Fig. 1 shows a photo of a circularly polarized textile antenna according to the invention
Fig. 2 depicts 3 basic shapes of circularly polarized textile patch antennas according to the invention.
Fig. 3 depicts the exact shape of an embodiment of a circularly polarized antenna
Fig. 4 shows a further shape of an embodiment of a circularly polarized antenna
Figs. 5a and 5b depict axial ratio and input matching values as a function of the frequency of the textile antenna of Fig. 1
Figs. 6a and 6b show the measured radiation patterns in two orthogonal planes for left-hand and right-hand circularly polarized radiation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A photograph of an embodiment of a textile antenna according to the invention is shown in Fig. 1. The antenna comprises anantenna patch 1 on atextile substrate 2, namely on a polyamid spacer fabric with a thickness of 6 mm. This fabric comprises a number of advantages. It is light, has a good drapability and is dimensionally stable in height. Due to its being very light and because it comprises a high fraction of air, its permittivity ε_r is close to 1. A measurement reveals a permittivity of around 1.15 at a frequency of 2.4 GHz. Underneath the fabric, the textile antenna comprises a ground plane (not visible), which is substantially larger than the patch and spans more than the entire surface of the section of the fabric shown in the photo. In theory, the ground plane's size is such that it may be considered to be infinite in all four directions of the plane defined by the fabric. In fact, the ground plane's size is at least the size of the patch, preferably at least four times or at least eight times its size.

The conductive material of theantenna patch 1 of Fig. 1 as well as of the ground plane is a conductive fabric, namely a nickel-plated woven textile. Nickel shows high resistance against oxidation and corrosion. The antenna patch and the ground plane are attached to the spacer fabric by ammonia-based textile glue. An electrical connection between the microstrip line 1.2 and the ground plane, respectively, and a transceiver electronics is established by conductive two-component glue.

As an alternative to Nickel, other conductive material may be used, for example Copper, Silver, alloys of these materials and of other metals, or any other conductive material. The thickness of the conductive layer is usually around 200 nm to 400 nm, which is smaller than the skin depth in the concerned materials at the above specified frequencies. Thus, by varying the thickness, damping properties may be controlled in a certain range.

Instead of plated textiles, also other thin conductive materials may be used, for example metal foils or, especially preferred, conductive paste which may be directly imprinted on the textile.

The conductivity of the conductive patch and ground plane material, if the material is a plated woven or knitted material or an imprint on a textile, is governed by the property of the conductive fabric. The resistance mainly arises from the contact resistance of transitions between crossed threads: The more transitions the current has to make in order to flow between two points, the higher the resistance between these two points. In order to determine a resistance, the path between two points is divided into squares to be crossed (the square sides being parallel to the threads). The resistance is measured in Ω/square. For the material used for the embodiment of Fig. 1, the resistance is about 5Ω/square.

As shown in the Figure, thepatch 1 comprises a main part 1.1 and a microstrip line 1.2. The main part has an essentially rectangular, almost quadratical shape, with truncated corners 1.3 opposing each other. The microstrip line 1.2 has an arbitrary length and contacts the rectangle 1.1 in the middle of one of its edges. The patch rectangle comprises three strip-shaped indentations 1.4, 1.5, 1.6: The first indentation 1.4 runs from a side of the rectangle towards the interior. The other two indentations 1.5, 1.6 are essentially parallel to the microstrip and effectively extend it into the rectangle. The dimensions of the patch correspond for example to the dimension of the patch indicated further below in Fig. 3.

When an alternating voltage of a frequency between 400 MHz and 20 GHz, for example of a frequency of at least 1 GHz and at most 10 GHz, depending on the patch's dimensions, is applied between the microstrip line 1.2 and the ground plane, a left-hand circularly polarized radio signal is generated. Other antenna sizes and designs would be useable for other frequency ranges, i.e. the MHz frequency range. The radiating edges of the patch are the edge in the main part 1.1 that is contacted by the microstrip line 1.2 and the opposing edge for a first radiation component and the other edges for a second radiation component. The truncated corners set off excitement of the mode causing the second component. The radiation components emitted by these edges (including the truncated corners) superpose in a manner that left-hand circularly polarized radiation is created.

Fig. 2 shows three basic shapes of patches of circularly polarized textile antennas of the invention. The invention is not restricted to these three basic shapes; rather these are mere examples of shapes on which an antenna design may be based on.

The firstbasic shape 11 corresponds to the shape of the antenna described with reference to Fig. 1. The main part 11.1 of the patch has the shape of a rectangle with two truncated corners opposing each other. The microstrip line 11.2 contacts the rectangle in the middle of one of its edges. According to the secondbasic shape 12 the patch essentially has the shape of a rectangle, the microstrip line 12.2 contacting the rectangle at one of its corners. The thirdbasic shape 13 comprises a main part 13.1 being a rectangle with a centrally located strip-shaped slot 13.3, the strip-shaped slot 13.3 running diagonally with respect to the rectangle (i.e. in an angle of about 45° with respect to the Cartesian coordinate system shown in the figure), the microstrip line 13.2 contacting the rectangle in the middle of one of its edges and having an angle of about 45° with respect to the slot 13.3.

All three basic shapes may be supplemented by preferably strip-shaped indentations which are not shown in Fig. 2 and which may be introduced for optimization purposes. More concretely, indentations may be used to amplify radiation components along a particular edge in order to tune the antenna. Further, indentations parallel to the microstrip line - for example of the manner shown in Fig. 1 - may be used to engineer the microstrip line's terminating impedance so that it matches the line impedance (or characteristic impedance) of the microstrip line in order to avoid undesired reflections which reduce the total efficiency of the antenna.

The exact shape of the patch of the textile antenna of Fig. 1 is shown inFig. 3. The figure includes indications of dimensions. For a textile substrate with the thickness and permittivity of the mentioned textile material (6 mm; the relative permeability may be assumed to be 1 for all discussed embodiments), a microstrip line with a standard characteristic impedance of 50 Ω (ignoring ohmic losses) would be rather wide (28 mm) compared to the patch dimensions. Since such a wide line would restrict the length of the radiating edges, it would then be difficult to achieve circularly polarized radiation. Therefore, the shown embodiment is a 75 Ω system resulting in a feed line width of 14 mm. Nevertheless, the indentations 1.5, 1.6 parallel to the microstrip line making the microstrip line an inset microstrip feed had to be used. These indentations make the circular polarization (CP) operation more difficult, since the necessary excitation of two orthogonal, near-degenerate resonant modes is disturbed by the indentations. This makes the presence of the additional indentation 1.4 being a perturbation slit beneficial.

The exact shape of apatch 21 of a second embodiment of a textile antenna is shown inFig. 4, which also comprises dimension values. Also this embodiment comprises an indentation 21.4 serving as perturbation slit.

Fig. 5a presents the measured axial ratio at an angle of 0° for the textile antenna of Figs. 1 and 3 as a function of the frequency. An axial ratio below 3 dB is obtained from 2.29 to 2.36 GHz. The bandwidth of circular polarization according to a first definition (axial ratio below 3 dB) is thus attained with a bandwidth of 3% with respect to a center frequency at 2.32 GHz. Nearly circular polarization according to the definition given above is achieved in the entire considered range between 2.2 GHz and 2.4 GHz.

Fig. 5b shows the measured input matching s₁₁ (solid curve) as a function of the frequency. The smaller the input matching (i.e. the lower the dB value) the higher the accepted power at a particular frequency. It can be seen in the Figure that the accepted power is at a steep maximum around frequencies of 2.2-2.3 GHz. The radiation efficiency, i.e. the ratio between emitted radiation power and the accepted power is usually in the range of 60% to 90%, depending on resistance losses and properties of the dielectric textile substrate.

The solid lines inFig. 6a andFig. 6b show the measured radiation intensity for left hand circular polarization as a function of the radiation direction. Figs. 6a and 6b depict the angular dependency in the y-z-plane and the x-z-plane, respectively, if the antenna is in the x-y-plane. Both figures, of course, show a somewhat reduced radiation intensity into a backward (or downward) direction but a relatively broad region with a high intensity in the forward direction. The figures also show the radiation proportion with right hand circular polarization (dashed lines). In the forward direction, the right-hand circular polarization is much weaker (by about 15 dB) than the left-hand circularly polarized radiation. It follows that a high proportion of the power going into the microstrip line is transformed into left-hand circularly polarized radiation in a forward direction.

It has been found that the efficiency of the antenna of Fig. 1 is not critically dependent on environmental parameters such as the surrounding air's humidity. The antenna can be tuned to fulfill the Bluetooth specification, i.e. a frequency range and antenna coverage of around 10 m for 1 mW power and around 100 m for about 20 mW power.

The above described embodiments are by no means the only ways to carry out the invention but may be altered in many ways. Next to the shapes of the patch, of course also the patch and ground plane materials and the textile materials may be varied. In principle, any electrically conductive and drapable material may be used for the ground plane and for the patch. Ground plane and patch may be provided on any textile material. The antenna design (i.e. the patch shape and microstrip width) has to be adapted to both, the textile material's permittivity and its thickness.