IE44158B1 - Multibandd antenna for window panes - Google Patents

Description

The present invention relates to a multiband receiving antenna ..- for a radio receiving set, the antenna being /supported on a windowpane, particularly on the windshield of a motor vehicle.

The term windowpane is intended to mean a pane of glass or '5 of plastics material.

The antenna may consist of conductors deposited by the silk screen process on the pane. In the case of an antenna supported on the . windshield of a motor vehicle it is preferred where the windshield is formed from tempered glass that the antenna is deposited on that face of 1Q:. the pane which, when fitted in the vehicle, is the internal one, and where the windshield is formed from two glass panes bonded together that the antenna is deposited on one of the faces of the pane which is in contact with another face. Instead of depositing the conductors by the • ' r silk screen process, a conducting metal wire may be used.

. - Obviously, an antenna may be supported on any window of a motor vehicle, although the windshield is the most suitable place.

The present invention further relates to an antenna for receiving radio signals lying in various wave bands, for example, in - 2 44158 the long wave band, the medium wave band, the short wave band, the metric or VHF wave band, the decimetric wave band, and the UHF wave band, and generally in all the wave bands used for sound and television transmission, including the frequencies reserved for radio amateurs.

In the case of a motor vehicle, an antenna supported on a windowpane, particularly on the windshield, is preferred to a conventional freely supported antenna because such antennas are subjected to various drawbacks such as:a) considerable vibration during driving which causes signal fluctuations, particularly when receiving distant stations with the receiver operating in threshold conditions; b) marked instability in their characteristics, such as increase in their resistance and consequent increase in their losses and changes in the antenna capacitance due to aging, and to water penetration into the bottom cylindrical element, which causes corrosion and oxidation of the tubular elements in the resulting damp atmosphere; c) in the case of monopole antennas, the fact that they project beyond the motor vehicle contours, which leads often to their breaking, for instance, when entering a garage or underpass, or damaging persons or goods if they are badly installed; d) furthermore a monopole antenna is also subject to being willfully broken by vandals.

For all these reasons, windshield antennas have been developed.

It is well known that the majority of radio-receiving sets for motor vehicles are provided with a single antenna socket, in contrast to domestic receivers which have one input for the medium wave band and another input for the metric wave band. Therefore a - 3 44158 problem which must be faced with antennas provided on motor vehicle windshields is that of obtaining good reception, for example, in the medium wave band as well as in the metric wave band with a single antenna socket provided in the radio-receiving set. Sj In the prior art, various shapes of antennas incorporated or . embedded in windshields have been suggested in an attempt to ensure good reception in all wave bands. For this purpose antennas have been T devised having one central and vertical straight monopole or T-shaped element, which provides good reception particularly in the metric wave T . band, and antennas have also been devised having elements of greater length which run along the rim of the window pane so as to form socalled rim conductors, which provide good reception in the medium wave band. However the problem in these types of antennas with separate receiving elements for the various frequency bands is that , er the signals received by the individual elements add at the single input socket of the receiving set and thus it is difficult to obtain good reception through all wave bands. For instance, an antenna designed to give good reception in the medium wave band generally does not have characteristics which gives it good reception in the metric wave band and vice versa. In the prior art there has been suggested a type of antenna which was supported on the windshield of a motor vehicle in which the part of the antenna which was suitable for a certain wave band formed an undesirable load when the antenna was operated in a different wave band and, furthermore, .in particular in the reception of the metric wave band the efficiency of this known type of antenna,varied with the direction of reception.

According to the present invention, there is provided a multi-band which is receiving antenna/supported on a windowpane comprises a monopole conductor having a free end for receiving radio signals lying in a first waveband and at least one conductor for receiving radio signals lying in a second waveband, the first waveband convering signals having a comparatively short wavelength and the - 4 44158 second waveband covering signals having a comparatively long wavelength, all conductors being connected to a common terminal located adjacent the rim of the windowpane, the or each conductor for receiving signals lying in the second waveband extending away from the common terminal, then doubling back to form a folded part, then running alongside the monopole conductor until approximately level with said free end, and then turning away from the monopole conductor and running alongside at least part of the rim of the windowpane, and the or each conductor for receiving signals lying in the second waveband also contributing to the reception of signals lying in the first waveband and the monopole conductor also contributing to the reception of signals lying in the second waveband.

Preferably, the first waveband is the metric waveband and the second waveband is the medium waveband.

The antenna of the present invention is capable of receiving with high efficiency both radio signals having a wavelength lying in the medium waveband (550-1600 KHz) and those having a wavelength lying in the metric waveband (87.5-108 MHz). Also, the electrical characteristics of the antenna of the present invention excellently satisfy those which are required by the majority of radio-receiving sets presently marketed, which require a very high antenna capacitance of 70-100 pF (a capacitance value which, added to the capacitance of an associated coaxial cable and an associated connector, permits by means of a trimmer provided in the receiving set the best possible tuning at a capacitance around 150 pP) with a high resistance (some hundred of kilohms) to losses in the medium waveband and an antenna impedance of approximately 150 ohm, which is prevalently resistive with a phase angle lying between plus and minus 30°, in the metric waveband.

In order to obtain good reception, the ideal would be to have the length of the antenna conductors equal to a well defined fraction of the wavelength, for example, A/2 or A/4 according to whether the antenna is of the symmetric or asymmetric type. - 5 44158 It is impossible for the medium waveband to have wires of a length equal to X/4 (187/4 - 570/4 meters) owing to the natural limitations inherent in motor vehicle/windows. Despite this, the antenna of the present invention provides high efficiency in the reception of signals in both the medium waveband and in the metric waveband.

This has been rendered possible by arranging the antenna of the present invention in such a manner that the monopole conductor part prevalently contributes to the reception of signals in one waveband and the remaining part 'prevalently contributes to the reception of signals in another waveband and each part also contributing to the reception of signals lying in the waveband prevalently received by the other part. Thus, instead of having as in the prior art two antenna parts, each of which is active in the reception of signals lying in an associated waveband while the other part is devoid of any use in this waveband or even becomes a parasitic load, in the antenna of the present invention both parts give an active contribution to the reception of signals and therefore the antenna of the present invention is actually a true multiband antenna. Also, its efficiency does not vary appreciably with the direction of the received signals.

This result has been obtained in the antenna of the present invention by giving it a geometry which satisfies extremely exacting requirements with regard to the impedance of the antenna circuit, and by giving the conductors a configuration and position with respect to the rim of the windowpane such that in reception of signals in the metric waveband the antenna has a prevalently resistance impedance approaching the optimum value of 150 ohms.

It is in order to obtain this result that the conductor which prevalently receives signals in the metric waveband is a monopole conductor and the or each conductor which prevalently receives signals in the medium waveband commences with a folded part. It is the folded part which gives the antenna an impedance approaching 150 ohms in the metric waveband. - 6 44158 Preferably, the length of the conductor forming the folded part is approximately equal to an uneven multiple of λ/4 for example % /4, where Λ corresponds to the central wavelength of the metric waveband.

The arrangement of the present invention also has the advantage of providing compensation for the reactive component of the impedance of the monopole over a wide range of frequencies.

The folded part of the or each conductor for receiving signals in the medium waveband also has the function of raising the minima of the directivity curves, thereby actively contributing to signal pickup, which is particularly valuable for those directions in which the pickup of the monopole conductor is minimal.

The overall impedance of an antenna having this arrangement varies in the frequency range from 87.5 to 110 MHz between 100 to 200 ohms and so transfers the maximum power to the imput of a radio-receiving set which requires an optimum impedance of 150 ohm.

With this arrangement, there is maximum power transfer from the antenna to the input of a receiving set. Also, during reception of signals in a particular waveband, the conductor or conductors which are not prevalently active in receiving signals in the particular waveband provided a contribution of the order of 10 to 30% which is added to the signal produced by the conductor or conductors which are active.

It has been found that the length of the monopole conductor of the antenna essentially depends on the size of the windowpane having regard to the condition that its length must be a well defined fraction of the wavelength, for example λ/4.

The length of the monopole also depends on whether it consists of a silver deposit applied to a pane of glass by the well-known silk screen process or whether it consists of a very thin wire, for example 0.1 to 0.2 millimeters, placed on a plastic sheet sandwiched between two glass panes so as to form safety glass. In fact the speed of propagation of electromagnetic waves depends on - 7 44158 whether reception occurs on the external surface of a pane of glass or on the interface between two panes of glass.

The correct length of the monopole is computed accordingly in each case so as to obtain reasonance at the desired frequencies so as to have in the reception of signals lying in the metric waveband an antenna impedance of around 150 ohms and therefore maximum transfer of signal power into the receiving-set.

For the conductor or conductors which contribute prevalently to the reception of signals in the medium waveband the shape and structure should be designed to give the maximum possible capacitance (approximately 100 picofarads) together with a high loss resistance so as to minimize signal loss and to transfer maximum possible power to the input of a receiving set.

It has been found that reception improves as the conductor or conductors which run alongside the rim of the windowpane are moved further away from the rim and therefore in selecting the spacing between this conductor or those conductors and the rim a compromise has to be found which provides both a good antenna capacitance and high pickup efficiency.

In receiving signals in the medium waveband with an antenna of the present invention it has been found that a good antenna capacitance is obtained when the conductor or conductors are spaced approximately 7 cm from the rim of the windowpane. In order to increase the capacitance, each of these conductors may include a second portion which doubles back on, and runs parallel to, the first position. Where the size of the windowpane permits it without interfering with the angle of visibility, it is advantageous to increase the spacing to approximately 9 to 10 cm.

The present invention will now be described in more detail, by way of example, with reference to the accompanying drawings in which:Figure 1 shows a first embodiment of the present invention; Figure 2 shows a modification of the embodiment shown in Figure 1; Figure 3 shows a second embodiment of the present invention; and - 8 44158 Figure 4 is a directivity diagram showing a comparison of an antenna of the present invention with that of a known T-type antenna.

Referring now to Figure 1 there is shown a multiband receiving antenna supported on a windowpane in the form of a motor vehicle windshield. The antenna , the terminal 1 being has a terminal 1 for connecting it to a motor vehicle receiving sei/located on the centre line of the windshield a few centimeters above the lower rim. A monopole conductor 2 extends vertically upwards from the terminal 2 and ends a few centimeters below the upper rim of the windshield. This conductor 2 prevalently receives signals in the metric waveband.

Two further conductors 3 and 4 branch in opposite directions from the terminal 1. Each of these conductors initially extends away from the monopole conductor 2 in a direction perpendicular thereto and runs parallel to the lower rim for a short distance and then doubles back running parallel to, and on the inside of the first stretch so as to form a folded part. At a short distance from the monopole conductor, each of the conductors 3 and 4 bends vertically upwards and runs parallel to the monopole conductor 2 until approximately level with the free end thereof. Thereafter, each conductor turns away from the monopole conductor 2 and runs alongside the windshield rim and terminates at a position short of the folded part. The conductors 3 and 4 prevalently receive signals in the medium waveband and the folded parts give the antenna a suitable impedance in the metric waveband.

In Figure 2 there is shown a modification of the antenna of Figure 1 in which each of the conductors 3 and 4 has a second portion which commences at said position short of the folded part and doubles back on the first portion running parallel thereto to the position where the first portion turns away from the monopole conductor 2. The two second portions increase the capacitance of the antenna which is useful where the windshield is so small that sufficient capacitance cannot otherwise be obtained. - 9 44158 ’ It has been found that optimum impedance is obtained in the metric waveband when the length (IJ of the folded parts of the conductors 3 and 4 lies in the range 28 to 35 cm so that the total length of the conductor forming the folded part is approximately equal to one quarter of the central wavelength of the metric waveband.

It has been found that the presence of the vertical stretches of conductors 3 and 4 which run parallel to the conductor 2, cause an improvement in respective ' the pickup of signals in the medium waveband. The distance between each/ vertical stretch of the conductors 3 and 4 and the conductor 2 should be approximately TO 7 cm but it may vary between 6 and 7 providing the visibility requirements of the windshield are not impaired.

In the modification shown in Figure 2, it is preferred that the first and second portions of the conductors 3 and 4 are spaced from each other by a distance lying in the range 0.2 to 1.2 cm; and that, for each of these conductors, the. distance between said position short of the folded part and the folded part lies in the range 0.2 to 1.2 cm.

An antenna of the type shown in Figure 1 was applied to a windshield having a size 60 x 130 cm with the terminal 1 positioned approximately 5 cm from the lower windshield rim and the length of the conductor 2 being 51 cm. The horizontal length (TJ of the folded part of each of the conductors 3 and 4 was . cm and the vertical stretches of these conductors were spaced 7 cm from the conductor 2. Conductors 3 and 4 ran alongside the windshield rim at a distance 7 cm therefrom and ended 1 cm 'from their respective folded parts.

This antenna had a capacitance of approximately 80 picofarads, which was optimum for receiving signals in the medium waveband, and the monopole conductor 2 was brought into resonance at 95 MHz, which represents the centre of the metric waveband. Thus, excellent pickup was obtained in the medium waveband.

An antenna of the type shown in Figure 2 was applied to a windshield having a size 60 x 130 cm and was tested by comparing the output voltage at terminal 1 with - 10 44158 the output voltage obtained from a conventional T-shaped antenna applied to the same windshield.

The decisive improvement in directivity characteristics of the antenna of Figure 2 over the conventional antenna can be seen in the diagram of Figure 4, in which the ordinate represents power in decibels and the abscissa represents orientation in degrees. The output of the antenna of Figure 2 is indicated by curve A whilst that of the conventional T-antenna is indicated by curve B. The arrow F indicates the direction of a transmitter broadcasting an FM signal at a frequency of 92.1 MHz.

Curve B shows that the conventional T-type antenna has minimum of sensitivity at 140° and 320° whilst A shows that the antenna of Figure 2 has a more constant response, especially around 300°, which is when the vehicle is facing the transmitter. 140° corresponds to the orientation of the vehicle in which the transmitter lies behind the windshield.

Another multi-band antenna supported on a motor vehicle windshield is shown in Figure 3 and instead of having three conductors, as is the case in the antennae shown in Figures 1 and 2, this antenna consists of two conductors, namely a monopole conductor 5 and another conductor 6, both of which are connected to an antenna terminal 1. The conductor 6 comprises four portions. The first portion extends away from the terminal 1 in a direction perpendicular to the monopole conductor 5, then doubles back to form a folded part, then runs alongside the conductor 5 to a position level with the free end thereof, and thereafter runs away from the conductor 5 /alongside the rim of the windshield to a position short of the folded part. The second portion doubles back on the first portion and runs parallel to it to a position level with the free end of the conductor 5 and the third portion runs from this position alongside the rim of the windshield to a position a short distance from the terminal 1. The fourth portion doubles back on the third portion and runs parallel thereto to a position adjacent the free end of the conductor 5.

In the antenna shown in Figure 3, it is preferred that the distance between the folded part and the end of the first portion has a value lying in the - 11 4 415 8 range 0.2 to 1.2 cm and that the first portion and third portions are spaced from the second and fourth, portion respectively by a distance lying in the range 0.2 to 1.2 cm.

- The antenna shown in Figure 3 possesses the advantageous features described with reference to the antennae shown in Figures 1 and 2.

Claims (13)

1. A multi-band receiving antenna/supported on a windowpane comprises a monopole conductor having a free end for receiving radio signals lying in a ../ first waveband and at least.one conductor for receiving radio signals lying in a 'second TO waveband, the first waveband covering signals having a comparatively short wavelength and the second waveband covering signals having a comparatively long wavelength, all conductors being connected to a common terminal located adjacent the rim of the windowpane, the or each conductor for receiving signals lying in the second waveband extending away from the common terminal, then doubling back 15 to form a folded part, then running alongside the monopole conductor until approximately level with said free end, and then turning away from the monopole conductor and running alongside at least part of the rim of the windowpane, and the or each conductor for receiving signals lying in the second waveband also contributing to the reception of signals lying in the first waveband and the 20 . monopole conductor also contributing to the reception of signals lying in the Second waveband.

2. An antenna according to Claim 1 in which the first wavebandis the metric waveband and the second waveband is the medium waveband.

3. An antenna according to Claim 2 in which the length of conductor forming 25 .the folded part is approximately equal to one quarter of the central wavelength of the metric waveband.

4. An antenna according to any one of the preceding claims in which there are two conductors for receiving radio signals lying in the second waveband, the conductors being on opposite sides of the monopole conductor, and each conductor 30 extending alongside the rim of the windowpane to a position short of the folded part. - 12 +41 S8

5. An antenna according to Claim 4 in which each conductor for receiving signals lying in the second waveband comprises a first portion extending from the common terminal to the position short of the folded part and a second portion which doubles back on the first portion and runs parallel to at least part of the first portion.

6. An antenna according to Claim 5 in which each second portion runs parallel to its associated first portion to the position where the first portion turns away from the monopole conductor.

7. An antenna according to Claim 1 or Claim 2 in which there is a single conductor for receiving signals lying in the second waveband, said single conductor comprising a first portion commencing at the common terminal and which, after forming said folded part and turning away from the monopole conductor, runs alongside the rim of the windowpane on one side of the monopole conductor to a position short of the folded part, a second portion which doubles back on the first portion to a position adjacent to the free end of the monopole conductor, a third portion which extends from the end of the second portion and runs alongside the rim of the windowpane on the other side of the monopole conductor to a position a short distance from the antenna terminal, and a fourth portion which doubles back on the third portion and runs parallel to the third portion to a position adjacent the free end of the monopole conductor.

8. An antenna according to any one of the preceding claims in which the distance between the monopole conductor and the part of the or each conductor for receiving signals lying in the second waveband which runs alongside the monopole conductor is between 60 and 70 mm.

9. An antenna according to Claim 5 in which the distance between the folded part and said position short of the folded portion has a value between 0.2 and 1.2 cm.

10. An antenna according to Claim 7 in which the distance between the folded part and the end of the first portion has a value between 0.2 and 1.2 cm. - 13 44158

11. An antenna according to Claim 5 in which the first portion is spaced 0.2 to 1.2 cm from the second portion, in the region where doubling back occurs. ·

12. An antenna according to Claim 7 in which the first portion is spaced'0,2 to 1.2 cm from the second portion and the third portion is spaced 0.2 to T.2 cm from 5 the fourth portion, in the region where doubling back occurs.

13. A multi-band receiving antenna supported on a windowpane substantially as hereinbefore described with reference to and as shown in Figure 1, or Figure 2, or Figure 3 of the accompanying drain ngs.