[0001] The present invention refers to a dielectric substrate single layer planar dipole
antenna.
[0002] More particularly the present invention relates to a dielectric substrate single
layer planar dipole antenna particularly suitable for wireless applications and remote
control systems.
[0003] Many single layer antennas are known in the art. They can be used for short-distance
radio-links, particularly for vehicular applications (locking and unlocking doors,
activating antitheft devices and searching the car on a crowed parking area). Most
of such antennas use the classical grounded monopole; alternatively a trace of the
rear window defroster can be used, if available. More simply the antenna can be realised
by a short piece of isolated conductor (wire) hidden inside the vehicle.
[0004] It is also possible to use the AM/FM whip on the car roof, if available, which radiates
an isotropic horizontal pattern; this solution has the drawback that such antenna
is sensitive only to vertical polarized waves.
[0005] It is worth noting that the wave radiated from the transmitter unit can be polarized
and that the polarization can be arbitraly vertical, horizontal or slanted, because
its polarization state depends on how said unit is oriented during the handling. Statistically,
however, the horizontal polarization prevails because it corresponds to the more natural
handling of the said transmitting unit.
[0006] When the receiving antenna is realised using a trace of the defrosting grid, the
polarization state is variable and it is mainly determined by the interaction of the
antenna with the vehicle metal body, which produces a remarkable depolarization effect.
Nevertheless, in order to privilege the horizontal polarization, the antenna structure
and position should be optimised during the design phase of the vehicle. This is often
considered a burden from the most car producers because optimization studies increase
the costs. Using instead a simple radiating wire inside the vehicle, the costs are
reduced but the antenna performances become poor and unpredictable because the metallic
body produces strong mismatching on a unbalanced antenna configuration.
[0007] The object of the present invention is to provide a simple, low cost highly efficient
and reproducible planar antenna to be installed inside the vehicle, for instance on
the dashboard, glasses, bumpers or other non-metallic parts of the car with minimal
mechanical and aesthetical constrains.
[0008] Further object of the present invention is to provide an antenna easily reconfigurable
and tuneable for operating on any vehicle type and installation place, offering good
radiation efficiency also in presence of near metallic parts.
[0009] Further object of the present invention is to provide a linear polarized antenna
easily to set for achieving a prevailing horizontal polarized pattern.
[0010] According to the present invention, these and other objects, which will be apparent
from the following description, are achieved by the features in the characterising
portion of claim 1.
[0011] Particularly, the planar antenna of the present invention is obtained by:
a) using a thin substrate layer as a support of a dipolar planar radiating element
comprising at least a couple of printed strips or wires having any shape such as linear,
spiral, meander or like, fed 180° out of phase to operate as a balanced dipole;
b) realising on the opposite side of the said substrate layer a thin metallized area
(patch) where a slot operating as balanced/unbalanced transformer (balun) is etched,
thus allowing an unbalanced feeding of the intrinsically balanced antenna by means
of a coaxial cable and simultaneously avoiding that the presence of said cable could
affect significantly the behaviour of the antenna;
c) connecting the inner ends of the dipole strips to said patch with two metallized
via-holes or rivets symmetrically disposed with respect to the slot centre and near
its main edges, in such a manner that the arms of the dipole are electrically in phase
opposition;
d) introducing a third planar strip on the same side of the dipole arms that bridges
the slot at its centre and is connected to the opposite patch through a via-hole or
rivet;
e) feeding the antenna with a coaxial cable whose inner conductor is connected to
said third strip and the outer conductor is connected to said patch;
f) providing that said via-holes can be replaced by lumped impedances and that other
lumped impedances can be placed in series to the feeding and radiating strips in order
to modify the electric parameters of the whole antenna;
g) providing that the feeding strip can be replaced by a coplanar waveguide (CPW)
etched on the opposite side of the planar dipole, thus making easier the connection
of the coaxial cable;
h) providing the possibility of varying shape, dimensions and configuration of the
patch supporting the slot and the slot itself in order to optimize the radiation efficiency;
i) providing the possibility of conforming the two strips of the balanced dipole as
spiral, meander or other shapes in order to reduce the overall dimensions of the antenna
or using multiple couples of strips having the same or different feeding points in
order to obtain a multiband/multifunction operation.
[0012] The main advantage of the present invention consists on the use of a simple slot
etched on a ground patch smaller than the wavelength to transform the balanced input
of a planar dipole into a unbalanced input, suitable for direct coaxial cable connection.
[0013] Moreover, by connecting the outer conductor of the coaxial cable with the point of
the patch surface where the RF current reaches its minimum, it is possible to reduce
the sensitivity of the antenna to the cable length and to the presence of near metallic
bodies.
[0014] A further advantage consists in that the dipolar planar antenna, while maintaining
its fundamental electromagnetic properties, can be properly designed to be optimally
fitted on different parts of the vehicle (for instance inside the rear-view mirror)
by assuming a rectangular or polygonal shape, planar or conformed to any curved surface.
[0015] A further advantage comes from the possibility of varying independently the length
of both dipole arms and the dimensions or the shapes of the slot in order to optimize
the antenna resonant frequency, bandwidth and impedance matching, after its placing
in the chosen position inside the vehicle.
[0016] A further advantage consists in the possibility of embedding the planar strips constituting
the dipole arms inside a multilayer of at least two layers of dielectric/magnetic
material, in order to increase the antenna compactness.
[0017] The invention is described in detail in the following, with reference to the figures
of the enclosed drawings that show some embodiments of the planar antenna of the present
invention, reported by way of non limiting examples and wherein:
Fig. 1 shows a schematic frontal and side view of the radiating element, for a planar
antenna according to the present invention, in a basic configuration constituted by
two thin metallic strips having a linear shape; in this case, the slot realizing the
balanced feeding has a rectangular shape and is tilted by 45° with respect to the
line of the radiating element in order to minimize the reactance associated to the
discontinuity of the lines constituting the radiating arms of the dipole,
Fig. 2 shows a schematic frontal view of the radiating element, according to the present
invention, in a further embodiment comprising two thin metallic spiral-shaped strips
and a ground plane of small dimensions where the balancing slot for the feeding (balun)
is etched, that in this example has the shape of a butterfly. The connection points
between the metallic strips and the lower ground plane are also visible, together
with the central metallic strip where the feeding cable is connected,
Fig. 3 shows a schematic frontal view of the radiating element for a planar antenna
according to the present invention, in a further embodiment comprising two thin metallic
strips with a meander shape,
Fig. 4 shows the behaviour of the input reflection coefficient as a function of frequency,
for the antenna of Fig. 2,
Fig. 5 shows the behaviour of the radiation diagram (gain) in the horizontal plane,
for the antenna of Fig. 2,
Fig. 6 shows, a schematic frontal view of an example of embodiment of the planar antenna,
according to the present invention, wherein two couples of metallic strips on the
upper side of the antenna are used and wherein the ends corresponding to the metallized
holes are in common, in order to achieve a double band behaviour,
Fig. 7 shows a schematic frontal view a further embodiment example of the planar antenna
according to the present invention, wherein the third metallic strip on the upper
side of the antenna has been removed and has been replaced with a coplanar line on
the bottom side, thus allowing to move on the same side all the connections to the
feeding cable.
[0018] With particular reference to the figures, the single-layer planar antenna, according
to the present invention, comprises at least one radiating dipole, constituted by
two thin metallic traces or strips 1 having any shape such as linear, spiral, meander
or like, fed in a balanced way by means of a slot 4 etched on a small ground plane
2 placed on the bottom layer of a dielectric supporting substrate 8.
[0019] The metallic traces or strips 1 are generally printed on the upper layer of said
single thin dielectric substrate 8, by using the well known etching techniques widely
used in the printed circuit boards production. Anyway, other techniques for producing
those metallic traces or strips can be used, as for example serigraphy technique with
a conductive paint; all the techniques are included in the scope of the present patent.
[0020] The metallic traces or strips 1 have an end connected to the lower ground plane 2
by means of metallized holes ("via-holes") or conductive rivets 3, directly or through
a lumped impedance. Optionally, the lumped impedances can be inserted in-series along
the metallic traces or strips 1, typically near the via-holes 3. Said metallized holes
3 are disposed symmetrically with respect to the centre of the slot 4 and near its
main edges in order to feed the two metallic traces or strips 1 in phase opposition,
as required for a dipole antenna.
[0021] Centrally, with respect to the slot 4, is present another metallic trace or strip
5 that is connected, near said slot, to the ground plane 2 by another via-hole or
conductive rivet 3 and is connected to the central conductor of a feeding coaxial
cable, on the opposite side of the same metallized hole with reference to the slot
4. Optionally, it is possible to realize the feeding metallic trace or strip on the
same side of the ground plane 2 using a coplanar technology, thus realizing a trace
or strip 7 that is bound directly to the external edge of the slot 4, in its central
position.
[0022] By appropriately varying the shape of the conductive metallic traces or strips 1
constituting the radiant element, it is also possible to insert further metallic traces
or strips, i.e. two strips 6, also these strips fed in phase opposition in a balanced
way. Such further metallic strips can be dimensioned in order to extend the working
bandwidth of the antenna or to realize a second band centred on a frequency different
from the first one. A typical example is a double band antenna, working respectively
in the ISM (Industrial Scientific Medical) bands assigned to the keyless-entry system
(434MHz and 868MHz).
[0023] Summarizing, while maintaining its fundamental characteristic of planarity together
with an electromagnetic behaviour typical of a classical balanced fed dipole, the
antenna can assume every shape such as rectangular, trapezoidal or generally polygonal,
planar or curved, in order to conform, also aesthetically, with the requirements and
bonds of the installation, for example, on a vehicle.
[0024] Independently of the shape of the metallic trace 1 constituting the radiating element
and of the thin dielectric substrate 8 that uphold it, the antenna of the present
invention shows a radiative behaviour completely similar to that of a classical dipole
operating in the same working conditions.
[0025] Even though the present invention has been described and illustrated here with reference
to some embodiments that are given only by way of non-limitative examples, it is clear
that various changes to layouts, to details, to configurations, to orientations, to
components and other combinations can be made by one skilled in the art in the light
of the above-mentioned description.
[0026] It is therefore understood that the present invention is meant to include all the
changes and variants falling within the spirit and the scope of the following claims.
1. A dielectric substrate single layer planar dipole antenna
characterized in that it comprises:
- a thin support substrate (8) of a dielectric material comprising an upper surface
and a bottom surface;
- a ground plane (2) placed on the bottom surface of said dielectric support substrate
(8) and having dimensions smaller than those of said substrate (8);
- a feeding comprising a slot (4) etched on said ground plain (2);
- at least one radiating dipole consisting of two thin metallic strips (1) printed
on the upper surface of said thin support substrate (8) fed in a balanced way by means
of said slot (4); said metallic strips (1) having one end connected to the lower ground
plane (2) by means of first metallized holes or conductive rivets (3) realized near
the slot (4) and placed along the longer dimension of said slot, symmetrically with
respect to the slot centre to feed the two metallic strips (1) in phase opposition;
- a third thin metallic strip (5), placed centrally with the respect to said slot
(4) and connected to the ground plane (2) by a second via-hole or conductive rivet
(3), and
- a feeding coaxial cable having inner conductor connected to said third thin metallic
strip (5) and an outer conductor fixed to the ground.
2. The dielectric single layer planar antenna according to claim 1, characterized in that the metallic printed strips (1) have linear, spiral or meander shape.
3. The dielectric single layer planar antenna according to anyone of the previous claims,
characterized in that the slot (4) is oriented and dimensioned to work as a unbalanced to balanced transformer
(balun).
4. The dielectric single layer planar antenna according to anyone of the previous claims,
characterized in that the non-radiating ends of the metallic strips (1), are connected to the ground plane
(2) by means of metallized holes (via-holes) or conducting rivets (3) symmetrically
disposed with respect to the centre of said slot (4) and near its main edges so that
said strips (1) constituting the dipole arms are electrically in phase opposition
placed close to the bigger sides of said slot (4), anti-symmetrically from an electric
point of view (phase opposition), with respect to its centre.
5. The dielectric single layer planar antenna according to anyone of the previous claims,
characterized in that the feeding strip is replaced by a coplanar waveguide etched on the opposite side
of the planar dipole.
6. The dielectric single layer planar antenna according to anyone of the previous claims,
characterized in that the metallized holes (via-holes) or conductive rivets (3), are replaced by lumped
impedances to reduce the dimensions of the antenna and to increase its operative impedance
bandwidth.
7. The dielectric simple layer planar antenna according to claim 6, characterized in that the lumped impedances are provided in series with the feeding strips.
8. The dielectric single layer planar antenna according to anyone of the previous claims,
characterized in that the external conductor of the feeding coaxial cable is connected to a point of the
patch surface where the amplitude of the radio frequency current is minimum.
9. The dielectric single layer planar antenna according to anyone of the previous claims,
characterized in that the metallic traces (1) on the upper side of the thin dielectric substrate (8), are
two couples with the feeding ends, corresponding to the metallized holes, in common.
10. The dielectric single layer planar antenna according to anyone of the previous claims,
characterized in that the planar antenna has dimensions even less than half a wavelength and a rectangular,
trapezoidal or polygonal, planar or curved shape.
11. The dielectric single layer planar antenna according to anyone of the previous claims,
characterized in that the metallic strips (1), are buried inside a multilayer constituted by at least two
thin substrate layer of dielectric and/or magnetic material.