Dual-polarization radiating element of a multiband antenna
CROSS-REFERENCE
This application is based on French Patent Application No. 10 54 150 filed
on May 28, 2010, the disclosure of which is hereby incorporated by reference thereto in
its entirety, and the priority of which is hereby claimed under 35 U.S.C. § 1 19.
TECHNICAL FIELD
The present invention pertains to the field of multiband antennas of base stations
for radiocommunications. These antennas are most commonly of a "panel" type and
comprise dual-polarization radiating elements which are normally aligned.
BACKGROUND
A dual-polarization radiating element generally comprises two dipoles (or systems
of dipoles) crossing one another at a 45° orthogonal polarization, one to generate the
first polarization signal (- 45°) and the other to generate the second polarization signal
(+ 45°). Techniques for constructing radiating elements are varied.
The main conditions for a radiating element, as used in base stations' panel
antennas, particularly include:
a) the radio performance of the radiating element (impedance, insulation between
the two polarizations, radiation pattern) must be good and stable over a very broad
frequency band,
b) the distribution surface area of the radio frequency current (RF) must be sufficient
to allow the use of a small-sized reflector for the antenna, with the accompanying
decrease in cost,
c) the structure for feeding the radiating element must be simple, such as a single
coaxial cable for feeding each polarization of the radiating element,
d) the structure of the radiating element must preferentially enable the use of
multiple radiating elements aligned along a common axis, in order to enable the
integration of multiband antennas,
e) the radiating element must be as low-cost as possible (using small quantities of
material, short assembly times, few parts, and moderate labor costs).
Several families of dual-polymerization radiating elements are already well known
and used by manufacturers of different types of antennas. However, none of the existing
radiating elements simultaneously and completely fulfills the five conditions described
above.
A first family comprises coaxial radiating elements, each formed of two orthogonal
half-wave dipoles. Provided that the shape of the dipoles is properly designed, the radio
performance of these radiating elements is good. However, all of these radiating
elements suffer from a limited surface area for distributing the RF current, which is only
concentrated on the two orthogonal half-wave dipoles. Consequently, a wide reflector is
necessary to achieve a given horizontal beamwidth on the antenna (65°, for example),
which leads to additional costs on the antenna's structure (larger radome, etc.). This first
family of radiating elements therefore does not meet condition (b) described above.
A second family comprises radiating elements, each formed of two half-wave
dipoles separated by a distance of approximately one-half the wavelength at the
operating frequency. The radio performance is good. The RF current's distribution
surface area is wide, making it possible to obtain the desired antenna beamwidth with a
limited-size reflector. However, the radiating elements must be fed at a four . (two points
for each polarization) leading to additional complexity and cost for the feeding network.
This second family of radiating elements therefore does not meet conditions (c) and (e)
described above. Some amount of surface area is available at the center of the radiating
element such that it is possible to add a radiating element for multiband operation in
order to satisfy condition (d).
There is an alternative radiating element that belongs to the second family. This
radiating element has a sufficient surface area to distribute RF current, and it is fed only
at two points (one point per polarization). The assembly time and cost of the material
may be kept under control, particularly as a result of the milling technique. A major
limitation of this type of radiating element is multiband integration. This is because
adding radiating elements for a high frequency band requires using the technique of
overlapping radiating elements. This means that the upper radiating element cannot use
the shared reflector to generate its radiation pattern. The lower radiating elements are
then used as reflectors, but their surface area is very low. This alternative from the
second family of radiating elements only partially meets condition (d) described above.
A third family comprises dual-polarization radiating elements of the patch type
(half-wave). The radio performance is not as good as for radiating elements formed of
dipoles, in particular in terms of bandwidth, so condition (a) is only partially satisfied. This
radiating element has a sufficient RF current distribution surface area, so that it can be
used with a reflector whose dimensions are small. The feeding structure is simple
because each dual-polarization radiating element can be fed with just two coaxial cables.
The patch radiating element may be designed to have a low cost. It is possible to add
another radiating element on top of the patch radiating element. In this situation, the
added radiating element must be fed through the patch element, which is not easy.
However, the upper radiating element cannot use the shared reflector to generate its
radiation pattern, but rather must use the patch radiating element located below it as a
reflector, with the drawback of a reduced surface area. This third family of radiating
elements therefore only partially meets condition (d) described above.
SUMMARY
It is a purpose of the present invention to propose a dual-polarization radiating
element for a multiband antenna, which simultaneously and completely fulfills all of the
conditions described above.
The object of the present invention is a dual-polarization radiating element for an
antenna, comprising
a support with a high dielectric constant whose shape is roughly cylindrical, having an
axis of revolution.
at least one first and one second pair of dipoles printed onto a first surface of the
support, the dipoles of the first pair being roughly orthogonal to the dipoles of the second
pair,
conductive lines, in order to feed each dipole, printed onto a second surface of the
support,
According to one aspect of the invention, the support is placed on a flat reflector,
with the cylindrical support's axis of revolution being perpendicular to the plane of the
reflector
The invention falls within the scope of directive antennas, meaning antennas
whose beamwidth in the horizontal plane is divided into sectors. The reflector, owing to
its flat shape and its placement perpendicular to the cylindrical support, makes it possible
to control the dividing of the pattern in the horizontal plane, meaning the value of its
beamwidth (-3dB).
Preferentially, the first surface supporting the dipoles is the outer surface of the
support.
According to a first aspect, the transversal axis passing through the middle of the
dipoles is a distance away from the reflector equal to about one-quarter the wavelength
at the central operating frequency.
According to a second aspect, the median axes passing through the middles of
two consecutive dipoles are about one half-wavelength apart from one another
According to a third aspect, the pair of dipoles is fed by a single coaxial cable.
According to a fourth aspect, the support is made up of a material with a high
dielectric constant, typically 2.5 to 4.5, and narrow thickness, typically 0.5 mm to 2 mm.
According to one embodiment, the radiating element comprises at least two
groups of dipoles. Each group of dipoles comprises at least one first and one seconds
pair of dipoles supported by the support, and each group of dipoles operates within a
different frequency band.
According to one variant embodiment, the support forms concentric cylinders
linked to one another, each cylinder supporting a group of dipoles and each group of
dipoles operating within a different frequency band.
According to one embodiment, the diameter of each of the concentric cylinders is
a function of the wavelength at the central operating frequency within each of the
frequency bands.
According to another embodiment, the concentric cylinders are connected to one
another by support parts that are free of dipoles, in order to form a spiral.
According to yet another embodiment, the first group of dipoles disposed on the
outer surface of the larger-diameter cylinder functions within the lower-frequency band,
and the last group of dipoles disposed on the outer surface of the smaller-diameter
cylinder functions within the higher-frequency band.
According to one particular embodiment, a first group of dipoles functions within
the GSM frequency band, a second group of dipoles functions within the DCS frequency
band, and a third group of dipoles functions within the LTE frequency band.
A further object of the invention is a multiband antenna comprising at least one
first radiating element, as previously described, operating within a first frequency band,
and at least one second radiating element operating within a second frequency band.
The second radiating element is disposed at the center of the cylinder formed by the
support of the first radiating element, the first and second radiating elements being
disposed on a shared flat reflector.
BRIEF DESCRIPTION OF THE DRAWINGS
Other characteristics and advantages of the invention will become apparent
while reading the following description of embodiments, which are non-limiting and given
for purely illustrative purposes, and in the attached drawing, in which:
- Figure 1 depicts a radiating element according to a first embodiment of the
invention,
- Figures 2a and 2b respectively show dipoles and feed lines of the radiating
element from Figure 1,
- Figure 3 depicts the standing wave ratio SWR of each pair of dipoles as a
function of the frequency F in MHz for the radiating element from Figure 1,
- Figure 4 depicts the decoupling K between the two pairs of dipoles in dB, as a
function of the frequency F in MHz for the radiating element from Figure 1,
- Figure 5 depicts a radiating element according to a second embodiment of the
invention,
- Figure 6 depicts a radiating element according to a third embodiment of the
invention,
- Figure 7 is a schematic perspective view of a radiating element according to a
fourth embodiment of the invention,
- Figures 8a and 8b respectively show dipoles and feed lines of the radiating
element from Figure 7.
DETAILED DESCRIPTION
In a first embodiment depicted in Figures 1, 2a, and 2b, the dual-polarization
radiating element 1 is formed of two half-wave dipoles 2 each comprising a conductive
feed line 3. The dipoles 2 are supported by a shared support 4 that is fastened to the
reflector 5. The radiating element 1 is constructed by forming the shared support 4 into a
cylindrical shape. The cylindrical support 4 thereby obtained is then positioned in a
perpendicular fashion onto a shared flat reflector 5 with multiple radiating elements 1.
In this example embodiment, the dipoles 2 are printed onto a first outer
surface 6 of the shared support 4. Each dipole 2 is fed by a conductive line 3 located on
the second inner surface 7 opposite the support 4 . Naturally, it is possible to print the
dipoles on the inner surface and the feed lines on the outer surface. The conductive feed
line 3 is, for example, a "microstrip" printed directly on the support 4. This shared
support 4 , whose circumference is about two wavelengths 2, is made of an insulating
material with a high dielectric constant (typically 2.5 to 4.5), with a narrow thickness
(typically 0.5 mm to 2 mm) and low cost. Alternatively, the air may also constitute a
support, in which case the dipoles and feed microstrips may be formed of metal plates
connected by insulating elements. Each pair of dipoles 2 is fed at a single point via
coaxial cable 8 passing through the reflector 5.
Thus, a group of two pairs of half-wave dipoles 2 at the central frequency of the
operating frequency band is achieved. The transversal axis 9 passing through the middle
of the dipoles 2 is located a distance L of about a quarter wavelength (/4) away, above
the surface of the reflector 5. The median axes 10 passing through the middle of the
contiguous dipoles 2 are separated from one another by a distance D of about a
half-wavelength (/2). The diagonal axis 1 1 passing through the middle of each of the
dipoles 2 of the first pair is positioned with a 45° angle relative to the longitudinal axis 12
of the reflector 5 in order to create the - 45° polarization, and the diagonal axis 13
passing through the middle of each of the dipoles 2 of the second pair likewise creates
the + 45° polarization.
The transmission and reflection parameters of the radiating element's two pairs
of dipoles, measured within the frequency band of 600 to 1100 MHz, are depicted in
Figures 3 and 4. These results show very stable characteristics within a large frequency
band.
Figure 3 detects the standing wave ratio SWR of each pair of dipoles as a
function of the frequency F in MHz. The standing wave ratio SWR is less than 1.5 for a
frequency domain F ranging from 650 to 1050 MHz, i.e. a bandwidth corresponding
to 47% of the central frequency of the frequency band.
Figure 4 depicts the decoupling K in dB between the two pairs of dipoles as a
function of the frequency F in MHz. The decoupling K is greater than 20 dB for a
frequency domain ranging from 650 to 1100 MHz.
Now consider Figure 5, which depicts another embodiment of a
dual-polarization radiating element 50, operating for example at a GSM frequency on the
order of 900MHz, making it possible to form an antenna that operates within a dual
frequency band.
The cylindrical shape of the support 5 1 of the radiating element 50 leaves a
large area 52 empty at its center. This free area 52 may be used to add, at the center of
the radiating element 50, another radiating element 53 operating within a greater
frequency than (DCS 1800MHz, in this example).
The radiating element 53 may be formed of two orthogonal half-wave dipoles.
This may, for example, be a radiating element belonging to the first family described
above, or a radiating element that may have any other shape. The height of this radiating
element 53 operating at high frequency band is about a quarter-wavelength (/4). As the
radiating element 53 with a high frequency band is placed above the shared reflector 54,
the characteristics of its radiation pattern are maintained.
Figure 6 depicts another embodiment of a dual-polarization radiating
element 60, operating for example at a CDMA frequency on the order of 800MHz,
making it possible to form an antenna that operates within a dual frequency band.
As the empty area 6 1 in the middle of the cylinder formed by the support 62 of
the radiating element 60 is very large, it is possible to insert a radiating element 63 into it
that operates at lower frequencies and has greater dimensions. The diameter of the
cylindrical support 62 depends on the wavelength at the central operating frequency in
the highest frequency band (in this example, 800 MHz). The radiating element 63, whose
type is called "butterfly", is formed of two dipoles crossing each other at an orthogonal
polarization ± 45°. The radiating element 63 inserted into the center of the cylindrical
support 62 operates within a low-frequency band (for example, LTE 700 MHz). It is a
thereby possible to construct an antenna operating within a dual band at relatively similar
frequencies, such as LTE 700 MHz and CDMA 800 MHz, working from the dualpolarization
radiating element 62. The two radiating elements 62 and 63, disposed
concentrically, use the shared reflector 64, and the antenna's width can consequently be
reduced.
Figures 7, 8a, and 8b depict a dual-polarization radiating element 70 capable of
operating within multiple frequency bands. The multiband radiating element 70 is
constructed of a single part. All the dipoles and feed lines needed for the radiating
element to operate 70 are supported by a shared support 7 1 fastened onto a shared
reflector 72. This substrate 7 1 may have a low cost and comprise a reduced quantity of
insulating material.
In this example, the radiating element 70 is a three-band element. Three
groups 73, 74, 75 of four dipoles each 73a... 73d, 74a... 74d, 75a... 75d are printed on a
first outer surface 76 of the shared support 7 1. Each group 73, 74, 75 corresponds to a
different frequency band. Each dipole 73a. ..73d, 74a. ..74d, 75a. ..75d is individually fed
by a microstrip line 73e...73h, 74e...74h, 75e...75h printed on the second lower
surface 77 opposite the shared support 7 1. Each group 73, 74, 75 of four dipoles is fed
by just two coaxial cables 78 crossing the reflector 72, leading to a total of six coaxial
cables 78 for the three-ban dual-polarization radiating element 70.
The single shared support 7 1 is formed by means of three cylindrical shapes of
different diameters such that the parts of the support 7 1 related to each group 73, 74, 75
form concentric cylinders whose diameters depend on the wavelength at the central
operating frequency in each of the frequency bands. The length of the support 7 1 is
calculated such that the three concentric cylinders are connected to one another by
support parts 79 that have no dipoles. The group 73 of dipoles 73a. ..73d disposed on
the outside of the largest-diameter cylinder operates at the lower frequency, and the
group 75 of dipoles 75a... 75d disposed on the inside of the smallest-diameter cylinder
operates at the highest frequency. Three groups 73, 74, 75 each of two pairs of
half-wave dipoles are therefore obtained, each at the central frequency of their
respective operating frequency bands, for example GSM 900 MHz (73), DCS 1800 MHz
(74) and LTE 2600MHz (75).
The transversal axis 80 passing through the middle of the dipoles of each group
is located at a distance L of about a quarter wavelength away (/4) at the central
operating frequency, above the surface of the reflector 72. The median axes 8 1 passing
through the middle of two consecutive dipoles are about a half-wavelength (/2) away
from one another at the central operating frequency. The
dipoles 73a...73d, 74a...74d, 75a...75d are positioned so as to create two orthogonal
polarization signals within each of three operating frequency bands.
If need be, frequency band separating devices may be printed on the inner
surface 77 of the shared support 7 1 supporting the microstrip
lines 73e...73h, 74e...74h, 75e...75h. These devices make it possible to use only two
coaxial cables in total, i.e. one cable per polarization, to feed the three-band dualpolarization
radiating element.
Naturally, the present invention is not limited to the described embodiments, but
rather is subject to many variants accessible to the person skilled in the art without
departing from the spirit of the invention. In particular, the principle described above for
three frequency bands may be extended to designing a multiband dual-polarization
radiating element operating on more than three frequency bands.
A dual-polarization radiating element for an antenna, comprising
- a support with a high dielectric constant whose shape is roughly cylindrical
having an axis of revolution,
- at least one first and one second pair of dipoles printed onto a first surface of
the support, the dipoles of the first pair being roughly orthogonal to the dipoles
of the second pair,
- conductive lines, in order to feed each dipole, printed onto a second surface of
the support,
wherein that support is placed on a flat reflector, with the cylindrical support's
axis of revolution being perpendicular to the plane of the reflector.
A radiating element according to claim 1, wherein the first surface supporting
the dipoles is the outer surface of the support.
A radiating element according to one of the claims 1 and 2, wherein the
transversal axis passing through the middle of the dipoles is a distance away
from the reflector equal to about one-quarter the wavelength at the central
operating frequency.
A radiating element according to one of the claims 1 to 3, wherein the median
axes passing through the middles of two consecutive dipoles are about one
half-wavelength apart from one another.
A radiating element according to one of the preceding claims, wherein the pair
of dipoles is fed by a single coaxial cable.
A radiating element according to one of the preceding claims, comprising at
least two groups of dipoles, each group of dipoles comprising at least a first and
a second pair of dipoles supported by the support, and each group of dipoles
operating within a different frequency band.
A radiating element according to claim 6, wherein the support forms concentric
cylinders linked to one another, each cylinder supporting a group of dipoles and
each group of dipoles operating within a different frequency band.
A radiating element according to claim 7, wherein the diameter of each of the
concentric cylinders is a function of the wavelength at the central operating
frequency within each of the frequency bands.
A radiating element according to one of the claims 7 and 8, wherein the
concentric cylinders are connected to one another by support parts that are free
of dipoles, in order to form a spiral.
A radiating element according to one of the claims 7 to 9, wherein the first group
of dipoles disposed on the outer surface of the larger-diameter cylinder
functions within the lower-frequency band, and the last group of dipoles
disposed on the outer surface of the smaller-diameter cylinder functions within
the higher-frequency band.
A radiating element according to claim 10, wherein a first group of dipoles
functions within the GSM frequency band, a second group of dipoles functions
within the DCS frequency band, and a third group of dipoles functions within the
LTE frequency band.
A multiband antenna comprising at least one first radiating element according to
the preceding claims operating within a first frequency band, and at least one
second radiating element operating within a second frequency band, wherein
the second radiating element is disposed at the center of the cylinder formed by
the support of the first radiating element, the first and second radiating elements
being disposed on a shared flat reflector.
13. A multiband antenna according to claim 12, wherein the second radiating
element is a radiating element according to one of the claims 1 to 7.