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 clip. 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, 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 substrate having a radiating element provided on a first side thereof; a first ground plane provided in proximity to the radiating element; a transmission line coupled to the radiating element and configured to link the radiating element to a transmitter and/or receiver circuit; and a second ground plane associated with the transmission line; wherein a switching means is provided for selective electrical isolation of the first ground plane from the second ground plane.

The present invention therefore provides an antenna in which the first ground plane can effectively be switched on and off. The result of this is that the impedance bandwidth of the device can be altered. The Applicants have found that embodiments of the invention can be arranged to reconfigure the operational bandwidth of the antenna. Thus, in particular embodiments, the antenna can be made to switch from operating over an Ultra Wide-Band (UWB) mode when the first ground plane is electrically isolated (for example, when the antenna is configured to operate as a monopole antenna) to operating over a narrowband of frequencies when the first ground plane is electrically connected. The Applicants have also found that in certain embodiments the switching means can be configured to shift the start and/or end frequencies of the UWB region to control the UWB bandwidth.

The fact that only a single antenna structure is required, which is switchable between different modes of operation, 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.

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).

References to the term electrical isolation should be interpreted as meaning no direct conducting path is provided.

The first ground plane may be provided on a second side of the substrate, opposite to the radiating element. Alternatively, the first ground plane may be provided on the first side of the substrate, adjacent to, but spaced from, the radiating element.

The radiating element may be constituted by a microstrip patch, a Planar Inverted F- Antenna (PIFA) or an Inverted F-Antenna (IFA). The shape of the radiating element is not particularly limited and may be, for example, square, rectangular, triangular, circular, elliptical, annular, star-shaped or irregular. Furthermore, the microstrip patch may include at least one notch or cut-out. It will be understood that the shape and configuration of the planar antenna will depend upon the desired characteristics of the antenna for the applications in question.

Similarly, the size and shape of the first ground plane may be varied to provide the optimum characteristics for both modes of the operation (i.e. when the first ground plane is switched on and when it is switched off). Accordingly, the first ground plane may be, for example, square, rectangular, triangular, circular, elliptical, annular or irregular. Furthermore, the first ground plane may include at least one notch or cut-out.

In a particular embodiment, the radiating element is a rectangular patch and the first ground plane is also rectangular and is disposed on a second side of the substrate, opposite to the radiating element. However, the Applicants have found that in one such embodiment the UWB operation of the device (i.e. when the first ground plane was disconnected) was degraded by the presence of a spurious resonance mode. The Applicants then found that by including a slit in the first ground plane, this spurious mode could be suppressed and the UWB performance improved. In this embodiment, the slit was provided mid-way along the longitudinal length of the first ground plane and extended over the entire width of the first ground plane.

The transmission line may be constituted by a microstrip comprising a conducting strip on the first side of the substrate and the second ground plane provided on the second side of the substrate, opposite to the conducting strip.

Alternatively, the transmission line may be constituted by a coplanar waveguide (CPW) comprising a conductor disposed midway through the second ground plane, with a pre- determined gap provided between the conductor and each half of the second ground plane. Accordingly, in the case of a CPW, all of the components of the transmission line are provided on the same side of the substrate as the radiating element. In this case, the first ground plane may also be provided on the same side of the substrate as the radiating element.

In either of the above embodiments, the shape and configuration of the transmission line may vary to provide the required impedance characteristics. For example, the conducting strip of a microstrip transmission line or the conductor of a CPW transmission line may split into two or more branches before coupling to the radiating patch. Alternatively, multiple conducting strips or conductors may be employed.

Furthermore, the transmission 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 transmission line may be coupled to the radiating element at a corner thereof.

The switching means may comprise at least one electrically activated switch between the first ground plane and the second ground plane. In a particular embodiment, a first electrically activated switch and a second electrically activated switch may be provided between the first ground plane and the second ground plane. The first switch may be spaced apart from the second switch and, in one embodiment, the first switch is positioned at the edge of the first ground plane and the second switch is located approximately three quarters of the way along a lower edge of the first ground plane from the first switch. It will be understood that the number and distribution of the electrically activated switches should be carefully chosen to maximise the performance of the antenna both when the switches are closed (i.e. the first ground plane is electrically connected) and when the switches are open (i.e. the first ground plane is electrically isolated).

In one embodiment, the first ground plane may comprise two or more distinct elements and further switches may be provided to connect the distinct elements together, for example, when the first ground plane is electrically connected to the second ground plane. An advantage of this embodiment is that the first ground plane can be configured from a plurality of distinct elements which are less likely to resonate at modes which might have a detrimental effect on the performance of the antenna when the first ground plane is electrically isolated.

In practice the switches may be realised using any suitable technology including PIN diodes, FETs and MEMs. The mechanism for controlling the switch states would depend largely on the application but could be achieved via a microprocessor, for example.

It will be understood that when the first ground plane is electrically isolated (i.e. the switches are open) and the second ground plane is positioned close to the bottom edge of the radiating element, extending at right-angles thereto, the radiating element will act as a monopole antenna in which multiple resonances are permitted, giving rise to UWB operation. Alternatively we can consider this as a structure in which there is very little "trapped" (or stored) energy thus giving rise to radiation over a broad range of frequencies.

When the switches are selectively closed and the first ground plane is electrically connected to second ground plane, the radiating element will act as a microstrip patch antenna which can be configured so that only a single mode of resonance will be supported and, as such, the antenna will operate in a narrowband of frequencies. The Applicants have also found that it is possible to alter the bandwidth (i.e. the start and/or end frequencies) of the UWB operation either by altering the number and positions of the switches which connect the second ground plane (i.e. the ground plane associated with the transmission line) to the first ground plane (i.e. the ground plane associated with the radiating element). By activating the switches in different combinations it may also be possible to reconfigure the frequency location of the UWB mode. An advantage of selectively activating different switches to achieve the desired start and/or end frequency of operation is that, with suitable control circuitry, the antenna could be capable of being reconfigured to different frequency bands whilst in the field.

The multifunctional antenna may be configured to be switchable from one mode of operation, suitable for one application, to another mode of operation, suitable for another application. Some possible applications for the UWB antenna include broadband wireless internet, spectrum scanning in cognitive radios, and wireless USB (i.e. the streaming of high bandwidth data from one device to another). Applications for the narrowband antenna would include any form of narrowband communication, such as wireless LAN, GSM, and the communications function within a cognitive radio system.

It will be understood that when optimising any particular design of the multifunctional antenna one would need to take into account the return loss performance of the device both when the first ground plane is electrically isolated (e.g. for UWB mode) and when the first ground plane is connected to the second ground plane (e.g. for narrowband mode).

For UWB operation, in order to ensure that the antenna yields a good impedance match over the desired range of operating frequencies, it will be necessary to adjust the length and width of the second ground plane together with the gap between the second ground plane and the radiating element. When a rectangular radiating element is employed, it may be necessary to use an off-set transmission line, or an inset feed to ensure that the antenna is well matched under a narrowband mode of operation (i.e. when the first ground plane is electrically connected to the second ground plane).

It will be understood that the size of the first ground plane must be large enough to sustain adequate performance under the narrowband mode of operation but not so large so as to degrade the UWB performance to an unacceptable level.

A parametric study may be undertaken to evaluate the optimum construction of a particular multifuntional 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 view of the top layer of a multifunctional antenna according to a first embodiment of the present invention, as printed onto a substrate (not shown);

Figure 2 illustrates a view of the bottom layer (i.e. ground plane configuration) of the multifunctional antenna shown in Figure 1 , from the same side as Figure 1 but with the top layer and substrate not shown;

Figure 3 illustrates a view similar to that of Figure 1 but also showing the bottom layer of Figure 2 underneath, as if the substrate were transparent;

Figure 4 shows a graph of both measured and simulated return loss against frequency for the antenna shown in Figures 1 to 3, when operating in UWB mode; Figure 5 shows a graph of both measured and simulated return loss against frequency for the antenna shown in Figures 1 to 3, when operating in narrowband mode;

Figure 6 illustrates 20 possible alternative shapes for the radiating element shown in

Figures 1 and 3;

Figure 7 a view similar to that of Figure 3 for a second embodiment of the invention, showing both the top and bottom layers, as if the substrate were transparent;

Figure 8 shows a graph of the return loss against frequency for the antenna shown in

Figure 7, when operating in three different modes; and Figure 9 shows a view of the top layer of a multifunctional antenna according to a third embodiment of the present invention, as printed onto a substrate (not shown).

Detailed Description of Certain Embodiments With reference to Figure 1, there is illustrated a top layer 10 of a multifunctional antenna 12 according to a first embodiment of the present invention. The top layer 10 comprises a patch antenna in the form of a rectangular radiating element or patch 14 which is fed at its base 15 by a rectangular section of microstrip transmission line 16. The microstrip transmission line 16 includes a conducting strip in the form of a feed- line 17 which is off-set to the left of the centre of the patch 14 to optimise the performance of the antenna 12. In this particular embodiment, the patch 14 is 16mm long and its base 15 is 12.45mm wide. The microstrip feed-line 16 is 2.46mm wide and 85.9 mm long and is positioned 2.09mm in from the left-hand corner of the base 15. Although not shown, the top layer 10 is printed onto a dielectric substrate to form the antenna 12.

Figure 2 shows a bottom layer 20 of the multifunctional antenna 12 taken from the same view point as Figure 1 but with the top layer 10 and substrate not shown. In other words, the bottom layer 20 is printed on the opposite side of the substrate to the top layer 10. The bottom layer 20 comprises a transmission line ground plane 22, a first additional section of ground plane 24 and a second additional section of ground plane 26. The transmission line ground plane 22 is of an elongate rectangular form (85mm wide and 30mm long) and is disposed orthogonally to the rectangular patch 14. The first additional section of ground plane 24 is constituted by a rectangular section of metalisation 7.8mm wide and 18mm long. The second additional section of ground plane 26 is constituted by a rectangular section of metalisation 9.2mm wide and 18mm long. Each of the first and second additional sections of ground plane 24, 26 are aligned with the transmission ground plane 22. The first additional section of ground plane 24 is disposed adjacent the centre of the transmission line ground plane 22 with a gap of approximately 0.6mm therebetween. The second additional section of ground plane 26 is disposed adjacent the first additional section of ground plane 24 with a gap of approximately lmm therebetween. Three electrically activated switches 30, 32, 34 are provided on the bottom layer 10. A first of these switches 30 is provided between the transmission line ground plane 22 and the first additional section of ground plane 24, at the far left-hand edge of the first additional section of ground plane 24. A second of these switches 32 is provided between the transmission line ground plane 22 and the first additional section of ground plane 24, approximately three quarters of the way along the first additional section of ground plane 24 from the first switch 30. A third of these switches 34 is provided between the first additional section of ground plane 24 and the second additional section of ground plane 26, at the far left-hand edge of the first and second additional sections of ground plane 24, 26. Each of the switches is approximately lmm wide and is configured to electrically connect the metalisation on either side of it when in a so- called closed position and to electrically disconnect the metalisation on either side of it when in a so-called open position. Although not shown, each of the switches 30, 32, 34 are in practice connected to switching circuits configured to selectively open or close each switch 30, 32, 34 according to user settings.

As shown in Figure 3, the transmission line ground plane 22 is provided underneath the feed-line 17 but with a gap of approximately lmm between the top surface 28 of the transmission line ground plane 22 and the base 15 of the rectangular patch 14. With the top layer 10 positioned centrally above the bottom layer 20, the first and second additional sections of ground plane 24, 26 are provided beneath the patch 14 and are arranged to extend outwardly therefrom.

In use, the antenna 12 is capable of operation as a UWB antenna or as a narrowband antenna. The chosen mode of operation is effected by opening or closing each of the switches 30, 32, 34. With each of the switches 30, 32, 34 open, the first and second additional sections of ground plane 24, 26 underneath the patch 14 are electrically isolated and this causes the antenna 12 to operate as a UWB monopole patch antenna. In order to reconfigure the antenna 12 for narrowband operation, the switches 30, 32, 34 are activated (i.e. closed) so as to electrically connect each section of metalisation on the bottom layer 10. The result of this is that the antenna 12 acts as a microstrip patch antenna whereby only a single mode of resonance is supported.

It should be noted that in the narrowband mode of operation the frequency response is largely determined by the positions of the active switches 30, 32, 34. In the particular embodiment described, the Applicants determined the most effective switch positions for narrowband operation from a parametric study conducted using a transient solver in CST Microwave Studio which employed The Finite Integration Method.

It should be noted that unlike for other known rectangular monopole designs, the present antenna 12 does not feature notches in the patch 14 or in any of the sections of ground plane 22, 24, 26. Consequently, the design and manufacture of the antenna 12 is simplified.

The overall dimensions of the antenna 12 described above (approximately 30 mm by 110 mm) are similar to the size of a modern mobile phone and so use of this antenna 12 in a mobile phone would not significantly increase its size.

In order to verify the performance of the above antenna 12, a prototype was fabricated on a Taconic TLY-5 dielectric substrate, which has a permittivity of 2.2 (± 0.02) and a loss tangent of 0.0009 (at 10 GHz). Strips of copper were used to represent high- performance (i.e. low-loss) microwave switches 30, 32, 34 and the results of a series of measurements on this device are shown in Figures 4 and 5 alongside simulated results obtained using the above software package.

From each of these graphs it can be seen that the antenna 12 performs well in both the UWB and narrowband mode. There is also a good standard of agreement between the results obtained through measurement and simulation and it is believed that any discrepancies between the results are largely due to fabrication errors. More specifically, it can be seen from Figure 4 that when operating in UWB mode the antenna 12 exhibits a measured return loss below -6.5 dB from 3.7 GHz to at least 11 GHz. Over the majority of this frequency range the curve remains below the commonly applied -10 dB bench mark with just a few exceptions. The most notable of the exceptions occurs at 6.27 GHz and 6.99 GHz, where the return loss is -7.75 dB and - 6.97 dB respectively.

Figure 5 shows the results when all three switches 30, 32, 34 are activated and the antenna 12 switches to a narrowband mode of operation at 7.62GHz. In this case, inspection of the measured results reveals an additional resonance, not predicted through simulation, at 3.4GHz. This additional resonance causes some deviation between the measured and simulated results in the frequency range 3 GHz to 5 GHz. The Applicants believe that this additional resonance may be in the ground plane metalisation but since it is highly likely that anyone choosing to use this antenna 12 would alter the length of the ground plane and/or use the ground plane as a surface on which to mount components for a radio front end (all of which would alter the range and frequency location modes that are supported on the ground plane), optimisation of the narrowband mode has not been fully conducted at this time.

Although the patch 14 in the antenna 12 described above is rectangular in shape, any suitable shape of patch may be employed to suit circumstances. For example, the shapes illustrated in Figure 6, which include triangles, polygons, ellipses, stars and other shapes, may be employed.

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.

In the antenna 12 described above, the gap between the first additional section of ground plane 24 and the second additional section of ground plane 26 effectively constitutes a slit in the ground plane located beneath the patch 14. This was provided in order to remove a resonant mode that would otherwise have degraded the UWB operation. For other antenna configurations such a slit may not be required. Where this feature is required, however, its location and width should be carefully selected to ensure that the spurious mode is adequately suppressed.

The position of each of the switches 30, 32, 34 was found to affect the narrowband performance and also to affect the frequency location of the UWB.

When designing the above antenna 12, the relative locations of the various elements that comprise the antenna 12 were carefully adjusted in order to optimise the return loss performance in both modes of operation and this was achieved through the parametric study described above.

Figure 7 shows a view similar to that of Figure 3 but for an antenna 38 in accordance with a second embodiment of the present invention. The antenna 38 of Figure 7 is of the same general construction as that of Figure 3 and so like reference numerals will be used where appropriate. In fact, the only difference between the antenna structure shown in Figure 3 and that of Figure 7 is in the number and position of the switches provided between the transmission line ground plane 22 and the first additional section of ground plane 24. In this embodiment, a first switch labelled "a" is provided between the transmission line ground plane 22 and the first additional section of ground plane 24, at the far left-hand edge of the first additional section of ground plane 24. A second switch labelled "b" is provided between the transmission line ground plane 22 and the first additional section of ground plane 24, approximately a fifth of the way along the first additional section of ground plane 24 from the first switch a and a third switch labelled "c" is provided between the transmission line ground plane 22 and the first additional section of ground plane 24, approximately a half-way along the first additional section of ground plane 24 from the first switch a. The switch 34 is still provided between the first additional section of ground plane 24 and the second additional section of ground plane 26, at the far left-hand edge of the first and second additional sections of ground plane 24, 26.

In use, the antenna 38 is capable of shifting the UWB frequency range in which it operates by selective activatiqn (or de-activation) of the switches a, b and c. In each case, the switch 34 remains closed and one of either switch a, switch b or switch c is also closed. A graph of the return loss against frequency is shown in Figure 8 for each of the 3 modes of operation. Thus, it can be seen that with switch a closed the antenna 38 operates from 3.16GHz to 7.62GHz, with switch b closed the antenna 38 operates from 4.17GHz to 7.84GHz and with switch c closed the antenna 38 operates from 6GHz to 10.102GHz.

Figure 9 shows the top layer of a multifunctional antenna 40 according to a third embodiment of the present invention, as printed onto a substrate (not shown). In this case, all of the metallisation is provided on the top layer of the substrate and so the radiating patch 14 is connected to a CPW transmission line 42 having an off-set feed- line 44 disposed part-way through a transmission line ground plane 46, with a gap 48 provided between the feed-line 44 and each portion of the transmission line ground plane 46. A further ground plane 50 is disposed around the radiating patch 14, spaced slightly therefrom. Two electrically activated switches 52 are provided between the transmission line ground plane 46 and the further ground plane 50. Accordingly, the further ground plane 50 can be electrically isolated from the transmission line ground plane 46 (in a similar manner to that described above), resulting in a change in the impedance bandwidth of the device. The switches 52 may either be activated alone or at the same time in order to provide further modes of operation.

A particular advantage of embodiments of the present invention is that they provide a multifuntional antenna which can operate in two or more bandwidths and which can be configured not take up an excessive amount of space. Accordingly, these factors should be taken into account when optimising the antenna design.

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.