Multiband antenna for window panes

A multiband antenna for window panes includes a frame-type antenna component and a fishpole-type antenna component. The two components have a common output terminal. The fishpole component is free of any cross bar. At least one leg of the frame-type component which runs along the lower rim of the window is provided with a succession of horizontally directed S-shaped loops, each loop having a length corresponding to an uneven multiple of wave lengths corresponding to the central frequency of the metric band.

BACKGROUND OF THE INVENTION 
The present invention refers to a radio-receiving multiband antenna 
supported on window panes, particularly for a windshield of motor 
vehicles. The term "window pane" is intended to mean in this connection a 
pane of glass or of plastic material and the antenna may consist of 
electric conductors deposited by the silk screen process on the pane, 
preferably on that face of the pane which, when fitted into the car, is 
the internal one; or else, if in lieu of tempered glass, two bonded 
together glass panes are used, applied to that face of the pane, which is 
in contact with the other face. Obviously, in lieu of conductors made by 
the silk screen process, also a conducting metal wire may be used. 
Obviously, such antenna may be applied to any window of a motor vehicle, 
although the windshield is the most suitable place. 
The antenna according to the present invention has been designed to receive 
radio-frequency signals in their various bands of transmissions, such as 
long waves, medium waves, short waves, metric or frequency modulation 
waves (FM) and VHF, decimetric waves and UHF and all the waves for sound 
and/or television information, included the frequencies reserved for radio 
amateurs. 
The antenna incorporated in the pane, particularly in the windshield, is 
preferred to the conventional, freely supported motorcar antennas, because 
they are subjected to various drawbacks, such as: 
A. CONSIDERABLE VIBRATIONS DURING DRIVING WHICH RENDER THE SIGNAL 
FLUCTUATING, TICULARLY WHEN RECEIVING DISTANT STATIONS AND THE RECEIVER 
OPERATES IN THRESHOLD CONDITIONS; 
B. MARKED INSTABILITY IN THEIR CHARACTERISTICS, SUCH AS INCREASE OF THEIR 
RESISTANCE AND CONSEQUENT INCREASE IN THEIR LOSSES, CHANGES IN THE 
CAITY OF THE ANTENNA, DUE TO ITS AGING, TO THE POSSIBILITY OF WATER 
PENETRATION IN THE CYLINDRIC BOTTOM ELEMENT, WHICH CAUSES CORROSION AND 
OXIDATION OF THE TUBULAR ELEMENTS IN A POLLUTING OR BRACKISH ATMOSPHERE; 
C. IN THE CASE OF FISHPOLE ANTENNAS, THE FACT THAT THEY STRONGLY PROJECT 
BEYOND THE MOTORCAR CONTOURS, WHICH LEADS OFTEN TO THEIR BREAKING, FOR 
INSTANCE WHEN ENTERING A GARAGE, AN UNDERPASS, ETC., OR DAMAGING PERSONS 
AND GOODS IF THEY ARE BADLY INSTALLED; 
D. FURTHERMORE THE FISHPOLE ANTENNA IS ALSO SUBJECT TO BE WILLFULLY BROKEN 
BY VANDALS. 
For all these reasons windshield antennas have been developed. 
It is well known that the major part of radio-receiving sets for motor 
vehicles is provided with a single aerial socket, differently from the 
domestic receivers which have an input for the medium waves and one for 
the metric waves (FM), therefore a problem which must be faced in the 
aerials embedded in motorcar windshields is that of obtaining good 
reception of the medium waves as well as of the metric waves in a single 
antenna socket of the radio-receiving set. 
In the prior art various shapes of antennas incorporated or embedded in 
windshields have been suggested, in an attempt to ensure a good reception 
in all wave bands. For this purpose antennas have been devised having one 
central vertical fishpole-type straight or T-shaped element, which afford 
a good reception particularly in the field of metric waves, and have also 
been devised antenna elements of greater length which run along the rim of 
the glass pane, forming so-called "rim" conductors, which afford a good 
reception in the field of medium waves. However, the problem in these 
types of antennas with the distinct receiving elements in the various 
frequency bands is that the signals received by the individual elements 
conjoin correspondingly to the single input of the radio receiver, and 
thus it is difficult to obtain a good reception throughout all wave bands, 
since an antenna built for instance to give a good reception in medium 
waves is generally not fitted with the characteristics which may confer to 
it a good yield also in the reception of metric waves and vice-versa. In 
the prior art there have been suggested types of antennas which were 
supported on the windshield of a motor vehicle, wherein that part of the 
antenna which was suitable for a certain frequency band, form an 
undesirable load when the antenna must operate for a different frequency 
band and furthermore, in particular in the reception of metric waves, 
these types of known antennas have a very variable efficiency in the 
various directions of reception. 
SUMMARY OF THE INVENTION 
According to the present invention, it has been found that some antenna 
structures are capable of receiving with an optimum efficiency both the 
signals in the range of the medium waves (550-1600 KHz) and those in the 
frequency modulation range (87.5-108 MHz). In fact, the electric 
characteristics of the windshield antenna according to the present 
invention excellently satisfy those which are required by the greater part 
of the radio-receiving sets presently marketed, which require a very high 
antenna capacity of 70-100 pF (a capacity value which, added to the 
capacity of the coaxial cable and of the connector permits, by means of 
the trimmer provided in the receiver, to obtain the best possible tuning 
between the antenna and the receiver at a capacity around 150 pF) with a 
high resistance to losses (some hundreds of kohm) in the medium waves band 
and an antenna impedance of approximately 150 ohm which is prevalently 
resistive and with a phase contained within .+-. 30.degree. within the 
band of metric waves. 
In order to obtain a good reception, the ideal would be to have the length 
of the antenna conductors equal to a well defined fraction of the wave 
length .lambda./2 - .lambda./4 according to whether the antenna is of the 
symmetric or assymmetric type. 
Since it is impossible, at least for the medium waves, to have wires of the 
length equal to .lambda./4 (187/4 - 570/4 meters) owing to the natural 
limitations inherent to the windows of a motor vehicle, an antenna has 
been designed which, although in its reduced development, insures an 
excellent efficiency of reception both in the medium wave band and in the 
frequency modulation band. 
This has been rendered possible, according to the invention, by adapting 
the antenna in such a manner that its one section prevalently contributes 
to the reception of the signal in a given frequency band and another 
section contributes prevalently to the reception of the signal of another 
frequency band, but each section contributes also to the section of the 
signal having a frequency included in the band which is that prevalently 
received by the other section. In such a manner, in lieu of having two 
antenna sections, each of which becomes active in the reception of a 
certain frequency band, while the other section is devoid of any utility 
or even a source of parasite load as it happens in the prior art -- in the 
antenna according to the invention both sections give an active 
contribution to the reception of the signal, and therefore this antenna is 
actually a true and real multi-band antenna which functions in an optimum 
manner for the most diverse frequency bands and in addition to it with 
respect to the known technique, it presents a convenient and regular 
efficiency of reception in all possible directions. 
This result has been obtained by an antenna having a geometry such as to 
satisfy extremely exacting requirements with regard to the impedance of 
the antenna circuit, by conferring a given configuration to the conductors 
of the antenna and positioning them with respect to the rims of the 
windshield in such a manner as to obtain, in the reception of the metric 
waves, a practically real magnitude of said impedance, approaching the 
optimum of 150 ohm. To this end the antenna according to the invention 
comprises a fishpole-type component and a frame-type component bordering 
the windshield rim, that leg of the frame-type component which borders the 
lower windshield rim forming a succession of horizontally directed loops, 
each loop having a length corresponding to an uneven multiple of 
.lambda./4, where .lambda. is the wave length corresponding to the central 
frequency of the metric wave band. 
This antenna configuration has the advantage of permitting the compensation 
of the reactive component of the impedance of the fishpole-type portion 
within a wide range of desired frequencies. The horizontal loops on the 
lower windshield rim have also the function of raising the minima of the 
directivity diagram by actively contributing to the signal pick-up, said 
contribution being particularly valuable for those directions, wherein the 
pick-up of the fishpole component is minimal. 
The total impedance of the antenna, once it has has been so matched, will 
vary within the frequency range from 87.5 - 110 MHz between 100 and 200 
ohm and transfer in this manner the maximum input to the car radio which 
requires an optimum impedance of 150 ohm. 
The term "matched" is intended to mean that, during reception, the 
contribution of the receiving element is prevalent, whereas the remaining 
portion of the antenna gives a contribution of the order 10 - 20.degree. 
which adds to the other element; in FM the prevalent receiving element is 
the central fishpole antenna, while in the medium waves the receiving 
element is the remaining portion of the antenna which runs along the rim 
of the glass pane, spaced a few centimeters therefrom; the optimum 
distance from the rim depends on the dimension of the glass pane. 
The principle underlying the configuration of the frame-type component for 
the reception of the medium waves is that of obtaining a maximum possible 
capacity of approximately 100 picofarad and a high loss resistance. 
The choice of the distance of the conductors of the frame-type component 
from the windshield rim is determined by the capacity which is necessary 
to minimize the partition of the signal picked up by the antenna and which 
is fed to the receiver, and is also a function of the windshield size. 
Since experience proves that the pick-up efficiency is improved when the 
conductors are spaced farther from the windshield rim it is convenient, 
when dimensioning the configuration of the antenna, to obtain a correct 
balance between a good antenna capacity value and a good pick-up 
efficiency. 
This experience has shown that in picking up medium waves with the antenna 
configurations according to the invention, a good antenna capacity is 
obtained when the conductors of its frame component are spaced 
approximately 7 cm. from the windshield rim; to further increase this 
capacity the conductor running along the windshield rim may be prolonged 
into an extension running parallel to said conductor but in the reverse 
direction. When the windshield size permits it and the visibility through 
the windshield is not impaired, it is advantageous to increase this 
spacing from the windshield rim to approximately 9 - 10 cm.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
With reference to the figures, all embodiments comprise the aforementioned 
fishpole-type component extending correspondingly to the vertical center 
line of the windshield and indicated at 2 in all figures, and the 
aforesaid frame component bordering all four windshield rims. Both 
components join at a common terminal 4 for the connection of the antenna 
to the radio-receiving set, which is located in the vicinity of the lower 
windshield rim. 
The frame component may consist of a single conductor 3, as shown in FIG. 3 
and 4. It forms a right lower leg bordering the right half of the lower 
windshield rim, a right lateral leg bordering the right lateral windshield 
rim, an upper leg bordering the upper windshield rim up to where, in the 
vicinity of the left windshield rim, it bends downward to form a left 
lateral leg and, in the vicinity of the lower windshield rim, it bends 
inward to form a left lower leg bordering the left lower half of the 
windshield rim to end short of the antenna terminal 4. 
As shown in FIG. 4, at the termination of said lower leg the conductor may 
be led upward in a hair-pin turn and backward to form an extension which 
runs parallel to the last mentioned three conductor legs to end short of 
the right lower leg of the frame conductor. 
Alternately, as in the embodiments of FIG. 1a and 2, said frame component 
may consist of two conductors 3, 3', one on each side of the fishpole-type 
component, which diverge from the antenna terminal 4 along two opposite 
paths, each conductor comprising a lower leg bordering one half of the 
lower antenna rim, an upwardly directed lateral leg bordering one lateral 
rim and an upper leg bordering part of the upper windshield rim to end 
short of the upper free and of the fishpole-type component 2. Similarly to 
this embodiment of FIG. 3, the length of the conductors 3, 3' may be 
increased by hair-pinning them upward and backward at their ends to form 
on each of them an extension running parallel to their respective upper 
and lateral legs. FIG. 1b shows a variant of FIG. 1a obtained in this 
manner. This arrangement is advantageous for small windshields. 
All embodiments shown, which represent basic forms of the antenna forming 
the invention, match the impedance value by two interconnected impedances 
or impedance groups formed by the lower frame conductor legs by conferring 
to them a looped course. In FIG. 1a and 1b each lower leg forms two double 
loops 7, 7' having the approximate shape of two oppositely facing 
flattened S's . In the embodiment of FIG. 2, each leg forms two double 
loops 7, 7' forming two flattened S's facing towards the antenna terminal, 
and in the embodiment of FIG. 4, all flattened S's face in the same 
direction. In the embodiment of FIG. 3, the right lower leg is identical 
to that of FIG. 1a and 1b, while the impedance of the rectilinear left 
lower leg is increased by the addition of a third straight conductor 8 
paralleling that part of the extension which adjoins said left lower leg. 
It has been found experimentally that the signal pick-up capacity of these 
embodiments improves if the level of the S's lies at a higher or at least 
at the same level with the terminal 4, while, if they are positioned at a 
lower level, these double loops would produce in an unsatisfactory manner, 
in the metric wave band, the compensation of the reactive component of the 
fishpole conductor 2. This represents a new criterion in antenna design. 
TEST RESULTS 
An antenna having the configuration shown in FIG. 1b is applied to a glass 
pane of 60 .times. 130 cm. and tested. The following characteristics have 
been found: 
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antenna capacity 
80 pF in the medium wave band 
resistance loss 
&gt;300 kohm in the medium wave band 
antenna impedance in 
100 - 150 ohm in the metric 
the 88 - 108 MHz range 
wave band 
phase angle .+-. 30.degree. in the metric wave band 
resonance at 95 MHz in the metric wave band 
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These results show that the invention provides an efficient multi-band 
antenna with excellent pick-up and directionality characteristics and 
which can be either made from a conducting wire sandwiched between glass 
or plastic panels or applied to one face of a panel by the silk screen 
process. 
It is clear that many changes and variants may be applied by an expert in 
the art to the above illustrated embodiments, as for instance in the 
number and mutual relationships of their loops. Obviously these and other 
changes and variants are all encompassed in the scope of the present 
invention.