WO2004017462A1 - Improvements relating to antenna isolation and diversity in relation to dielectric antennas - Google Patents

Improvements relating to antenna isolation and diversity in relation to dielectric antennas Download PDF

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Publication number
WO2004017462A1
WO2004017462A1 PCT/GB2003/003588 GB0303588W WO2004017462A1 WO 2004017462 A1 WO2004017462 A1 WO 2004017462A1 GB 0303588 W GB0303588 W GB 0303588W WO 2004017462 A1 WO2004017462 A1 WO 2004017462A1
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dielectric
antenna
antennas
substrate
surface
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PCT/GB2003/003588
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French (fr)
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Simon Philip Kingsley
Scott William Spencer Tyler
Rebecca Thomas
Steven Martin
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Antenova Limited
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    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01QANTENNAS, i.e. RADIO AERIALS
    • H01Q21/00Antenna arrays or systems
    • H01Q21/24Combinations of antenna units polarised in different directions for transmitting or receiving circularly and elliptically polarised waves or waves linearly polarised in any direction
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01QANTENNAS, i.e. RADIO AERIALS
    • H01Q1/00Details of, or arrangements associated with, antennas
    • H01Q1/52Means for reducing coupling between antennas; Means for reducing coupling between an antenna and another structure
    • H01Q1/521Means for reducing coupling between antennas; Means for reducing coupling between an antenna and another structure reducing the coupling between adjacent antennas
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01QANTENNAS, i.e. RADIO AERIALS
    • H01Q9/00Electrically-short antennas having dimensions not more than twice the operating wavelength and consisting of conductive active radiating elements
    • H01Q9/04Resonant antennas
    • H01Q9/0485Dielectric resonator antennas

Abstract

There are disclosed various techniques for improving isolation between dielectric antennas mounted on a small substrate, for example a PCB in a mobile communications device or WLAN access point or the like. The techniques include arranging the antennas in various orthogonal configurations so that nulls in their radiation patterns coincide and optionally making use of polarisation diversity. Further techniques include the provision of conductive barriers between the antennas and feeding one antenna with part of a signal from another antenna in anti-phase. The present invention allows a number of antennas to be provided in a small area so as to provide good operational diversity.

Description

IMPROVEMENTS RELATING TO ANTENNA ISOLATION AND DIVERSITY IN RELATION TO DIELECTRIC ANTENNAS

The present invention relates to techniques for the provision of antenna isolation and diversity in small spaces, with particular reference to the use of dielectric antennas and especially dielectric resonator antennas (DRAs).

Suitable dielectric antennas include, but are not limited to, dielectric resonator antennas (DRAs), high dielectric antennas (HDAs), dielectrically loaded antennas (DLAs) and dielectrically excited antennas (DEAs). However, DRAs are particularly preferred, since they exhibit good linear polarisation.

Dielectric antennas are antenna devices that radiate or receive radio waves at a chosen frequency of transmission and reception, as used for example in mobile telecommunications. In general, a DRA consists of a volume of a dielectric material (the dielectric resonator) disposed on or close to a grounded substrate, with energy being transferred to and from the dielectric material by way of monopole probes inserted into the dielectric material or by way of monopole aperture feeds provided in the grounded substrate (an aperture feed is a discontinuity, generally rectangular in shape, although oval, oblong, trapezoidal 'H' shape, '<->' shape, or butterfly/bow tie " shapes and combinations of these shapes may also be appropriate, provided in the grounded substrate where this is covered by the dielectric material. The aperture feed may be excited by a strip feed in the form of a microstrip transmission line, grounded or ungrounded coplanar transmission line, triplate, slotline or the like which is located on a side of the grounded substrate remote from the dielectric material). Direct connection to and excitation by a microstrip transmission line is also possible. Alternatively, dipole probes may be inserted into the dielectric material, in which case a grounded substrate may not be required. By providing multiple feeds and exciting these sequentially or in various combinations, a continuously or incrementally steerable beam or beams may be formed, as discussed for example in the present applicant's co-pending US patent application serial number US 09/431,548 and the publication by KL GSLEY, S.P. and O'KEEFE, S.G., "Beam steering and monopulse processing of probe-fed dielectric resonator antennas", JJEE Proceedings - Radar Sonar and Navigation, 146, 3, 121 - 125, 1999, the full contents of which are hereby incorporated into the present application by reference.

The resonant characteristics of a dielectric antenna depend, ter alia, upon the shape and size of the volume of dielectric material, the shape, size and position of the feeds thereto and also on the shape, size and position of the groundplane. It is to be appreciated that in a dielectric antenna, it is the dielectric material that radiates when excited by the feed. This is to be contrasted with a dielectrically loaded antenna (DLA), in which a traditional conductive radiating element is encased in a dielectric material that modifies the resonance characteristics of the radiating element. As a further distinction, a DLA has either no, or only a small, displacement current flowing in the dielectric whereas a DRA or HDA has a non-trivial displacement current.

Dielectric antennas may take various forms, a common form having a cylindrical shape or half- or quarter-split cylindrical shape. The resonator medium can be made from several candidate materials including ceramic dielectrics.

Dielectric resonator antennas (DRAs) were first studied systematically in 1983 [LONG, S.A., McALLISTER, M.W., and SHEN, L.C.: "The Resonant Cylindrical Dielectric Cavity Antenna", IEEE Transactions on Antennas and Propagation, AP-31, 1983, pp 406-412]. Since then, interest has grown in their radiation patterns because of their high radiation efficiency, good match to most commonly used transmission lines and small physical size [MONGIA, R.K. and BHARTIA, P.: "Dielectric Resonator Antennas - A Review and General Design Relations for Resonant Frequency and Bandwidth", International Journal of Microwave and Millimetre- Wave Computer-Aided Engineering, 1994, 4, (3), pp 230-247]. A summary of some more recent developments can be found in PETOSA, A., ITTIPIBOON, A., ANTAR, Y.M.M., ROSCOE, D., and CUHACI, M.: "Recent advances in Dielectric-Resonator Antenna Technology", LEEE Antennas and Propagation Magazine, 1998, 40, (3), pp 35 - 48.

A variety of basic shapes have been found to act as good dielectric resonator structures when mounted on or close to a ground plane (grounded substrate) and excited by an appropriate method. Perhaps the best known of these geometries are:

Rectangle [McALLISTER, M.W., LONG, S.A. and CONWAY G.L.: "Rectangular Dielectric Resonator Antenna", Electronics Letters, 1983, 19, (6), pp 218-219].

Triangle [ITTIPIBOON, A., MONGIA, R.K., ANTAR, Y.M.M., BHARTIA, P. and CUHACI, M.: "Aperture Fed Rectangular and Triangular Dielectric Resonators for use as Magnetic Dipole Antennas", Electronics Letters, 1993, 29, (23), pp 2001- 2002].

Hemisphere [LEUNG, K.W.: "Simple results for conformal-strip excited hemispherical dielectric resonator antenna", Electronics Letters, 2000, 36, (11)].

Cylinder [LONG, S.A., McALLISTER, M.W., and SHEN, L.C.: "The Resonant Cylindrical Dielectric Cavity Antenna", IEEE Transactions on Antennas and Propagation, AP-31, 1983, pp 406-412].

Half-split cylinder (half a cylinder mounted vertically on a ground plane) [MONGIA, R.K., ITTIPIBOON, A., ANTAR, Y.M.M., BHARTIA, P. and CUHACI, M: "A Half-Split Cylindrical Dielectric Resonator Antenna Using Slot-Coupling", IEEE Microwave and guided Wave Letters, 1993, Vol. 3, No. 2, pp 38-39].

Some of these antenna designs have also been divided into sectors. For example, a cylindrical DRA can be halved [TAM, M.T.K. and MURCH, R.D.: "Half volume dielectric resonator antenna designs", Electronics Letters, 1997, 33, (23), pp 1914 - 1916]. However, dividing an antenna in half, or sectoring it further, does not change the basic geometry from cylindrical, rectangular, etc.

High dielectric antennas (HDAs) are similar to DRAs, but instead of having a full ground plane located under the dielectric resonator, HDAs have a smaller ground plane or no ground plane at all. DRAs generally have a deep, well-defined resonant frequency, whereas HDAs tend to have a less well-defined response, but operate over a wider range of frequencies. HDAs can take the same variety of preferred shapes as DRAs. However, any arbitrary dielectric shape can be made to radiate and this can be useful when trying to design the antenna to be conformal to its casing.

In both DRAs and HDAs, the primary radiator is the dielectric resonator. In DLAs the primary radiator is a conductive component (e.g. a copper wire or the like) and the dielectric modifies the medium in which the antenna operates, and generally makes the antenna smaller. Dielectrically excited antennas (DEAs) are similar to DLAs in that the primary radiator is a conductive component (such as a copper dipole or patch), but unlike DLAs they have no feed mechanism. DEAs are parasitic conducting antennas that are excited by a nearby dielectric or dielectric antenna having its own feed mechanism.

In telecommunications and radar applications it is often desirable to have two or more antennas that give a different or diverse 'view' of an incoming signal. Generally speaking, the different views of the signal can be combined to achieve some optimum or at least improved performance such as maximum or at least improved signal to noise ratio, minimum or at least reduced interference, maximum or at least improved carrier to interference ratio, and so forth. Signal diversity using several antennas can be achieved by separating the antennas (spatial diversity), by pointing the antennas in different directions (beam diversity) or by using different polarisations (polarisation diversity). A significant problem arises when diversity is required from a small space or volume such that the antennas have to be closely spaced. An example of this is when a PCMCIA card, inserted into a laptop computer, is used to connect to the external world by radio. Most high data rate radio links require diversity to obtain the necessary level of performance, but the space available on a PCMCIA card is generally of the order of about 1/3 of a wavelength. At such a close spacing, most antennas will couple closely together and will therefore tend to behave like a single antenna. In addition, there is little isolation between the antennas and, consequently, there is little diversity or difference in performance between the antennas. As a rule, about -20dB coupling (isolation) is the target specification between antennas operating on the same band for a PCMCIA card. For access points (in WLAN and the like applications), which are rather like micro-base stations, even greater isolation is required, about -40dB being desirable. Such high isolation is extremely hard to achieve with conventional antennas when the access points are the size of domestic smoke alarms and less than a wavelength across. Similarly with laptop computers, isolation between WLAN and Bluetooth® antennas of -40dB or more is seen as desirable.

hi order to address these problems, the present applicant has developed a series of techniques that, individually but preferably in various combinations, enable significant antenna diversity in a small space. The techniques make use of the properties of dielectric antennas, in particular but not exclusively DRAs.

Technique 1 : High stored energy

According to a first aspect of the present invention, there is provided an antenna arrangement comprising at least two dielectric antennas each formed as a dielectric resonator mounted on one surface of a common dielectric substrate and provided with a feeding mechanism for transferring energy into and out of the dielectric resonators, characterised in that the dielectric antennas are mounted generally orthogonal to each other on the dielectric substrate such that radiation patterns generated by the dielectric antennas are also generally orthogonal.

Advantageously, the at least two dielectric resonators are mounted 100mm or less apart from each other on the substrate, preferably 60mm or less apart, more preferably 40mm or less apart and most preferably 12mm or less apart. In some embodiments, the dielectric resonators may be 6mm or less apart, and may even touch each other, while still displaying low mutual electromagnetic coupling

The significance of the closeness is perhaps best demonstrated by comparison with dipoles. A pair of conventional half-wave dipoles carefully disposed co-linearly (end to end) will exhibit good isolation (better than -20dB) when they are a wavelength λ apart. This means there is a gap between them of λ/2, because each arm is λ/4 long. However, this arrangement gives no beam or polarisation diversity and only weak spatial diversity. Disposing dipoles orthogonally to each other and obtaining good isolation is extremely difficult.

In contrast to dipoles, when 2.4GHz dielectric resonators are mounted λ/3 apart (40mm), an isolation of-40dB can be obtained. When the resonators are λ/10 apart (12mm), it is still possible to achieve -15 to -20dB isolation. When the resonators are λ/20 apart (6mm), isolation of about -9 or -lOdB is achieved. This is very good compared to dipoles, and the whole arrangement is much smaller. When the resonators touch each other, isolation of about -5 or -6dB can be obtained.

Technique 2: Use beam and polarisation diversity

According to a second aspect of the present invention, there is provided an antenna arrangement comprising at least two dielectric antennas each formed as a dielectric resonator mounted on one surface of a common dielectric substrate and provided with a feeding mechanism for transferring energy into and out of the dielectric resonators, characterised in that the dielectric antennas are each configured to generate a radiation pattern with a generally figure-of-eight shape in a plane parallel to the dielectric substrate, and in that the dielectric antennas are mounted generally orthogonal to each other on the dielectric substrate such that radiation patterns generated by the dielectric antennas are also generally orthogonal.

The dielectric resonators may each have a half-split cylindrical configuration, or may be each have a quarter-split cylindrical configuration with a metallised layer on an exposed generally rectangular surface representing a mirror plane. Alternatively, one of the resonators may be a half-split cylinder and the other may be a quarter-split cylinder with a metallised layer on one surface as described above.

Technique 3: Create suitable nulls

According to a third aspect of the present invention, there is provided an antenna arrangement comprising at least first and second dielectric antennas each formed as a dielectric resonator mounted on one surface of a common dielectric substrate and provided with a feeding mechanism for transferring energy into and out of the dielectric resonators, characterised in that the first dielectric antenna is configured to generate a radiation pattern with a null in a first direction in a plane parallel to the substrate and with a polarisation vector substantially along the first direction in the plane, and in that the second dielectric antenna is configured to generate a radiation pattern with a null in a second direction substantially opposite to the first direction in the plane and with a polarisation vector in the plane substantially orthogonal to the polarisation vector of the first dielectric antenna.

In one preferred embodiment, the first dielectric antenna comprises a half-split cylindrical dielectric resonator disposed with its generally rectangular surface on the dielectric substrate such that its radiation pattern has opposed nulls parallel to the plane of the substrate along a longitudinal extent of the rectangular surface and a polarisation vector also pointing along the longitudinal extent in the plane. The second dielectric antenna comprises a cylindrical dielectric resonator with one of its generally round surfaces on the dielectric substrate and provided with a feed adapted to generate a radiation pattern with opposed nulls in the same directions in the plane as the radiation pattern of the first dielectric antenna but with a polarisation vector generally orthogonal to that of the first dielectric antenna in the plane. The second dielectric antenna is disposed on the plane such that one of the nulls of its radiation pattern directly faces one of the nulls of the radiation pattern of the first dielectric antenna.

In an alternative embodiment, the second dielectric antenna also comprises a half- split cylindrical dielectric resonator disposed with one of its generally semicircular surfaces on the dielectric substrate and with a metallised layer on its generally rectangular surface, the arrangements of nulls and polarisation vectors being as before.

Alternatively or in addition, the first dielectric antenna may be configured as a quarter-split dielectric resonator with one of its generally rectangular surfaces being provided with a metallised layer as discussed in connection with the second aspect of the present invention.

Technique 4: Three or more pellets

According to a fourth aspect of the present invention, there is provided an antenna arrangement comprising at least first, second and third dielectric antennas each formed as a dielectric resonator mounted on one surface of a common dielectric substrate and provided with a feeding mechanism for transferring energy into and out of the dielectric resonators, characterised in that at least two of the dielectric antennas are disposed in a mutually orthogonal configuration.

Each of the first, second and third dielectric antennas preferably has a longitudinal axis in a plane of the substrate, and is preferably symmetrical about its longitudinal axis in a plane perpendicular to that of the substrate. In one preferred embodiment, the first and third dielectric antennas are mounted generally parallel to each other, with the second dielectric antenna mounted between the first and third dielectric antennas so as to be generally orthogonal to both. The first and third antennas may be mounted with their longitudinal axes generally parallel and separated, with a line containing the longitudinal axis of the second antenna intersecting both the first and third antennas. Alternatively, the first and third antennas may be mounted such that their longitudinal axes lie along a single line, with the longitudinal axis of the second antenna being generally perpendicular to this single line.

In a second preferred embodiment, the first and second dielectric antennas are mounted generally orthogonal to each other on either side of a line of reflection such that the longitudinal axis of each of the first and second dielectric antennas subtends an angle of around 45 degrees to the line of reflection (i.e. the first and second dielectric antennas are mounted such that their longitudinal axes form a 'V shape with the line of reflection passing between arms of the 'V'). The third dielectric antenna is then mounted with its longitudinal axis along the line of reflection, either between the arms of the 'V (this is preferred for space-saving reasons) or on the side of the point of the 'V.

In a third preferred embodiment, similar to the second, the first and second dielectric antennas are again mounted with their longitudinal axes forming a 'V shape about the hne of reflection, but with the third dielectric antenna mounted on the line of reflection with its longitudinal axis generally perpendicular thereto.

In a fourth preferred embodiment, the first and second antennas are combined to form a single cross-shaped dielectric antenna element, with the third antenna disposed either in a first position with its longitudinal axis pointing directly towards the combined first and second antennas, or in a second position orthogonal to the first. The first and second dielectric antennas may be configured as WLAN antennas, for example operating at around 2.44GHz, and the third antenna may be configured as a Bluetooth® antenna, for example also operating at around 2.44GHz.

Technique 5: Further isolation techniques

According to a fifth aspect of the present invention, there is provided an antenna arrangement comprising at least first and second dielectric antennas each formed as a dielectric resonator mounted on one surface of a common dielectric substrate and a conductive groutidplane on an opposed surface of the substrate and provided with a feeding mechanism for transferring energy into and out of the dielectric resonators, characterised in that the dielectric resonator elements are mounted on the substrate so as to be as distantly spaced from each other as possible for any particular shape of substrate.

According to a sixth aspect of the present invention, there is provided an antenna arrangement comprising at least first and second dielectric antennas each formed as a dielectric resonator mounted on one surface of a common dielectric substrate and a conductive groundplane on an opposed surface of the substrate and provided with a feeding mechanism for transferring energy into and out of the dielectric resonators, characterised in that the conductive groundplane is only provided on regions of the opposed surface of the substrate corresponding to locations on the one surface of the substrate where the dielectric resonators and their feeding mechanisms are located and not on other regions of the opposed surface.

According to a seventh aspect of the present invention, there is provided an antenna arrangement comprising at least first and second dielectric antennas each formed as a dielectric resonator mounted on one surface of a common dielectric substrate and a conductive groundplane on an opposed surface of the substrate and provided with a feeding mechanism for transferring energy into and out of the dielectric resonators, characterised in that the dielectric substrate includes a conductive barrier within a thickness of the substrate in a region between the dielectric resonator elements.

The conductive barrier may be formed as a solid conductive wall within the thickness of the substrate, or as a series of pins or plated holes or the like. Where pins or plated holes or the like are provided in the thickness of the substrate, they may be joined by a conductive element either on the one surface or the opposed surface or both surfaces of the substrate. One, two or more such barriers maybe provided.

According to an eighth aspect of the present invention, there is provided an antenna arrangement comprising at least first and second dielectric antennas each formed as a dielectric resonator mounted on one surface of a common dielectric substrate and a conductive groundplane on an opposed surface of the substrate and provided with a feeding mechanism for transferring energy into and out of the dielectric resonators, characterised in that the dielectric substrate includes a conductive barrier on either or both of the surfaces of the substrate in a region between the dielectric resonator elements.

The conductive barrier may be formed as an elongate strip or a series of conductive elements or patches arranged generally along a line. The barrier is preferably located on the surface of the substrate on which the dielectric resonators are mounted.

The conductive barrier may lie substantially flush with the surface or surfaces of the substrate, or may project therefrom, for example taking the form of a wall that is generally perpendicular to the substrate or arranged at some other angle relative thereto. Alternatively, the conductive barrier may be formed as a ridge or crest with a generally 'A' shape or with a 'T' section.

The eighth aspect of the present invention may also be combined with the seventh aspect. Technique 6: Using cancellation techniques

According to a ninth aspect of the present invention, there is provided an antenna arrangement comprising at least first and second dielectric antennas each formed as a dielectric resonator mounted on one surface of a common dielectric substrate and a conductive groundplane on an opposed surface of the substrate and provided with a feeding mechanism for transferring energy into and out of the dielectric resonators, characterised in that part of a signal generated by the first dielectric antenna is fed in anti-phase to the second dielectric antenna.

This may be achieved by way of a coupling line configured as a directional coupler between the dielectric antennas. The coupling line may be provided on one or other of the surfaces of the substrate. The coupling line may be surface mounted, and may be printed on the substrate. Alternatively, the coupling line may be a coaxial cable or other type of RF transmission line.

Alternatively, each dielectric antenna may be provided with a low-efficiency conductive antenna element, the conductive antenna elements being configured to transmit the required anti-phase signal from one dielectric antenna to the other.

For a better understanding of the present invention and to show how it may be carried into effect, reference shall now be made by way of example to the accompanying drawings, in which:

FIGURE la shows a general half-split cylindrical DRA mounted on a dielectric substrate with a direct microstrip feedline;

FIGURE lb shows a radiation pattern of the DRA of Figure 1;

FIGURE lc shows two DRAs of Figure la mounted in a mutually orthogonal configuration and the associated radiation patterns; FIGURE Id shows a general quarter-split cylindrical DRA mounted on a dielectric substrate with a direct microstrip feedline and having one rectangular surface provided with a metallised layer;

FIGURE 2a shows two DRAs of Figure la mounted in a parallel configuration and the associated radiation patterns;

FIGURE 2b shows two DRAs of Figure la mounted in a linear configuration and the associated radiation patterns;

FIGURE 2c shows two DRAs of Figure la mounted in a mutually orthogonal configuration and the associated radiation patterns;

FIGURE 3a shows a first DRA of Figure la and a second cylindrical DRA both mounted on a substrate and the associated radiation patterns and polarisation vectors;

FIGURE 3b shows a first DRA of Figure la and a second half-split cylindrical DRA with its rectangular surface provided with a metallised layer both mounted on a substrate and the associated radiation patterns and polarisation vectors;

FIGURE 4a shows two WLAN antennas and a Bluetooth® antenna mounted in a first arrangement on a substrate;

FIGURE 4b shows two WLAN antennas and a Bluetooth® antenna mounted in a second arrangement on a substrate;

FIGURE 4c shows two WLAN antennas and a Bluetooth® antenna mounted in a third arrangement on a substrate; FIGURE 4d shows two WLAN antennas and a Bluetooth® antenna mounted in a fourth arrangement on a substrate;

FIGURE 4e shows two WLAN antennas combined in a cross-shaped configuration and a Bluetooth® antenna mounted on a substrate;

FIGURE 5a shows two DRAs of Figure la mounted as far apart as possible on a substrate with a conductive groundplane provided over an entire opposed surface of the substrate;

FIGURE 5b shows the arrangement of Figure 5a, but with a conductive groundplane provided only underneath the DRAs and not in other regions of the opposed surface of the substrate;

FIGURE 5c shows the arrangement of Figure 5b with additional conductive barriers formed within a thickness of the substrate;

FIGURE 5d shows the arrangement of Figure 5c with an additional conductive barrier formed on a surface of the substrate;

FIGURE 5e shows a barrier of Figure 5c in more detail;

FIGURE 5f shows a cross-section of a 'Λ'-shaped conductive barrier;

FIGURE 6a shows the arrangement of Figure 5a with an additional coupling line for feeding part of a signal from one DRA to the other DRA in anti-phase; and

FIGURE 6b shows the arrangement of Figure 5a with additional small lines and antennas for transmitting part of a signal from one DRA to the other DRA in antϊ- phase. Technique 1: High stored energy

It is hard to get isolation or separate patterns between antennas when they are operating in the near-field of each other. An example will be given for radio systems working at 2.44GHz, a frequency band much used by WLAN (Wireless Local Area Network) systems such as the 802.11b protocol, or the Bluetooth® 'cable replacement' radio protocol. For most conventional resonant antennas (e.g. dipoles) operating at 2.44GHz, the near-field extends up to about 60rnm from the antenna surface, this being derived from a formula 2D2/λ where λ is the radio wavelength and D is the largest dimension of the antenna, taken here to be λ/2. At 2.44GHz, two antennas need to be around 120mm apart to obtain good diversity. On a typical PCMCIA card there is only a space of typically 40mm x 30mm on which two or three diverse antennas must be located. Even if the antennas are operating with nominally different polarisations, the coupling between them will tend to make received signals from each antenna very similar if the antennas lie within the near field of each other.

Dielectric antennas, in particular DRAs, have very high stored energy within the dielectric part of the antenna and so are less susceptible to the influences of other antennas or objects placed in the near field. At 2.44GHz, DRAs can be placed a few millimetres apart without significantly adverse interactions, and so they represent a good starting point when designing a high isolation antenna pair or group. The electromagnetic fields are so strongly contained within the material of the dielectric resonator that coupling between two orthogonally disposed DRAs on a groundplane can be as low as -lOdB even when they are nearly touching, and -20dB when the separation between them is a few millimetres.

A further advantage of the high stored energy in dielectric antennas is that features of the far field of the antenna pattern such as nulls, that do not necessarily exist in the near field, can be observed within a few millimetres of the antenna. This is extremely useful for technique 3 below. Technique 2: Use beam and polarisation diversity

The coupling between two antennas can be minimised or at least reduced, and the diversity between them increased, by making use of pattern diversity and polarisation diversity. For example, a half-split cylindrical DRA 1 on a non-conducting substrate 2 (with a conducting layer 3 on the underside) can be directly fed from a microstrip 4 so as to create a resonant mode. A sketch of this arrangement is shown in Figure la. In this mode the antenna pattern, when measured around the edge of the substrate, is revealed to be a cosine or figure of eight pattern 5 with a null 6 lying along the long axis 7 of the DRA 1, as shown in Figure lb. When a second antenna 8 is placed on the same substrate 2 and disposed orthogonally to the first, this antenna will have a pattern 9 facing 90 degrees away from the first 5, as shown in Figure lc. When both antennas 1, 8 are used, either simultaneously or by switching between them, one antenna or the other will preferentially pick up different multipath signals arriving from different directions, thereby giving pattern diversity.

The pattern diversity described above is measured around a plane containing the substrate 2. Although the antenna 1, 8 does radiate in this plane, it is not the direction of maximum radiation which is generally vertically up from the substrate 2. When viewed from above, the two orthogonally disposed antennas 1, 8 have - orthogonal polarisations. Even on small substrates 2, the difference in gain between an antenna when viewed cross-polar and when viewed copolar is of the order of 17- 20dB. This effect gives rise to significant polarisation diversity that is extremely useful in wireless LAN and 3G data communication applications. At all other viewing angles, i.e. between looking down vertically from above and looking horizontally, a mixture of polarisation and pattern diversity is observed.

A variation of the antenna shown in Figure la is shown in Figure Id. Here a quarter- split cyhndrical DRA 10 has been used with its vertical wall 11 made into a conducting surface (for example by way of metallisation). This use of symmetry halves the size of the antenna element 10 without changing the radiation patterns greatly. The advantage of this is that on a board or substrate 2 of a given size, smaller elements 10 can be spaced further apart, thereby increasing the isolation further.

Technique 3: Create suitable nulls

A further technique for creating isolation and diversity between antennas is to make use of one or more nulls in the antenna patterns to reduce the coupling between them. An example of doing this can be made with reference to the antenna described above in relation to Figure la, the half-split cylindrical DRA 1 on a non-conducting substrate 2 (with a conducting layer 3 on the underside) fed directly from a microstrip 4. Figure 2a shows two antenna elements 1, 1' placed side by side - this is the worst case for coupling as the DRAs 1, 1' radiate into each other from one main lobe 5 into another 5'. Figure 2b shows an optimum case for isolation - the two antenna elements 1, 1 ■ in line - but here they have the same polarisation and pattern 5, 5' alignment and there is no diversity beyond a very small amount of spatial diversity created by the small separation between the DRAs 1, 1 '. Figure 2c shows a compromise situation where there is diversity and yet one null 6 is used to improve the isolation between the DRAs 1, 1'.

An ideal situation would be to have one antenna with nulls at the end of the element and another with nulls at the side, yet both with the same polarisation vector. In other words, this would be an arrangement of two dielectric resonators each having a major axis, the resonators being placed parallel with each other such that the polarisation vectors of the resonators are parallel with the major axes, but with the resonators operating in different resonant modes such that one has nulls along its major axis and the other has nulls orthogonal to its major axis. If such an arrangement could be created, it would be possible to turn one of the resonators through 90 degrees so as to give substantially orthogonal polarisation vectors but with the radiation patterns of the resonators being such that the nulls of the patterns face each other. Li this situation, when the two elements are disposed orthogonally to give diversity advantages shown in Figure 2c, the nulls in the patterns face each other and give the isolation advantages of Figure 2b. A way of achieving this is shown in Figure 3 a. The left hand element 20 is the same half-split cylindrical DRA as above, with polarisation vector 25, whereas the right hand element 21 is a cylindrical DRA, resting horizontally on the substrate 2 and probe or slot fed and having a polarisation vector 25'. The elements 20, 21 give rise to the patterns 22, 23 shown. Both elements 20, 21 in Figure 3a can be halved using vertical conducting surfaces 24 and Figure 3b shows this for the right hand element 21. It now becomes clear that the right hand element 21 is similar to the left hand element 20 (both are half-split cylinders), but has been 'pushed over' to lay flat on the substrate 2. The vertical polarisation 25 of interest in the left hand element 20 becomes horizontal 25' in the right hand element 21 where it is not of interest. Likewise the horizontally polarised radiation pattern (not plotted for the left hand element 20 because they are not of interest) becomes important in the right hand element 21. In other words, rotating an element 90 degrees with respect to another rotates the polarisation 90 degrees and allows two elements 20, 21 to have the same polarisation but with nulls 6 facing each other.

Technique 4: Three or more pellets

All the advantages and techniques discussed above can be used to create isolation and diversity with more than two antennas. An example is when 2.44GHz WLAN systems such as the 802.11b protocol requires two antennas for diversity and must operate simultaneously with a single 2.44GHz Bluetooth® radio system in a small space such as on a PCMCIA card or access point. Figures 4a, 4b, 4c, 4d and 4e show a number of ways in which the techniques described above can be employed to dispose elements 30, 31, 32 so as to maximise WLAN diversity and minimise coupling between all three elements 30, 31, 32. The elements are designated "WLAN" (for wireless LAN) and "BT" (for Bluetooth®) in the drawings. The prefened technique depends on the board 2 area available and on the shape and size of the elements 30, 31, 32 used.

i) On a very small card or board 2 area, the arrangement shown in Figure 4a works well in practice, but does not give the best combination of isolations, in that the two outside elements 30, 32 have the worst isolation (having their main lobes facing each other).

ii) It would be advantageous to have a pair of orthogonal elements 31, 32 for WLAN with fair isolation between them (preferably better than lOdB), and then much better isolation between these two elements 31, 32 and a third antenna element 30 used for Bluetooth®. This greater isolation is needed so that the two protocols (WLAN and Bluetooth®) do not interfere with each other. The arrangement in i) above does not achieve this. iii) Accordingly, if sufficient space is available on the substrate or board 2, then the arrangement shown in Figure 4b is preferred. The two elements 31, 32 are the WLAN elements and the element 30 is for Bluetooth®.

iv) Figure 4e is a variant on the arrangement of Figure 4b in which the two separate orthogonal WLAN elements 31, 32 have been combined into a single cross- shaped element 33 with two mutually orthogonal feed points. This has been tested experimentally and works well. If the Bluetooth® system also requires two diversity antennas, then two cross-shaped antennas could be fitted onto a single board 2, each having two feed points.

v) Figure 4c is a variant on the arrangement of Figure 4b.

vi) Figure 4d is a variant on the arrangement of Figure 4a, but with the elements 30, 31, 32 each turned through 90 degrees in the plane of the board 2. This appears to offer good isolation between all three elements.

vii) It will be apparent that there are a number of other specific ways of arranging three elements 30, 31, 32 on a board 2, with two elements being orthogonal. All of these arrangements have various advantages (usually space-saving) and disadvantages (usually one or more isolations being poor). In all these arrangements, the basic principles of aligning nulls and utilising orthogonality will tend to apply, and no further detailed discussion is therefore required.

Technique 5: Further isolation techniques

The present applicant has determined a number of ways to improve the isolation between two, or more, antennas 40, 41 on a single substrate 2, these are:

i) Place the antenna elements 40, 41 as far apart as possible on the substrate 2, for example as shown in Figure 5a. In Figure 5a, the substrate 2 is provided with a conductive groundplane 3 on its underside, and the elements 40, 41 are fed by microstrip feeds 42, 43.

ii) Isolate the groundplanes 3, 3' of the antennas 40, 41, that is to say, ensure that the underside of the substrate 2 is only conducting 3, 3' underneath each antenna 40, 41 and is non-conducting in between antennas 40, 41, see Figure 5b.

iii) Currents flowing in the substrate 2, 3, 3' and contributing to coupling can be reduced by building vertical conducting walls into the substrate 2, 3, 3'. This can be achieved by, for example, drilling a row of holes 44 through the board 2 and placing conducting pins 45 in the holes 44, or through plating the insides of the holes 44. A conductive strip 3" may then be attached under the substrate 2 so as electrically to connect the pins 45 and/or the holes 44. Figure 5c shows this construction.

iv) Placing conducting strips 46 on either side of the substrate 2 between the antenna elements 40, 41, and preferentially disposed substantially orthogonally to the elements 40, 41 as shown in Figure 5d, also helps to reduce coupling. The technique works best for conducting strips 46 on the upper surface than the lower surface. Even small patches of metal, rather than strips, helps to reduce coupling.

v) Vertical conducting walls between the antennas 40, 41 help to reduce coupling. Such walls can be constructed as a conductive bridging extension 47 between the pins 45 described at iii) above (see Figure 5e), or as an extension of the conductive strips 46 described at iv) above, for example by constructing a conductive ridge or crest 48 (see Figure 5f). Various alternative configurations for the conducting walls may be used, including vertical or near- vertical walls, walls angled at an obtuse or acute angle to the substrate 2, walls having a 'T' section and others.

Technique 6: Using cancellation techniques

Cancellation of signals common to two antennas can be achieved by feeding part of the signal from one antenna in anti-phase into the other. Figure 6a shows a method of achieving this for two DRAs 40, 41 on a substrate 2 using a microstrip, coplanar or stripline feed 49 having ends 50, 51 configured as directional couplers in relation to the elements 40, 41. The coupling line 49 may be located on an upperside or an underside of the board 2. Such cancellation has been tested using printed directional couplers and -40dB isolation was achieved. Surface mounted or other discrete directional couplers can also be used in this application. The physical link 49 between the antennas could also be a co-axial cable or any other type of RF transmission line. It is also possible to use very small antenna elements 52, 52' (Figure 6b) of low efficiency to radio the cancellation signal between two antennas 40, 41. With this latter configuration, it may not always be necessary to include the directional couplers as the correct level of coupling might be achieved by using the small radio link antennas 52, 52' to radiate with the appropriate phase directly into the main antenna elements 40, 41 that are to be isolated. Figure 6b shows this latter arrangement.

Cancellation techniques can also be arranged to work with an arrangement of three antennas. In this case, two directional couplers per antenna are used.

The preferred features of the invention are applicable to all aspects of the invention and may be used in any possible combination. Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", mean "including but not limited to", and are not intended to (and do not) exclude other components, integers, moieties, additives or steps.

Claims

CLAIMS:
1. An antenna arrangement comprising at least two dielectric antennas each formed as a dielectric resonator mounted on one surface of a common dielectric substrate and provided with a feeding mechanism for transferring energy into and out of the dielectric resonators, characterised in that the dielectric antennas are mounted generally orthogonal to each other on the dielectric subsfrate such that radiation patterns generated by the dielectric antennas are also generally orthogonal.
2. An antenna arrangement as claimed in claim 1, wherein the at least two dielectric resonators are mounted 100mm or less apart from each other on the substrate, preferably 60mm or less apart, more preferably 40mm or less apart and most preferably 12mm or less apart.
3. An antenna arrangement as claimed in claim 1, wherein the at least two dielectric antennas touch each other.
4. An antenna arrangement comprising at least two dielectric antennas each formed as a dielectric resonator mounted on one surface of a common dielectric substrate and provided with a feeding mechanism for transferring energy into and out of the dielectric resonators, characterised in that the dielectric antennas are each configured to generate a radiation pattern with a generally figure-of-eight shape in a plane parallel to the dielectric substrate, and in that the dielectric antennas are mounted generally orthogonal to each other on the dielectric substrate such that radiation patterns generated by the dielectric antennas are also generally orthogonal.
5. An antenna arrangement as claimed in claim 4, wherein the dielectric resonators each have a half-split cylindrical configuration.
6. An antenna arrangement as claimed in claim 4, wherein the dielectric resonators each have a quarter-split cylindrical configuration with a metallised layer on an exposed generally rectangular surface thereof.
7. An antenna arrangement as claimed in claim 4, wherein one of the at least two dielectric resonators has a half-split cylindrical configuration and another of the at least two dielectric resonators has a quarter-split cylindrical configuration with a metallised layer on an exposed generally rectangular surface thereof.
8. An antenna arrangement comprising at least first and second dielectric antennas each formed as a dielectric resonator mounted on one surface of a common dielectric substrate and provided with a feeding mechanism for fransferring energy into and out of the dielectric resonators, characterised in that the first dielectric antenna is configured to generate a radiation pattern with a null in a first direction in a plane parallel to the substrate and with a polarisation vector substantially along the first direction in the plane, and in that the second dielectric antenna is configured to generate a radiation pattern with a null in a second direction substantially opposite to the first direction in the plane and with a polarisation vector in the plane substantially orthogonal to the polarisation vector of the first dielectric antenna.
9. An antenna arrangement as claimed in claim 8, wherein the first dielectric antenna comprises a half-split cylindrical dielectric resonator disposed with its generally rectangular surface on the dielectric substrate such that its radiation pattern has opposed nulls parallel to the plane of the substrate along a longitudinal extent of the rectangular surface and a polarisation vector also pointing along the longitudinal extent in the plane.
10. An antenna arrangement as claimed in claim 8, wherein the first dielectric antenna comprises a quarter-split dielectric resonator disposed with one of its generally rectangular surface on the dielectric substrate and with another of its generally rectangular surfaces being provided with a metallised layer, such that its radiation pattern has opposed nulls parallel to the plane of the substrate and a polarisation vector pointing in the same direction as the nulls along the plane.
11. An antenna arrangement as claimed in claim 9 or 10, wherein the second dielectric antenna comprises a cylindrical dielectric resonator with one of its generally round surfaces on the dielectric substrate and provided with a feed adapted to generate a radiation pattern with opposed nulls in the same directions in the plane as the radiation pattern of the first dielectric antenna but with a polarisation vector generally orthogonal to that of the first dielectric antenna in the plane.
12. An antenna anangement as claimed in claim 9 or 10, wherein the second dielectric antenna comprises a half-split cylindrical dielectric resonator disposed with one of its generally semicircular surfaces on the dielectric substrate and with a metallised layer on its generally rectangular surface and provided with a feed adapted to generate a radiation pattern with opposed nulls in the same directions in the plane as the radiation pattern of the first dielectric antenna but with a polarisation vector generally orthogonal to that of the first dielectric antenna in the plane.
13. An antenna arrangement comprising at least first, second and third dielectric antennas each formed as a dielectric resonator mounted on one surface of a common dielectric substrate and provided with a feeding mechanism for fransferring energy into and out of the dielectric resonators, characterised in that at least two of the dielectric antennas are disposed in a mutually orthogonal configuration.
14. An antenna arrangement as claimed in claim 13, wherein each of the first, second and third dielectric antennas has a longitudinal axis in a plane of the substrate.
15. An antenna arrangement as claimed in claim 14, wherein each antenna is substantially symmetrical about its longitudinal axis in a plane perpendicular to that of the substrate.
16. An antenna arrangement as claimed in any one of claims 13 to 15, wherein the first and third dielectric antennas are mounted generally parallel to each other, with the second dielectric antenna mounted between the first and third dielectric antennas so as to be generally orthogonal to both.
17. An antenna arrangement as claimed in claim 16 when depending on claim 14 or 15, wherein the first and third antennas are mounted with their longitudinal axes generally parallel and separated, with a line containing the longitudinal axis of the second antenna intersecting both the first and third antennas.
18. An antenna arrangement as claimed in claim 16 when depending on claim 14 or 15, wherein the first and third antennas are mounted such that their longitudinal axes lie along a single line, with the longitudinal axis of the second antenna being generally perpendicular to this single line.
19. An antenna arrangement as claimed in claim 14 or 15, wherein the first and second dielectric antennas are mounted generally orthogonal to each other on either side of a line of reflection such that the longitudinal axis of each of the first and second dielectric antennas subtends an angle of around 45 degrees to the line of reflection and wherein the third dielectric antenna is mounted with its longitudinal axis along the line of reflection.
20. An antenna anangement as claimed in claim 14 or 15, wherein the first and second dielectric antennas are mounted generally orthogonal to each other on either side of a line of reflection such that the longitudinal axis of each of the first and second dielectric antennas subtends an angle of around 45 degrees to the line of reflection and wherein the third dielectric antenna mounted on the line of reflection with its longitudinal axis generally perpendicular thereto.
21. An antenna arrangement as claimed in claim 14 or 15, wherein the first and second antennas are combined to form a single integrated cross-shaped dielectric antenna element, with the third antenna disposed either in a first position with its longitudinal axis pointing directly towards the combined first and second antennas, or in a second position orthogonal to the first.
22. An antenna arrangement as claimed in any one of claims 13 to 21, wherein he first and second dielectric antennas are configured as WLAN antennas and the third antenna is configured as a Bluetooth® antenna.
23. An antenna arrangement comprising at least first and second dielectric antennas each formed as a dielectric resonator mounted on one surface of a common dielectric subsfrate and a conductive groundplane on an opposed surface of the substrate and provided with a feeding mechanism for transferring energy into and out of the dielectric resonators, characterised in that the dielectric resonator elements are mounted on the substrate so as to be as distantly spaced from each other as possible for any particular shap e of substrate.
24. An antenna anangement comprising at least first and second dielectric antennas each formed as a dielectric resonator mounted on one surface of a common dielectric substrate and a conductive groundplane on an opposed surface of the substrate and provided with a feeding mechanism for transferring energy into and out of the dielectric resonators, characterised in that the conductive groundplane is only provided on regions of the opposed surface of the subsfrate corresponding to locations on the one surface of the substrate where the dielectric resonators and their feeding mechanisms are located and not on other regions of the opposed surface.
25 An antenna arrangement comprising at least first and second dielectric antennas each formed as a dielectric resonator mounted on one surface of a common dielectric substrate and a conductive groundplane on an opposed surface of the substrate and provided with a feeding mechanism for transferring energy into and out of the dielectric resonators, characterised in that the dielectric subsfrate includes a conductive barrier within a thickness of the substrate in a region between the dielectric resonator elements.
26. An antenna arrangement as claimed in claim 25, wherein the conductive barrier is formed as a solid conductive wall within the thickness of the substrate.
27. An antenna arrangement as claimed in claim 25, wherein the conductive barrier is formed as a series of pins or plated holes within the thickness of the substrate.
28. An antenna arrangement as claimed in claim 27, wherein the pins or plated holes are joined by a conductive element either on the one surface or the opposed surface or both surfaces of the subsfrate.
29. An antenna anangement comprising at least first and second dielectric antennas each formed as a dielectric resonator mounted on one surface of a common dielectric substrate and a conductive groundplane on an opposed surface of the subsfrate and provided with a feeding mechanism for transferring energy into and out of the dielectric resonators, characterised in that the dielectric subsfrate includes a conductive barrier on either or both of the surfaces of the substrate in a region between the dielectric resonator elements.
30. An antenna anangement as claimed in claim 29, wherein the conductive banier is formed as an elongate strip.
31. An antenna arrangement as claimed in claim 29, wherein the conductive barrier is formed as a series of conductive elements or patches arranged generally along a line.
32. An antenna arrangement as claimed in any one of claims 29 to 31, wherein the barrier is located on the surface of the subsfrate on which the dielectric resonators are mounted.
33. An antenna anangement as claimed in any one of claims 29 to 32, wherein the conductive barrier lies substantially flush with the surface or surfaces of the subsfrate.
34. An antenna anangement as claimed in any one of claims 29 to 32, wherein the conductive barrier projects from the surface or surfaces of the substrate.
35. An antenna arrangement as claimed in claim 34, wherein the conductive barrier is formed as a ridge or crest.
36. An antenna arrangement comprising at least first and second dielectric antennas each formed as a dielectric resonator mounted on one surface of a common dielectric subsfrate and a conductive groundplane on an opposed surface of the subsfrate and provided with a feeding mechanism for fransferring energy into and out of the dielectric resonators, characterised in that part of a signal generated by the first dielectric antenna is fed in anti-phase to the second dielectric antenna.
37. An antenna anangement as claimed in claim 36, wherein the part of the signal is fed to the second dielectric antenna by way of a coupling line configured as a directional coupler between the dielectric antennas.
38. An antenna anangement as claimed in claim 37, wherein the coupling line is provided on one or other of the surfaces of the substrate.
39. An antenna anangement as claimed in claim 36, wherein each dielectric antenna is provided with a low-efficiency conductive antenna element, the conductive antenna elements being configured to transmit the required anti-phase signal from one dielectric antenna to the other.
40. An antenna arrangement substantially as hereinbefore described with reference to or as shown in Figures lc, 2b, 2c, 3 a, 3b, 4a, 4b, 4c, 4d, 4e, 5a, 5b, 5c, 5d, 5e, 5f, 6a and 6b of the accompanying drawings.
PCT/GB2003/003588 2002-08-15 2003-08-15 Improvements relating to antenna isolation and diversity in relation to dielectric antennas WO2004017462A1 (en)

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