1. Field of the Invention
The present invention relates to a transponder label.
2. The Prior Art
Transponder labels, generally also called RFID labels, which have a resonant circuit with a coil element and a condenser element, which can be shifted into resonance in a radio frequency range in order to detect the label, and, likewise also to supply a semiconductor chip mounted on the label with energy, are widely distributed. They are used, among other things, for the protection of goods and in many other applications, above all, in the field of logistics. In addition to the pure detection of the labels or the goods, on which these are mounted, also the job of contact-free data transmission is common, for example, in order to read out a product code or further product specifications stored on a chip mounted on the label. Typically, detection or reading apparatuses used for this purpose operate today with a frequency of 13.56 MHz. Also, the resonance frequency of the transponder resonant circuit of the respectively used FRID labels lies in this range. In addition, also transponder systems with an operating frequency of 100 to 135 kHz are common, but which are characterized by a well defined transmission rate and large manufacturing cost of the corresponding label, because of the commonly used wire coils.
Common transponder labels of the described type cannot be used for identification of metallic objects, since they no longer can be detected or read after mounting on an electrically conductive surface. This is based on the fact that the high frequency magnetic alternating field, which is required for excitation of the transponder resonant circuit and thus, for inductive energy transmission in the transponder label, induces eddy currents in the electrical-conductive surface, which, in turn, produces an oppositely oscillating magnetic field. Near the conducting surface, therefore, an intense weakening of the incidental field takes place.
From DE-A-195 16 448, a flexible plastic film is known, in which ferrite particles are incorporated as magnetically soft material, as well as its use for shielding of an HF coil against metals. Also, already label-type transponder structures with semiconductor chips exist, which have such a ferrite film for shielding metallic against metallic subsurfaces. The ferrite film, on which antenna coils and chips are applied, always has a thickness of at least 1.5 mm, in order to achieve a sufficient shielding against the conductive bedding layer, whereby the possible reading ranges already are greatly reduced with the noted minimal thickness. The film thickness of 1.5 mm, therefore, also is not fallen short of, since instead underlying metallic substrate affects too great of a detuning of the transponder to frequencies that are too high; that is, the resonance frequency of the transponder resonance circuit increases. The energy supply principle and the data interchange principle of the magnetic coupling, however, functions only based on the resonance characteristics of the transponder resonance circuit. If the actual resonance frequency of the transponder resonance circuit lies too far outside of a narrow target frequency range at the operating frequency of the detection or reading apparatus, the currents induced in the transponder resonance circuit are too small to make possible a sufficient transmission range.
From the need for larger minimum film thicknesses with common label-type transponder structures, substantial problems for practical use are provided. In particular, application on highly curved surfaces in a typical labeling manner by means of adhesives is made much more difficult, since the thick film structure produces a relatively high restoring moment upon bending. Generally, a corresponding structural thickness also is undesired, because long-extending projections on the surface of the object to be marked would limit its use or be disadvantageous on aesthetic grounds. An application on common, non-protected transponder labels is largely missing.