[0001] The present invention relates to a planar antenna for motor-vehicles. More particularly,
the present invention relates to a microstrip (patch) planar antenna, particularly
suitable for use in motor-cars both for cell-phones and for different applications.
[0002] The planar antenna of the present invention is preferably located on the windows
of motor-vehicles, in particular on the rear window. It may be utilized both in the
cell-phones of the present frequency field of 800/900 MHz (GSM, ETACS, AMPS, PCD)
up to the higher frequencies of 2.5 GHz (DCS, UMTS, PCN, PDC, 1.5) and for applications
other than telephony, such as for instance: reception in L, DAB, GPS band and the
like, and at higher frequencies up to 6 GHz, as well as for Telepass type transponder
systems of the, and the like.
[0003] Radiophony systems for motor-vehicles are known that utilize frequencies in the UHF
band; at present systems are widespreading that use higher frequency systems, suitable
to obtain an increasing number of transmission channels.
[0004] The present antennas adopted in such systems are of the stylus type; they are installed
on the vehicles' metal roofs and provide acceptable performances, with an almost omnidirectional
radiation and vertical polarization. The metal roof acts as a mass plane and provides
a good shielding for the interior compartment from the electromagnetic radiation emitted
by the antenna. In spite of this, said antennas, while having proved to have satisfactory
radioactive characteristics, have some drawbacks. In fact, for their installation
it is necessary to make a through-hole in the car-body, with ensuing possible problems
of seal against wheathering, and their external location, besides modifying aesthetically
the line of the vehicles, constitutes a precarious exposure condition, which causes
them to be subject to damages, tampering and/or vandalisms.
[0005] In order to avoid such holes, there have been adopted the so-called "capacitive coupling"
antennas, which are made by an element applied on the inside of the windscreen or
the rear window of vehicles, connected with the transceiver apparatuses by means of
a coaxial cable that transfers the electromagnetic energy through the glass to a counterpart
applied on the outside of said glass. On the same external part, a stylus-like structure
is applied which is responsible for the electromagnetic radiation.
[0006] However, also these antennas, while allowing the application without making holes
in the body, comprise always an external protruding part subject to damages, tampering
and/or vandalisms.
[0007] More recently, antennas have been realized that are directly mounted on the windscreen
or the rear glass, in the inside of the passengers' compartment. They are constituted
of flat leads having various shapes similar to monopoles or dipoles which however,
because of their elongated shape, are mainly installed in an horizontal position,
with ensuing distortion of the polarization of the magnetic field and a severe degradation
of the radiation diagram, because of the electromagnetic coupling with the metal structures
of the vehicle. Besides, their emission takes place indifferently both towards the
inside and the outside of the passengers' compartment, exposing said passengers to
high levels of electromagnetic field.
[0008] Planar antennas suitable for the frequencies of the radiotelephone systems, capable
of obviating the above listed drawbacks, can be derived from the theory of the so-called
patch-antennas, that consist essentially of two conductive surfaces, one of which
has generally a size greater than the other one, aligned in parallel at a distance
shorter than the wave length; between the two leads a dielectric material may be interposed
or, more simply, the leads are maintained in the position by insulating spacers exploiting
air as a dielectric. This class of antennas includes also the one called "QWSCM" (Quarter
Wave Short Circuited Microstrip Antenna) wherein the resonating length of the upper
lead is reduced by λ/2 to λ/4, shorting a radiant edge. This type of antenna has been
proposed for use in the field of motor-cars; however, these proposals proved to be
substantially specific, applicable, for instance, only to vehicles with non-metallic
roofs, or so configured as not to allow the application on all the models of motor-cars
or also configured with specific and limited characteristics, such as for instance
a wave length <4% with VSWR = 2 (Voltage Standing Wave Ratio) and a prevailingly isotropic
radiation diagram. The characteristics clash with the requirements of the present
cellular communication systems which utilize a frequency band of at least 10% and
with the increasingly contingent need of protecting passengers against the exposure
to the electromagnetic fields emitted by the antennas.
[0009] Object of this invention is to eliminate the above drawbacks.
[0010] According to the present invention, these and other aims that will result from the
following description are achieved by a planar antenna for motor-vehicles having the
features of the characterizing part of claim 1.
[0011] In the whole, the antenna of the present invention has a low profile with contained
dimensions, is of simple construction and low cost, and can be easily installed in
the inside of vehicles' compartments.
[0012] The advantages achieved by the antenna of the present invention lie essentially in
that it has a band length equal to or higher than 10%; it can irradiate uniformly
throughout the horizontal plane with a prevailingly vertical polarization of the electric
field, as it may be compared to a horizontal radiant slot (magnetic current) and allows
an adequate containment of the radiation emitted in the inside of passengers' compartment,
avoiding the exceeding of the electromagnetic field limits indicated by the norms
in force. It may be installed, on a prior simple tuning, on any type of vehicle.
[0013] The invention is described in detail in the following, with reference to the figures
of the attached drawings that show some embodiments reported by way of non limiting
example and wherein:
Figures 1A and 1B show respectively the schematic plan and side views of an example
of the antenna of the present invention;
Figure 2 shows the schematic side view of two possible arrangements of the antenna
of Figure 1 on the rear window of a vehicle;
Figure 3 shows the perspective view of an embodiment of the planar antenna of the
present invention, inductively charged by orthogonal metal strips;
Figure 4 shows the transmission line model of the antenna of Figure 3, which illustrates
the concept of the resonant length reduction;
Figure 5 shows the perspective view of another embodiment of the planar antenna of
the present invention, having a further reduced resonant length, thanks to a higher
inductive charge;
Figure 6 shows the perspective view of a further embodiment of the planar antenna
of the present invention;
Figure 7 shows the VSWR diagram of a planar antenna of the present invention, and
Figure 8 shows the radiation diagram on the horizontal plane.
[0014] The figures refer to a planar patch-antenna for use in motor-car applications of
cellular telephony, of the known type indicated by the initials "QWSCM".
[0015] With reference to the figures, the microstrip planar antenna (patch) of the present
invention comprises a radiant element 1, that adheres to glass 2 constituting the
windshield or the rear window of a vehicle: a mass plane 8 parallel to said radiant
element 1 and a continuous striped metal lead 9 connecting said radiant element to
said mass plane 8, and constituting the mass shorting element.
[0016] Said antenna is fed by a coaxial cable 5 fixed to a capacitive coupling 4 applied
to the mass plane 8 and spaced from the radiant element 1 by a portion which depends
on the operating band of the antenna.
[0017] According to a first simplified and economical embodiment of the antenna of the present
invention, shown in Figures 1A and 1B, the radiant element 1, the shorting lead 9
arranged orthogonally and the mass plane 8 are realized by punching and subsequent
bending of one only continuos wall (9) which is oriented in a substantially orthogonal
manner with respect to the radiant element 1 and the mass plane 8, parallel with each
other.
[0018] The antenna is fed through a flat armor 13 capacitive coupling 4.
[0019] The lower distance of armor 13 from the upper radiant element 1 can be adjusted in
order to optimize the radioactive and circuit characteristics of the antenna, in particular
to optimize its band width.
[0020] In order to obtain a significant reduction in the emissions in the inside of motor-vehicles
20, the mass plane 8 may be possibly extended by means of a suitable transparent metallization
of the low resistance, high optical transmittance rear window. To this purpose, transparent
conductive films 2' applied on normal glasses may be utilized, or glasses already
metallized by vacuum evaporation or sputtering, such as those already mounted on many
models of motor-vehicles. With these solutions, the back radiation can he reduced
by up to 100 times, equal to 20 dB.
[0021] The main characteristics of the antenna of the present invention, which distinguish
it from the known ones, are basically the following:
a) The glass 2 of the rear window or the windshield constitute the over-layer of the
quarter-wave radiant element and therefore contributes to determine the radioactive
characteristics of the antenna, being the seat of surface waves.
b) The tapered geometric configuration of the radiant element 1 allows to obtain a
widening of the operating band of the antenna; it may be quadrangular, or have sequential
steps 3 decreasing towards the shorting extreme or may tend towards a practically
trapezoid shape 3'.
c) The feeding of the antenna is obtained through a capacitive coupling 4 where cable
5 abuts, possibly associated to a connector 6. The position of the connector for the
maximum impedance adaptation may be adjusted through the sliding and subsequent tie
along a slot 7 obtained on the mass plane 8. The capacitive coupling determines a
further widening of the operating band of the antenna, as it tunes the inductance
associated to lead 9 substantially orthogonal to the radiant element 1 and the mass
plane 8.
d) The radiant element 1 is associated to a mass plane (8) of reduced size. The radiant
element 1 and the mass plane 8 are mutually engaged through said continuous orthogonal
9 or striped 9' lead, which constitutes the mass shorting element of the antenna.
[0022] Such characteristics cause the antenna of the present invention to be substantially
universal for applications on any types of motor-cars and allow to operate in a band
width equal to or higher than 10%, with a uniform irradiation on the horizontal plane,
with a prevailingly vertical polarization of the electric field and boundary emissions
of the electromagnetic field in the inside of the vehicle comprised within those indicated
by the norms in force.
[0023] According to a second embodiment of the antenna of the present invention, illustrated
in Figure 3, the mass shorting wall, instead of being constituted of a continuous
lead 9, is constituted of several suitably spaced strips 9' whose number and shape
or size are characterizing parameters of the project of the antenna, depending on
the determination of the overall entity of the inductive charge and the entity of
the reduction in the physical size of the antenna. The width determines the equivalent
inductance associated to each strip.
[0024] Figure 4 schematizes the circuit configuration equivalent to a transmission line,
wherefrom one understands that the inductive charging, realized by means of a plurality
of strips 9' spaced by gaps, allows to shorten the resonant length of the antenna
and, as a consequence, to reduce the size thereof. The broken lines represent the
width of RF voltage within the antenna.
[0025] A further increase in the inductance of the antenna and therefore a greater compactness
can be obtained with the embodiment shown in Figure 5, wherein strips 9', that constitute
the body of mass shorting, instead of being rectilinear, are arc-shaped 9''. This
solution can also be utilized to realize a simple tuning, by cutting one or more strips
9''.
[0026] A further embodiment, realizable even more easily and which is more suitable for
mass industrialization, is obtained by forming the mass plane 8 of the antenna from
a printed circuit board metallized on one side only, for instance by means of a surface
coppering, and having a coplanar transmission line printed or etched as shown in Figure
6. In this way, the feeding line can be realized in the form of a coplanar microstrip,
with the following advantages:
· structural simplicity, which makes fabrication easier.
· perfectly symmetrical configuration, with minimal overall dimensions.
· minimal electromagnetic perturbation,
· transition simplicity at the coaxial cable 5,
· possibility of side entrance of cable 5, through both the open side 10 of the board
and side 11 of the mass shorting with straight 9' or arc-shaped 9'' strips,
· mechanical strength,
· possibility of tuning.
[0027] In the latter realization, the radiant element 1 that comprises arc-shaped strips
9'' is directly fastened to the conductive layer 12 of the mass plane 8 of the board
by means of welding or rivets. The fastening is carried out along the lower ends of
said strips 9''. In the same way, the capacitive coupling 4' is realized by means
of a suspended foil 13, also provided with an arc-shaped strip 14 whose lower end
is welded or tied with rivets or the like, at an end of the coplanar transmission
line whose other end is connected to the connector or, more simply, to the coaxial
cable 5.
[0028] Figure 7 shows the diagram of SWR or ROS (Stationary Waive Ratio) of an antenna according
to the invention, measured with a 50 ohm standard instruments: as can be observed,
its band with SWR < 2 is given by frequencies (700-1130) MHz and that the percent
value with respect to the central frequency is > 20%.
[0029] Such percentage is much higher than that which is obtained with a traditional normal
antenna of the QWSCM type.
[0030] The diagram of Figure 8 of radiation on the horizontal plane shows the measure of
realization of an antenna with the gain value indicated in dB.
[0031] The planar antenna of the present invention is applied in particular in cellular
telephony, from the present frequencies of 800/900 MHz (GSM, ETACS, AMPS, PCD) up
to the higher ones of 2.5 GHz (DCS, UMTS, PCN, PCD 1.5); besides, it may be utilized
for applications other than telephony, such as L, DAB, GPS band reception and the
like and at higher frequencies up to 6 GHz, also for Telepass type transponder systems,
and the like.
[0032] While the present invention has been described and illustrated according to some
embodiments reported by way of non limiting examples, it is obvious that many alternatives
and variants will he evident to those skilled in the art, in the light of the above
description.
[0033] Therefore, the present invention intends to cover all the alternatives and variants
that fall within the spirit and the protection scope of the appended claims.
1. A planar patch antenna, suitable to be applied to windscreens or rear window of vehicles,
characterized in that it comprises:
- a radiant element (1) adhering to glass (2) constituting the windscreen or the rear
window;
- a mass plane (8) parallel to said radiant element (1);
- a mass shorting element comprising a metal lead (9, 9'') that connects said radiant
element (1) to said mass plane (8), and
- a feeding means comprising a capacitive coupling 4 fastened to a coaxial cable 5;
said capacitive coupling 4 being applied to the mass plane (8) and spaced from the
radiant element 1 of a portion that depends on the operating band of the antenna.
2. The planar antenna for motor-vehicles according to claim 1, characterized in that
the capacitive coupling 4 is constituted of an armor (13) whose size depends on the
band width of the antenna.
3. The planar antenna for motor-vehicles according to claim 1 o 2, characterized in that
glass (2) of the windscreen or the rear window constitutes the overlayer of the radiant
element (1).
4. The planar antenna for motor-vehicles according to any of the preceding claims, characterized
in that the dimensions of the mass plane (8) are extended by means of a transparent
metallization of the low resistivity, high optical transmittance rear window.
5. The planar antenna for motor-vehicles according to any of the preceding claims, characterized
in that the radiant element (1) has a quandrangular shape or a profile with sequential
steps (8) or a trapezoid shape (3') decreasing towards the open part.
6. The planar antenna for motor-vehicles according to any of the preceding claims, characterized
in that the mass shorting body (9) is orthogonal to the radiant element (1) and the
mass plane (8).
7. The planar antenna for motor-vehicles according to any of the preceding claims, characterized
in that the shorting lead (9) is constituted of several spaced orthogonal straight
(9') or arc-shaped (9'') strips.
8. The planar antenna for motor-vehicles according to any of the preceding claims, characterized
in that the mass plane (8) of the antenna is obtained by means of a printed circuit
board. metallized on one only side and provided with a printed or etched line of coplanar
transmission.
9. The planar antenna for motor-vehicles according to claim 8, characterized in that
the radiant element (1) comprising bent strips (9'') is directly associated to the
conductive layer (12) of the printed circuit (8) along the lower ends of said strips
(9'') and that the capacitive coupling (4') is realized with a foil (13) provided
with a bent strip (14) whose lower end is fastened to an end of the coplanar transmission
line of the mass plane (8).
10. The planar antenna for motor-vehicles according to any of the preceding claims 1-7,
characterized in that the mass plane (8) is provided with a slot (7) wherein a lead
(6) slides which is associated to the coaxial cable (5) for the adjustment of the
capacitive coupling (4) with respect to the radiant element (1).
11. The planar antenna for motor-vehicles according to any of the preceding claims 1-7
and 10, characterized in that the radiant element (1), the shorting lead (9) and the
mass plane (8) are realized by punching and subsequent bending of one only metal plate.
12. Use of the planar antenna according to any of the preceding claims 1 to 11 in cellular
telephony with frequencies comprised between those of 800/900 MHz (GSM, ETACS, AMPS,
PCD) and the higher ones of 2.5 GHz (DCS, UMPTS, PCN, PDC 1.5).
13. Use of the planar antenna according to any of the preceding claims 1 to 11 in the
reception in L, DAB and GPS band, and at higher frequencies up to 6 GHz.
14. Use of the planar antenna according to any of the preceding claims 1 to 11 in the
Telepass type transponder systems.