WIDEBAND, DIRECTIONAL, LINEARLY POLARIZED ANTENNA HAVING
HIGH POLARIZATION PURITY
The present invention relates to a wideband , directional, linearly
polarized antenna having high polarization purity.
In the field of wideband directional antennas employing linear
polarization , and in particular in the context of amplitude goniometry systems,
5 there is generally observed , with antennas of this type, degraded accuracy of
the measurement of the D.O.A ("direction of arrival") of targets. In this case,
polarization purity defects lead to deformation of the radiation diagrams (this
phenomenon is known as "clouding ") that increases with the elevation,
inducing degraded detection system location performance.
10 This problem is currently solved with the aid of empirical solutions that
cannot be generalized , for example the addition of arrays of metal wires in
front of the antenna.
The subject matter of the present invention is a wideband (the
frequency band possibly exceeding a decade), directional antenna having
15 high polarization purity , of the printed circuit type , which antenna can be
integrated into a dual polarization antenna and , when it is used in a location
system , enable improvement of the location performance thereof, particularly
at non-zero elevations.
If this antenna is of the linearly polarized type, its theoretical
20 copolarization is defined relative to the geometry of the radiating circuit. In
practice, the real copolarization differs from the theoretical copolarization.
The polarization purity is defined as being the difference between the
theoretical polarization and the real copolarization. It may be measured using
the "copolarization level/cross-polarization level" ratio in the geometrical
25 definition plane of the antenna. If the antenna is perfect, this ratio is infinite.
In practice, what is looked for is a ratio generally between 15 dB (for a logperiodic
type antenna) and 20 dB (for a "sinuous" antenna).
The antenna of the invention , of the plane support type, is a wideband,
directional , linearly polarized antenna having high polarization purity, having
W® 2011/128243 2 PCT/EP2011 /056419
at least one pair of radiating elements printed on one face of a printed circuit,
the two elements being symmetrical to each other with respect to the center
of the antenna and delimited in their angular extent by two virtual straight
lines passing through the center of the antenna, and is characterized in that it
5 includes radiating elements printed on the other face of the support, these
elements being identical to those of the first face, and being deduced
therefrom by a rotation of 1800 about an axis passing through the center of
the antenna , and which is the bisector of the angle at the center of said pair
of elements , this angle at the center being that formed by said two virtual
1o straight lines, this rotation being followed by a translation over a distance
equal to the thickness of the printed circuit.
The present invention will be better understood on reading the
detailed description of one embodiment considered by way of nonlimiting
example and shown in the appended drawing, in which:
15 - figure 1 is a plan view of a prior art antenna, and
- figure 2 is a plan view of an antenna conforming to the present
invention.
The present invention is described hereinafter with reference to an
antenna operating with linear polarization and having radiating elements of
20 "sinuous" shape inscribed within a circle, but it is to be clearly understood
that it is not limited to any such type of antenna , and that it applies to any
antenna with plane radiating elements, radiating with linear polarization,
having a wire geometry, where the aim is to improve the polarization purity,
the cmpolarization of which antenna is assumed to be linear, wideband or
25 otherwise , which may, where necessary, be the basic element for the design
of a dual polarization antenna.
The type of antenna from which the invention stems is generally that
produced with the aid of a single-sided printed circuit fabrication technology.
One example of a prior art antenna 1 of this type is shown in figure 1. It
30 essentially comprises two sinuous radiating elements or branches 2, 3
symmetrical to each other with respect to the geometric center 0 of the
system. It is to be clearly understood that this antenna could include two
WO 20111128243 3 PC T/EP2011/055419
other branches. The layout of the radiating elements of a so-called "sinuous"
antenna being well known, for example from US patent 4 658 262, it will not
be described in more detail here. It will be specified here only that the two
arms 2, 3 are symmetrical to each other with respect to the center 0. These
5 two arms are delimited in their angular extent a by two virtual straight lines A,
B passing through the center 0 of the antenna.
The antenna 4 of the invention, as represented in figure 2, is of the
double-sided printed circuit type. The representation in figure 2 is as if the
printed circuit on which the radiating elements are formed were transparent.
10 The first face, which is assumed to be the anterior face, includes the same
branches 2, 3 as in figure 1. The posterior face of the printed circuit includes
the branches 4,5 the shapes and dimensions of which are identical to those
of the branches 2, 3.
In the figure 2 view, the location of the branches 4, 5 is deduced from
15 that of the branches 2, 3 by rotation to 180° about an axis C passing through
the center 0 and which is the bisector of the angle a at the center of the
branches 2, 3. In reality, it would be necessary to add to this rotation a
translation over a distance equal to the thickness of the printed circuit (from
the anterior face to the posterior face of this printed circuit). In other words,
20 the layout of the branches 4, 5 is obtained by rotation of the branches 2, 3
about the center 0 through an angle having a value equal to (180° + a), and
then by the same translation. In the case of an antenna with four branches
(two pairs of branches), only one of the pairs is considered for this rotation.
The invention enables improvement of the polarization purity of the
25 antenna by more than 10 dB compared to the geometry on a single-sided
printed substrate. More generally, it enables improvement of the polarization
purity of all plane wire antenna geometries (log-periodic and other type
antennas). Applied to dual polarization antennas, it improves the coupling
between the two radiating elements.

CLAIMS
1. A wideband, directional, linearly polarized antenna having a plane
support and high polarization purity, having at least one pair of radiating
elements (2, 3) printed on one face of a printed circuit, the two elements
being symmetrical to each other with respect to the center (0) of the antenna
5 and delimited in their angular extent by two virtual straight lines (OA, OB)
passing through the center of the antenna, characterized in that it includes
radiating elements (4, 5) printed on the other face of the support, these
element.r.> being identical to those of the first face, and being deduced
therefrom by a rotation of 1800 about an axis (OC) passing through the
1o center of the antenna and which is the bisector of the angle (a) at the center
of said pair of elements, this angle at the center being that formed by said
two virtual straight lines (bA, OB), this rotation being followed by a translation
over a distance equal to the thickness of the printed circuit.
2. The antenna claimed in any one of the preceding claims,
15 characterized in that each face of the printed circuit includes two arms of
sinuous shape.
3. The antenna claimed in either of claims 1 or 2, characterized in that
each face of the printed circuit includes two arms of log-periodic shape.
4.The antenna claimed in claim 1, characterized in that it is a dual
20 linear polarization antenna.