WO2010029305A1

WO2010029305A1 - Band-notched wideband antenna

Info

Publication number: WO2010029305A1
Application number: PCT/GB2009/002176
Authority: WO
Inventors: James Kelly; Peter Hall
Original assignee: The University Of Birmingham
Priority date: 2008-09-12
Filing date: 2009-09-10
Publication date: 2010-03-18
Also published as: GB0816746D0

Abstract

The present invention provides a wideband antenna (10) comprising: a substrate (13); a radiating element (12) supported on the substrate and coupled to a transmission feed-line (14); and a resonator (20) mounted in proximity to the radiating element and occupying an area that is covered at least in part by the radiating element. In embodiments of the invention, the resonator (20) is configured to resonate at a predetermined frequency within an ultra-wideband spectrum so as to suppress transmission of signals by the antenna in a notch band centred on the predetermined frequency.

Description

Band-Notched Wideband Antenna

Filed of the Invention

The present invention relates to a wideband antenna, and more particularly to a band-notched antenna incorporating a resonator.

Background

In recent years, the transmission speed and information capacity of portable wireless communications devices has increased at an exponential rate, driven by an increasing demand for multimedia wireless communications, and wireless internet. In February 2002 the USA Federal Communications Commissions (FCC) allocated a band of frequencies from 3.1 to 10.6 GHz for licence exempt ultra-wideband communication systems. The FCC stipulated a very low transmit power for UWB in order to ensure that they could successfully share the spectrum with existing licence holders. UWB systems would support high-data rate communications over a short range. This is an important advantage given the increasing need to transfer large amounts of data (e.g. music and video) between consumer electronic devices. In addition, the Institute of Electrical and Electronic Engineering (IEEE) also proposed a new standard, IEEE 802.15 WPAN (wireless personal area network), for mobile communications consumer devices to provide high-speed and low- power UWB communications. In the future UWB WPANs will probably replace wired USB in most multimedia applications (e.g. streaming video from a dvd player or camcorder to a TV).

In the following 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).

UWB systems will require a sensitive receiver. For this reason a strong narrowband signal (e.g. one used in a WLAN - wireless local area network) could quite easily overload the receiver. One solution to this problem would be to connect an external band-stop filter after the antenna in order to prevent the narrowband signal from entering the receiver. This approach, however, increases the production cost and the design complexity of the system circuitry. A much better alternative is to integrate some form of band-stop filter into the radiating element of the antenna.

As an alternative to the use of an external filter circuit, UWB antennas may be configured with a notch at the frequency of the strong narrowband signal (e.g. WLAN). Although there is a large body of literature on this topic most of the proposed solutions suffer from at least one of the following limitations: 1) a relatively low quality factor associated with the notch band resonance, 2) poor isolation at the notch frequency, and 3) complex geometry which would be difficult to redesign for operation at a different frequency. For example, US6774859 discloses UWB monopole and dipole antennas that incorporate a metal plate having one or more slits and/or one or more curved narrow slots. The antenna exhibits multiple notch bands in which destructive interference is used to cast out frequencies which overlap with other communications systems. A major disadvantage of this antenna is that it requires a very large metal plate and is difficult to integrate with the ground plate of the antenna's RF circuitry. US7061442 discloses a planar UWB antenna that has a band- notched function for suppressing interference. The antenna incorporates a metal plate that is located inside a slot, or opening in the ground plate. A further U-shaped slot is provided in the metal plate. Although this configuration could be integrated more readily with the transceiver circuitry, both of these prior art antennas suffer from the problems common to most of the existing solutions. These designs do not exhibit a very sharp profile of the destructive band (notch), but tend to reject (or suppress) neighbouring frequencies as well. As a result these antennas have a characteristic more similar to that shown in Figure 1a, which shows a graphical representation of the gain of the antenna, plotted as a function of the frequency. As can be seen, the presence of a notch band causes a drop in the gain, but with these prior art antennas this drop is spread across a relatively wide band of frequencies. Designs that have a sharper profile (i.e. a narrower notch band), typically utilise a fairly complex structure which means that they cannot easily be modified or reconfigured, to change the notch band frequencies. Also there is scope for improving the level of rejection afforded by many of the prior art antennas at the centre of the notch band.

Summary of the Invention

The present invention adopts a new approach in order to address the above limitations. The principle is illustrated in Figure 1b. The gain remains high across most of the UWB spectrum, but exhibits a drop at a narrow band of frequencies (referred to as the notch band) that corresponds to the WLAN bandwidths.

According to the present invention there is provided a wideband antenna comprising: a substrate; a radiating element supported on the substrate and coupled to a transmission feed-line; and a resonator mounted in proximity to the radiating element and occupying an area that is covered at least in part by the radiating element.

In embodiments of the invention, the resonator is configured to resonate at a predetermined frequency within an ultra-wideband spectrum so as to suppress transmission of signals by the antenna in a notch band centred on said predetermined frequency. Preferably, the resonator is a microstrip resonator.

It is an advantage that the resonator is integrated with the antenna so that the antenna rejects the frequencies at which the resonator resonates (i.e. in the notch band). Thus, when operating as a receiver, and receiving signals with a frequency in the notch band, the resonator will resonate, causing those frequencies to be suppressed (i.e. a substantial portion of the signal strength is reflected) rather than transferred onto the feed-line. Equally, when operating as a transmitter, frequencies within the notch band will be suppressed due the resonator and not transmitted by the antenna. It is a further advantage that the resonator may be formed as a microstrip, and easily mounted to the same dielectric substrate as the wideband antenna. In a currently preferred embodiment of the invention, the microstrip resonator is of an open-loop type. The microstrip resonator may comprise two microstrip gaps. The distance between these microstrip gaps and/or the sizes of the gaps may be selected to set the required notch band frequency. However a wide variety of other resonators may also be suitable and could be employed.

In embodiments of the invention, the resonator is positioned directly above a junction between the radiating element and the transmission feed-line.

In embodiments of the invention, the radiating element is supported on a first side of the substrate and the resonator is mounted on a second side of the substrate. It is an advantage that the resonator can be mounted within the same footprint as the radiating element of the antenna, thereby leaving free space for other components.

Preferably, the transmission line comprises a coplanar wave guide (CPW), which includes a signal conductor and a ground plane for the antenna.

In embodiments of the invention a plurality of resonators may be mounted in proximity to the radiating element and occupying areas that are covered at least in part by the radiating element. One advantage of this arrangement is that it can be used to improve the characteristics (e.g. the sharpness) of the notch band. Alternatively, the resonators may be configured to create multiple notch bands located at different frequencies throughout the UWB band.

In certain embodiments, two or more resonators may be stacked one above the other. For example, a first microstrip resonator may be provided on one side of a further substrate and a second microstrip resonator may be provided on the opposite side of the further substrate. In this case, the frequency separation between the resulting two notch bands can be varied by altering the relative permittivity of the further substrate. It is also possible to alter the frequency of each notch band individually by altering the dimensions of the associated resonator. It is noted that if the frequency separation is reduced sufficiently, the two (narrow) notch bands will merge into one (wider) notch band.

In another embodiment, the radiating element may further include a slot resonator. The slot resonator may be of a similar size and/or shape as the aforementioned resonator. Furthermore, the slot resonator may be laterally offset from the resonator. The offset distance may be varied in order to control the performance of the antenna.

Brief Description of the Drawings

Figure 1a is graphical representation of the gain of an antenna having a notch band. The response is typical of many prior art antennas.

Figure 1b is a graphical representation of the gain of an antenna having a narrow notch band.

Figure 2a is an illustration, in plan view, of an antenna in accordance with a first embodiment of the invention.

Figure 2b shows a side elevation of the antenna of Figure 2a.

Figure 2c shows more detail of a microstrip resonator forming part of the antenna of Figures 2a and 2b.

Figure 3 is a graph showing the return loss as a function of frequency for the antenna of Figures 2a-2c.

Figure 4 is an illustration, in plan view, of an antenna in accordance with a second embodiment of the invention.

Figure 5 is an illustration showing a variety of different shapes that may be used as an alternative to the wideband antenna of Figures 1 and 4. Figure 6 is an illustration showing a variety of different configurations for alternative resonators that may be used in the antennas of Figures 1 and 4.

Figure 7 is an illustration of an alternative microstrip resonator that may be employed in the antennas of Figures 2a and 4.

Figure 8A shows a side elevation view of an antenna in accordance with a third embodiment of the invention.

Figure 8B shows an isometric view of the antenna of Figure 8A.

Figure 9 shows a graph of return loss as a function of frequency for the antenna of Figures 8A and 8B, for two different dimensions of the second resonator.

Figure 10 shows a plan view of an antenna in accordance with a fourth embodiment of the invention.

Figure 11 shows a graph of return loss as a function of frequency for the antenna of Figure 10.

Detailed Description

Figures 2a and 2b illustrate a construction of an antenna 10 in accordance with a first embodiment. In Figure 2b, the thicknesses of the components have been exaggerated to show these more clearly. The antenna 10 includes a wideband antenna radiating element in the form of a monopole disc 12, which is supported on one side of a substrate 13 (not shown in Figure 2a). Also supported on the substrate 13 is a section of coplanar wave guide (CPW) transmission line made up of a central signal feed-line 14 separated by narrow gaps 15 from planar portions 16. The planar portions 16 provide a ground plane for the entire antenna structure. A gap 18 is provided between the planar portions 16 and the antenna disc 12. A microstrip resonator 20 is mounted on the other side of the substrate to the antenna disc 12. The resonator 20 is mounted so that it occupies an area that is covered (on the other side of the substrate) by part of the antenna disc 12, and is also close to the junction between the antenna disc 12 and the feed-line 14.

The substrate 13 is a low-loss, high permittivity microwave substrate having a thickness of 0.635 mm. The antenna components (disc 12, CPW transmission line and resonator 20) may be printed onto the substrate material. For example, a demonstration prototype was fabricated using standard printed circuit board developing processes, which would be familiar to anyone with experience in this area. The substrate was a commercially available product called 3010 supplied by Rogers Corporation, and having a permittivity of 10.2. Equivalent alternatives are also available from other manufactures (e.g. CER10 from Taconic). These materials are based on PTFE or "Teflon" which is suitably loaded, during manufacture, in order to increase the dielectric constant. Materials with higher permittivities would be advantageous where available. Materials with lower permittivities could be employed in applications that could tolerate a notch-band having a lower quality factor. Fabrication technologies include Low Temperature Co-fired Ceramics (LTCC), which is a multi-layer ceramic technology. It is important to stress that the sharpness of the notch band is in part due to the use of a high permittivity substrate material, and in this respect this is an important feature of the design. The thickness of the substrate is also an important design feature. The use of a thin substrate material enables one to achieve a strong high coupling of electromagnetic fields from the disk to the resonator. This leads to a high level of reflection at the notch band centre frequency.

In a typical construction of the antenna 10, the antenna disc 12 has a diameter of 25mm. The feed-line 14 has a width of 1.93mm, the narrow gaps 15 are each 0.5mm, the planar portions 16 extend for a length of 10mm, and the overall width of the CPW section is 25mm. This arrangement provides an impedance of about 50Ω. The radius of the disc 12, the width of the ground plane (planar portions 16 of the CPW), and the gap 18 between ground plane and antenna disc must all be selected carefully in order to ensure that the antenna yields a good standard of impedance match over the desired range of operating frequencies.

Some typical dimensions of the microstrip resonator 18 are shown in Figure 2c. The geometry of the resonator 20 is based around that of an open-loop resonator. However, in this embodiment, there are several important differences, including the use of two microstrip gaps 22 having a separation 23. The distance separating these two gaps controls the precise value of the notch band centre frequency. Specifically the frequency reduces as the separation distance 23 between the two gaps 22 is reduced. The width of the gaps 22 also has a similar effect (i.e. the notch band centre frequency reduces as the gap widths are reduced). For this reason these dimensions should be chosen carefully in order to achieve the required notch-band frequency.

The resonant frequency of the open-loop type resonator 20 will be linked to its dimensions through a series of simple mathematical relationships. These relationships may be determined empirically or through computer simulation, and used during the design of the antenna 10. The use of a low-loss, high permittivity, microwave substrate material ensures that the notch-band resonance has a high-quality factor, as mentioned earlier.

In use, the frequencies at which the resonator resonates will provide a notch band of frequencies within the UWB spectrum of the antenna 10, at which transmission/reception of signals will be suppressed. In other words, when the antenna 10 receives signals with a frequency in the notch band, the resonator 20 will resonate, causing those frequencies to be suppressed because a substantial portion of the signal is reflected rather than transferred onto the feed-line 14. Equally, when operating as a transmitter, frequencies in the notch band of a signal provided through the feed-line 14 will be suppressed due the resonator 20 resonating at those frequencies, so that the strength of radiation from the antenna, at those frequencies is suppressed. In order to ensure adequate electromagnetic coupling between the UWB antenna disc 12 and the resonator 20 it is important to position the resonator directly above the disk-feedline junction. The coupling is also enhanced by using a thin microwave substrate material. Care should also be taken to prevent misalignment between the printed layers on the substrate 13 as the antenna's return loss performance is quite sensitive to these factors.

Figure 3 shows a comparison of a laboratory measurement 34 against a result obtained through computer simulation 32 of return loss for a range of UWB frequencies for an antenna constructed as described above and shown in Figures 2a-2c. The return loss may be related to the antenna gain because a low (close to 0 dB) value of return loss implies that a high proportion of the signal is reflected rather than being allowed to pass through (i.e. transmitted or received by) the antenna 10. Thus, as can be seen in Figure 3, the antenna has a notch band at just above 5GHz, where the return loss is very low (ca. -1.14dB according to measurement).

Figure 4 illustrates a construction of an antenna 40 in accordance with a second embodiment. As with the antenna 10 of Figures 2a-c, the antenna 40 includes a monopole antenna disc 42, which is supported on a first side of a substrate (not shown) and a section of coplanar wave guide (CPW) transmission line 44. A microstrip resonator 50 is mounted on the reverse, second side of the substrate so that it occupies an area that is covered (on the first side of the substrate) by part of the antenna disc 42, and is also close to the junction between the antenna disc 42 and the transmission feed-line. In addition three further microstrip resonators 52, 54, 56 are mounted on the reverse, second side of the substrate, each occupying an area that is covered (on the first side of the substrate) by part of the antenna disc 42. Each of the microstrip resonators 50, 52, 54, 56 is essentially identical so that each will resonate at the same notch band frequency. The use of multiple resonators creates a filter (in the classical sense) thus giving a more sharply defined notch band. A further possibility is for the plurality of resonators 50, 52, 54, 56 to have a different resonant frequency, thereby providing a plurality of notch bands at different frequencies in the UWB spectrum.

In the first and second embodiments described above in connection with Figures 2a to 4, the wideband antenna uses a monopole disc 12, 42 as the radiating element. However, it is not necessary for this component to be in the shape of a disc. For instance, any of the shapes illustrated in Figure 5 may be employed. These include squares, triangles, polygons, ellipses, stars, and other shapes. Indeed, almost any suitable shape of monopole formed on the substrate could be employed. Furthermore, this radiating element may include one or more notches or cut-outs. The literature contains many examples of this type, and this is often used as a mechanism for impedance matching over a broad band of frequencies.

As a further alternative one might chose not to use a wideband monopole but instead utilise some other form of planar wideband antenna. This would include variants currently described in the literature or yet to be devised. The band notch technique presented here would be equally applicable to this scenario.

In the embodiments described above, the resonator is based on an open-loop microstrip type. However, it is not essential that the resonator is of this form, provided that the operating resonant frequency of the resonator can be accurately and reliably determined. Figure 6 illustrates a variety of alternative configurations of resonator, which are further described in Table 1 below:

Table 1

Figure 7 shows a further open-loop microstrip resonator 70 that could be employed in the antennas of Figures 2a and 4. Essentially, the resonator 70 is a simplified version of the resonator 20 in that it incorporates only a single microstrip gap 72. The resonator 70, as viewed in Figure 7, therefore has the appearance of a C-shaped strip 74 laid on its back. It will be understood that the resonant frequency can be varied by altering the dimensions of the resonator 70, as described above.

Figures 8A and 8B illustrate a third antenna 80 in accordance with a further embodiment of the invention. The basic structure of the antenna 80 is the same as that described above in relation to the antenna 10 of Figures 2a to 2c except that the resonator 20 of Figure 2c is replaced by a (first) resonator 70 of Figure 7. Like reference numerals will therefore be employed where appropriate. In addition to the above, a second substrate 82 is provided with a first side adjacent the first resonator 70 on the substrate 13. A second resonator 70', is provided on a second side of the second substrate 82, opposite to the first resonator 70. The second resonator 70', as shown in Figures 8A and 8B, is generally of the same size and shape as the first resonator 70. Thus, the antenna 80 incorporates a pair of microstrip open- loop resonators 70, 70' stacked one above the other.

Each of the resonators 70, 70' in this design can produce a single notch band and so the antenna 80 can be used in environments where two different narrow-band frequencies require suppressing. It has been found that the frequency separation of the notch-bands can be controlled by altering the relative permittivity of the second substrate 82. It has also been found that the frequency location of each individual notch band can be varied by the altering the dimensions of the corresponding resonator.

Figure 9 shows a graph of return loss as a function of frequency for the antenna 80, both in the case where the second resonator 70' has a length d₂ of 11.5mm (the same as the length di of the first resonator 70) and in the case where the second resonator 70' alone has an increased length d₂ of 15mm. It can therefore be seen that increasing the length d₂ of the second resonator 70' reduces the frequency separation between the two individual notch bands 84, 86 such that they begin to merge into a single notch band 88. In this particular example, the single notch band 88 has a 3dB bandwidth which is approximately 5% (i.e. around 300MHz) wider than the first notch band 84.

Figure 10 shows a plan view of a fourth antenna 90 in accordance with another embodiment of the invention. The basic structure of the antenna 90 is again the same as that described above in relation to the antenna 10 of Figures 2a to 2c except that, in this case, the radiating element 12 includes a slot resonator 92. Like reference numerals are therefore employed where appropriate.

The slot resonator 92 is of the same size and shape as the microstrip open- loop resonator 20 but is laterally offset therefrom by a vertical distance of 6mm (as viewed in Figure 10).

Thus, the antenna 90 of Figure 10 illustrates an alternative configuration capable of yielding a pair of notch bands. It will be understood that the frequency of each notch band is determined by the dimensions of each of the resonators 20, 92.

Figure 11 shows a graph of return loss as a function of frequency for the antenna 90 of Figure 10, thereby illustrating the two notch-bands 94, 96.

The antennas of the present invention offer significant advantages. Firstly, there is strong discrimination between the wanted and unwanted signals due to the very low return loss (high reflection) at the notch band centre frequency. The notch band characteristic also has a sharply defined transition between the frequencies that are passed through the antenna, and those that are suppressed. This also enables one to make the notch band very narrow (i.e. it has a high quality factor). Secondly, the antenna is simple to design because it can employ a simple monopole shape, having well established characteristics, together with a simple variation of a popular, and readily available type of microstrip resonator. The resonant frequency can readily be determined from the geometry of the resonator. Thirdly, the antenna is compact because the resonator components are integrated with the antenna, and occupy space within the same footprint as the monopole. This means that no further space is required for additional circuitry etc. to provide the notch band characteristics.

Claims

1. A wideband antenna comprising: a substrate; a radiating element supported on the substrate and coupled to a transmission feed-line; and a resonator mounted in proximity to the radiating element and occupying an area that is covered at least in part by the radiating element.

2. The antenna of claim 1 wherein the resonator is configured to resonate at a predetermined frequency within a wideband spectrum so as to suppress transmission of signals by the antenna in a notch band centred on said predetermined frequency.

3. The antenna of claim 1 or claim 2 wherein the resonator is a microstrip resonator.

4. The antenna of claim 3 wherein the microstrip resonator is of an open- loop type.

5. The antenna of claim 4 wherein the microstrip resonator comprises two microstrip gaps.

6. The antenna of claim 5 wherein a distance between the microstrip gaps and/or sizes of the gaps is selected to provide the predetermined frequency.

7. The antenna of any preceding claim wherein the resonator is positioned directly above a junction between the radiating element and the transmission feed-line.

8. The antenna of any preceding claim wherein the radiating element is supported on a first side of the substrate and the resonator is mounted on a second side of the substrate.

9. The antenna of any preceding claim wherein the transmission line comprises a coplanar wave guide, CPW, which includes a signal conductor and a ground plane for the antenna.

10. The antenna of any preceding claim wherein the substrate comprises a thin and/or low-loss, high permittivity microwave substrate material.

12. The antenna of any preceding claim wherein the radiating element and/or the resonator are printed onto the same substrate material.

13. The antenna of any preceding claim wherein the wideband antenna is in the shape of a disc.

14. The antenna of any preceding claim comprising a plurality of resonators mounted in proximity to the radiating element and occupying areas that are covered at least in part by the radiating element.

15. The antenna of claim 14 wherein the plurality of resonators have substantially the same resonant frequency.

16. The antenna of claim 14 wherein the resonators have different resonant frequencies to create multiple notch bands located at different frequencies throughout a wideband spectrum.

17. The antenna of any preceding claim wherein two or more resonators are stacked one above the other.

18. The antenna of any preceding claim wherein the radiating element further comprises a slot resonator.

19. The antenna of claim 18 wherein the slot resonator is laterally offset from the resonator.