Patent Publication Number: US-9899737-B2

Title: Antenna element and antenna device comprising such elements

Description:
This application is the U.S. national phase of International Application No. PCT/GB2012/053196 filed 19 Dec. 2012 which designated the U.S. and claims priority to GB 1122324.5 filed 23 Dec. 2011, the entire contents of each of which are hereby incorporated by reference. 
     FIELD 
     The present invention relates to the field of compact antenna elements and miniaturised antenna devices comprising such elements. 
     BACKGROUND 
     Over the past few years, there has been an increasing worldwide research interest in multiple-input-multiple-output (MIMO) systems, also known as multiple-element antenna (MEA) systems, as they have been shown to have the potential for improved capacity, spectral efficiency and reliability as compared to single-antenna communication systems. MIMO technology is a breakthrough in the field of modern wireless communications, and is poised to play a significant role in the implementation of next generation&#39;s wireless products and networks. 
     MIMO systems consist of multiple antennas on both the transmitter and receiver ends. It is regarded as an extension to Smart Antenna systems, which consist of multiple antennas controlled by algorithms optimising their spectral and spatial efficiency. 
     The antenna is an extremely important component in any wireless appliance because it transmits and receives radio waves. An antenna operates as a matching device from a transmission line to free space and vice versa. An ideal antenna radiates the entire power incident from the transmission line feeding the antenna from one or more predetermined direction. Performance of the antenna dictates performance of most wireless devices and hence is a critical part of the system. 
     In current mobile and handheld devices, miniaturised antennas are formed by providing conductive patches and or traces on one of both sides of an insulating (or electric) substrate. The substrate may be part of a circuit board carrying other components, or part of the housing or chassis. This type of antenna may be regarded as a patch antenna, optionally with microstrip antenna features. 
     A number of antennas and RF circuits may be necessary to cover all the bands from 700 MHz up to 5 GHz, appropriate for applications ranging from GSM, CDMA, 3G, WiFi, Bluetooth and GPS. This not only increases the real estate required but also increases the chance of free space coupling and interference between antennas, thereby reducing the efficiency of the antenna. 
     Spiral antennas are well known in the art as means to provide coverage over a broad range of frequencies. The most popular configurations in the past have been Archimedean and Log spiral antennas. These have rarely made it to mobile devices in a form that has been easy to integrate either onto circuits or the chassis. 
     A miniaturised spiral antenna element is described in U.S. Pat. No. 6,791,497 (WO 02/29928A2), wherein the spiral antenna exists on one side of a substrate, whilst the second surface contains a planar balun and feed point which feeds the antenna element through a slot. This is said to cover a range of 800 MHz to 3 GHz. Slow wave structures are introduced as part of the spiral wind, which are said to make the device geometrically smaller for the same wavelength. 
     On the other hand, U.S. Pat. No. 6,295,029 B1 describes a microstrip antenna element which is formed as a rectangular spiral. This enables miniaturisation and maintains the broadband characteristics. At the end of the antenna an open stub is formed to assist in antenna matching. A ground plane for this antenna element is on the same side as the antenna, and a hole is punched in the ground to provide frequency shifting and a smaller size antenna element. However there is no discussion of the gain of the antenna, nor the impact of any neighbouring devices or other antenna elements that may affect the efficiency of the antenna element concerned. 
     WO 03/094293A1 discloses a miniaturised resonant slot antenna element which is fed from the back side of the substrate. Such a design is said to enhance impedance matching and reduce the physical size of the antenna, but may not improve the bandwidth of the antenna. US20060038724 describes a modified version of the same antenna, again fed from the back side of the antenna, with additional sub-slots, running clockwise and counter-clockwise, to improve the bandwidth of the antenna. The two multiple spiral slot antenna elements seen above mention the reduction of antenna size and slight improvement in the bandwidth. However it is believed that, when applied in a MIMO configuration, these implementations will not be efficient enough to provide a higher bandwidth as expected in modern portable communication systems. 
     U.S. Pat. No. 5,892,482 describes a circuit meant to reduce the mutual coupling between two patch antennas in an antenna for a cellular radio base station. A capacitor-inductor-capacitor network is used to couple neighbouring patch elements together, so as to inject into each one a signal in antiphase to that in its neighbour. 
     MIMO is expected to be a key technology to deliver the next generations of mobile devices, wherein a large number of protocols will be expected to run in parallel. This is envisaged to enable higher throughput of the devices within the limited channel bandwidth. The primary criteria of the antenna in these devices will be to reduce the co-channel interference and free-space coupling, also known as mutual coupling. There is another parameter known as envelope correlation, or cross correlation, which couples the radiation pattern of two such antennas when placed in proximity. The cross correlation can be reduced by decoupling the polarisation and placement of the antennas and by reduction of the beam width of individual antennas so they do not couple in free space, again relating to mutual coupling. However, for miniature equipment such as a mobile phone, the freedom of placement is very limited. Current design guidance is to place antenna elements at least a half wavelength apart in order to avoid grating lobes in the radiation pattern. A well-known reference in this area is R. King (1958) “Linear arrays: Currents, impedances, and fields, I”. 
     A particular challenge is therefore to reduce the electromagnetic interaction in the near field with neighbouring antenna elements or devices which may be amplifiers, filters, GPS receivers, camera modules, casings, battery modules, memory, sensors, and other electronic components. 
     WO2011102143(A1) proposes to use additional circuitry to try and adjust the mutual coupling between two antennas. The present inventors prefer to mitigate the need for such external circuitry by virtue of the antenna design itself. 
     SUMMARY 
     The invention provides antenna elements and devices as defined in the appended claims. 
     The spiral slot, reduces the Q of the antenna, thereby increasing its bandwidth and reducing the antenna&#39;s susceptibility to interference and mutual coupling, whilst still maintaining an omnidirectional pattern. Further adjustment of frequency response can be made by secondary slots in the active patch, along with the location of the ground lines and notches. 
     The parasitic ground enables an enhanced antenna radiation pattern enabling diversity and a reduced mutual coupling in the near field, thereby allowing compact antenna arrays. The antenna in one embodiment consists of a linear polarised patch antenna, with a spiral slot and balanced ground feed. 
     Further use of a parasitic ground patch with a slot or aperture increases directivity, thereby allowing size reduction of multi-element antenna devices, and enhances the gain of the antenna. In embodiments illustrated below, the ground patch is not connected conductively to the signal ground, only passively (parasitically) connected. 
     Compact antenna elements according to the invention can be designed for mobile devices to cover a wide frequency range, for example from 400 MHz up to 8.4 GHz, or even down to 100 MHz and/or up to 60 or 80 GHz. 
     Where the description and claims refer to “central” and “peripheral” portions of a patch, it is to be understood that no strict definition of “central” applies. The central portion is merely a portion of the patch that is generally surrounded by an outer or peripheral portion. Similarly, where the description and claims refer to a conductor as a “strip”, “line” or “patch”, no strict definition of these terms is intended, other than what the context requires. 
     The present invention will be apparent in its novelty and other characteristics after reading the detailed description and preferred embodiment in reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described, by way of example only, by reference to the accompanying drawings, in which: 
         FIG. 1  shows (a) a top perspective view and (b) a transverse cross section of an antenna element according to a first embodiment of the present invention; 
         FIG. 2  shows (a) a rear perspective view of the same antenna element, and (b) the same view with the substrate removed; 
         FIG. 3  is a schematic view of an antenna device comprising an array of novel antenna elements of the type shown in  FIGS. 1 &amp; 2 ; 
         FIG. 4  is a schematic view of another antenna device comprising an array of novel antenna elements of the type shown in  FIGS. 1 &amp; 2 ; 
         FIG. 5  illustrates performance requirements of an example antenna device comprising two antenna elements of the type shown in  FIGS. 1 &amp; 2 ; 
         FIGS. 6, 7, 8 and 9  illustrate modifications of an active patch of an antenna element, for use in alternative embodiments of the present invention; 
         FIGS. 10, 11, 12 and 13  illustrate modifications of a ground patch of an antenna element, for use in alternative embodiments of the present invention; 
         FIG. 14  illustrates a coordinate system for the discussion of radiation patterns; 
         FIGS. 15 to 20  present (a) return loss against frequency and (b) radiation patterns at and example frequency 2.45 GHz, for a variety of antenna designs selected from the above examples; 
         FIGS. 21 and 22  present radiation patterns showing the directivity introduce by a slot in the parasitic ground patch; 
         FIGS. 23 and 24  present radiation patterns for two example multi-element antenna devices at three different frequencies; 
         FIGS. 25 and 26  present simulated return loss RET and mutual coupling CPL for a pair of antenna elements at spacing 4 cm and 2 cm respectively; and 
         FIGS. 27 and 28  present measured mutual coupling between pairs of elements in four-element antenna devices made according to embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIG. 1  shows (a) a top perspective view and (b) a transverse cross section of an antenna element  100  according to a first representative example.  FIG. 2  shows (a) a rear perspective view of the antenna element  100 , and (b) the same view with the substrate removed. The antenna is made by forming conductive patches or traces on front and rear surfaces of an insulating substrate  102 . Conductors on the front surface generally in a layer  104 , form an ‘active patch’ of the antenna. Conductors  106  on a rear surface form a modified passive or parasitic ground patch. The substrate may be a circuit board of materials such as glass epoxy like FR4, or it may be a more particular material such as ceramic, alumina or quartz, silicon etc. In practice, the substrate can extend beyond the dimensions of the antenna, as will be seen in later examples. The conductors on both sides may be metal, for example copper, aluminium, gold, or any other suitable conductive layer. 
     On the front surface shown in  FIG. 1( a )  an active patch of the antenna comprises various conductive portions  110   a  to  110   h  in a generally square area. These elements are formed by modifying the conductive layer  104  with one or more spiral slots  112 , which is formed (i) so as to form a square spiral of different-length segments  110   e ,  110   f  etc, and (ii) to leave relatively broad peripheral patch portions  110   a  to  110   d , surrounding the square spiral area. To one side of the active patch, a co-planar feed conductor  114  is formed, which is flanked by signal ground strips formed by patches  116 ,  118 . Small extensions  110   g ,  110   h  from the square patch area extend towards the signal ground patches  116 ,  118  respectively, leaving gaps  120 ,  122  in between. These extensions enable a lower Q and lower mutual coupling in both the directional and omni-directional variants of the antenna. On the rear side of the substrate, as seen better in  FIGS. 2( a ) and ( b ) , the second conductive layer  106  is formed into a ground patch having peripheral portions including  130   b  and  130   d , but being interrupted by an aperture or slot  132 . In  FIG. 2( b ) , it can be seen that slot  132  is formed at a location generally behind the spiral traces  110   e ,  110   f  etc of the active patch antenna on the front surface. 
     More detail and variations of the ground patch design will be discussed further below. While a generally square patch antenna with square spiral slot is used as an example, this square form is only an example and any rectangular, or more generally polygonal, geometry can be considered as well. Smoother curved shapes are also possible, but rectangular/polygonal designs tend to be easier to design and manufacture. While a spiral slot is used in the examples, the principles of the invention can be extended to other forms of convoluted slot, such as serpentine or linear or segmented linear. 
     The active patch of the antenna element has been modified with spiral slot  112  so as to give the antenna element wideband performance. Further, optional modifications to be described later provide not just for the broadband performance but also for an increased gain, a highly directive radiation pattern and reduced mutual coupling between multiple antenna elements. These different embodiments of the antenna element include for example additional slots to give additional frequency response. In the square patch the slot induces the current to cross larger electrical lengths thereby covering larger frequency range. The directions of the currents induced into the device and the aperture are selected in a manner that the electromagnetic waves radiated into space will introduce less mutual coupling amongst other neighbouring device. The thicker external patch formed by portions  110   a  to  110   d  protects the inner currents from external sources. The passive (parasitic) ground patch  130  also plays a key role. The ground patch with aperture  132  is strategically placed under the active path, in order to balance the fringing fields and enhance the radiation pattern of the antenna element. The antenna is also balanced fed, using signal ground strips (patches  116 ,  118 ), making it less susceptible to proximity effects, and ideal for embedded applications such as handheld mobile devices. 
     By adjusting the design of the slots in the antenna elements and associated parasitic elements, individual parameters can be varied to provide enhanced performance, suitable for compact antenna arrays. In particular, the directional property of the individual antenna element allows the overall antenna array to have a higher gain, lesser mutual coupling, lesser envelop correlation and large bandwidth from 400 MHz to 8.4 GHz. 
     By applying the principles described above one can provide a UHF antenna, which is less than 1/16 th  the size of its traditional counterparts working at the same frequency from 300 MHz up to 3 GHz in UHF (Ultra High Frequency) bands, and 3 GHz up to 30 GHz in SHF (Super High Frequency) bands. This allows for a compact integrated element and element arrays suitable for modern portable communication systems. All this is achieved solely on the basis of patch design and not by having any additional circuitry attached to the antenna. Examples of the novel antenna have been successfully tested in the range 0.3 GHz to over 8 GHz, but the principles of its design can be applied in other frequency ranges. In general terms, we would anticipate covering roughly a decade of frequencies with a single design, for example, 0.3 to 3, 3 to 30 or 8 to 80 GHz. We have examples that cover significantly more than a decade, however. Other forms of convoluted slot are possible. In alternative embodiments, where wide bandwidth is not required, a simpler slot shape could be used, while maintaining the other features and benefits of the embodiments to be described below. 
     In multi-element applications, the available bandwidth may depend also on an acceptable mutual coupling between elements. Where a lower degree of mutual coupling is desired, then for a given geometry less of the spectrum may lie under that dB threshold. 
     As explained before, MIMO requires that the elements should have a low mutual coupling less than for example −10 db, −13 dB or −15 dB or −20 dbB. The invention by virtue of its design and control of the radiation pattern enables an ultra wideband characteristics ideal for a smart antenna system. Examples illustrated below provide a mutual coupling of −16 dB to −67 dB, over the entire spectral range of 400 MHz up-to 8.4 GHz. This makes it very interesting for the next generation of mobile MIMO devices. The concept of having the least electromagnetic interaction in the near field and maximum interaction in the far field allow beam forming if desired. 
     As is known, a single sided patch antenna lacks in bandwidth and has an omni-directional radiation pattern. The addition of a parasitic ground patch or reflector on the back side of the substrate increases the efficiency of radiation (reduces return loss). In some embodiments of the invention the parasitic ground patch  130  is modified to create a slot  132 . This allows the designer to blend higher gain and efficiency with a highly directive pattern. A higher directivity means the antenna radiates or focuses its energy in a particular direction compared to the average of all other directions. More focused means less energy wasted in other directions and saves radiated power and reduces noise. In the case of cellular communications, for example, it also allows a particular cell to be chosen in preference to others, and for multiple reflections to be eliminated, ensuring that only one desired signal is selected from a range of interferers. 
     As a key application for the antenna elements are to be designed is for a modern system which enables MIMO, the inventors have studied the effects of the antenna elements combined into an array form. The active patch without the ground patch exhibits significant coupling between two adjacent antenna elements. However with the modified ground patch and feed structure, the coupling was significantly reduced. Over the entire range from 300 KHz to 3 GHz in one example, the two elements displayed a coupling between −15 dB to −67 dB. 
     For a typical MIMO application the free space coupling should be low, for example below −10 dB, or even lower and experiments with the novel element have proved that this performance is achievable, even with elements in fairly close proximity. 
     Consequently, having a preferred balance between a modified slot in the square patch and the ground, leads to a higher directivity, higher gain, and more focused radiation pattern and reduces mutual coupling between a plurality of elements. 
       FIG. 3  shows schematically an antenna device  200  comprising an array of novel antenna elements  202 - 1  to  202 - 4 , each of the form described above. These elements may be employed as a “smart antenna” device, or in MIMO applications. The antenna elements, four in this example, are formed on a common substrate  208 . As described above, the novel elements have enhanced ultra-wideband (UWB) characteristics of the antenna, including reduced free space coupling. In the case of a smart antenna, the overall gain and directivity of the antenna can be controlled by controlling the amplitude, phase and electrical characteristics of a signal as it is fed to each element. The smart antenna, in a transmitting mode, receives a single RF input signal via the feed arrangement (not shown), and emits a beam  212  with a desired direction and directivity. In a receive mode, which may be operating simultaneously, the directional beam  212  represents a direction from which radiation is preferentially received, and a received RF signal is output via the same or a different arrangement. This creates a novel smart antenna in which spectral and spatial diversity, for example, can be tailored to an application. With active control of the phases between elements, the beam can be digitally controlled using a software algorithm. 
       FIG. 4  illustrates a multiple input, multiple output (MIMO) antenna array  220  comprising antenna elements  222  of the novel form described above, formed on a common substrate  228 . Instead of a single RF input, each element has an associated RF device (switch, filter, matching circuit, etc.) with an input connector  230 , so that multiple RF signals can be can be emitted or received and manipulated in parallel via multiple beams  232 . The MIMO array is exploited to provide parallel data channels than can deliver enhanced data rates. The concepts of MIMO and smart antenna arrays can be combined, if desired. 
     There are three main ways of using MIMO systems: spatial multiplexing, beam forming and spatial diversity. Spatial multiplexing is a concept of particular interest, and is the main way MIMO systems differ from smart antenna systems. Beam forming is a concept common to both conventional smart antenna and MIMO systems. It refers to the concentrating of energy towards certain directions in such a way as to increase the signal-to-noise ratio (SNR) or signal-to-interference-and-noise ratio (SINR). For example, the receive array can form an array radiation pattern where the main beams are directed towards the transmitters to obtain a higher received signal strength, while nulls are introduced in the directions of interfering signals. This is done by weighting (either in amplitude and/or phase) the signals received by different antenna elements appropriately to maximise the contribution of the desired signal and concurrently minimise that of the interfering signals. Another key concept of smart antenna is spatial diversity, where the use of multiple antenna elements improves the reception/detection of signals. The presence of multiple antenna elements can counteract the effect of random fading due to multipath propagation. As a result, the probability of signal loss decreases exponentially with the number of uncorrelated antenna elements. MIMO systems harness the spatial diversity benefits of array antennas as well. However, MIMO systems have an added advantage over smart antennas in that they can enjoy both transmit and receive diversity gain. 
     In spatial multiplexing, the transmit data is split or multiplexed into several parallel streams and each stream is transmitted simultaneously across the channel by different transmit antenna elements (or channel Eigen modes, if the channel is known at the transmitter). The signals received on the multiple receiver antennas are thus a combination of the transmit data streams. 
       FIG. 5( a )  illustrates some parameters of dimensions of the individual antenna elements, and space in between them in a multi-element antenna device. As mentioned, the key characteristic of the novel antenna elements is their degree of mutual coupling with neighbouring devices, and two antenna elements  100 - 1  and  100 - 2  illustrated for this discussion. The degree of mutual coupling between those devices, and/or between these devices and neighbouring circuitry, is key to their utility in compact antenna arrays, whether for “smart antenna” applications (as in  FIG. 3 ) or MIMO applications ( FIG. 4 ). The elements are shown formed on a common substrate  234 . As seen in  FIG. 5( b ) , their ground patches  130 - 1  and  130 - 2  on the back side of the substrate may be continuous with one another, rather than discrete. Each ground patch is identifiable by its slot  132 - 1 ,  132 - 2 , positioned behind the spiral slot portion of the corresponding active patch. 
     “Mutual coupling” refers to the electromagnetic interactions between the elements of an antenna array, or between independent antennas in close proximity to each other. Some of the energy transmitted by a transmit antenna element is transferred to the other elements. Correspondingly, a portion of the energy in the incident field of a receive antenna element is transferred to the nearby elements. As a result, the feed current on each transmit antenna element in an antenna array does not solely consist of the current as when they are transmitting in isolation, but also of the current induced by the other antenna elements in proximity. The same argument follows for the induced current on the receive elements of the array. 
     Another way of describing the effect of mutual coupling is that the electric field generated by one element alters the current distribution, as well as distorts the radiation/reception pattern of the other elements as compared to their isolated radiation/reception patterns. The amount of mutual coupling depends on the separation between antenna elements. It generally increases the closer the antenna elements are to each other, but the relationship will be complex and frequency-dependent The extent of mutual coupling also depends on the element radiation patterns and the array geometry, for example the relative orientation and location of the antenna elements. The direction of arrival (DOA) of the incident field also affects mutual coupling. Due to this, mutual coupling plays an important factor in the design of smart antennas and multi-element antennas in general. 
     Referring again to  FIG. 5( a ) , the active patch on the front side of substrate  234  for each antenna element  100 - 1 ,  100 - 2  has a spacing D from the edge of the active patch of the neighbouring element. The active patch has an outer dimension W 1  in the direction of distance D, while the spiral slot portion within the active patch has an outer dimension W 2 . A pitch dimension P is defined between the centres of the active patches. In the central portion of the active patch, dimension WS is defined as the width of the spiral slot  112 , while WT is the width of a trace  110   e ,  110   f  etc formed by the spiral slot. In a rectangular, rather than square, embodiment, each of these widths may be defined separately in different directions. Each element  100 - 1 ,  100 - 2  receives its own radiation beam  236 - 1 ,  236 - 2 . A near field mutual coupling between the elements is designated  238 . Where the elements are positioned end-to-end (as is the case for elements  202 - 1  and  202 - 2  for example in  FIG. 3 ), the distance D can include the feed arrangement. 
     In embodiments of the invention, it has been mentioned that the elements can be placed closer together than known antenna designs, without undue mutual coupling. For example, the outer dimension W 1  of the active patch may be between 10 mm and 20 mm, for example between 14 and 18 mm. The inner dimension W 2  may be between one third and three quarters of the outer dimension. The spacing D may be in the range 2-50 mm, for example being less than 50 mm, 40 mm, 30 mm, 25 mm, 20 mm, 15 mm or 10 mm. The spacing D may be less than 2 times the outer dimension W 1 , or less than 1.5 or 1 times. The spacing may be selected also to provide a mutual coupling across a range of operating frequencies that is less than −10 dB, less than −14 dB or less than −18 dB, for example. The spacing D and/or the pitch P may be less than 0.5 times the wavelength λ at a highest operating frequency for which the antenna is designed (D&lt;=λ/2). At lower operating frequencies, the wavelengths will be longer and so the gaps between element will be much shorter, in wavelength terms. When embodiments of the invention are properly designed, satisfactory mutual coupling performance can be obtained when the spacing D is as low as λ/100, λ/300 or even λ/400. It may be envisaged that an antenna element may operate at several frequency bands in a given product application. The mutual coupling is only an important performance criterion in those bands where smart antenna and/or MIMO functions are being implemented. 
       FIGS. 6 to 8  illustrate variations of the active patch design, forming alternative embodiments of antenna element  340 ,  350 ,  360  and  370 , based on the general form of element  100  shown in  FIGS. 1 and 2 . Throughout  FIGS. 6 to 9 , the same reference signs  110   a  etc are used for the same parts as in the example of  FIG. 1 . The following discussion will highlight only the additional features specific to these modified embodiments. 
     In  FIG. 6 , antenna element  340  is the same as element  100 , except for an additional or secondary slot  342  which extends the spiral slot  112  so as to disconnect outer patch portions  110   b  and  110   c . From experimental results it is noticed that the frequency of operation is varied slightly,  FIG. 6 . The patch edge is also varied by a small extension  110   i , which is an additional piece of conductive material near the ground strip gap  122  at one side of the active patch only. The extension which improves coverage at higher frequencies and also improves (reduces) mutual coupling, by the effect of capacitance with the ground patch. The need for separate capacitive circuitry such as in U.S. Pat. No. 5,892,482 is reduced. Additional extensions could be provided at either side or both sides. 
       FIG. 7  shows modified element  350 , which is the same as element  100  except for an added or secondary slot  352  which bisects peripheral patch portion  110   a  to form a separate portion  110   j , to one side of the feed strip  114 . The outer edge of peripheral portion  110   c  is modified by a slot  354 . This modification alters the frequency response of the antenna, for example by adding additional resonances. It can therefore be employed to support and additional frequency band or bands. 
       FIG. 8  shows a further modified example element  360 . This is the same as element  350  of  FIG. 7 , except that the secondary slot  362  is in a slightly different position, extending from one corner of the spiral slot  112 . 
     In each of these variations, we see the active patch of the antenna element being modified with further slots. This helps to extend the UWB characteristics of the antenna element. The position of the slot will define new boundaries and lengths for the electrical path and hence require current to travel further. This forms effectively inner and outer loops or segments of the antenna element, each working as an independent spiral antenna with its own resonant behaviour. Adjusting the position of secondary slots  342  or  352  can tune this resonant behaviour by altering the absolute and relative length of the segments. It also helps to overcome any interference from neighbouring radiating antenna elements thereby reducing the mutual coupling and interference posed by other electrical devices. 
       FIG. 9  shows an element  370  which is the same as element  100  except for the provision of slot  374 , similar to slot  354 , at the peripheral portion  110   c  of the active patch, opposite the feed strip  114 . 
       FIGS. 10 to 13  show the back side of modified antenna elements  400 ,  410 ,  420  and  430 . Different shapes of slot  132  are provided in the modified ground patch  130  on the opposite side of the substrate from the active patch spiral slot. Each shape of ground patch slot adds a distinctive property to the overall antenna element. The alternative active patch designs of  FIGS. 1, 6 to 9  can be combined freely with the modified ground patch designs FIGS.  10  to  13 , to permit a wide choice of characteristics. This enables individual antenna elements and an antenna array fabricated by combing multiple antenna elements to function better in certain applications. 
     Numerous permutations and variations of the features described above can be deployed to customise an antenna element for its particular application. It may be assumed from the above description that elements in an array will be identical, but this is not necessarily the case. The spiral slots, secondary slots, extensions, notches etc. of the active patch and/or the ground patch may be varied according to the position of each element in an array, for example, or to provide different operating bands in a switchable array. Secondary slots can be deployed in the peripheral portion of the ground patch, not only the active patch. The spiral slot in the active patch may be swapped for a different design of convoluted slot. 
       FIG. 14  illustrates a coordinate system for the presentation of directional radiation patterns.  FIG. 15  onward show spectral performance and directionality characteristics for various antenna elements and arrays made according to the principles described above. IN  FIG. 14  the Z axis is defined as normal to the active patch with fields FZ (forward) and FZ′ (rear). The X axis extends in the plane of the patch and transversely with respect to the feed direction and with fields FX and FX′. The Y direction extends in the plane of the patch but parallel with the feed direction, with fields FY and FY′ as illustrated. As in the case of a simple dipole antenna, this direction tends to be a null in the radiation pattern, so the fields FY and FY′ are shown in broken lines. 
       FIG. 15( a )  shows return loss against frequency for an element having the spiral slot and a plain ground patch  130  (without slot  132 ). The addition of the spiral slot reduces the Q of the antenna. This in turn increases the bandwidth of the antenna and allows for multiple resonant bands. As the skilled reader knows, the lower the return loss (higher negative dB value), the better the antenna is at radiating a signal at the frequency in question. A return loss below −7 or −9 dB indicates a usable antenna. Sub-bands B 1  and B 2  are indicated in  FIG. 15( a ) , where the return loss is below −10 dB, and consequently transmission is certainly usable. The extremes of these bands cover frequencies of 1.4 GHz and 8.4 GHz, a factor of six apart. 
     We can describe an antenna element as ultra wideband (UWB), even if its return loss is unacceptable at some points across the spectrum. A multi-purpose antenna can be designed to carry different signal types which might have widely different frequencies, but parts of the spectrum in between are unused. For the purpose of this description, we can define an UWB antenna or antenna element for example as one having usable spectral regions that are more than 5 GHz apart, or more than two octaves apart. We can define usable regions as those having a return loss better than −7 dB, −9 dB or −10 dB, for example. Examples according to the embodiments described above have two or more regions at least 1 GHz in width, possibly 2 GHz in width. The sub-band B 1  with two resonant peaks in it covers a range of more than an octave. 
     The parasitic ground patch without a slot results in an omnidirectional radiation pattern as shown in  FIG. 15( b ) . This toroidal radiation pattern is similar to that of a classic dipole antenna, with nulls along the Y axis. 
       FIG. 16  illustrates (a) the return loss and (b) the radiation pattern of a similar antenna element, but with the notch  110   i  such as that illustrated in  FIG. 6 . Introducing the notch  110   i  produces a slight shift in the frequency of spectrum features. 
       FIG. 17  shows the performance of a third variation of the antenna where the edge of the patch is being cut further in, as seen at  354  in  FIG. 9 . Comparing the previous graphs, this variant shows a shift in the operational frequency to a higher bandwidth. 
     The next few variations will show the effect of additional secondary slots in the active patch of the antenna, of the type shown in  FIGS. 6, 7 and 8  above. 
       FIG. 18  shows return loss and radiation pattern for the example of  FIG. 8  above, when the peripheral conductor portion  110   a  is cut diagonally it separates the square patch into two spiral antennas. What is noticed is that the ultra wideband antenna which originally produced a 3 GHz bandwidth now breaks down into three sub-bands B 1 -B 3  of roughly 1 GHz bandwidth. However it now supports a higher order of frequencies in a sub-band B 4 , which go beyond 12 GHz. The original square patch being dissected close to the feed point as shown in  FIG. 8  no longer supports a balanced feed and there is a mismatch at the feed. This causes the gain to reduce thereby reducing the antenna efficiency. However there are techniques to improve upon this. 
       FIG. 19  shows the effect of variation of the secondary slot to the position shown in  FIG. 7 . What is noticed that performance in the higher frequencies (B 2 , B 3 ) is improved compared with  FIG. 18 . 
       FIG. 20  shows the effect of moving the position of the secondary slot to that shown at  342  in  FIG. 6 . Near the feed  114 , the peripheral portion  110   a  of the active patch is very much complete, yielding a full balanced feed. The bandwidth is in each sub-band is still around 1 GHz, but we have (a) improved performance especially in sub-band B 1  and (b) a higher directivity and gain from this variation. 
       FIGS. 21 and 22  show the change in the radiation pattern (directivity) achievable by introducing the rear slot  132  in the ground patch. The active patch in this example has the same form as the one simulated in  FIG. 15 . It is found that return loss remains substantially the same, but comparing  FIG. 21 or 22  with  FIG. 15( b )  one can see that the directivity increases as a greater part of the radiation is reflected back into the forward normal (Z) direction. By optimising the shape of the slot in a particular geometry, as illustrated in  FIGS. 2 and 10 to 13 , a desired directional radiation pattern is achieved. In  FIG. 21 , a large, square slot  132  is provided, which is substantially larger than the spiral slot area on the front side of the substrate and leaves only a narrow peripheral portion of ground conductor under the active patch. In  FIG. 22 , a more optimised design yields even greater directivity. The slot  132  is again square, but very similar in size to the spiral slot area, for example a 5 mm square. 
     This improved directivity, which may be for example greater than 3 db or greater than 4 dB at a given operating frequency, allows for diversity. It also enables the antennas to be placed closer together so as to create compact antenna arrays. The antennas by virtue of their lower Q are also less susceptible to interference and mutual coupling. As the slot dimension affects the gain and the Q of the antenna, a balance between a higher directivity and a reasonable gain can be achieved through iterative design of the antenna geometry. 
       FIG. 23  shows the radiation pattern for an array of two antenna elements, as might be used for MIMO applications. The antenna elements are arranged end-to-end as in the case of  202 - 1  and  202 - 1  in  FIG. 3 , which gives a higher directivity. The radiation pattern is different at different frequencies, and frequencies measured here are (a) 700 MHz, (b) 2.45 GHz and (c) 5.0 GHz. The beam width narrows (directivity increases) as the frequency goes up. This is due to the inter-element distance being greater in terms of the wavelength, which is shorter at higher frequencies. 
       FIG. 24  shows the radiation pattern of a 2×2 array of antenna elements such as that shown in  FIG. 3 . Again the frequencies shown are (a) 700 MHz, (b) 2.45 GHz and (c) 5.0 GHz. The beam width narrows (directivity increases) as the frequency goes up, as before. This is due to the inter-element distance being greater in terms of the wavelength, which is shorter at higher frequencies. 
       FIGS. 25 &amp; 26  show simulated spectra of return loss RET and mutual coupling CPL between two of the novel antenna elements when in a compact array, and the effect of the gap (D in  FIG. 5 ) between elements. In  FIG. 24 , D=40 mm and the mutual coupling is less than −50 dB even at its highest point on the spectrum. In  FIG. 26  the gap is reduced to 20 mm the coupling is less than −27 dB. Clearly the reduced distance and frequency have increased the coupling between the elements, but this level is still reduced from the what one would normally achieve by the current state-of art antenna. 
     The spacing D can be expressed in terms of the free space wavelength λ at a given operating frequency. The spacing D can be as little as λ/400 at the lowest designed operating frequency of the antenna array, and as high as λ/2 at the highest operating frequency. As mentioned already, conventional wisdom would be to separate the elements by at least λ/2 at any operating frequency, so as to avoid undue mutual coupling. Acceptable levels of mutual coupling for a MIMO application or other application might be for example −10 dB, −12 dB, −15 dB or even −20 dB in practice. Where an array has more than two elements, the coupling will exist between every pair of elements, but one would expect that it will generally be strongest between the immediate neighbour elements. This is confirmed in simulation and experiment. 
       FIGS. 27 and 28  shows the mutual coupling actually measured between pairs of elements in a four-element array such as the one shown in  FIG. 4  using the novel element design. In  FIG. 27 , the spacing D between elements was just 4 mm, with a pitch P around 20 mm. In  FIG. 28  the spacing D was increased to 20 mm, with a pitch P around 35 mm. In both cases, the mutual coupling is lower than −40 dB across the entire spectrum from 0.70 GHz to 5 GHz, indicating very suitable characteristics for use in MIMO and/or smart antenna arrays and UWB applications. 
     Polarisation is another characteristic of the electromagnetic wave generated from the antenna which then travels in air. Most spiral antennas generate circular polarisation. In our designs as covered by the present application it is possible for a suitable choice of geometric configuration to be chosen to yield the particular polarisation required by a particular communication protocol. For example, linear polarisation, particularly a vertical polarisation is the choice in cellular mobile communication systems, where the plane is normal to the substrate. 
     Having a vast number of variations is ideal as one can customise the characteristics of the antenna depending on the performance sought. Hence for an omnidirectional element a full ground patch/reflector is utilised, whilst for a highly directional element a slotted ground is utilised.