Some materials have metallic
contents or coatings that need to be considered. Rice, for example, has been stated as
having a high mineral iron content that affects RFID performance (Clarke et al., 2005).
The choice of void ?¬?ll can affect RFID. Bubble wrap and loose Styrofoam void ?¬?ll have
very little effect on RFID, whereas dense foam will absorb some RF energy. Crushed or
formed paper, corrugate cardboard will have little effect unless it is very densely packed
(and then may pose the potential of the liquid problem).
While it may seem obvious that certain products will have an adverse effect on RFID, it
is possible to design antennas that take advantage of the nature of the surrounding
material properties. There are RFID tag designs that can be placed directly on ?¬‚at metal
surfaces. These tags employ a relatively thin layer of dielectric insulation between the tag
and the metal surface. This effectively turns the metal surface into part of the antenna
(a ?¬?nite ground plane) and can signi?¬?cantly improve performance by using the metal to
re?¬‚ect the RF signal back to the interrogator that would otherwise radiate into the item.
An example of an antenna design for tagging metallic objects can be found in Ranasinghe
et al. (2004) which resulted from the investigation into the tagging of drill strings employed
in oil rigs.
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