearance of areas without communication (blind spots)
Anticipated field intensity
curve according to
theoretical free space loss
Field intensity curve
measured
150
200
250
300
Distance to the antenna in cm
Area without
communication
350
400
450
500
21
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