AUTOMOBILE ANTENNA SYSTEM

Abstract: An automobile antenna system which requires no pole an­ tenna or the like projecting outwardly from the automobile body. The antenna system has a high-frequency pickup includ­ ing a loop antenna and this pickup is incorporated within a por­ tion of the vehicle body. The high-frequency pickup is secured to the rear window frame and the longitudinal side of the loop antenna which is partially exposed through the casing of the high-frequency pickup is disposed in the vicinity of an edge-­ moulding mounting retainer. The loop antenna picks up high-­ frequency surface currents which are caused by broadcast waves and concentratively flow in the edge-moulding mount­ ing retainer.

Description: The present invention relates to automobile antenna systems and, more particularly, to an improved automobile antenna system for effectively detecting broadcast waves received by the automobile body and then supplying signals thus detected to various kinds of receivers mounted in the automobile. Antenna systems are indispensable for automobiles required to receive various broadcast and communication waves such as those for radio, television or telephone by using receivers mounted in the automobile, and these types of antenna system are also important in allowing communication to take place between automobiles and other stations, for example, transmitting and receiving citizen band radio waves. Therefore, such antenna systems play a major role in communication functions which will henceforth usually be incorporated in automobiles. One conventional type of common antenna system is known as a pole antenna. Although the pole antenna which projects outwardly from the body of an automobile exhibits desirable performance in terms of reception, it has always been the fate of the pole antenna to be treated as a nuisance from the viewpoint of vehicle body design. Furthermore, the pole antenna involves various problems in that it is exposed to damage such as breakage or bending in actual service and in that it becomes the target of mischief or theft and, additionally, it generates unpleasant noise during high-speed running. For these reasons, there has heretofore been a strong demand for eliminating such pole antennas. In particular, since frequency bands for broadcast or communication waves received in the interior of an automobile have recently increased in number, it is necessary to install a plurality of antennas corresponding to each of the frequency bands received. Consequently, the installation of a plurality of antennas involves disadvantage in that the aesthetic appearance of an automobile is spoiled and the reception performance of the various antennas deteriorates due to the mutual electrical inteference caused therebetween. Several efforts have previously been made to replace the above-mentioned pole antenna system or to conceal the system from the exterior. As an example, a means of applying a length of antenna wire to the rear windshield of an automobile has been put into practical use. As another conventional means of solving the above-noted problems, proposals have been made to detect surface currents which are induced by broadcast waves on the vehicle body itself. Although it is considered that, seemingly, the utilization of currents flowing on the vehicle body might be expected to be the most positive and efficient means of reception, the experiments carried out to date have shown disappointing results . One of the reasons why the surface currents induced on a vehicle body by broadcast waves have not been utilized is that the values of the surface currents are not so large as was expected. Although the prior art mainly uses surface currents induced on the roof plate of the vehicle, it was still impossible to obtain a detected output showing a utilizable level. Another reason is that interference at a high level of noise is mixed in the surface currents. Such noise is mainly generated by the engine ignition system and the battery charging regulator system. As long as the engine is operating, such noise continues to leak into the vehicle body, thus preventing any clear reception of broadcast waves at a practicable level. Several proposals have heretofore been made in an attempt to cope with these adverse conditions. A conventional type of antenna system using currents induced on the vehicle body by broadcast waves is disclosed in Japanese Patent Publication No. 22418/1978 in which electrical insulation is formed at a portion of the vehicle body on which electric currents are concentrated and the currents flowing between the opposite ends of the insulation are directly detected by a sensor. It is true that this conventional antenna system suggests that it can provide a detected signal of a utilizable level which is superior in S/N ratio. However, since a pick-up structure therefor requires a cut-out in a portion of the vehicle body, it cannot be applied to normal mass-production types of automobiles. Another conventional antenna system is disclosed in Japanese Utility Model Publication No. 34826/1978 in which an antenna including a pick-up coil for detecting a current flowing in the pillar of a vehicle body is proposed. This prior art was useful in anticipating the course of development of systems based on incorporation of an antenna into a vehicle body. However, it is of no practical use for the pick-up coil to be located in the vicinity of a pillar in a direction perpendicular to the longitudinal axis thereof. In addition, since this pick-up arrangement is not capable of providing any antenna output of utilizable level, it has been regarded merely as a casual idea. As described above, the prior-art antenna systems have not necessarily been successful in detecting currents induced on the vehicle body by broadcast waves. In particular, no solution has heretofore been found to various problems which arise with a pick-up structure for effectively detecting currents induced by broadcast waves which are conducted by the vehicle body and a pick-up arrangement capable of obtaining a utilizable S/N ratio. Rather, the results of various kinds of experiments have suggested that it might be theoretically impossible to use an antenna system which utilizes currents flowing on the vehicle body. The prior art antenna systems have mainly been designed to receive AM band waves to meet the needs of the times. Consequently, since the wavelengths of those broadcast waves are too large to be received, antenna systems based on the detection of the vehicle body currents cannot achieve proper reception characteristics. The inventors took notice of this frequency-dependent property and, in the present invention, selected 50 MHz or greater which is above the FM frequency band as broadcast waves being received, thereby enabling remarkably effective reception from vehicle body currents which has conventionally been considered impossible. US-A-4317121 describes an automobile antenna system comprising a pick-up mounted on a portion of an automobile body to detect radio frequency surface currents induced in said body portion by broadcast radio frequency signals. The present invention is characterized in that: said pick-up is adapted to detect said surface currents at a frequency above 50 MHz which have a concentrated flow along peripheral edge portions of the automobile body; said pick-up comprises a metal casing having an opening and an elongate loop antenna disposed within said casing with one longer side thereof externally exposed through said opening; said pick-up being mounted with said exposed longer side of said loop antenna extending along and adjacent to the peripheral edge of a said peripheral edge portion; and said peripheral edge portion being an elongate edge-moulding mounting retainer extending along and separated from the rear windshield frame of the automobile body by a distance substantially equal to 2 x 10⁻³ x wavelength, the wavelength being that of a broadcast wave to be picked up. EP-A-181120 is a document of the type mentioned in Article 54(3) of the EPC and discloses and claims an automobile antenna system comprising a pick-up mounted on a portion of an automobile body to detect radio frequency surface currents induced in said body portion by broadcast radio frequency signals; characterized in that: said pick-up is adapted to detect said surface currents at a frequency above 50 MHz which have a concentrated flow along marginal edge portions of the automobile body; and said pick-up comprises elongate pick-up means extending along a said marginal edge portion substantially parallel to the edge of the marginal edge portion and is arranged to be mounted to said marginal edge portion at a distance from said edge which is less than the distance given by the formula: 12 x 10⁻³c/f(m) where c = the velocity of light and f = the carrier-frequency of a broadcast wave to be picked up. EP-A-183443 is partially a document of the type mentioned in Article 54(3) of the EPC. Figs 13 to 15 thereof are similar to Figs. 1 to 3 herein. An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Fig. 1 is a perspective view showing the general construction of a preferred embodiment of automobile antenna system in accordance with the present invention, in which an electromagnetic coupling type of pick-up is mounted on the rear periphery of the roof plate of an automobile; Fig. 2 is a plan view showing in detail a state wherein the pick-up shown in Fig. 1 is fixed; Fig. 3 is a cross-sectional view of the essential portion of the general construction shown in Fig. 1; Fig. 4 illustrates surface currents I induced on the vehicle body B by external waves W; Fig. 5 illustrates a probe for detecting the distribution of surface currents on the vehicle body and having the same construction as that of the pick-up used in the present invention, and a circuit for processing signals from the probe; Fig. 6 illustrates the electromagnetic coupling between the surface currents I and the pick-up loop antenna; Fig. 7 illustrates the directivity of the loop antenna shown in Fig. 6; Fig. 8 illustrates the distribution characteristics of the intensity of the surface currents; Fig. 9 illustrates the directions of flow of the surface currents; Figs. 10, 11 and 12 are graphs showing the distribution of surface currents at various points on the automobile body shown in Fig. 8 along the longitudinal axis thereof, respectively; and Fig. 13 is a graph showing variations in antenna sensitivity with respect to the gap between the rear windshield frame and the retainer. Preferred embodiments of the automobile antenna system in accordance with the present invention will be described below with reference to the accompanying drawings. Figs. 4 to 12 illustrate a process of finding the most efficient antenna mounting position in terms of reception through the distribution characteristics of high-frequency currents. Fig. 4 shows that when external electromagnetic waves W such as broadcast waves pass through a vehicle body B made of conductive metal, surface currents I are induced at various points of the vehicle body B at levels corresponding to the intensities of electromagnetic waves passing therethrough. The present invention is aimed only at electromagnetic waves which belong to relatively high frequency bands of 50MHz or greater, such as FM waves, television waves and others. The distribution of the surface currents induced on the vehicle body by electromangetic waves within the above described particular wave bands is measured with a view to identifying a position on the vehicle body which is high in surface current density and low in noise and at which a pick-up may be located. The distribution of surface currents is determined by simulation using a computer and also by measuring actual intensities of surface currents at various positions on a vehicle. The measurement is carried out by the use of a probe which can operate in accordance with the same principle as that of a pick-up actually located on the vehicle body at the desired position. Such a probe is moved on the vehicle body over the entire surface thereof to measure the level of surface currents at various positions on the vehicle body. Fig. 5 shows the general construction of such a probe P which is constructed in accordance with substantially the same principle as that of the pick-up described hereinafter. The probe P is composed of a casing 10 of electrically conductive material for preventing interference by external electromagnetic waves and a loop coil 12 is secured to the interior of the casing 10. The casing 10 includes an opening 10a formed therein through which a portion of the loop coil 12 is exposed to the exterior. The exposed portion of the loop coil 12 is positioned in close proximity to the surface of the vehicle body B to detect magnetic flux induced by surface currents on the vehicle body B. Another portion of the loop coil 12 is connected to the casing 10 through a short-circuiting line 14. The loop coil 12 further includes an output end 16 connected to a core 20 in a coaxial cable 18. Still another portion of the loop coil 12 includes a capacitor 22 for causing the frequency of the loop coil 12 to resonate relative to the desired frequency to be measured so as to increase the efficiency of the pick-up. Thus, when the probe P is moved along the surface of the vehicle body B and also angularly rotated at various points of measurement, the distribution and direction of surface currents on the vehicle body can be accurately measured. In Fig. 5, the output of the probe P is amplified by a high-frequency voltage amplifier 24 and the resulting output voltage is measured by a high-frequency voltmeter 26. This coil voltage output is read through a value indicated by the high-frequency voltmeter 26 and a voltage corresponding to the value indicated by the voltmeter 26 is recorded by an XY recorder 28 to provide the distribution of surface currents at the various positions on the vehicle body. Signals indicative of various positions are fed from a potentiometer 30 to the input of the XY recorder 28, whereby the high-frequency surface currents at each of the positions thereof can be ascertained. Fig. 6 shows a declination ϑ which is formed by the high-frequency surface currents I and the loop coil 12 of the pick-up, and, as shown in this figure, a magnetic flux  caused by the currents I crosses the loop coil 12 so as to generate a detected voltage V at the loop coil 12. As shown in Fig. 7, when ϑ is 0, that is, when the surface currents I are parallel to the loop coil 12 of the pick-up, the maximum level of voltage can be obtained and the direction in which the surface currents I flow when the maximum voltage is obtained can be found by rotating the probe P at the respective points of measurement. Figs. 8 and 9 shows the magnitude and the direction of the high-frequency surface currents generated at the respective positions on the vehicle body at a frequency of 80MHz which is found on the basis of both the results obtained from measurement by the probe P and a computer simulation. As shown in Fig. 8, the magnitude of the surface currents is distributed in such a manner that current density is high at the portions along the edge of each flat portion of the vehicle body while it is extremely low at the center of each flat portion thereof. As indicated by the arrows (showing the direction of each current flow) shown in Fig. 9, it will be understood that the respective currents are concentrated in a direction parallel to each edge of the vehicle body or along each portion which forms a junction between the flat portions. When the distribution of the currents which are induced on the metal portion of the vehicle is examined in detail along the one-dot chain line in Fig. 8 which runs lengthwise over the vehicle, distribution characteristics as shown in Figs. 10 to 12 are obtained. Fig. 10 is a graph showing the distribution of the surface currents which appear along the line between points A and B on the trunk (boot) lid. As clearly shown in this figure, the distribution characteristics are such that the maximum level of currents flow at the opposite ends of the lid while the current value decreases from the ends of the lid to the center thereof. As can be seen from Fig. 10, the disposition of the pick-up in the vicinity of a peripheral edge portion of the trunk lid enables the detection of the currents concentratedly flowing near this peripheral edge portion. Likewise, Fig. 11 shows the current distribution along the chain line shown in Fig. 8 running over the roof plate of the vehicle body and Fig. 12 shows that along the same chain line where it runs over the bonnet of the engine compartment. As will be understood from the two figures, the maximum level of current is conducted at both ends of the roof plate and the engine compartment bonnet, respectively, and, conversely, the current values decrease toward the center thereof. Accordingly, it will be readily understood that broadcast waves can be picked up with proper sensitivity in the vicinity of each peripheral edge of the vehicle body. It is a matter of course that the pick-up mounting position is not limited to the bonnet or the roof plate noted above and the system may be applied to a pillar or a fender (wing) in a similar manner. Furthermore, the pick-up is mounted in such a manner that, as an example, the length of the loop antenna is located in close proximity to and along the peripheral edge portion of a vehicle body portion). In this case, it is preferable that the pick-up mounting zone along the peripheral edge is set within a range which is dependant upon the carrier frequencies of broadcast waves whereby excellent sensitivity can be achieved in practical terms. The distribution characteristics shown in Figs. 10 to 12 are those of the vehicle body currents with respect to an FM broadcast frequency of 80 MHz in which, as described above, the value of each surface current is lowered in accordance with the distance from the end or edge of each vehicle body portion. As can be seen from these characteristics, since a satisfactory sensitivity can be actually achieved within a current lowering range of 6 dB or less, remarkably excellent sensitivity can be accomplished within 4.5 cm from the edge. Therefore, it is possible to obtain a satisfactory antenna system in practical terms by disposing the pick-up within 4.5 cm from the edge of a vehicle body portion with respect to a carrier frequency of 80 MHz. In addition, the results of a computer simulation and various experiments clearly show that this utilizable margin depends upon carrier frequencies and it has been recognized that the utilizable margin decreases as the frequencies increase. Accordingly, on the basis of the above mentioned facts, namely that the utilizable margin at a carrier frequency of 80 MHz is 4.5cm and is inversely proportional to any increase in frequency, if a pick-up mounting range from the edge of the flat metal portion of the vehicle body is set within the following dimension: 12 × 10 ⁻³ c/f(m), proper reception can be achieved in correspondence with any desired carrier frequency (where, c = the speed of light and f = the carrier frequency). As described above, the pick-up is disposed in close proximity to an edge portion of a metal vehicle body portion, and is preferably located within the above-noted margin from the edge, thereby accomplishing a proper reception effect. The actual margin in each case depends on the particular frequency. As an examples at a carrier frequency of 100 MHz, the pick-up may be located within 3.6 cm from the edge of the vehicle body portion. Hence, as the carrier frequency f increases, the pick-up mounting position is limited to a progressively narrower area extremely close to the edge. Figs. 1 to 3 show one preferred embodiment in accordance with the present invention in which the pick-up is disposed in close proximity to the rear edge of the roof plate. Fig. 1 shows a roof plate 32 in an exposed state. The metal roof plate 32 with a rear window frame 34 constituting the peripheral edge is connected to a rear glass 36. As shown in detail in Fig. 2, a pick-up 38 includes a casing 40 made of metal for preventing external magnetic flux from reaching the interior, while the interior of the casing 40 contains a loop antenna 42, thus constituting an electromagentic coupling type of pick-up. The construction is analogous to the foregoing probe containing a loop coil which, as described above, is used to examine the surface current distribution on the vehicle body. Fig. 3 is a lateral cross-sectional view in which the pick-up 38 is secured to the roof plate 32. The roof plate 32 includes a roof panel 44 and the rear window frame 34 is secured to one end of the roof panel 44. The rear glass 36 is secured to the roof panel 44 by a fastener 46 and a dam 48 which airtightly adhere to each other by the medium of an adhesive 50. Also, a moulding 52 is secured between the roof panel 44 and the rear glass 36. Furthermore, a roof trim (garnish) 64 is secured to the roof panel 44 inward of the rear window frame 34 of the roof plate 32 (inside the vehicle body), and an edge moulding 66 is fixed to the ends of the roof trim 64 and the rear window frame 34 in combination. An edge-moulding mounting retainer 68 for mounting the edge moulding 66 is disposed in the space between the rear window frame 34 and the roof trim 64. The retainer 68 is separated from the rear window frame 34 by spacers 70 and 72 thereby facilitating the concentration of surface currents. Fig. 13 is a graph showing variations in antenna sensitivity with respect to the gap between the rear window frame 34 and the retainer 68, that is, the varied values of a surface current density which reaches the maximum at about 2 × 10⁻³ × wavelength. On the basis of this result, the degree of concentration of surface currents flowing at each edge of the vehicle body can be enhanced by separating the retainer 68 from the rear window frame 34 by a distance corresponding to the above mentioned gap. An opening 34a is formed in a portion of the rear window frame 34 and the casing 40 of the pick-up 38 is inserted into the opening 34a in such a manner that the loop antenna 42 of the pick-up 38 is disposed in face-to-face relationship with the edge of the retainer 68. Specifically, the the loop antenna 42 of the pick-up 38 is disposed in close proximity to the edge of the retainer 68 and along the length thereof. The retainer 68 is separated from the rear window frame 34 by a dimension approximately equivalent to (2 × 10⁻³ × wavelength). The longitudinal length is made substantially equal to a wavelength in the television band, for example, about half a wavelength with respect to a low frequency band (1 to 3 chs in Japan) of the VHF band, about one wavelength with respect to a high frequency band (4 to 12 chs in Japan) of the same band and about (2 to 4 × wavelength) with respect to the UHF band. Therefore, concentration of surface currents in these frequency bands is further facilitated, whereby high sensitivity reception can be accomplished. As illustrated in detail in Fig. 3, the casing 40 of the pick-up 38 has an opening 40a through which the longitudinal side of the loop antenna 42 is exposed, and the portion of the loop antenna 42 which is exposed through the casing 40 made of electrically conductive material is disposed in close proximity to and in face-to-face relationship with the edge of the edge-moulding mounting retainer 68. Therefore, magnetic flux induced by high-frequency surface currents flowing in the edge portion of the retainer 68 is positively captured by the loop antenna 42 within the casing 40, and additionally, the casing 40 positively prevents external magnetic flux from reaching the interior thereof, thereby enabling the currents induced on the vehicle body to be detected with high sensitivity by the pick-up 38. In order to positively position and secure the casing 40 of the pick-up 38 relative to the edge-moulding mounting retainer 68, L-shaped brackets 54 and 56, as shown in Fig. 2, are connected to the opposite ends of the casing 40 by using fastening means such as bolts and the brackets 54 and 56 are threadedly secured to the rear window frame 34. A circuit section 58 which is connected to the loop antenna 42 is incorporated into the casing 40 of the pick-up 38 and a detected signal is processed by a preamplifier or similar arranged in the circuit section 58. Furthermore, the high-frequency detected signal thus obtained is taken off through a coaxial cable 60 and is processed by the same circuit as that used in the above-mentioned measurement of surface current distribution. The circuit section 58 is supplied with electricity and signals for controlling the circuit section 58 through a cable 62. The loop antenna 42 is constituted by a single-turn antenna and has a structure in which the coil thereof is insulation-coated so that it may be disposed in physical contact with the edge-moulding mounting retainer 68 in electrically isolated relationship therebetween, being forced against the end surface of the retainer 68. This further strongly forces magnetic flux caused by surface currents to cross the loop antenna 42. In accordance with this embodiment, the side of the loop antenna 42 which is exposed through the casing 40 is disposed within 4.5 cm from the edge of the retainer 68, thereby enabling the detection of vehicle body surface currents flowing in the edge portion of the retainer 68 due to the induction caused by broadcast waves of 50 MHz or greater, particularly FM-band or TV-band frequencies. Fig. 9 clearly shows the direction of flow of the vehicle body surface currents in this instance in which the currents flow along the edge portions of the vehicle body. Therefore, in accordance with this embodiment, the loop antenna 46 is disposed such that the length thereof extends along the edge of the retainer 68. As described above, surface currents flowing in the edge portions of a vehicle body, particularly the edge of the roof plate, are electromagnetically detected by the pick-up, whereby it is possible to positively receive a high-frequency band without exposing any portion of the antenna system to the exterior. It is evident from this advantage that the system is remarkably useful as an automobile antenna system. As described above, a broadcast wave receiving antenna for relatively high frequency bands such as the VHF band is formed by utilizing high-frequency surface currents developed at particular portions of a vehicle body, specifically in the peripheral edge of the edge-moulding mounting retainer whose longitudinal length is substantially equal to the wavelength of the television band, thus enabling a high-density, high-quality and low-noise detection. Moreover, it is possible to obtain a small-size and high-performance automobile antenna system without any need for a conventional pole antenna or similar exposed to the exterior.

Claims: An automobile antenna system comprising a pick-up (38) mounted on a portion of an automobile body to detect radio frequency surface currents induced in said body portion by broadcast radio frequency signals; characterized in that: said pick-up (38) is adapted to detect said surface currents at a frequency above 50 MHz which have a concentrated flow along peripheral edge portions of the automobile body; said pick-up (38) comprises a metal casing (40) having an opening (40a) and an elongate loop antenna (42) disposed within said casing with one longer side thereof externally exposed through said opening; said pick-up (38) being mounted with said exposed longer side of said loop antenna (42) extending along and adjacent to the peripheral edge of a said peripheral edge portion; and said peripheral edge portion being an elongate edge-moulding mounting retainer (68) extending along and separated from the rear windshield frame (34) of the automobile body by a distance substantially equal to 2 x 10⁻³ x wavelength, the wavelength being that of a broadcast wave to be picked up. An automobile antenna system according to claim 1 wherein said edge-moulding mounting retainer (68) is secured (70,72) to the roof plate (32) of the automobile body to extend along an edge of the roof plate substantially parallel to and separated from the rear windshield frame (34). An automobile antenna system according to claim 1 or claim 2 wherein said edge-moulding mounting retainer (68) has a length substantially equal to the wavelength of the broadcast wave to be picked up.

Inventors: KONDO, HIROSHI, OHE, JUNZO
Applicants: TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1986
Main classification: H01Q1
IPC codes: H01Q1, B60R11