Field of the Invention The invention relates to a multifunctional antenna. Particularly, but not exclusively, the invention relates to a multifunctional antenna for use in a portable device such as a mobile phone, personal digital assistant (PDA), radio or laptop.

Background to the Invention There is growing demand for multifunctional devices which are capable of transmitting and/or receiving wireless signals for a number of different applications operating over a number of different frequency bands. For example, mobile phones are often required to operate in different countries where the communication frequencies and standards are different. The phone may also require GPS, Bluetooth connectivity, and wireless internet access. Traditionally, this means that a number of different antennas are required with corresponding circuitry and this has implications on the size of the device and its styling, both of which are considered by end users as important factors.

Similar considerations also arise in relation to the concept of Cognitive Radio (CR) and in particular Spectrum Sensing Cognitive Radio (SSCR) which aims to provide the user with an improved and more reliable service by making more efficient use of the frequency spectrum. It is envisaged that a CR device would change its communication frequency whenever necessary - for example, to avoid interference and spectrum "traffic jams" or when more bandwidth is needed such as to send a video file. A CR device would need to work alongside existing (so-called legacy) radio users. Consequently, in order to make informed decisions about the choice of operating frequency the CR must scan the frequency spectrum listening for legacy users and available bandwidth.

Although the architecture of a CR has not yet been standardised some suggest that an

Ultra Wide-Band (UWB) antenna could be used for performing the sensing function and a separate frequency reconfigurable narrowband antenna could then handle the required communications. However, it will also be important for these antennas to be capable of sufficient filtering of undesired outband signals so that the CR device is immunised against strong interferer signals. Furthermore, as above, the space available for these antennas and their supporting circuitry will be limited in a portable CR device.

It is therefore an aim of the present invention to provide a multifunctional antenna which helps to address the above-mentioned problems.

Summary of the Invention

According to a first aspect of the present invention there is provided a multifunctional antenna comprising a radiating element; a microstrip feed line coupled to the radiating element, the microstrip feed line comprising a conductive strip disposed on a first side of a substrate and a ground plane, disposed on a second side of the substrate, extending beneath the conductive strip; and at least one slot, provided in the ground plane, which extends at least in part beneath the conductive strip; wherein at least one switch is provided in the ground plane and is configured to selectively open and close the at least one slot in the region beneath the conductive strip.

The Applicants have found that embodiments of the invention can be arranged to provide a compact antenna having a reconfigurable operational bandwidth. Providing at least one open (i.e. activated) slot in the ground plane beneath the conductive strip ensures that frequencies proportional to the slot length are blocked (i.e. filtered) and so they do not propagate along the microstrip feed line. The at least one slot behaves in a similar manner to a microstrip resonator except that, rather than passing resonant frequencies, the slot suppresses or blocks resonant frequencies. The Applicants have found that the strongest coupling between the slot and the feed line occurs in the region therebetween. Consequently, when the slot is closed (i.e. deactivated) in the region beneath the conductive strip, the coupling between the slot and the feed line is greatly reduced and so the filtering effect of the slot is also greatly reduced. Accordingly, the Applicants have found that selective opening and closing (or activating and deactivating) of the slot can serve to alter the operational bandwidth of the antenna. In particular embodiments, the antenna can be configured to switch from operating over an Ultra Wide-Band (UWB) mode when the slots are deactivated to operating over a narrowband range of frequencies when the slots are activated.

It will be understood that the term Ultra Wide-Band (UWB) is used throughout to denote a relatively large frequency range and is not limited to a specific range of frequencies such as those defined as UWB by the US Federal Communications Commission (FCC).

The fact that only a single antenna structure is required, which is switchable between different modes of operation (e.g. allowing transmission of different frequencies or frequency ranges), means that the overall size of a device incorporating the antenna can be reduced since there is no need for separate antennas to perform each function. In addition, the size of the antenna is not affected by inclusion of a filtering means since the slots which perform the filtering function are integrated into the ground plane of the feed line.

It is significant to note that, contrary to traditional techniques, the filtering in the present invention is effected on the ground plane of the feed line and not on the radiating element itself. This is advantageous in that larger slots/filters (providing greater frequency suppression) can be used due to the fact that the ground plane is usually larger than the radiating element. Furthermore, better performance can be achieved with the switches positioned outside of the radiating element since transient radiation caused during the opening/closing of the switches will not be transmitted by the radiating element but instead they will be absorbed by the ground plane.

A further ground plane may be provided either adjacent or beneath the radiating element.

The shape of the radiating element is not particularly limited and may be, for example, square, rectangular, triangular, circular, elliptical, annular, star-shaped, funnel-shaped or irregular. Furthermore, the radiating element may include at least one notch or cut-out. It will be understood that the shape and configuration of the radiating element will depend upon the desired characteristics of the antenna for the applications in question. For example, the radiating element may be configured as a slot resonator or a Vivaldi resonator.

Similarly, the size and shape of the ground plane may be varied to provide the optimum characteristics for both modes of the operation (i.e. when the slot is activated and when it is deactivated). Accordingly, the ground plane may be, for example, square, rectangular, triangular, circular, elliptical, annular or irregular.

The ground plane may be disposed at right angles to the radiating element, to form a monopole antenna.

The radiating element may be provided on a first side of a substrate and the ground plane provided on the opposite, second, side thereof.

The shape and configuration of the feed line may vary to provide the required impedance characteristics. For example, the feed line may comprise a single strip or it may be split into two or more branches before coupling to the radiating element. Alternatively, multiple feed lines may be employed.

Furthermore, the feed line may be coupled to the radiating element along an edge thereof and may be disposed centrally along the edge or offset to one side. Alternatively, the feed line may be coupled to the radiating element at a corner thereof.

In one embodiment, a single slot is provided in the ground plane to effectively create a notch in the antenna response at a frequency which is proportional to the slot length.

The shape of the at least one slot is not particularly limited and may, for example, be straight, L-shaped, U-shaped, H-shaped, T-shaped, square, rectangular, triangular, circular, elliptical, or annular, or may comprise any combination of these shapes. The shape of the slot may be chosen to minimise the space it requires.

The at least one switch may be electrically activated. In practice the at least one switch may be realised using any suitable technology including PIN diodes, FETs and MEMs. The mechanism for activating and deactivating the switches would depend largely on the application but could be achieved via a microprocessor, for example.

One or more further switches may be provided to alter the length or configuration of the at least one slot. Altering the length of the slot will alter the frequency suppressed by the slot and so it will be possible to 'tune' or vary the frequency suppressed by selectively activating and deactivating the one or more further switches. Moreover, providing one or more further switches configured to selectively open and close further regions of a single slot will enable a further reduction in the coupling effect between the slot and the feed line to be realised, resulting in improved antenna performance when the slot is closed.

Two or more slots may be provided in the ground plane. Each of the two or more slots may extend at least in part beneath the conductive strip.

In a certain embodiment, two or more slots are disposed in the ground plane in proximity to one another. It has been found that, in such a configuration, the frequencies suppressed by each slot are coupled together. More specifically, it has been found that two slots create two different blocking resonances and that the frequency of these blocking resonances varies depending on the distance between the slots. When the distance between the slots is increased the blocking resonances are moved towards each other (i.e. the two frequencies move inwardly on the spectrum) and when the distance between the slots is decreased the blocking resonances are moved away from each other (i.e. the two frequencies move outwardly on the spectrum). Between the two blocking resonances there is a frequency band-pass whereby all the energy entering from one side of the feed line is transmitted to the other side. Thus, the antenna can be configured to pass frequencies in a particular band and suppress those around the desired band. In other words, where two or more slots are disposed in the ground plane in proximity to one another, the slots will serve as a bandpass filter. An antenna having such a configuration, and having two or more switches provided in the ground plane, configured to selectively open and close each slot in the region beneath the conductive strip, can therefore be made to selectively operate over a narrowband range of frequencies (when the switches are open) or an UWB range of frequencies (when the switches are closed).

In further embodiments, one or more further switches may be arranged to effectively alter the distance between the two or more slots, thereby altering the narrowband frequency range of the antenna when the slots are open.

Thus, in embodiments of the present invention, it will be possible to control the signal filtering to provide a frequency reconfigurable narrowband antenna which is integrated with a UWB monopole antenna. Such an antenna would be ideally suited to CR applications since it would be able to perform the sensing, tuning and transmitting functions required, in a compact device.

The antenna response may be impedance matched around a desired frequency band so that the frequencies outside this band are suppressed.

In embodiments of the present invention, it may be possible to achieve a higher quality factor (Q) than that which could normally be achieved by an unfiltered monopole antenna, operating in the desired frequency band. Furthermore, it may be possible to achieve this increase in performance with little or no added cost since the antenna dimensions in each case are equivalent and the production costs would not be likely to be affected by inclusion of the slot filters.

A parametric study may be undertaken to evaluate the optimum construction of a particular multifunctional antenna according to an embodiment of the present invention. Brief Description of the Drawings

Certain embodiments of the present invention will now be described with reference to the accompanying drawings in which: Figure 1 illustrates a perspective view from above of a portion of an antenna according to a first embodiment of the present invention wherein a microstrip feed line is positioned centrally over a ground plane which includes a central U-shaped slot filter;

Figure 2 shows a graph of simulated S ₁₂ scattering parameter (i.e. the reverse voltage gain) for a range of frequencies for two different lengths of slot in the structure shown in Figure 1;

Figure 3 illustrates a view from above of a structure similar to that shown in Figure 1 but with an additional U-shaped slot provided in the ground plane adjacent to the central

U-shaped slot, as per a second embodiment of the present invention;

Figure 4 shows a graph of simulated S ₂₁ scattering parameter (i.e. the forward voltage gain) for a range of frequencies for three different distances between the two U-shaped slots shown in Figure 3;

Figure 5 illustrates a view from above of an antenna according to a third embodiment of the present invention, in which the slots beneath the feed line are open (i.e. the antenna is operating in narrowband mode); Figure 6 shows a graph of both simulated and measured S] i scattering parameter (i.e. the input port voltage reflection coefficient) for a range of frequencies for the structure shown in Figure 5;

Figure 7 illustrates a view from beneath the antenna shown in Figure 5, in which the slots beneath the feed line are closed (i.e. the antenna is operating in UWB mode); and Figure 8 shows a graph of both simulated and measured Sn scattering parameter (i.e. the input port voltage reflection coefficient) for a range of frequencies for the structure shown in Figure 7.

Detailed Description of Certain Embodiments With reference to Figure 1, there is illustrated a portion of an antenna 10 according to a first embodiment of the present invention in which an elongate microstrip feed line 12 is provided on a first side of a dielectric substrate (not shown) and is positioned centrally over a rectangular ground plane 14 which is provided on an opposite, second, side of the substrate. Note that the substrate has been omitted from each of the Figures for clarity and, consequently, the Figures have been drawn as if the substrate were transparent.

The ground plane 14 includes a central U-shaped slot filter 16. As viewed in Figure 1, the U-shaped slot 16 is inverted such that a horizontal portion 18 of the slot 16 is positioned under the upper portion of the feed line 12. In this particular embodiment the slot 16 has a total length (L) of 26mm (the horizontal portion 18 being 10mm long and each leg 20 of the slot being 8mm long, as measured from the underside of the horizontal portion 18).

Although not shown, in use, a switch will be provided to activate and deactivate the slot 16; for example, by providing either a gap or a connection in the ground plane 14 beneath the feed line 12.

A graph of the simulated S ₁₂ scattering parameter (i.e. the reverse voltage gain) for the structure illustrated in Figure 1 is shown in Figure 2 for a frequency range of 1 to 12 GHz. In addition, the results for a similar structure but wherein the slot 16 has a total length (L) of 40mm (the horizontal portion 18 being 20 mm long and each leg 20 of the slot being 10 mm long, as measured from the underside of the horizontal portion 18) are presented on the graph.

From Figure 2, it can be seen that where the slot 16 has a length of 26mm, a blocking resonance is produced at around 4.2GHz and where the slot 16 has a length of 40mm, a blocking resonance is produced at around 2.7GHz. It is therefore clear that the frequency response of the feed line 12 is proportional to the length of the slot 16.

Figure 3 illustrates a structure similar to that shown in Figure 1 but for a second embodiment of the present invention in which an additional U-shaped slot 22 is provided in the ground plane 14 adjacent to the central U-shaped slot 16. As viewed in Figure 3 (i.e. from above the feed line 12), the additional slot 22 is provided to the right- hand side of the slot 16. Also, the additional slot 22 is inverted such that a horizontal portion 24 of additional slot 22 is aligned with the horizontal portion 18 of the slot 16. In this particular embodiment the additional slot 22 has the same dimensions as the slot 16 of Figure 1. Between the slot 16 and the additional slot 22 there is a distance (d) of lmm.

As for Figure 1 , although not shown, in use, a switch will be provided to activate and deactivate the slot 16; for example, by providing either a gap or a connection in the ground plane 14 beneath the feed line 12.

A graph of the simulated S _2J scattering parameter (i.e. the forward voltage gain) for the structure of Figure 3 is shown in Figure 4 for a frequency range of 2 to 5 GHz. In addition, the results for two similar structures wherein the distance (d) between slot 16 and additional slot 22 is 0.5mm and Omm, respectively, are presented on the graph.

From Figure 4, it can be seen that each structure produces two blocking resonances and that the frequency of each of these resonances is dependent on the distance (d) between the two slots 16, 22. In other words, the blocking resonances of the two slots 16, 22 are coupled together. More specifically, when the distance (d) between the two slots 16, 22 is increased, the blocking resonances are moved closer together (i.e. inwardly on the frequency spectrum). Conversely, when the distance (d) between the two slots 16, 22 is decreased, the blocking resonances are moved further apart (i.e. outwardly on the frequency spectrum). Between the two resonances there is a frequency pass-band where virtually all energy entering from one end the feed line 12 is transmitted to the other end.

It will be noted that in embodiments of the present invention, providing at least one further switch between the two slots 16, 22 could serve to effectively alter the distance between the two slots 16, 22 and in this way it will be possible to re-configure the frequency range of the pass-band. An antenna 30 according to a third embodiment of the present invention is shown in Figure 5. The antenna 30 includes a monopole antenna structure comprising a circular radiating element 32 disposed on a first side of a dielectric substrate 34 and a rectangular ground plane 36 disposed horizontally adjacent to the radiating element 32, on an opposite second side of the substrate 34. An elongate microstrip feed line 38 is provided on the first side of the substrate 34 and is positioned centrally over the ground plane 36, extending from the central edge 40 of the radiating patch 32 to above the lower edge 42 of the ground plane 36.

A first slot filter 44 is provided in the centre of the ground plane 36 beneath the upper portion 46 of the feed line 38. The first slot filter 44 is composed of three horizontally adjacent U-shaped sections 48, the first and third U-shaped sections being inverted (when viewed from above the feed line 38, as in Figure 5). Each of the three U-shaped sections 48 is arranged to touch the adjacent U-shaped section 48 so as to form a continuous slot 44. A second slot filter 50 is provided in the centre of the ground plane 36 beneath the lower portion 52 of the feed line 38 and is formed as a horizontal mirror- image of the first slot filter 44. Although not shown in Figure 5, the first and second slot filters 44, 50 are both open (i.e. active) beneath the feed line 38.

In the embodiment shown in Figure 5, the radiating element 32 has a radius of 20mm and the ground plane 36 has a width of 20mm and a length of 50 mm. The feed line 38 has a length of 20.4mm and a width of 2.6 mm. Each of the U-shaped sections 48 has a height of 7mm, a slot width of lmm, and a distance of 4mm between each leg 54 of the U-shaped section 48.

A graph of both the simulated and measured S ₁₁ scattering parameter (i.e. the input port voltage reflection coefficient) for the structure shown in Figure 5 is shown in Figure 6 for a frequency range of 1 to 12GHz. Thus, it can be seen that the measured results substantially correspond with the simulated results and that the antenna 30 is operating in a narrowband mode (when its slot filters 44, 50 are open) passing frequencies between 5 and 6GHz and generally suppressing those outside this region.

Figure 7 shows a view from beneath the antenna 30 of Figure 5 but with the two slot filters 44, 50 being closed beneath the feed line 38 by switches in the form of metal strips 56 of approximately 1 mm wide. The presence of the metal strips 56 deactivates the slot filters 44, 50 to recover the UWB operating mode of the monopole antenna.

It will be understood that each of the switches 56 will be configured to electrically connect the metalisation of the ground plane 36 on either side of the slots 44, 50 when in a so-called closed position and to electrically disconnect the metalisation on either side of the slots 44, 50 when in a so-called open position. Although not shown, each of the switches 56 will in practice be connected to switching circuits configured to selectively open or close each switch 56 according to user settings or other conditions.

A graph of both the simulated and measured Sn scattering parameter (i.e. the input port voltage reflection coefficient) for the structure shown in Figure 7 is shown in Figure 8 for a frequency range of 1 to 12GHz. Thus, it can again be seen that the measured results substantially correspond with the simulated results and that the antenna 30 is now operating in a UWB mode (when its slot filters 44, 50 are closed) passing frequencies between 2 and 10GHz and generally suppressing those outside this region.

In accordance with the above it is therefore possible to configure a multifunctional antenna which can operate in a narrowband mode when the slot filters are activated (i.e. switches open) and a UWB mode when the slot filters are deactivated (i.e. switches closed). It is also possible to 'tune' the narrowband response by including further switches that can be selectively employed to vary the distance between two or more slots to thereby shift the blocking resonances and the resulting narrow frequency pass- band. Consequently, the antenna may be ideally suited to cognitive radio applications. The overall dimensions of the antenna 30 described above (approximately 50 mm by 50 mm) are similar to the size of a traditional modern mobile phone antenna and so use of this antenna 30 in such a device would not significantly increase its size.

Although the radiating element 32 in the antenna 30 described above is circular in shape, any suitable shape of radiating element may be employed to suit circumstances. For example, the shape of the radiating element may comprise triangles, polygons, ellipses, stars and other shapes.

Similarly, different shapes of ground plane metalisation and microstrip feed-line may be employed.

When designing an antenna according to an embodiment of the present invention it is important to try to achieve a good standard of return loss performance under both the narrowband and UWB mode of operation. Thus, after making a significant change to improve performance in the narrowband mode of operation, for example, the designer should switch to the UWB mode and check that the change does not have a detrimental effect there. Conversely, after making a change which improves the UWB operation he/she should check that the narrowband performance remains acceptable.

A particular advantage of embodiments of the present invention is that they provide a multifunctional antenna which can operate in two or more bandwidths and which can be configured not take up an excessive amount of space.

It will be appreciated by persons skilled in the art that various modifications may be made to the above-described embodiments without departing from the scope of the present invention.