Technical field

The present invention relates to an UHF RFID tag on a substrate, which tag comprising dipole antenna, an inductor feed loop, a tag chip that is galvanically connected to the inductor feed loop and a coupling section that connects the inductor feed loop to the dipole antenna.

In the following the expression RFID (Radio Frequency Identification) tag will be frequently used. An RFID tag is a tag that is intended to be attached onto objects to be identified in a radio-frequency identification system. An RFID tag comprising an RFID antenna and an RFID IC (integrated circuit or microchip), which IC is

electrically connected onto the antenna.

The RFID in this context is a UHF antenna and operates in the frequency range 860-960 MHz.

Background - Problem

The most widely used antenna for RFID tags operates in the UHF band and is a half-wave dipole antenna.

Typically, the tag consists of four main components, with reference to Figure 1: Dipole antenna (A), coupling section (B) , inductor loop (C) and tag chip or IC(104) . The inductor loop together with the coupling section match the impedance of the dipole antenna (~70W) to the impedance of tag chip (parallel resistance ~1.2 kH and capacitance (C) ~1-1.5 pF) . The inductance (L) of

inductor (feed) loop and capacitance (C) of tag chip set the resonance frequency of the dipole tag antenna where power transfer is maximum.

The dipole tag antenna resonance frequency is

proportional to 1/VLC, which means that if the

capacitance (C) increases the inductance (L) must

decrease to maintain the same resonance frequency. The inductance (L) of inductor loop is proportional to the length of the inductor loop. If the capacitance increases the inductor loop must be physically shorter to maintain the same resonance frequency. The problem is that

physically shorter loop is found less efficient in power transfer between dipole antenna and tag chip. It is typical that especially older generation or low-end tag chips have high capacitance and low sensitivity which makes the design of high performing dipole tag antenna difficult .

Object of invention

An object with the invention is to provide an improved RFID tag that solves the above-mentioned problems.

Summary of the invention

In accordance of the invention, the RFID tag is

characterized in that the tag further comprising a parasitic inductor loop, in parallel with the inductor feed loop, wherein the parasitic inductor loop is not galvanically connected to the tag chip or the inductor feed loop or the dipole antenna.

Detailed description of the invention The invention will now be described more with reference to figures 2-6:

Figure 2 discloses an RFID tag in accordance with the invention, which comprising a parasitic inductor loop 5.

Figure 3 discloses current distribution of the dipole tag antenna 1 with and without the parasitic inductor loop 5.

Figure 4 discloses radiation and total efficiency of the dipole tag antenna with and without the parasitic

inductor loop.

Figure 5 discloses antenna Gain of the dipole antenna tag with and without the parasitic inductor loop.

Figure 6a, 6b and 6c disclose three different embodiments of the arrangement of the parasitic inductor loop

relative the inductor feed loop on a substrate.

Figures 2 and 6a-c disclose the inventive UHF RFID tag on a substrate 6. The RFID tag comprising a dipole antenna 1, an inductor feed loop 2, a tag chip 3 that is

galvanically connected to the inductor feed loop 2 and a coupling section 4 that connects the inductor feed loop to the dipole antenna.

The inventive UHF RFID tag further comprising a

continuous parasitic inductor loop 5, in parallel with the inductor feed loop, wherein the parasitic inductor loop is not galvanically connected to the tag chip or the inductor feed loop or the dipole antenna. In tests this antenna solution has been proven to

increase the power transfer between the dipole antenna and tag chip.

Figure 3 discloses the current distribution of the dipole tag antenna with parasitic inductor loop (right side) and without parasitic inductor loop (left side) . Part of the RF signal is coupled to parasitic inductor loop which enhance the power transfer between the dipole antenna and the tag chip, which can be seen by comparing the dipole antenna to the right with the dipole antenna to the right .

Figure 4 discloses the dipole tag antenna radiation- and total efficiency with parasitic inductor loop and without parasitic inductor loop in decibels [dB] . The radiation efficiency of the dipole antenna tag with parasitic inductor loop is up to 2.5 dB higher that the same tag without the parasitic inductor loop.

Graph 10 discloses Radiation Efficiency [dB] without parasitic inductor loop.

Graph 11 discloses Radiation Efficiency [dB] with

parasitic inductor loop.

Graph 12 discloses Total Efficiency [dB] without

parasitic inductor loop.

Graph 13 discloses Total Efficiency [dB] with parasitic inductor loop. Figure 5 discloses the dipole tag antenna maximum Gain [dBi] with and without parasitic inductor loop. The antenna maximum Gain of the dipole antenna tag with parasitic inductor loop is higher than the Gain of the same tag without the parasitic inductor loop.

Graph 14 discloses antenna Gain [dBi] without parasitic inductor loop.

Graph 15 discloses antenna Gain [dBi] with parasitic inductor loop.

The parasitic inductor loop 5 arrangement may be arranged in different ways on the substrate relative to the inductor feed loop. Figures 6a, 6b and 6c disclose three different embodiments within the scope of the invention.

Figure 6a discloses a first embodiment of the invention, wherein the parasitic inductor loop 5 is located on the same side of the substrate as the inductor feed loop 2 and the dipole antenna.

Figure 6b discloses a second embodiment of the invention, wherein the parasitic inductor loop 5 is located on the opposite side of the substrate as the inductor feed loop 2 and the dipole antenna.

Figure 6b discloses a third embodiment of the invention, wherein the parasitic inductor loop 5 is located on the opposite side of the substrate as the inductor feed loop 2 and the dipole antenna and wherein the loops 2, 5 at least partly overlapping each other. A benefit with the invention is that inventive antenna design improves the performance of UHF RFID tags by adding a parasitic inductor loop close to feed area. RFID product development is often chase of best performance, so this idea has real-life use cases for example in retail area.

The substrate is preferably made of paper or cardboard However, a skilled person realizes that other non- conductive materials are possible such as plastic.

In the foregoing, the invention has been described on basis of some specific embodiments. However, a person skilled in the art realizes that other embodiments and variants are possible within the scope of the following claims .