WO2012140586A1

WO2012140586A1 - Omnidirectional antenna with a null in a selected direction

Info

Publication number: WO2012140586A1
Application number: PCT/IB2012/051782
Authority: WO
Inventors: Gordon MAYHEW-RIDGERS; Paul Andries VAN JAARSVELD
Original assignee: Vodafone Ip Licensing Limited
Priority date: 2011-04-12
Filing date: 2012-04-12
Publication date: 2012-10-18
Also published as: ZA201202632B; ZA201404722B

Abstract

An antenna arrangement 10 comprises an antenna array 12 comprising a first antenna 14 having a port 15 and a second antenna 16 spaced from the first antenna and having a port 17. A decoupler 18 is mounted between the first antenna 14 and the second antenna 16 on an imaginary line 20 extending between the first antenna and the second antenna. An exciter 22 is connected to the port 15 of the first antenna and to the port 17 of the second antenna. The exciter 22 is configured to drive the array 12 so that in a radiation pattern 26 associated with the array, there is at least a partial null 24 on the line 20.

Description

INTRODUCTION AND BACKGROUND

This invention relates to antenna arrangements and more particularly to an antenna arrangement, antenna array, feed network for the array and a method of generating a radiation pattern, which are suitable for use at a base station of a cellular network, for example.

Normally, in rural areas, the radiation pattern of a base-station antenna of a cellular network is omnidirectional in order to provide optimal coverage for mobile customers. However, there are cases where it is necessary to suppress radio-frequency (RF) radiation in a certain direction, for example to avoid RF interference in that direction. One example is where a radio telescope, normally being extremely susceptible to RF interference, is located in the direction. In such a case, radiation in that direction may typically have to be reduced significantly, around 30 dB or more, whilst maintaining the radiation pattern, and therefore coverage, in other directions. One solution to this problem may be to replace the omnidirectional antenna with sector antennas and to point them away from the aforementioned direction. However, it has been found that as soon as more than one sector antenna is used at a base station, it becomes very difficult if not impossible to reduce the signal in one direction by 30 dB or more, while maintaining the radiation pattern in other directions. In theory, a single null can be produced in an otherwise omnidirectional radiation pattern by an antenna array comprising of two omnidirectional antennas with a quarter-wavelength separation between their phase centres and a 90 degree phase difference between their excitations. However, the physical diameter of actual omnidirectional base-station antennas, the rather small quarter-wavelength separation requirement and the excessive mutual electromagnetic coupling between the closely spaced antennas, normally makes a physical implementation of the aforementioned theoretical solution impossible.

OBJECT OF THE INVENTION

Accordingly, it is an object of the present invention to provide an antenna arrangement, an antenna array, a feed network for the array and a method of generating a radiation pattern with which the applicant believes the aforementioned disadvantages may at least be alleviated, or which may provide a useful alternative for the known apparatus and methods.

SUMMARY OF THE INVENTION

According to the invention, there is provided an antenna arrangement comprising:

- an antenna array comprising a first antenna having a port; a second antenna spaced from the first antenna and which second antenna has a port; and a decoupler mounted between the first antenna and the second antenna on an imaginary line extending between the first antenna and the second antenna; and

- an exciter connected to the port of the first antenna and to the port of the second antenna and which exciter is configured to drive the array, so that in a radiation pattern associated with the array, there is at least a partial null on the imaginary line.

The first antenna and the second antenna may each be in the form of an omnidirectional antenna. The first and second antennas may each comprise an elongate omnidirectional antenna.

The first and second antennas are preferably mounted in juxtaposition, parallel to one another and orthogonally to the line. The antennas may be spaced more than a quarter wavelength apart, but preferably not more than a half wavelength apart. When reference is made to wavelength in this context in this specification, it should be understood as the wavelength associated with a centre frequency of a frequency band of interest.

The decoupler may comprise an elongate member and may be made of a dielectric material. In other embodiments, the decoupler may comprise an elongate member made of a conductive material and having any suitable shape in transverse cross section, such as flat, angled or curved.

In some embodiments, the decoupler may comprise a length of an angled member comprising first and second limbs at an angle of less than 180 degrees, preferably 90 degrees, relative to one another and the member may be mounted between the first and second antennas with a vertex of the member on the imaginary line and facing the first antenna and said angle facing the second antenna.

Preferably, the member has a length equal to that of the two antennas.

In some embodiments, the exciter may comprise a passive feed network, the feed network comprising a first signal path extending between a feed network port and the port of the first antenna, a first phase shifter connected in the first signal path, a second signal path extending between a coupler associated with the first signal path and the port of the second antenna, and a second phase shifter in the second signal path.

The first and second signal paths may comprise stripline and the coupler may comprise adjacent cooperating sections, typically quarter-wavelength sections, of the stripline of the first and second signals paths respectively. The sections msy be tunes bio or 3djust3 le, for exsmple m3nu3lly, to adjust the coupling between the first and second signal paths.

Each of the first phase shifter and the second phase shifter may comprise a section, typically a quarter-wavelength section, of the stripline of the relevant signal path and may be tuneable or adjustable, for example manually, to adjust the phase of a signal in the first signal path and a signal in the second signal path, respectively.

In other embodiments, the exciter may comprise a transceiver arrangement comprising a first transceiver having a first port which is connectable to the port of the first antenna, a second transceiver having a second port which is connectable to the port of the second antenna, a computerized controller and a computer program running or executing on a processor or computer of the controller for configuring the magnitude and phase of a signal at the first port of the transceiver arrangement and for configuring the magnitude and phase of a signal at the second port of the transceiver arrangement, so that there is at least the partial null in the radiation pattern on the imaginary line.

Further included within the scope of the present invention is an antenna array comprising a first antenna; a second antenna spaced from the first antenna; and a decoupler mounted between the first antenna and the second antenna on an imaginary line extending between the first antenna and the second antenna.

Still further included within the scope of the present invention is a passive feed network comprising a first signal path extending between a first feed network port and a second feed network port, a first phase shifter connected in the first signal path, a second signal path extending between a coupler associated with the first signal path and a third feed network port, and a second phase shifter in the second signal path.

Yet further included within the scope of the present invention is a method of generating an omnidirectional radiation pattern having at least a partial null in a selected direction, the method comprising the steps of:

- utilizing a first antenna and a second antenna mounted in spaced juxtaposition on an imaginary line extending in the selected direction;

- at least partially electromagnetically decoupling the first and second antennas from one another; and

- exciting the first and second antennas such that at least a partial null is formed on the imaginary line on another side of the second antenna as the first antenna. The electromagnetic decoupling may be such that it assists in introducing the partial null into the radiation pattern of the first antenna in the direction of the second antenna. The step of electromagnetic decoupling may comprise reflection of electromagnetic waves originating from either or both of the first antenna and the second antenna.

Still further included within the scope of the present invention is a computer-readable medium comprising a computer program which when executing on a computer configures the magnitude and phase of a signal at a first port of a transceiver arrangement and which port is connectable to a first antenna of an antenna array and configures the magnitude and phase of a signal at a second port of the transceiver arrangement and which port is connectable to a second antenna of the array, so that there is at least a partial null in a radiation pattern of the array on an imaginary line extending between the first antenna and the second antenna.

BRIEF DESCRIPTION OF THE ACCOMPANYING DIAGRAMS

The invention will now further be described, by way of example only, with reference to the accompanying diagrams wherein:

figure 1 is a high-level block diagram of a first example embodiment of an antenna arrangement; figure 2 is a diagrammatic perspective view of the antenna arrangement;

figure 3 is another high-level block diagram of the antenna arrangement;

figure 4 is a diagrammatic perspective view of relevant parts of a feed network forming part of the antenna arrangement;

figure 5 is a basic circuit diagram of a coupler forming part of the feed network;

figure 6 is a diagrammatic perspective view of a stripline implementation of the coupler;

figure 7 is a basic circuit diagram of a phase shifter forming part of the feed network;

figure 8 is a diagrammatic perspective view of a stripline implementation of the phase shifter;

figure 9 is a polar plot of a radiation pattern of the antenna arrangement having a deep null on a line extending through the first and second antennas;

figure 10 is a rectangular plot of the radiation pattern; and

figure 1 1 is a block diagram of another example embodiment of the antenna arrangement. DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

An antenna arrangement is generally designated by the reference numeral 10 in figures 1 , 2, 3 and 1 1.

The antenna arrangement 10 comprises an antenna array 12 comprising a first antenna 14 having a port 15, a second antenna 16 spaced from the first antenna and which second antenna has a port 17, and a decoupler in the form of a reflector 18 mounted between the first antenna 14 and the second antenna 16 on an imaginary line 20 extending between the first antenna and the second antenna. Hence, the decoupler 18 is located directly in line and between the first antenna 14 and the second antenna 16. The antenna arrangement 10 further comprises an exciter 22 connected to the array 12 and which exciter is configured to drive the array 12, so that in a radiation pattern 26 (shown in figures 9 and 10) associated with the array 12, there is at least a partial null 24 on the imaginary line 20. The null is preferably a deep null.

In the example embodiments shown, the first antenna 14 is an elongate omnidirectional antenna. The second antenna 16 is also an elongate omnidirectional antenna. The first and second antennas are mounted in spaced, juxtaposition and parallel to one another. The decoupler 18 is equal in length to the first antenna and the second antenna and comprises a length of a 90 degree angled conductive member comprising a first limb 18.1 and a second limb 18.2 at 90 degrees relative to one another. A vertex 18.3 of the member 18 is on imaginary line 20 and faces the first antenna 4, while the 90 degree angle 25 faces the second antenna 16.

As shown in figure 2, the array 12 is mounted in any suitable manner on a known tower 27 of a known base-station 28 of a known cellular network (not shown).

In the example embodiment of figure 1 1 , the exciter 22 comprises a transceiver arrangement 70 comprising a first transceiver 72 having a first port 73 and a second transceiver 74 having a second port 75. The first and second ports 73, 75 are connectable to the ports 15 and 17 of the antennas. The exciter 22 further comprises a computerized controller 76 comprising a computer 78. A computer program 80 executes on the computer for generating excitation parameters for configuring the magnitude and phase of a signal at the first port 73 and for configuring the magnitude and phase of a signal at the second port 75, so that there is at least the partial null 24 in the radiation pattern 26 on the imaginary line 20, as will be described in more detail below.

In the example embodiment shown in figure 1 , the exciter 22 comprises a passive feed network 23, as will be described in more detail below. The feed network 23 is mounted in a suitable hermitically sealed housing 30 on the tower.

Referring to figures 1 and 4, the feed network 23 comprises a first feed network port 32 for a signal to or from a transceiver 29 forming part of the base-station 28, a second feed network port 34, which is connectable to the port 15 of the first antenna 14 and a third feed network port 36, which is connectable to the port 17 of the second antenna 16. A first signal path 38 comprising stripline 40 (shown in figures 4, 6 and 8) extends between the first feed network port 32 and the second feed network port 34. A first phase shifter 42 is provided in the first path 38. A second signal path 44 comprising stripline 46 extends between a directional coupler 48 cooperating with the first path in known manner and the third feed network port 36. A second phase shifter 50 is provided in the second signal path 44. As shown in figures 3 and 4, a port 54 for a termination element 56 of the second signal path is provided on the housing 30.

A circuit diagram of the coupler 48 is shown in figure 5 and it comprises a quarter-wavelength section 40.1 of stripline 40 and a quarter-wavelength section 46.1 of stripline 46. As shown in figure 6, the coupler is adjustable or tuneable in that the distance or a gap 47 between the sections 40.1 and 46.1 is manually adjustable by threaded members 60 made of a low-loss dielectric material. The threaded members cooperate with complementary threaded holes in the housing 30. The coupler 48 preferably employs broadside coupling, which is more sensitive than edge coupling and also less complex to implement in an adjustable and tuneable way.

The first phase shifter 42 and second phase shifter 50 are similar in configuration and operation and therefore only the first phase shifter 42 will be described in more detail hereinafter. A circuit diagram of the first phase shifter is shown in figure 7 and comprises a quarter-wavelength section 40.2 of stripline 40 between a first capacitor 62 and a second capacitor 64. This is a loaded-line phase shifter, which produces low return-loss values. As shown in figure 8, each of the capacitors comprises a gap 66 between a conductive threaded member 68 and the section 40.2 of the stripline. The phase is adjustable by changing the value of the capacitance of capacitors 62, 64 by manual manipulation of the conductive threaded member 68 and hence the size of the gap 66. This arrangement allows for a limited number of moving parts.

The example implementations of the coupler 48, as shown in figure 6, and the phase shifters 42, 50, as shown in figure 8, are expected to allow for nearly independent adjustability and tubeability, more particularly fine- tuning, of the magnitude and phase of the complex excitation parameters associated with the two antennas 14 and 16. The contactless implementation of the conductive moveable members and/or section is also expected to reduce the risk of passive intermodulation in the feed network 23.

Assume that Ei is the radiated electric far field of the first antenna 14 for unity excitation while in the presence of the second antenna 16 and the decoupler 18, but with the port 17 of the second antenna 16 terminated. Similarly, E₂ is the radiated electric far field of the second antenna 16 for unity excitation while in the presence of the first antenna 14 and the decoupler 18, but with the port 15 of first antenna 14 terminated. The values of Ei and E₂ are typically measured on a suitable antenna test range. When using an array of two antennas, complex excitation parameters ai and a₂. associated with the first antenna 14 and second antenna 16 respectively, may be found by choosing them such that the far fields of the two antennas cancel out in the direction of the required null. In the direction of the null, this may be expressed as

If, for example, the value of ai is chosen beforehand, the value of a₂ may be calculated as

a₂ = - a-|Ei / E₂

The feed network 23 or computer program 80, as the case may be, is then designed to produce the complex excitation parameters ai and a₂ (magnitude and phase) at ports 34, 36 and 73, 75, respectively. In the case of embodiments comprising the passive feed network 23, due to measurement and manufacturing tolerances, fine tuning of the coupler 48 and phase shifters 42, 50 may normally be required to set these excitation parameters accurately.

It is believed that the decoupler 18 introduces a partial null into the radiation pattern of the first antenna 14 in the direction of the second antenna 16. A relatively small current in the second antenna 16, proportional to excitation parameter a₂, is then required to cancel the far field from the first antenna 14 in the direction of the required null. This implies that the magnitude of the excitation parameter a₂ is smaller than the magnitude of the excitation parameter a-i and explains the presence of the coupler 48 in the feed network 23. The coupler 48 couples off a part of the signal from the port 32, proportional to the magnitude of a₂, towards the port 36 to the second antenna 16, while the rest of the input signal, proportional to the magnitude of a-i, is fed through to the port 34 to the first antenna 14. The first signal path 38 and the second signal path 44 typically have different lengths to provide for the difference in the phases of the complex excitation parameters ai and a₂. The first phase shifter 42 and the second phase shifter 50 are required to fine-tune the required phase difference between the complex excitation parameters

and a₂. Due to mutual electromagnetic coupling, the input impedance of each antenna is a function of the electric current that flows on the antenna itself as well as the electric current that flows on the other antenna. By reducing the mutual electromagnetic coupling between the antennas 14 and 16, the input impedance of each antenna becomes less dependent on the ratio of the currents that flow on them and mostly depend on the input impedance of the antenna itself in the presence of the decoupler 18. The decoupler 18, in the form of a 90 degree angled member, dramatically reduces the mutual electromagnetic coupling between the first antenna 14 and the second antenna 16. Due to the magnitude of the complex excitation parameter ai being much larger than the magnitude of the complex excitation parameter a₂, the electric current flowing on the first antenna 14 will be much larger than the electric current flowing on the second antenna 16. As such, the input impedance at the port 32 to the feed network 23 will be more sensitive to an impedance mismatch at port 15 of the first antenna 14 than to an impedance mismatch at port 17 of the second antenna 16. It is therefore important that the input impedance of the first antenna 16 is well matched to the port 34 of the feed network 23 or port 73 of the transceiver arrangement 70 as the case may be. By using a 90 degree angled member for the decoupler 18, with the 90 degree angle 25 facing towards the second antenna 16, the first antenna 14 essentially faces the vertex 18.3 of the 90 degree angled member, which has a minimal effect on its input impedance. This is due to the radiated fields from the first antenna 14 mostly being reflected away from the decoupler by the vertex 18.3 of the decoupler. The second antenna 16 faces the 90 degree angle 25 of the 90 degree angled member, which has a more significant effect on its input impedance. However, as has been stated, the overall input impedance at the port 32 to the feed network 23 is less dependent on this mismatch and therefore it does not pose a significant problem. It is possible to use alternative cross-sectional shapes for the decoupler, such as flat or curved. A decoupler 18 with a wider cross section is expected to provide more decoupling between the antennas 14 and 16 than one with a narrower cross section. The cross-sectional shape of the elongate member 18 may hence take different suitable shapes or configurations, but care must be taken that a shape is not used that reflects radiated fields back into the first antenna 14, thereby affecting its input impedance.

The phase centre of an elongate omnidirectional antenna normally lies in the middle of a line that runs axially through the centre of the antenna. When a decoupler 18 is used in the array 12, the separation distance between the antennas 14 and 16 may be a half wavelength, which is double the separation distance of the theoretical arrangement referred to in the introduction of this specification and wherein no decoupler is used. It has been determined that the phase centres of the two antennas 14 and 16 should not be more than a half wavelength apart, to ensure that the radiation pattern 26 is smooth without excessive ripple. It has further been found that the arrangement 10 is capable of producing a null of more than 30 dB deep over about a 20 degree sector and in a desired frequency range. A decoupler 18 with a fairly narrow cross section will produce a wider main lobe opposite to the null, while a wider cross section will produce a narrower main lobe opposite to the null.

Hence, in use, the array 12, when used at a cellular base station 28 may be set up with the second antenna 16 and the imaginary line 20 in line with any radio-frequency (RF) sensitive area that needs protection from the cellular network. Such an area may be a core or any other part of a radio- astronomy site. On the other hand, the arrangement 10 or array 12 may be used to protect cellular networks against RF interference from external sources, such as high-power radar systems. Furthermore, the arrangement may be deployed next to a border area where a cellular operator may want to limit cross-border RF spillage from a base station 28, but still provide RF coverage on his own side of the border. Still furthermore, for a two-sector cellular base station, the array 12 may be deployed on a base-station tower together with a sector antenna (not shown), which points in the direction of the null 24, providing one wide sector and one narrow sector with minimal overlap between the sectors. It will further be appreciated that the arrangement, array, feed network, computer program and method may also have other applications, different to that in this field of cellular communications, for example in general wireless broadcasting.

Claims

1. An antenna arrangement comprising:

- an exciter connected to the port of the first antenna and to the port of the second antenna and which exciter is configured to drive the array, so that in a radiation pattern associated with the array, there is at least a partial null on the line.

2. An antenna arrangement as claimed in claim 1 wherein the first antenna and the second antenna are each in the form of an omnidirectional antenna.

3. An antenna arrangement as claimed in any one of claims 1 and 2 wherein the first antenna and the second antenna each comprises an elongate antenna element and wherein the first antenna and the second antenna are mounted in spaced juxtaposition, parallel to one another and orthogonally to the line.

4. An antenna arrangement as claimed in claim 3 wherein the first antenna and second antenna are spaced more than a quarter wavelength and up to and including a half wavelength apart.

5. An antenna arrangement as claimed in claim 3 wherein the decoupler comprises an elongate member and is made of an electrically conductive material.

6. An antenna arrangement as claimed in claim 5 wherein the decoupler comprises a length of an angled member comprising first and second limbs which are at an angle of 90 degrees relative to one another and wherein the member is mounted between the first and second antennas with a vertex of the member facing the first antenna.

7. An antenna arrangement as claimed in any one of claims 3 to 6 wherein the decoupler, first antenna element and second antenna element are of equal length.

8. An antenna arrangement as claimed in any one of claims 1 to 7 wherein the exciter comprises a passive feed network, the feed network comprising a first signal path extending between a feed network port and the port of the first antenna, a first phase shifter connected in the first signal path, a second signal path extending between a coupler associated with the first signal path and the port of the second antenna and a second phase shifter in the second signal path.

9. An antenna arrangement as claimed in claim 8 wherein the coupler comprises stripline and is tuneable or adjustable, to adjust the coupling between the first and second signal paths.

10. An antenna arrangement as claimed in claim 8 or claim 9 wherein each of the first phase shifter and the second phase shifter comprises stripline and is tuneable or adjustable, to adjust the phase of a signal in the first signal path and a signal in the second signal path, respectively.

1 1 . An antenna arrangement as claimed in any one of claims 1 to claim 7 wherein the exciter comprises a transceiver arrangement having a first port which is connectable to the port of the first antenna, a second port which is connectable to the port of the second antenna, a computerized controller and a computer program executing on the controller for configuring the magnitude and phase of a signal at the first transceiver port and for configuring the magnitude and phase of a signal at the second transceiving port, so that there is at least the partial null in the radiation pattern on the imaginary line.

12. An antenna array comprising a first antenna; a second antenna spaced from the first antenna; and a decoupler mounted between the first antenna and the second antenna on an imaginary line extending between the first antenna and the second antenna.

13. A passive feed network comprising a first signal path extending between a first feed network port and a second feed network port, a first phase shifter connected in the first signal path, a second signal path extending between a coupler associated with the first signal path and a third feed network port, and a second phase shifter in the second signal path.

14. A method of generating an omnidirectional radiation pattern having at least a partial null in a selected direction, the method comprising the steps of:

- at least partially electromagnetically decoupling the first and second antennas from one another; and exciting the first and second antennas such that at least a partial null is formed on the imaginary line on another side of the second antenna as the first antenna.

15. A computer-readable medium comprising a computer program which when executing on a computer configures the magnitude and phase of a signal at a first port of a transceiver arrangement and which port is connectable to a first antenna of an antenna array and configures the magnitude and phase of a signal at a second port of the transceiver arrangement and which port is connectable to a second antenna of the array, so that there is at least a partial null in a radiation pattern of the array on a line extending between the first antenna and the second antenna.