WO2008145978A1

WO2008145978A1 - Beam steerable antenna

Info

Publication number: WO2008145978A1
Application number: PCT/GB2008/001787
Authority: WO
Inventors: Amit Mehta
Original assignee: Uws Ventures Limited
Priority date: 2007-05-26
Filing date: 2008-05-23
Publication date: 2008-12-04
Also published as: GB0807942D0; GB0710142D0; GB2449736A

Abstract

An antenna has a generally star-shaped conductive element for generating a radiation beam. Feeding ports are provided on the conductive element for selective excitation by a signal. The feeding ports are positioned such that excitation results in deflection of the radiation beam so as to steer the radiation beam in the direction of deflection. The antenna is particularly suitable for use in wireless telecommunications applications.

Description

Beam Steerable Antenna

This invention relates generally to a beam-steerable antenna and, more particularly, to a general purpose, relatively simple and low-cost directional antenna with accurate beam steering capability, especially (but not necessarily exclusively) suitable for use in wireless communications applications.

An antenna is a device that transmits and/or receives electromagnetic waves, often referred to as radio waves. More specifically, an antenna is a transducer that converts radio frequency current to electromagnetic waves that are then radiated into space and/or converts electromagnetic waves received from space into radio frequency current.

Wireless communication is becoming increasingly widespread and sophisticated systems are being developed to facilitate increasing demand. For example, in 2002, a large, contiguous part of the radio spectrum between 3.1 GHz and 10.6 GHz was made available, license-free, for low power, high bit-rate use in the USA. The EU followed with similar specifications late in 2003, and Ultra Wide Band (UWB) radio systems that can make use of this have been developed accordingly. In these, and other wireless communication applications, it is often required to direct a beam in a particular direction, or receive a beam only from a particular direction, relative to an antenna. Changing the direction of the main lobe of a radiation pattern is known as beam steering, which may be accomplished, in conventional systems, by switching antenna elements or by changing the relative phases of the RF signals driving the elements. For example, a phased array antenna is composed of a plurality of radiating elements, each with a phase shifter. Beams are formed by shifting the phase of the signal emitted from each radiating element, to provide constructive/destructive interference so as to steer the beams in the desired direction.

However, since the advent of array theory and development of the early beam- steerable phased arrays in the 1960s, phase shifters have been widely recognised as the most complex, sensitive and expensive parts of the phased array systems. Indeed, beam steerable antennas using a phased array concept have been successfully demonstrated for wireless communications. However, this type of antenna is only practical for base stations where stringent requirements for phase shifters, space for multiple antenna elements, signal loss and cost can be borne. The same technology cannot be implemented in handsets or small wireless transceivers due to space limitations.

International Patent Application No. WO-A-062024 describes an antenna suitable for a mobile telephone or other such communication device comprising a spiral transmission element supported on a dielectric layer. Switching elements in the form of, for example, microelectromechanical (MEM) switches or PIN diodes are provided on the transmission element for selectively short or open circuiting the element (21). The excitation of the switches either open or short the antenna arm, so as to vary the current flow on the outer circumference of the antenna, thereby allowing the orientation of the radiation pattern to be altered as required and providing beam steerability.

The above-mentioned antenna eliminates the need to employ phase shifters and enables a single antenna element to be used for beam steering. However, due to the deployment of switches on the spiral antenna arm, when a switch is excited, there is a change in the configuration (and therefore the current distribution) of the antenna arm which results in a change in the antenna polarization, thereby causing polarization randomness. In other words, the antenna beam can have a dominant polarization (Eø/Eφ or circular polarization) under one switching configuration which may then change drastically under another switching configuration. This means that, if the communication is linear polarized, nil signal will be received in the event of a polarization mismatch or, in the case of a circularly polarized link, a weak signal will be received (up to 3dB received signal loss) in the event of polarization mismatch. In addition, other antenna parameters (e.g. gain, VSWR, etc) may vary with each switching configuration.

It is therefore an object of the present invention to provide a relatively simple, accurate, relatively high bandwidth beam-steerable antenna in which the above- mentioned problems are overcome and in respect of which the radiation beam can be switched in different directions whilst maintaining constant antenna parameters. In accordance with the present invention, there is provided an antenna comprising a generally star-shaped conductive element for generating a radiation beam, a plurality of feeding ports being provided on said conductive element for selective excitation by a signal, said feeding ports being positioned such that excitation thereof results in deflection of said radiation beam so as to steer said radiation beam in the direction of deflection

In a first exemplary embodiment, the antenna may comprise a closed loop comprising at least four equi-distant, substantially V-shaped conductive arms coupled together in a generally star-shaped configuration. In this case, a feeding port is preferably provided at the apex of each conductive arm. Each port may be excited separately or, alternatively, means may be provided for selectively exciting one or more of the ports at a time. A significant advantage of this configuration is that the number of arms and the angle at the apex of each arm can be varied to vary the gain and bandwidth of the antenna. Thus, an increase in the angle at the apex of each conductive arm will result in an increased bandwidth and decreased gain. Another advantage of the star configuration is that, the electrical length of the antenna is large enough to result in a travelling wave structure which can further increase gain.

In a second exemplary embodiment, the antenna may comprise at least four, equidistant, substantially triangular shaped, conductive sections coupled so as to provide said generally star-shaped configuration. In this case, a feeding port is beneficially provided at a distal end of each conductive section and a feeding port is preferably provided at a centre point of the antenna. Beneficially, a feeding port is provided on a centreline of one conductive section, one quarter of the distance from said distal end to said centre point.

In either of the first or second embodiment, each port may be excited separately or, alternatively, means may be provided for selectively exciting one or more of the ports at a time. The antenna is preferably provided on a dielectric substrate. Beneficially, at least two conductive sections provide current paths that are substantially symmetrical about an axis of said antenna. Due to the fact that the feeding ports can be suitably far apart, even in small applications, construction is relatively simple and practical.

These and other aspects of the present invention will be apparent from, and elucidated with reference to, the embodiments described herein.

Embodiments of the present invention will now be described by way of examples only and with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram illustrating a star antenna according to a first exemplary embodiments of the invention;

Figure 2 is a graphical representation of the simulated return loss of the antenna of Figure 1 for configuration A;

Figure 3 illustrates radiation patterns of the antenna of Figure 1 for configuration A in respect of (a) Φ variation at θmax = 42° and (b) θ variation at Φmax = 15°;

Figure 4 illustrates IdB radiation cones for the antenna of Figure 1 for configurations (a)A, (b)B, (c)C and (d)D;

Figure 5 illustrates schematically the current distribution in relation to the antenna of Figure 1 for configuration A; Figure 6 is a schematic diagram illustrating a star antenna according to a second exemplary embodiment of the invention; Figure 7 is a graphical representation of the simulated VSWR for tilted, axial and doughnut beam of the antenna of Figure 6; Figure 8a illustrates radiation patterns of the antenna of Figure 6 for configuration

A in respect of θ variation at θ max = 0°and θ variation at Θ max = 42°; Figure 8b illustrates current distribution in the antenna of Figure 6 for configuration A; Figure 9a illustrates radiation patterns of the antenna of Figure 6 for configuration

E in respect of θ variation at Φ = 0° and at Φ = 90 °; Figure 9b illustrates current distribution in the antenna of Figure 6 for configuration E; Figure 10a illustrates radiation patterns of the antenna of Figure 6 for configuration

F in respect of θ variation at Φ = 0° and Φ variation at θ max = 42 °; and Figure 10b illustrates current distribution in the antenna of Figure 6 for configuration FG.

Figure 1 shows a first exemplary embodiment of the invention in the form of a star antenna which is composed of a total of eight conductor arms 14 (strips). Each two arms merge at an apex 16 with a selectable angle, e.g. 10 degrees in the illustrated example. The conductor arms 14 are etched on a dielectric substrate 18 which is backed by a conducting ground plane (not shown). The antenna has a thickness of substantially h=12mm, an arm width of substantially w=1.5mm, and a substrate permittivity of 3.45. The length of a single arm is substantially L=30mm. The present invention is in no way intended to be limited in this regard and all dimensions and materials used can be selected according to design requirements.

The feeding points for the antenna are A, B, C and D which are preferably apexes 16 of the antenna. It will be appreciated that by altering the angle between merging conductor arms 14 at the apex 16, the overall gain and bandwidth of the antenna can be changed. Another way to increase bandwidth and decrease gain is to increase the number of conductor arm pairs from 4 to say 8 or 12, as long as a symmetrical structure is maintained.

By referring to the antenna feeding points A, B, C and D, a total of four antenna configurations are analysed below. Therefore, if the antenna is fed at point A, the configuration is referred to as A. However, it will be appreciated that not only can the feed points be executed individually to attain respective antenna configurations, but means may also be provided for selectively exciting combinations of feed points to give different antenna configurations. It will be understood that with a single port excitation (of either A, B, C or D), the beam may be steered at a given area of the complete coverage area in front of the antenna. By use of simultaneous switching, an even higher gain beam in a given direction can be achieved. For the avoidance of doubt, it should be noted that an antenna arm may have more than one feeding point.

The antenna can be fed from the bottom, using for example a coplanar waveguide incorporating switches and a particular feeding port out of the four may be selected electronically. This technique provides a simple implementation and does not hamper the antenna radiation pattern, as would be in the case in prior art systems whereby the switches are incorporated on the top of the antenna arm.

The star antenna specified above can be analysed and for configuration A, its typical return-loss (Sl 1) is shown graphically in Figure 2. A person skilled in the art will be fully familiar with the concept of return loss being a measure of the power lost to the load and thus not reflected) and, as such, this concept will not be discussed more fully herein.

Examining the radiation patterns corresponding to the resonances observed on the Sl 1 plot, it is observed that at the operating frequency 4.5 GHz, the antenna generates a tilted beam in a quadrant (Φ_max = 15° and θ_max = 42°) as shown in the radiation patterns in Figure 3. This tilted beam is due to a combination of radiations from its two main fed arms and the other six auxiliary arms connected to the main arms.

As seen in Figure 2, the antenna shows a bandwidth of 500MHz for a 10 dB criterion and a bandwidth of IGHz for a 6 dB criterion. Since the antenna and feeding structure are symmetrical, the values of Sl 1 for the remaining three configurations (B, C and D) are identical to that of configuration A. Upon changing the feeding point, the direction of the tilted beam moves to a different quadrant. Since there is no switching involved on the antenna arm, the antenna polarization and other characteristics stay the same for all the tilted beam directions. The radiation patterns in Figure 3 shows the vertical (elevation) and conical (azimuth) radiation pattern cuts for configuration A in the direction of maximum radiation. The radiation pattern Φ = Φ_max (15°) as a function of θ reveals that the half-power beam- width is less than 75°, yielding high gain beam. Due to the antenna and its feeding being symmetrical the radiation patterns are the same for the other three configurations (B to D).

Figure 4 shows the 3 -dimensional radiation patterns (1 dB cone; E_net = (|E_Θ|²+|E_Φ|²)^1/2) for all the four configurations. The beam for configuration A has #_max- 42° and Φ_max=15°, for configuration B has θ_max= 42° and Φ_max=285⁰, for configuration C has <W= 42° and Φ_max=195⁰, and for configuration D has θ_max= 42° and Φ_max=195°. Hence, each of the four configurations radiates a tilted beam in a different space quadrant, thus realising a switched beam antenna. The axial ration (AR) and gain for all the four cases are approximately 12.12 dB and 8.2 dBi respectively, which are not a function of the switching.

Figure 5 shows the current distribution on the antenna arms for the configuration A, at the resonant frequency. As pointed by the arrows in the figure, the currents on arm C are equal in magnitude but opposite in direction; hence no radiation occurs from arm C. On the contrary, currents on arms A, B and D are in the same direction, thus yielding effective radiation from the antenna with the polarization dominance in the Eθ direction. It should be noted that even though arm C does not contribute to radiation in configuration A, it does, however, contribute to the overall resonance of the antenna.

In conclusion, a star antenna with four symmetrical feeding points has been described above for switching beam applications. The feeding points are located on the four apexes 16 of its 8 arms 14 and when excited one by one, generate four tilted beams of the same characteristics in different respective quadrants, thus yielding a beam steerable antenna. Also, the return losses of the antenna under different feed configurations remain the same. The gain and axial ratio (AR) for the antenna for each of the feed configuration are 8.2 dBi and 12.12 dB, respectively, with polarization dominance in the Eθ direction. An advantage of the star configuration is that, the electrical length of the antenna is great enough to result in a travelling wave structure that can further increase gain.

Referring now to Figure 6 of the accompanying drawings, a star antenna according to a second exemplary embodiment of the invention is shown comprising four generally triangular conductor arms 20 arranged in a star configuration. The conductor arms 20 are sections of electricity conductive material, such as copper or any other suitable material. The centre-line of a conductor arm 20 is generally aligned with and arranged substantially parallel to that of an opposite arm 20 and substantially perpendicular to that of each adjacent arm 20, defining a star antenna having four ends 22 and four conductor arm intersection points 26. The rectangular area between the conductor arms is a conducting base, to which each conductor arm 20 is coupled, thus forming a solid star-shaped antenna. When the base is a non-square rectangle, the star antenna may be symmetrical about at least one axis (i.e. an axis defined by the combined centre-lines of opposing conductor arms 20). When the base is square, the antenna may be symmetrical about at least two, orthogonal, axis.

In this exemplary embodiment, the conductor arms 20 are etched as strips onto a dielectric substrate 24 having a thickness of 12mm and a relative permittivity of 3.45. The substrate 24 is backed by a conducting ground plane (not shown), many realisations of which will be apparent to one skilled in the art. Each conductor arm 20 has a generally flat end 22 having a width of substantially 1.5mm. The end 22 may in other embodiments be curved, so as to define a single apex, or the end 22 may be of very small length, e.g. 0.1mm, so as to define a point. Because in this embodiment the base is square, the distance between the centre of the star antenna and an end 22 is substantially 34mm. Opposing sides of a conductor arm 20 each extend from its end 22 towards the base of an adjacent conductor arm 20 at an angle of substantially 10° to a centreline of the respective conductor arm 20 from which they extend. Each side has a length of substantially 28.2mm, from a first end at which it connects to a respective end 22, to a second end defining a conductor arm intersection point 26.

As described, the star antenna is preferably substantially symmetrical about one axis and beneficially symmetrical about at least two, perpendicular, axis. This provides for efficient current cancellation in opposing arms 20, adjacent the arm being fed, so as to substantially cancel any radiation there from. One skilled in the art will appreciate that, when using the term symmetrical, the antenna configuration may vary from perfect symmetry by a small degree without adversely affecting the operational characteristics of the antenna.

The antenna according to the second exemplary embodiment has six feeding points for excitation by a signal. Feeding points A, B, C and D are arranged towards or at the end 22 of the four conductor arms 20, respectively, on or near the centreline of a respective conductor arm. Feeding point F is arranged on the same centreline as feeding point C, at a distance of substantially 8.5mm from the end 22 of that conductor arm 20 or from feeding point C. Feeding point E is arranged at the centre of the antenna, which in this embodiment is the point where the centreline of each conductor arm 20 intersects. At least six antenna configurations are obtainable by way of individual excitation of each of the six feed points A, B, C, D, E and F. However, it is envisaged that combinations of feeding points may be fed contemporaneous Iy .

When the antenna is fed at feeding point A, it will be referred to as configuration A, feeding point B will be referred to as configuration B and so fourth. Configurations A, B, C and D each provide a tilted beam in a respective quadrant of the antenna, as described below with respect to Figure 7. Configuration E provides an axial beam, as described below with respect to Figure 8 and configuration F provides a pseudo- doughnut-shaped pattern, as described below with reference to Figure 9. The skilled person will appreciate that the antenna according to this exemplary embodiment is able to provide axial and doughnut-shaped beams, at least in part, due to the fact that the conductor arms 20 are whole between the sides thereof, or solid, so as to provide for a larger conducting surface than in earlier described embodiments, which conducting surface can accommodate feeding points E and F in the arrangements herein described.

Although the six feeding point configurations demonstrate wideband input impedance characteristics in the Ultra Wide Band (UWB) region of the spectrum, their common radiation pattern bandwidth is restricted to 600MHz, as shown in Figure 7. For the pseudo-doughnut-shaped pattern of configuration F, the radiation pattern bandwidth is from 2.5GHZ to 5.5GHZ. For the tilted beam patterns of configurations A to D, the radiation pattern bandwidth is from 2.7GHZ to 3.5GHZ and for the axial beam pattern of configuration E, the radiation pattern bandwidth is from 2.9GHZ to 3.5GHZ. The large radiation pattern bandwidth for the pseudo-doughnut shaped pattern of configuration F is attributable to the fact that when the substantially symmetrical star antenna is fed via point F, i.e. at its central position, the phase variations due to the shift in the frequency are equal in all directions. Thus, any deviations from the original radiation pattern are cancelled out. In many ways, the behaviour of the antenna for configuration F is similar to that of a top loaded monopole.

The antenna test frequency is kept in the middle of the radiation pattern bandwidth of the axial beam pattern of configuration E, i.e. at 3.2GHz. The generator impedances are equal to 87 + j 40 Ω for configurations A, B, C and D, equal to 28 + j 90 Ω for configuration E and equal to 80 + j 8 Ω for configuration F. These values are the complex conjugate of the input impedances of the various configurations of the antenna at the resonant frequency of 3.2GHz. Figure 7 shows the voltage standing wave ratio (VSWR) for the tilted beam of configurations A to D, the axial beam of configuration E and doughnut shaped pattern of configuration F. It will be noted that for each, the VSWR is less than 2, which provides for a desirable return loss. Compared to the antenna of the first embodiment which has resonating frequencies at 5.2GHz and 4.5GHz, the resonating frequency of the antenna according to this second exemplary embodiment is 3.2GHZ, which is considerably lower. One skilled in the art will appreciate that this is desirable for a lower propagation loss, a lower shadow fade margin and the like.

Referring now to Figure 8a, the radiation pattern of a beam according to configuration A is shown, tilted in a quadrant. (Φ_max = 0 ° and #_max= 42°). This tilted beam is at least in part due to flow of a phase lagging current towards the direction of the tilt (from A to C), similar to the situation with a forward pointing travelling wave antenna. This is shown in the current distribution in the Figure 8b. In addition, equal and opposite currents travelling in the side arms (towards B and D) cancel any radiation from them, thus the side arms may, in some circumstances, only contribute towards the antenna resonance. Hence, with use of feeding points A, B, C and D, four tilted beams of θ_max= 42° can be obtained in 4 different quadrants Φ_max = 0 ° ; ° 270 °; 180 °; and 90 °). The axial ratio (AR) and gain for configurations A-D are substantially 50 dB and 7.8dBi.

Feeding configuration E provides an axial beam pattern, as shown in Figure 9a. As the feeding point is moved inwards from a strip end, the beam tilt starts to decrease and at a distance of 1/4* from the end an axial radiation is achieved. The current distribution is shown in Figure 9b. It will be appreciated that in the axial beam configuration, there is a backward flow of currents towards the corner from the feeding point E, which is not present in the tilted beam configuration. This current difference in effect results in an axial pattern. The AR and gain for configuration E substantially 83 dB and 7.3 dBi, respectively.

Referring now to Figure 10a, the radiation pattern for feeding configuration F is shown providing a pseudo-doughnut pattern. In this case, as the feeding point is further moved inwards at the centre from the point E, the pattern experiences a cancellation effect in the bore sight i.e. a null in the z-direction. The current distribution for this configuration is shown in Figure 10b. It demonstrates that, while the centre patch section strongly resonates, there are equal and opposite currents in the four star arms. This makes configuration F similar to that of a loaded monopole antenna and hence, a doughnut shape pattern (#_max = 42° for all values of Φ) is generated. Advantageously, even though the pattern is similar to that of a loaded monopole, the gain offered is significantly higher. The AR and gain for configuration F is substantially 45 dB and 6.2dBi. This higher gain, enables the antenna system to have a larger signal to noise ratio (SNR) which in turn will reduce the system bit error rate (B.E.R.).

Thus, in summary, upon feeding from any of the four edges, (configurations A to D) the antenna radiates a tilted beam. When fed from a distance of 8.5mm (l/4^th of distance between patch centre and strip end) from the end corner of the star (configuration E), the pattern takes and axial shape and upon feeding in the middle of the star (configuration F) a doughnut pattern is realised. Hence, when these feeding points are excited, one by one, most of and in some circumstances, the whole of the space in front of antenna can be scanned. The gain and CPAR (circular polarization axial ration) for the three beams is above 6.2dBi and 45dB, respectively, with polarization dominance, in the EΦ direction

Thus, the disclosed invention provides an antenna wherein the beam direction may be steered without requiring phase shifting equipment, multiple antennas or the like. Such an antenna is useful in many applications, e.g. mobile UWB wireless communications, fixed UWB wireless communications and the like.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be capable of designing many alternative embodiments without departing from the scope of the invention as defined by the appended claims. In the claims, any reference signs placed in parentheses shall not be construed as limiting the claims. The word "comprising" and "comprises", and the like, does not exclude the presence of elements or steps other than those listed in any claim or the specification as a whole. The singular reference of an element does not exclude the plural reference of such elements and vice-versa. The invention may be implemented by means of hardware comprising several distinct elements. In a device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

Claims;

1. An antenna comprising a generally star-shaped conductive element for generating a radiation beam, a plurality of feeding ports being provided on said conductive element for selective excitation by a signal, said feeding ports being positioned such that excitation thereof results in deflection of said radiation beam so as to steer said radiation beam in the direction of deflection.

2. An antenna according to claim 1, wherein said conductive element comprises a plurality of conductive sections, each coupled to at least one other conductive section.

3. An antenna according to claim 2, wherein said conductive sections are coupled together in a closed loop configuration, comprising at least four, equi-distant, substantially V-shaped conductive sections coupled together in said generally star-shaped configuration.

4. An antenna according to claim 3, wherein a feeding port is provided at the apex of each conductive section.

5. An antenna according to claim 2, wherein said conductive element comprise at least four, equi-distant, substantially triangular shaped, conductive sections coupled so as to provide said generally star-shaped configuration.

6. An antenna according to claim 5, wherein a feeding port is provided at a distal end of each conductive section.

7. An antenna according to any of claims 5 to 6, wherein a feeding port is provided at a centre point of said antenna.

8. An antenna according to claim 7, wherein a feeding port is provided on a centreline of one conductive section, one quarter of the distance from said distal end to said centre point.

9. An antenna according to any preceding claim, wherein said antenna is provided on a dielectric substrate.

10. An antenna according to any preceding claim, wherein at least two conductive sections provide current paths that are substantially symmetrical about an axis of said antenna.

11. An antenna according to any preceding claim, wherein each port is excited separately.

12. An antenna according to any of claims 1 to 10, comprising means for selectively exciting one or more of the feeding ports at a time.

13. An antenna substantially as herein described with reference to the accompanying drawings.