ANTENNA ELEMENT FOR TWO ORTHOGONAL POLARIZATIONS

ELEMENT D'ANTENNE DONNANT DEUX POLARISATIONS ORTHOGONALES

English Abstract

An antenna element intended for transmission and
reception of orthogonal polarisations having a good
polarisation purity of electromagnetic radiation is
described. It comprises a ground plane and at least one
radiation disc connected to two ports for the supply of a
desired electromagnetic radiation, preferably in the
microwave range. The distance between the ground plane and
the radiation disc is a fraction of the wavelength .lambda. of the
radiation. The antenna element is in particular
characterized in that the ports are located at an electric
distance of 90° from each other and that a quarterwave
transformer is added between the ground plane and the
radiation disc that is shaped so that the transmission line
impedance between the ports is reduced in order to make the
ports radiationally independent of each other.

French Abstract

La présente invention concerne un élément d'antenne conçu pour l'émission et la réception de polarisations orthogonales avec une bonne pureté de polarisation du rayonnement électromagnétique. L'élément comprend un plan de sol (2) et au moins un disque de rayonnement (1) connectés à deux orifices (4, 5) fournissant un rayonnement électromagnétique, de préférence dans la gamme des micro-ondes. La distance entre le plan de sol (2) et le disque de rayonnement (1) est une fraction de la longueur d'onde lambda du rayonnement. Cet élément d'antenne est notamment caractérisé par une position des orifices (4, 5) comportant entre eux un écart électrique de 90 DEG , et l'adjonction d'un transformateur quart-d'onde (6) entre le plan de sol (2) et le disque de rayonnement (1), lequel transformateur est configuré de façon que l'impédance propre à la ligne de transmission entre les orifices (4, 5) est réduite, ce qui permet de rendre les orifices indépendants l'un de l'autre en matière de rayonnement.

Claims

4~~
The embodiments of the present invention in which an exclusive property or
privilege is claimed are defined as follows:
1. An antenna element intended for transmission and reception of two
orthogonal
polarisations having good polarisation purity of electromagnetic radiation,
comprising a
ground plane and at least one radiation disc connected to two ports for the
supply of a
desired electromagnetic radiation, the distance between the ground plane and
the
radiation disc being a fraction of the wavelength .lambda. of the radiation;
wherein the ports are located at an electrical distance of 90° from
each other and a
quarterwave transformer is located between the ground plane and the radiation
disc that
has such a shape that the transmission line impedance between the ports is
reduced to
make the ports radiationally independent of each other.
2. An antenna element according to claim 1, wherein the quarterwave
transformer is
located inside the 90° sector between the ports.
3. An antenna element according to claim 1, wherein the quarterwave
transformer is
located inside the 270° sector opposite the ports.
4. An antenna element according to claim 1, wherein the quarterwave
transformer
consists of two parts, symmetrically located inside the 90° sector
between the ports and
inside the 270° sector opposite the ports.
5. An antenna element according to any one of claims 1 to 4, wherein the
quarterwave transformer is formed by steps in the ground plane located more
closely to
the radiation disc than the rest of the ground plane.
6. An antenna element according to any one of claims 1 to 4, wherein the
quarterwave transformer is formed by steps in the radiation disc located more
closely to
the ground plane than the rest of the radiation disc.
7. An antenna element according to any one of claims 1 to 6, wherein the
electromagnetic radiation is in the microwave range.

5
8. An antenna element for transmission and reception of two orthogonal
polarisations of electromagnetic radiation having good polarisation purity,
comprising:
a ground plane;
at least one radiation disk connected to the ground plane;
two ports for supplying microwave range electromagnetic radiation connected to
the at
least one radiation disk, the two ports are arranged at an electrical distance
of 90° from
each other; and
a quarterwave transformer arranged between the ground plane and the at least
one
radiation disk;
wherein the ground plane and the at least one radiation disk are separated by
a distance
of a fraction of a wavelength of the microwave range electromagnetic
radiation.
9. The antenna element according to claim 8, wherein the radiation disk has a
circular shape.
10. The antenna element according to claim 8, wherein the radiation disk has a
quadratic shape.
11. The antenna element according to claim 8, 9 or 10, wherein the quarterwave
transformer is arranged inside the 90° distance between the two ports.
12. The antenna element according to claim 8, 9 or 10, wherein the quarterwave
transformer is arranged inside a 270° distance opposite the two ports.
13. The antenna element according to any one of claims 8 to 12, wherein the
quarterwave transformer comprises one part.
14. The antenna element according to claim 8, 9 or 10, wherein the quarterwave
transformer comprises two parts, a first part arranged inside the 90°
distance between the
two ports and a second part arranged inside a 270° distance opposite
the two ports.

6
15. The antenna element according to any one of claims 8 to 14, wherein the
quarterwave transformer comprises a plurality of steps in the ground plane
arranged
closer to the at least one radiation disk than a remainder of the ground
plane.
16. The antenna element according to any one of claims 8 to 14, wherein the
quarterwave transformer comprises a plurality of steps in the at least one
radiation disk
arranged closer to the ground plane than a remainder of the at least one
radiation disk.

Description

WO 96/35240 PCT/SE96/00561
CA 02219925 1997-10-30
ANTENNA ELEMENT FOR TWO ORTHOGONAL POLARIZATIONS
The present invention relates to an antenna element intended for transmission
and reception
of two orthogonal polarisations having a good purity of polarisation, of
electromagnetic
radiation, comprising a ground plane and at Ieast one radiation disc connected
to two ports
for supplying a desired electromagnetic radiation, preferably within the
microwave range, the
distance between the ground plane and the radiation disc being a fraction of
the wavelength
of the radiation.
Antenna elements of this kind are called micro strip antennas or patch
antennas and constitute
an all round class of antenna elements. Hence, the basic building block is a
radiation disc in
the shape of a flat, metallic plate having a shape that in principle is
arbitrary, i.e. a patch,
separated from a ground plane by means of air or a dielectric. By means of a
correct choice
of the dimensions of the plate it is made resonant for a chosen
electromagnetic radiation
frequency, usually in the microwave range, and can be used for absorbing or
radiating
ervrgy at this frequency. It is often desirable that the antenna absorbs or
radiates radiation
sciely having a particular polarisation. It is required that the antenna has a
good purity of
polarisation (a low cross polarisation, a low axial ratio). It is often
required that the antenna
can obtain two orthogonal polarisations corresponding to two antenna ports,
i.e. two linear or
opposed elliptical, in particular circular polarisations, having a good purity
of polarisation. It
is clear that deviations from a geometrical symmetry can cause degradation of
the
polarisation purity. The supply itself to the radiation disc can cause such an
asymmetry. The
problem related to polarisation purity then becomes pronounced in the case
when the antenna
element is made type broad band, since the effect from the supply in this case
becomes
marked.
A number of different solutions of the problem relating to polarisation purity
in antenna
elements of the above type has been suggested.
The problem of geometrical asymmetry caused by the supply of the radiation
disc can be
solved by means of balanced supply, but this will add supply points. This is a
complication
that can not always be accepted. Balanced supply results in a doubling of the
number of
supply points for the antenna element, an increased complexity and the problem
with
potential build-up.
Cutting-outs and tongues at the edges of the antenna elements are often used
to obtain a
circular polarisation using only one supply point. In the case of two
polarisations they can, in

WO 96!35240 ~ PCT/SE96/00561
CA 02219925 1997-10-30
principle, be used in order to improve the polarisation purity. In practice it
seems to be hard
to design such radiation discs having cutting-outs and tongues since they also
modify the
electric dimensions of the antenna.
If the antenna element is to generate a circular, single or double (i.e. right
and left rotating)
polarisation and a quadrature hybrid is used, unbalance in the hybrid provides
a method of
improving the polarisation purity. Unless circular polarisation is required
and the hybrid
akeady is a part of the antenna the use of unbalanced hybrids seems to be of
little value.
Sequential rotation of and supply to elements can be used as a method of
improving the
polarisation purity in an array of antenna elements. It is a supplement to
other methods and
only works in the case ~ when the antenna elements are used together with
other elements in
an array where cross-polarisation of different antenna elements can be made to
cancel each
other at angles close to the broad side direction.
The object of the invention is to provide an antenna element of the kind,
mentioned in the
introduction, which has a high polarisation purity but still has a simple
construction.
According to the invention such an antenna element is primarily characterised
in that the
ports are located at an electrical distance of 90° from each other and
that a quarterwave
transformer is added between the ground plane and the radiation disc, that is
so designed that
the transmission line impedance between the ports is reduced to make the ports
radiationally
independent of each other.
The quarterwave transformer can be designed in different ways. It can be
located as one unit
inside the 90° sector between the ports or inside the 270°
sector opposite to the ports. It can
also consist of two parts, symmetrically located within the 90° sector
between the ports and
within the 270° sector opposite the ports. In a preferred embodiment
the quarterwave
transformer is formed of steps in the ground plane at a smaller distance from
the radiation
disc than the rest of the ground plane.
The quarterwave transformer can also consist of steps in the radiation disc
located at a '
smaller distance from the ground plane than the rest of the radiation disc.
The principle underlying the invention can be described in terms of a
transmission model of
the radiation disc, for example for a circular radiation disc, which is fed at
two points, that
are located geometrically 90° apart from each other. At resonance of
the base mode TMll

w0 96/35240 PCT/SE96/00561
CA 02219925 1997-10-30
(H11) the two ports are also electrically separated 90° due to the
circumferential eke
dependence of the electrical field. The two ports, matched to their
transmission lines result in
high impedance resistive loads on the low impedance loss radiation disc
transmission line.
The impedance at one port is higher as seen in ~ a direction towards an
adjacent port than
away therefrom, since the load impedance is transformed via the
quarterwavelength, which
separates the ports. This has the effect that less energy is distributed to
the adjacent port than
what is distributed in the opposite direction. The a+~8 and a ~e components
are excited to
different amplitudes, i.e. an elliptical polarisation is excited by one port,
which in an ideal
case would provide a linear polarisation. .
r
As a non-limiting exemplary embodiment of an antenna element according to the
invention,
an antenna element having a circular radiation disc is described below, but in
principle the
radiation disc in an antenna element according to the invention can have other
shapes, such
as for example rectangular, in particular quadratic shapes, the quarterwave
transformer being
adapted to the shape of the radiation disc.
In Figure 1 an antenna element is shown in a perspective view, in Figure 2 the
same antenna
element is shown in a plane view and in Figure 3 in a side view.
In the figures a circular radiation disc is denoted 1, a ,;round plane is
denoted 2 and a host of
_ . . y _ r ___ _
a dielectric or metallic material is denoted 3. The latter one maintains the
radiation disc at a
distance from the ground plane, which is a fraction of the wavelength ~ of the
radiation.
Two probes 4, and 5 constitute ports for the supply of the present signal to
the radiation disc
1. These ports are located geometrically and thereby electrically at a 90
° distance from each
other. In the sector between them, the ground plane is elevated to form an
impedance
reducing step 6.
By supplying and receiving the corresponding signals respectively through the
two ports 4
and 5 a very high polarisation purity in all desired orthogonal polarisations
is obtained.
An important advantage of the quarterwave transformer, designed as a step in
the region
between the radiation disc and the ground plane, is that the radiation disc
can be adapted and
optimized with regard to other parameters before the antenna element is
optimized with
regard to the requirements of polarisation purity. The reason for this is that
the impedance
steps in general do not effect the electrical magnitude of the radiation disc.
However, if
instead tongues and cutting-outs were used on the radiation disc in order to
achieve
polarisation purity, all other parameters must be optimized simultaneously.

Representative Drawing