Title: A microstrip patch antenna

The invention relates to a microstrip patch antenna, comprising a substantially rectangular electrically conducting area connected to a transmission line, the conducting area having a length and a width wherein the length substantially equals half a wavelength of a radio wave in the substrate to be received by the microstrip patch antenna. Such a microstrip patch antenna is e.g. known from the US patent application US 2004119645 for providing a relatively small, flat and relatively cheap antenna. Such antennas are widely used for mobile communication devices.

However, microstrip patch antennas provide inherently a narrow bandwidth. The microstrip patch antenna disclosed in US 2004119645 comprises a sandwich layer structure to enhance bandwidth properties. Other techniques to improve the poor bandwidth of patch antennas encompass providing self supporting structures, employing short circuit pins or walls, using one or a multiple number of feeding probes extending through a substrate on which the patch antenna is formed, providing a relatively thick dielectric, non-planar antennas, using wirebond techniques, using a metal box, and/or providing a complex matching network that is larger than the antenna. All these techniques enlarge production costs significantly.

It is an object of the invention to provide a microstrip patch antenna according to the preamble, wherein the disadvantages identified above are reduced. In particular, the invention aims at obtaining a microstrip patch antenna according to the preamble wherein a bandwidth is increased while production costs do not significantly increase. Thereto, according to the invention, a width to length ratio of the conducting area is larger than approximately 1.5.

By designing the substantially conducting area such that a width to length ratio of the conducting area is larger than approximately 1.5, not only a desired electromagnetic resonance mode of the antenna operating frequency occurs in the conducting area, also other resonance modes may occur. The invention is at least partly based on the insight that by simultaneously exciting at least two resonance modes of the conducting area, the bandwidth of the patch antenna increases significantly while the input impedance of the patch antenna may remain constant. Obviously, costs for manufacturing the microstrip patch antenna may substantially remain the same. In designing a microstrip patch antenna according to the invention, a prejudice of the person skilled in the art that the width to length ratio should always be smaller than approximately 1.5. in order to suppress higher order modes occurring in the conducting area of the patch antenna, is overcome. The microstrip patch antenna according to the invention can advantageously be used for mobile communication devices, especially for relatively wideband wireless communication applications. Examples of relatively wideband wireless communication devices are PDA's or mobile telephones.

By applying the width to length ratio of the conducting area as a design parameter for optimizing the bandwidth of the microstrip patch antenna the bandwidth of the antenna can increase significantly.

Further, by employing the interconnection location of the conducting area and the transmission line as a design parameter for selecting an input impedance of the conducting area, an impedance match with the transmission line can be obtained without reducing the bandwidth of the patch antenna. This way, the bandwidth of the microstrip patch antenna can be optimized. In a preferred embodiment, the transmission line is implemented using microstrip technology, so that the patch antenna and the transmission line can be made using the same production steps, thereby decreasing manufacturing costs of the microstrip patch antenna. It is noted, however, that

also other types of transmission lines might be used, such as a coax cable element.

Other advantageous embodiments according to the invention are described in the following claims. By way of example only, embodiments of the present invention will now be described with reference to the accompanying figures in which

Fig. 1 shows a first embodiment of a microstrip patch antenna according to the invention;

Fig. 2 shows a reflection graph of the patch antenna of Fig. 1; Fig. 3 shows an additional reflection graph of a patch antenna similar to the one shown in Fig. 1;

Fig. 4 shows a second embodiment of a microstrip patch antenna according to the invention; and

Fig. 5 shows a third embodiment of a microstrip patch antenna according to the invention.

The figures show merely preferred embodiments according to the invention. In the figures, the same reference numbers refer to equal or corresponding parts.

Figure 1 shows a first embodiment of a microstrip patch antenna 1 according to the invention.

The antenna 1 comprises a substantially rectangular electrically conducting area 2 having a length 1 and a width w. The conducting area is connected to a transmission line 3 for further transmission at an interconnection location 4. The length of the conducting area 2 substantially equals half a wavelength of a radio wave in the substrate to be received by the microstrip patch antenna 1. Further, the width w of the conducting area is larger than approximately 1.5 to admit at least two resonance modes in the conducting area 2 excited by an incidence radio wave.

The transmission line 3 and the conducting area 2 are realized using microstrip technology. In doing so, a ground plate of a printed circuit board

serves as the earth, while on the opposite side of the board an electrically conducting pattern is printed. The printed circuit board has dielectric properties to electromagnetically couple the printed pattern and the ground plate. In particular, the transmission line 5 is formed as a printed track, while the antenna patch is realized for optimal receipt and transmission of radio waves around a predetermined central frequency fo, e.g. 2.45 GHz. It is noted, however, that also other techniques could be used, such as etching techniques for forming the structures on the printed circuit board.

The material of the printed circuit board is the widely used FR4. Obviously, also other printed circuit board types could be used, such as a microwave laminated substrate.

The length 1 to width w ratio of the substantially rectangular conducting area 2 is a design parameter for optimizing the bandwidth of the microstrip patch antenna 1. Figure 2 shows a reflection graph 5 of the patch antenna 1 wherein a reflection parameter R is depicted as a function of frequency f. The graph is substantially constant for relatively low and high frequencies at a constant level ro and has a dip at the central frequency fo where the patch antenna 1 is operable. The bandwidth of the patch antenna is defined as frequency band where the graph 5 is 10 dB below the constant level ro. In practice, prior art patch antennas have a bandwidth of approximately 1%.

However, by according to the invention using the length 1 to width w ratio of the conducting area 2 as a design parameter for optimizing the bandwidth of the microstrip patch antenna 1, a bandwidth of approximately 3% can be obtained.

Fig. 3 shows an additional reflection graph 5 of a patch antenna 1 similar to the antenna 1 shown in Figure 1. Due to the non-symmetrical behaviour of the reflection graph a subtle choice of the length 1 to width w ratio of the conducting area 2 might lead to a relatively large bandwidth.

Further, the interconnection location 4 of the conducting area 2 and the transmission line 3 might be used as a design parameter for selecting an input impedance of the conducting area 2. In doing so, impedances of the conducting area 2 and the transmission line 3 can optimally be matched, thereby reducing matching losses while maintaining the bandwidth of the patch antenna. As an example, the input characteristic impedance of the conducting area 1 may be chosen as circa 50 ω to match common transmission line characteristics.

Optionally, the impedance of the antenna is matched with a complex impedance of an integrated circuit. Therefore, in an advantageous way, an intermediate matching circuit between the antenna and an integrated circuit connected to the antenna becomes superfluous, thereby making the system more compact and cheaper.

As shown in Figure 1, the interconnection location 4 of the conducting area 2 and the transmission line 3 is near an edge 6 of the conducting area 2 so that a simple layout is obtained.

Figure 4 and 5 show a second and a third embodiment of a microstrip patch antenna 1 according to the invention wherein the interconnection location 4 of the conducting area 2 and the transmission line 3 are elsewhere, viz. in an inner region of the conducting area 2. In particular, in Figure 4, the transmission line 3 enters the inner region of the conducting area 2 via a gap 7 in the conducting area 2. In Figure 5, the transmission line enters the inner region of the conducting area 2 via a path outside the plane wherein the conducting area 2 on the printed circuit board extends. The invention is not restricted to the embodiments described herein.

It will be understood that many variants are possible.

As an example, geometries of the antenna patches can also be designed for performing wireless communication using radio waves having other frequencies, e.g. more or less than 2.45 GHz.

Other such variants will be obvious for the person skilled in the art and are considered to lie within the scope of the invention as formulated in the following claims.