Page 334 - Mechatronics for Safety, Security and Dependability in a New Era

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Roll-like bulk goods, such as paper or cardboard rolls, have to be identified with 100% reliability when
the roll is handled at a factory, warehouse, when loading a conveyer chain or at the warehouse of
a printing house. A roll is identified in a controlled situation, wherein the position of the roll with
respect to its cylinder axis is known, i.e. the roll is either in a vertical or in a horizontal position. As far
as the antenna of the identification device is concerned, this means that the polarization plane of the
antenna is known. In contrast, the position angle of the roll around the cylinder axis is not known. In
other words, when the identification device to be arranged in the roll uses a directional antenna
element, the direction of the maximum of the antenna radiation beam is not known. If an identification
device arranged on the surface of the roll is used in this kind of a situation, in the worst case the
identification device is on the opposite side of the roll and the direction of the radiation beam of the
antenna of the identification device is opposite to the direction from which the reader makes the
identification. This means that reliable identification is very unlikely in such a situation.

The dipole and folded dipole antennas generally used in radio frequency identification devices are
usually omnidirectional, i.e. they emit electromagnetic radiation in all directions. However, these
antenna types have low amplification. Furthermore, the frequency bands used by radio frequency
identification devices have an officially regulated highest permitted transmission power, i.e. directional
antenna structures can be used for improving the transmission of an identification device, if required.
The use of directional, i.e. amplifying antenna structures, such as a microstrip antenna or an antenna
array, allows the electromagnetic radiation power transmitted by the antenna to be directed more
efficiently in the desired direction. This improves the coupling between the identification device and
the reader antennas in the direction of the maximum of the radiation beam of the directional antenna
compared with omnidirectional antennas, whereas the coupling is weaker outside the radiation beam
than with omnidirectional antennas.

DIELECTRIC PROPERTIES OF PAPER
The relative permittivity in copy paper or in other paper qualities consisting mostly of wood fibers is
typically from 2 to 4 decreasing with frequency. In coated paper the permittivity increases even up to 8
due to high amount fillers like CaCO3 added. The change in the moisture content of paper doesn't
change much the dielectric constant of paper itself, though the dielectric constant in water is 80. This is
because in paper, water molecules are associated with polysaccharide chains and cannot rotate freely.
Rotation is possible only if the field is parallel to the chain axis. Because of the chain orientations in
paper are random, only a small fraction of the paper molecules have perfect alignment with the electric
field. This makes the effective dielectric constant much smaller than it would be in liquid water,
Niskanen (1998). However, the increase in moisture content increases dielectric losses in paper. In
paper with anisotropic fiber orientation, the dielectric constant is largest in the direction of the fiber
orientation angle i.e. typically in the planar directions. In z-direction the dielectric constant is smaller.
See Figure 1.
The real part of the relative permittivity s t of paper increases with increasing density p and the
behavior follows with reasonable accuracy the Clausius-Mossotti relation, Niskanen (1998)

^ 7 ^ (1)

The imaginary part of the relative permittivity, loss tangent tang, increases linearly with density.
Factors effecting on the electrical properties of paper are given in Matsuda (2002). Dielectric constant
is affected by paper density, fiber orientation, crystalline cellulose and pulp components (lignin,
hemicellulose, etc). The amount of dielectric loss depends on ionic conduction losses, inclusion of