Radio-frequency electronic device

Abstract

The invention relates to a radio-frequency electronic device comprising at least one antenna (100) made of continuous conductive metal wire, in particular based on copper, the wire (100) being sheathed in a sheath (101) made of an electrically insulating hydrophobic material, the antenna being produced by winding turns of the continuous wire (100), part of the winding of the antenna corresponding to a capacitive part thereof, the antenna being sandwiched between two planar layers of paper (200); the device is essentially characterized in that: the dielectric space between two adjacent turns of the capacitive part of the antenna is an inter-track area containing a hydrophobic material (300) the electric permittivity of which is greater than a predetermined threshold value.

Description

[0001] Radiofrequency electronic device

[0002] The present invention relates to the field of wire antennas, in particular those used in passports.

[0003] Currently, most passports include an antenna. This antenna is embedded in a paper, cellulose, or more commonly synthetic paper backing, in this case usually Teslin (registered trademark).

[0004] The wire antenna, and hereinafter "antenna" for brevity, has an initial resonant frequency which must meet legal or normative obligations. This initial resonant frequency must be located in a determined range centered around a predetermined frequency FO.

[0005] The initial resonant frequency FO at which an antenna resonates is determined by the antenna design itself. However, under operating conditions, the effective resonant frequency F1 at which the antenna resonates may vary from the initial resonant frequency FO.

[0006] If the effective resonant frequency F1 of the antenna is below or above the determined range, then the antenna is no longer qualified and the passport may be considered a fake, which can cause problems for both the antenna manufacturer and the passport holder.

[0007] There may therefore be a frequency shift between the effective resonant frequency F1 and the initial resonant frequency FO. This frequency shift is due to the conditions of use of the antenna, in particular the humidity of the atmosphere in which the antenna is located.

[0008] Thus, an official passport may be recognized as authentic in a hot, dry country but recognized as inauthentic in a cold, wet country, or vice versa, which is not acceptable.

[0009] An antenna is an RLC resonant circuit, which has a capacitive part (capacitance C) and an inductive part (inductance L). According to a study by the applicant, the resonant frequency shift, i.e. the difference between FO and F1, comes from the paper in which the antenna is integrated, the permittivity of which varies according to humidity. The paper absorbs humidity, like a sponge. The result of this study is illustrated in Figure 3: by abruptly varying the humidity H, the resonant frequency F varies abruptly.

[0010] By absorbing moisture, the dielectric constant of the paper changes, therefore the capacitance changes, therefore the resonant frequency changes. As a result, the passport may no longer meet legal or regulatory constraints.

[0011] But in a passport, paper is mandatory, often synthetic paper.

[0012] The present invention aims to solve this problem.

[0013] In this context, the present invention relates to a radiofrequency electronic device comprising at least one antenna made of a continuous wire (100) of conductive metal, in particular based on copper, the wire (100) being sheathed by a sheath (101) made of an electrically insulating hydrophobic material, said antenna being produced by winding said continuous wire (100) in turns, a part of the winding of the antenna corresponding to a capacitive part thereof, the antenna being sandwiched between two flat layers of paper (200). The radiofrequency electronic device is essentially characterized in that: the dielectric space between two adjacent turns of the capacitive part of the antenna is an intertrack which comprises a hydrophobic material (300) whose electrical permittivity is greater than a predetermined threshold value, said hydrophobic material (300) being in the form of a continuous wire.

[0014] For brevity, the hydrophobic material (300) whose electrical permittivity is greater than a predetermined threshold value is called hydrophobic material (300).

[0015] It can be provided that the predetermined threshold value of the electrical permittivity of the hydrophobic material (300) of the intertrack is greater than that of the paper, in particular that of the cellulosic or synthetic paper (200). It can be provided that the equivalent diameter of the continuous wire of hydrophobic material (300) is greater than or equal to the equivalent diameter of the wire (100) of conductive metal

[0016] The hydrophobic material (300) can be expected to be flexible.

[0017] It can be provided that the hydrophobic material (300) is a polymer-based material, in particular plastic-based, in particular polyurethane, PTFE, a tropicalizing varnish or a tropicalizing resin.

[0018] The intertrack distance can be expected to be less than 500 pm, and preferably between 40 pm and 150 pm.

[0019] According to another of its objects, the invention relates to a passport comprising a radiofrequency electronic device according to the invention.

[0020] Finally, the invention also relates to a method of manufacturing a radiofrequency electronic device according to the invention, comprising steps consisting of:

[0021] • Create an antenna by depositing a continuous wire (100) of conductive metal,

[0022] • Either directly on a first flat layer of paper (200),

[0023] • Either directly on a flat layer of a hydrophobic material, in particular based on plastic, and in particular polyurethane or PTFE, said layer of hydrophobic material being deposited beforehand or subsequently on a first flat layer of paper (200), the wire (100) being sheathed by a sheath (101) made of an electrically insulating hydrophobic material, the antenna comprising: a capacitive part, by winding the continuous wire (100) in turns in a first direction, and in a second direction opposite to the first direction, the dielectric space between two adjacent turns of the capacitive part of the antenna being an intertrack; and an inductive part, by winding the continuous wire (100) in turns;

[0024] • sandwiching the antenna between the first flat layer of paper (200) and a second flat layer of paper (200); characterized in that it comprises a step consisting of:

[0025] • arranging in the intertrack, in the form of a continuous wire, a hydrophobic material (300) whose electrical permittivity is greater than that of the paper (200). Preferably, the step of arranging in the intertrack a hydrophobic material (300) is implemented prior to the step of sandwiching the antenna between the first flat layer of paper (200) and a second flat layer of paper (200).

[0026] It can be provided that the step of arranging a hydrophobic material (300) in the intertrack comprises at least one of the following steps:

[0027] • pre-lamination of the antenna on a plastic-based material, in particular polyurethane or PTFE, prior to the step of depositing the antenna on a first flat layer of paper (200);

[0028] • pre-lamination on the antenna of a plastic-based material, in particular polyurethane or PTFE, preferably after the step of depositing the antenna on a first flat layer of paper (200);

[0029] • spraying a tropicalizing varnish onto the antenna, prior to the step of sandwiching the antenna between the first flat layer of paper (200) and a second flat layer of paper (200);

[0030] • in which the deposition of a continuous wire (100) of a hydrophobic material (300) in the intertrack is prior to the step of sandwiching the antenna between the first flat layer of paper (200) and a second flat layer of paper (200).

[0031] Other characteristics and advantages of the present invention will appear more clearly on reading the following description given by way of illustrative and non-limiting example and made with reference to the appended figures.

[0032] Figures are not to scale.

[0033] DESCRIPTION OF THE DRAWINGS

[0034] [Fig. 1] illustrates a partial cross-section of the deposition of an antenna on a first flat layer of paper,

[0035] [Fig. 2] illustrates the antenna of Fig. 1 covered with a second flat layer of paper, according to the prior art,

[0036] [Fig. 3] illustrates the temporal variation of the resonant frequency (F) of an antenna in conjunction with the variation of humidity (H),

[0037] [Fig. 4] illustrates a partial cross-section of an embodiment of a radiofrequency electronic device according to the invention,

[0038] [Fig. 5] illustrates a partial cross-section of another embodiment of a radiofrequency electronic device according to the invention, [Fig. 6] illustrates a partial cross-section of another embodiment of a radiofrequency electronic device according to the prior art.

[0039] Detailed description

[0040] The present invention is advantageously implemented for passport antennas, the only example described here, but not limiting.

[0041] For example, it is possible that other wood-based media, i.e. those containing cellulose, absorb moisture and face the same frequency shift problem, which can occur in some smart cards.

[0042] It is known to produce a radiofrequency electronic device which comprises at least one antenna made of continuous wire of conductive metal, in particular based on copper.

[0043] The wire 100 is sheathed by a sheath 101 made of an electrically insulating hydrophobic material, typically polyurethane (PU). For example, the wire 100 of conductive metal has a diameter D of the order of 80 μm, and the sheath 101 has a thickness E of the order of 5 μm.

[0044] For brevity, the numerical reference 100 here indistinctly represents the metal wire 100 and the metal wire 100 sheathed by the sheath 101.

[0045] The antenna is made by winding continuous 100 wire in turns. It includes a capacitive part and an inductive part.

[0046] The capacitive part of the antenna is made by a first winding of the continuous wire 100 in turns in a first direction (illustrated by the sign "+" in Figure 4 and Figure 5), and a second winding of the continuous wire 100 in a second direction (illustrated by the sign "-" in Figure 4 and Figure 5), opposite to the first direction, the second winding being nested in the first winding so that in cross-section, one turn turns in one direction and the adjacent turn turns in the other direction, which is known per se.

[0047] The dielectric space between two adjacent turns of the capacitive part of the antenna is called "intertrack" I, it is the space from sheath 101 to sheath 101 (see figure 1). Preferably, the intertrack I is less than 500 pm, and preferably between 40 pm and 150 pm. Conventionally, the antenna is sandwiched between two flat layers of paper 200, as illustrated in figure 2. The antenna is therefore previously deposited on a first flat layer of paper 200.

[0048] For purely illustrative purposes, reference is made to an antenna shown in Figure 4. The (purely illustrative) operating frequency value of this antenna is 14 MHz. Such an antenna faces a frequency variability of approximately 300 kHz. The final board including such an antenna will therefore have an operating frequency between 13.7 and 14.3 MHz.

[0049] When the intertrack is made of paper (Figure 2), this also causes a frequency shift due to the humidity of the paper. This variation is approximately + / - 1 MHz, which can take the antenna out of its operating range due to this variability in the permittivity value of the dielectric, in this case paper.

[0050] According to the invention, the insertion of a hydrophobic material 300 in the intertrack makes it possible to reduce this nuisance of frequency shift due to humidity.

[0051] For example, if 50% of the original dielectric (i.e. paper) is replaced with a hydrophobic 300 dielectric, then 50% of the capacitance value is no longer dependent on humidity and the humidity-related variations are only 50% of 1 MHz, or + / - 500 kHz, which is suitable for the above operating range. In Figure 4, approximately 60% of the original dielectric (i.e. paper) is replaced with a hydrophobic 300 dielectric.

[0052] According to the invention, it is provided that the intertrack, that is to say the dielectric space between two adjacent turns of the capacitive part of the antenna, comprises a hydrophobic material 300 whose electrical permittivity is greater than a predetermined threshold value.

[0053] Preferably, the predetermined threshold value is greater than the electrical permittivity of the paper 200, in particular cellulosic paper or synthetic paper, in particular Teslin (registered trademark).

[0054] Preferably, the hydrophobic material 300 is plastic-based. For example, polyurethane, PTFE or a tropicalizing varnish.

[0055] Preferably, the hydrophobic material 300 is flexible. In a first embodiment, illustrated in FIG. 4, the intertrack is made of said hydrophobic material 300.

[0056] In a first variant, it is possible to provide for pouring a layer of hydrophobic material 300 onto the antenna, in this case polyurethane, the antenna thus covered being subsequently deposited on a first flat layer of paper 200.

[0057] In a second variant, unlike the first variant, it is possible to first provide the step of depositing the antenna on a first flat layer of paper 200 and then to pre-laminate on the antenna a hydrophobic material 300, in this case based on plastic, in particular polyurethane or PTFE.

[0058] In a third variant, it is possible to spray a tropicalization varnish onto the antenna, after the step of depositing the antenna on a first flat layer of paper 200 and before the step of sandwiching the antenna between the first flat layer of paper 200 and a second flat layer of paper 200.

[0059] In the prior art, illustrated in Figure 6, it is possible to integrate the antenna into a layer of hydrophobic material 300 based on plastic, in particular polyurethane or PTFE, then to sandwich the antenna thus integrated between two flat layers of paper 200.

[0060] In a second embodiment, the intertrack comprises said hydrophobic material 300, and comprises another material, in particular paper 200.

[0061] In a variant of this second embodiment, illustrated in FIG. 5, provision may be made to deposit a continuous wire of a hydrophobic material 300 based on plastic, in particular polyurethane or PTFE, in the intertrack, prior to the step of sandwiching the antenna between the first flat layer of paper 200 and a second flat layer of paper 200.

[0062] Preferably, the equivalent diameter of the continuous wire of a plastic-based hydrophobic material 300 is greater than or equal to the equivalent diameter of the conductive metal wire 100, so as to limit or even block parasitic interactions between two adjacent wires. These parasitic interactions depend on the dielectric between two adjacent wires and influence the value of the capacitance of the electronic circuit comprising the antenna. These parasitic interactions are substantially planar in the plane of the antenna. They are illustrated by the dotted rectangle in Figure 5, and are responsible for the frequency shift in the absence of hydrophobic material 300.

[0063] Thanks to the present invention, since the intertrack comprises a hydrophobic material 300, the antenna is much less sensitive to the ambient humidity level, so that a passport comprising such an antenna is less sensitive to humidity variations, therefore its resonant frequency is much more stable.

[0064] A comparative test was carried out by the applicant. In this test, an antenna was embedded in PU, according to the invention; and the same antenna design was embedded in paper.

[0065] With an antenna embedded in PU according to the invention, the resulting resonant circuit is much less sensitive to humidity: for a humidity variation going from 99% to 60%, a variation in the frequency F0 of the order of 2% is observed when the antenna is embedded in PU, whereas for a humidity variation going from 75% to 50%, a variation in the frequency F0 of the order of 5.6% is observed when the same antenna is embedded in paper.

[0066] Furthermore, the difference in resonance frequency between the two solutions is 1.2 MHz, for a frequency F0 of 15 MHz, thus, thanks to the present invention, there is an increase in capacity of approximately 15%, therefore the possibility of advantageously reducing the length of the copper wire of the antenna by approximately 15% when it is embedded in PU.

[0067] Preferably, the thickness of the layer of hydrophobic material, in particular based on plastic, and in particular polyurethane or PTFE, is between 100 μm and 150 μm.

[0068] Nomenclature

[0069] 100 Conductive metal wire of the antenna

[0070] 101 Antenna wire sheath 100 metal

[0071] 200 Paper 300 Hydrophobic material

Claims

Claims 1. Radiofrequency electronic device comprising at least one antenna made of continuous wire (100) of conductive metal, in particular based on copper, the wire (100) being sheathed by a sheath (101) made of an electrically insulating hydrophobic material, said antenna being produced by winding said continuous wire (100) in turns, a part of the winding of the antenna corresponding to a capacitive part thereof, the antenna being sandwiched between two flat layers of paper (200), characterized in that: the dielectric space between two adjacent turns of the capacitive part of the antenna is an intertrack which comprises a hydrophobic material (300) whose electrical permittivity is greater than a predetermined threshold value, said hydrophobic material (300) being in the form of a continuous wire.

2. Device according to claim 1, in which the predetermined threshold value of the electrical permittivity of the hydrophobic material (300) of the intertrack is greater than that of the cellulose or synthetic paper (200).

3. Device according to any one of claims 1 or 2, in which the equivalent diameter of the continuous wire of hydrophobic material (300) is greater than or equal to the equivalent diameter of the wire (100) of conductive metal.

4. Device according to any one of the preceding claims, in which the hydrophobic material (300) is flexible.

5. Device according to any one of the preceding claims, in which the hydrophobic material (300) is a plastic-based material, in particular polyurethane, PTFE or a tropicalizing varnish.

6. Device according to any one of the preceding claims, in which the intertrack is less than 500 pm, and preferably between 40 pm and 150 pm.

7. Passport comprising a radiofrequency electronic device according to any one of the preceding claims.

8. Method of manufacturing a radiofrequency electronic device according to any one of claims 1 to 6, comprising steps consisting of: • Create an antenna by depositing a continuous wire (100) of conductive metal, • Either directly on a first flat layer of paper (200), • Either directly on a flat layer of a hydrophobic material, in particular based on plastic, and in particular polyurethane or PTFE, said layer of hydrophobic material being deposited beforehand or subsequently on a first flat layer of paper (200), the wire (100) being sheathed by a sheath (101) made of an electrically insulating hydrophobic material, the antenna comprising: a capacitive part, by winding the continuous wire (100) in turns in a first direction, and in a second direction opposite to the first direction, the dielectric space between two adjacent turns of the capacitive part of the antenna being an intertrack; and an inductive part, by winding the continuous wire (100) in turns; • sandwiching the antenna between the first flat layer of paper (200) and a second flat layer of paper (200); characterized in that it comprises a step consisting of: • place in the intertrack, in the form of a continuous wire, a hydrophobic material (300) whose electrical permittivity is greater than that of the paper (200).

9. Manufacturing method according to claim 8, in which the step of arranging in the intertrack a hydrophobic material (300) in the form of a continuous wire is carried out prior to the step of sandwiching the antenna between the first flat layer of paper (200) and a second flat layer of paper (200).

10. Manufacturing method according to claim 8, wherein the step of arranging in the intertrack a hydrophobic material (300) comprises at least one of the following steps: • pre-lamination of the antenna on a plastic-based material, in particular polyurethane or PTFE, prior to the step of depositing the antenna on a first flat layer of paper (200); • pre-lamination on the antenna of a plastic-based material, in particular polyurethane or PTFE, preferably after the step of depositing the antenna on a first flat layer of paper (200); • spraying a tropicalizing varnish onto the antenna, prior to the step of sandwiching the antenna between the first flat layer of paper (200) and a second flat layer of paper (200); • in which the deposition of a continuous wire of a hydrophobic material (300) in the intertrack is prior to the step of sandwiching the antenna between the first flat layer of paper (200) and a second flat layer of paper (200).

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