AUTOMOBILE ANTENNA SYSTEM

Abstract: An automobile antenna system integrally mounted on the vehicle body detects the high-frequency surface currents induced on the vehicle body by broadcast waves. The antenna system includes a high-frequency pickup having a loop antenna (42) and a core (44) around which the loop antenna is wound. The pickup is secured to a predetermined position of the vehicle body by fixture means, and a predetermined side of the loop antenna is opposed to a marginal edge portion of the vehicle body.

Description: The present invention relates to an automobile antenna system and, more particularly, to an improved automobile antenna system for effectively detecting broadcast radio waves received by the vehicle body and then transferring detected signals to various receivers located in the vehicle. Antenna systems are indispensable to modern automobiles which must positively receive various broadcast waves such as those for radio, television and telephone at the receivers located within the vehicle. Such antenna systems are also very important for citizen band transceivers. One of the conventional antenna systems is known as a rod or pole-type antenna which projects outwardly from the vehicle body of an automobile. Although such a pole antenna is superior in performance in its own way, it always remains a nuisance from the viewpoint of vehicle body design. Furthermore, the pole antenna is disadvantageous in that it is subject to damage, tampering or theft and also in that the antenna acts to generate noises during high-speed driving. For these reasons, there has heretofore been a strong desire to eliminate the need for such pole antennas. With the enlargement of the frequency bands for broadcast or communication waves received at automobiles in recent years, a plurality of pole antennas have been required in accordance with each frequency band. This brings about other problems; a plurality of pole antennas damages the aesthetic appearance of the automobile and the receiving performance is greatly deteriorated by electrical interference between the antennas. Efforts have been made to eliminate the pole antenna system or to conceal the antenna from the exterior. One of such proposals has been to apply a length of antenna wire to the rearwindow glass of an automobile, and this proposal has been put into practical use. Another proposal has been to detect surface currents which are induced by broadcast waves on the vehicle body of an automobile. This means seems to be the most positive and efficient for receiving broadcast waves, but the experiments carried out to date have not provided any satisfactory results. One of the reasons why surface currents induced on the vehicle body by broadcast waves have not been utilized well is that their induced value is not as large as expected. Although the prior art mainly uses surface currents induced on the roof panel of the vehicle body, no surface currents of a satisfactory level have been obtained. Another reason is that surface currents contain noises of a very high level. Such noises are mainly generated by the engine ignition system and the battery charging regulator and cannot be eliminated unless the engine is stopped. Noises transmitted to the interior of the vehicle make it impossible to effect any practicably clear reception of broadcast waves. In such a situation, some proposals have been made to overcome the above problems. One of such proposals is disclosed in Japanese Patent publication No. 22418/1978 in which an electrical insulation is formed at a portion of the vehicle body on which currents are concentrated, with the currents being detected directly by a sensor between the opposite ends of the insulation. Although such structure can detect utilizable signals which are superior in S/N ratio, a pickup used therein requires a particular cutout in the vehicle body. This cannot be accepted in the mass-production of automobiles. Another proposal is disclosed in Japanese Utility Model Publication 34826/1978 in which an antenna including a pickup coil for detecting currents in the pillar of a vehicle body is provided. This is advantageous in that the antenna can be disposed completely within the vehicle body. However, it is not practical because the pickup coil used therein has to be located adjacent to the vehicle pillar in a direction perpendicular to the longitudinal axis of the pillar. Thus, it also appears that this arrangement cannot pick up any utilizable output from the antenna. As has been described above, the conventional antenna systems have not been successful in efficiently detecting currents induced on the vehicle body by broadcast waves. No effective measure has heretofore been proposed for overcoming the above-described principal problems of the conventional art in providing, in particular, a pickup structure for effectively detecting currents induced on the vehicle body by broadcast waves and a pickup arrangement capable of obtaining a utilizable S/N ratio. The results of various kinds of experiments show that it might in fact be basically impracticable to use an antenna system which utilizes currents flowing on the vehicle body. In view of the above-described problems of the prior art, it is an object of the present invention to provide a small-sized improved antenna system for automobiles which is capable of effectively detecting currents induced on the vehicle body by broadcast waves and then transferring detected signals to various receivers located in the vehicle and which is so designed as to facilitate mounting of a high-frequency pickup in a systematic assembling operation and to improve the sensitivity of the pickup. The prior art antenna systems were mainly intended to receive AM broadcast waves. As a result, good reception performance was not obtained since the wavelength of AM broadcast waves to be received by the prior art antenna systems is too long. A further object of the present invention is to make it possible to provide an automobile antenna system whereby surface currents induced on the vehicle body by broadcast radio frequency signals at a frequency in the FM frequency band, normally above 50 MHz, can efficiently be detected. DE-A-1949828 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; said pick-up comprising an elongate loop antenna extending around a core of material of high permeability. In that prior art system the pick-up is mounted on the unbroken outside surface of a hollow pillar of the automobile body. In addition, that system is not adapted for reception of FM signals. The present invention is characterized in that: said pick-up is adapted to receive signals at a frequency above 50MHz; said pick-up includes a casing of electrically conductive material having an opening; said loop antenna is disposed within said casing with one side thereof externally exposed through said opening and the remainder of said loop antenna is shielded from external electromagnetic fields by said casing; said loop antenna is a wound antenna having a plurality of turns, said externally exposed side thereof being wound to lie within the depth of a groove extending along at least one side surface of said core; and mounting means mounting said casing to said automobile body portion so that said groove in said core fits over a marginal edge portion of the automobile body to locate said externally exposed side of the loop antenna in a predetermined position extending lengthwise of and closely adjacent to an end surface of said marginal edge portion. Our co-pending European Patent Applications under publications Nos. EP-A-180462; EP-A-181120; and EP-A-182614 are documents of the type mentioned in Article 54(3) of the European Patents Convention. Each said document describes an automobile antenna system and briefly describes an alternative embodiment being a high-frequency pick-up of a coil-type having a ferrite core which is arranged so that the core will be parallel to and in close proximity with the marginal edge of certain vehicle body portions, and a coil would about the ferrite core is used to acquire the induced currents. Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Figure 1 illustrates surface currents I induced on a vehicle body B by external waves W; Figure 2 illustrates a probe for detecting the distribution of surface currents on the vehicle body and having the same construction as that of a high-frequency pick-up used in the present invention, and a circuit for processing signals from the probe; Figure 3 illustrates the electromagnetic coupling between the surface currents I and the pickup loop antenna; Figure 4 illustrates the directional pattern of the loop antenna shown in Figure 3; Figure 5 illustrates the intensity distribution of the surface currents; Figure 6 illustrates the directions of flow of the surface currents; Figures 7, 8 and 9 are graphs showing the distribution of surface currents at various locations of the vehicle body shown in Figure 5 along the longitudinal axis. Figure 10 is a perspective view of a high-frequency pickup in accordance with the present invention, mounted on the roof panel of an automobile; Figure 11 illustrates the appearance of an automobile antenna system according to the present invention; Figure 12 is a sectional view of the antenna shown in Figure 11; and Figure 13 is a fragmentary sectional view of the pickup shown in Figure 10; Preferred embodiments of the automobile antenna system according to the present invention will be described hereinunder with reference to the accompanying drawings. Figures 1 to 9 illustrate a process of examining the distribution characteristics of high-frequency currents to know a location at which an antenna system can operate most efficiently on the vehicle body of an automobile. Figure 1 shows that when external electromagnetic waves W, such as broadcast waves, pass through the vehicle body B of conductive metal, surface currents I are induced at various vehicle locations at levels corresponding to the intensities of electromagnetic waves passing therethrough. The present invention aims at only electromagnetic waves which belong to relatively high frequency bands in excess of 50 MHz, such as FM broadcast waves, television waves and others. The distribution of the surface currents induced on the vehicle body by electromagnetic waves within the above-described particular wave bands is measured so as to seek a location on the vehicle body which is higher in surface current density and lower in noise and at which a pickup used in the present invention is to be located. The distribution of surface currents is determined by a simulation using a computer and also by measuring actual intensities of surface currents at various locations on a vehicle. In accordance with the present invention, the measurement is carried out by the use of a probe which can operate in accordance with the same principle as that of a high-frequency pickup actually located on the vehicle body at the desired location, as will be described later. Such a probe is moved on the vehicle body throughout the entire surface thereof to measure the level of surface currents at various locations of the vehicle body. Figure 2 shows an example of such a probe P which is constructed in accordance with substantially the same principle as that of the high-frequency pickup described hereinafter. The probe P is composed of a casing of electrically conductive material 10 for preventing any external electromagnetic wave from transmitting to the interior thereof and a loop coil 12 rigidly located within the casing 10. The casing 10 includes an opening 10a formed therein through which a portion of the loop coil 12 is externally exposed. 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 with the casing 10 through a short-circuiting line 14. The loop coil 12 further includes an output end 16 connected with a core 20 in an coaxial cable 18. Still another portion of the loop coil 12 includes a capacitor 22 for causing the frequency in the loop coil 12 to resonate relative to the desired frequency to be measured to increase the efficiency of the pickup. Thus, when the probe P is moved along the surface of the vehicle body B and also angularly rotated at various locations of measurement, the distribution and direction of surface currents can accurately be determined at each of the vehicle locations. In Figure 2, 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 output voltage is read at the indicated value of the high-frequency voltmeter 26 and also is recorded by an XY recorder 28 to provide the distribution of surface currents at various vehicle locations. The input of the XY recorder 28 receives signals indicative of various vehicle locations from a potentiometer 30 to recognize the value of high-frequency surface current at the corresponding vehicle location. Figure 3 illustrates an angle ϑ of deflection between the high-frequency surface currents I and the loop coil 12 of the pickup. As is clear from the drawing, magnetic flux  intersects the loop coil 12 to generate a detection voltage V in the loop coil 12. As shown in Figure 4, when the angle ϑ of deflection is equal to zero, that is, the surface currents I are parallel to the loop coil 12 of the pickup, the maximum voltage can be obtained. The direction of the surface currents I when the probe P is rotated to obtain the maximum voltage can also be known. Figures 5 and 6 respectively show the magnitude and direction of high-frequency surface currents induced at various different locations of the vehicle body at the frequency of 80 MHz, the values of which are obtained from the measurements of the probe P and the simulation effected by the computer. As can be seen from Figure 14, the distribution of surface currents has higher densities at the marginal edge of the vehicle body and lower densities at the central portions of the flat vehicle panels. It will also be apparent from Figure 6 that the surface currents are concentrated in the direction parallel to the marginal edge of the vehicle body or in the direction along the connections of various flat panels. Carefully studying the distribution of surface currents induced at various metallic vehicle portions along the longitudinal axis of the vehicle body as shown in Figure 5 , distribution characteristics such as those shown in Figures 7 to 9 can be obtained. Figure 7 shows a distribution of surface currents along a trunk lid between two points A and B on the longitudinal axis. As can be seen from this drawing, the surface currents attain very high levels at these points A and B and decrease toward the central portion of the trunk lid from the opposite points thereof. Thus, if a high-frequency pickup is disposed near the marginal edge of the trunk lid, the currents concentrating thereon can be detected. Similarly, Figure 8 shows the distribution of surface currents along the roof panel of the vehicle body while Figure 9 shows the distribution of surface currents along the engine hood of the vehicle body. As is apparent from these drawings, surface currents of a very high level flow at the marginal edges of the roof panel and the engine hood, respectively. The value of the surface currents decreases toward the central portion of each panel area of the vehicle sections. It is thus understood that the pickup should be disposed at or near the marginal edge of each panel area of the vehicle body in order to catch broadcast waves with high sensitivity. It goes without saying that the high-frequency pickup can similarly be located on one of pillars and fenders as well as on the trunk lid, the engine hood and the roof panel in the present invention. Although the loop antenna of the high-frequency pickup is arranged longitudinally adjacent to and along the marginal edge of each vehicle panel area in accordance with the present invention, this loop antenna is preferably positioned within a range determined depending upon the carrier frequency of broadcast waves in order to obtain sensitivity suitable for practical use. The distribution of currents shown in Figures 7 to 9 relate to the currents induced on the vehicle body by FM broadcast waves having the frequency of 80 MHz. The value of surface currents decreases in accordance with the distance between the position of the surface currents and the marginal portions of the vehicle. Considering that good sensitivity can actually be obtained in the range of decreased currents below 6 dB, it is understood that such sensitivity may be realized if the pickup is located within a distance of 4.5 cm from each marginal edge of the vehicle. Thus, a satisfactory antenna system can be provided in accordance with the present invention if a high-frequency pickup is arranged within a distance of 4.5 cm away from a marginal vehicle portion for the carrier frequency of 80 MHz. It is found from the computer's simulation and experimental measurements that the above distance which is suitable for practical use depends upon the carrier frequency used therein. It is also recognized that the distance is decreased as the value of the carrier frequency is increased. From the fact that the suitable distance of 4.5 cm from the corresponding marginal vehicle portion is inversely proportional to the value of the carrier frequency, good results can be obtained relative to the respective values of the carrier frequency if the high-frequency pickup is spaced away from the marginal edge of a metallic vehicle panel within a distance represented by the following formula: 12 × 10⁻³ c/f(m) where c = the velocity of light and f = carrier frequency. In this manner, the present invention provides an improved high-frequency pickup which is located adjacent to the marginal edge of each panel area of the metallic vehicle body and which is preferably disposed within said range from that marginal edge. For example, where a carrier frequency equal to 100 MHz is to be caught, a high-frequency pickup may be disposed at a vehicle location spaced away from a desired marginal edge of the vehicle body within a distance of 3.6 cm. It will be apparent that as the value of the carrier frequency f is increased, the distance between the high-frequency pickup and the corresponding marginal edge of the vehicle body will be decreased. Figure 10 shows a high-frequency pickup according to the present invention mounted near the rear marginal edge of the roof panel. In the drawing, a roof panel 32 is illustrated in the exposed state, and the metallic roof panel 32 is connected to a rearwindow glass 36 with a rear window frame 34 as its marginal edge. In this embodiment, a high-frequency pickup 38 is disposed within a distance of 4.5 cm inward of the rearwindow frame 34. Figure 11 shows the external appearance of a high-frequency pickup. The high-frequency pickup 38 includes a metallic casing 40 for shielding it from undesirable external electromagnetic flux and a core 44 located within the casing 40 and with a loop antenna 42 wound around. Therefore, this pickup is of an electromagnetic coupling type similar to the aforementioned probe including its loop coil for measuring the distribution of surface currents on the vehicle body. The core 44 is made of a material of high permiability such as ferrite, and a groove for containing the loop antenna 42 in a wound form is formed on at least one side surface of the core 44. In this embodiment, a grooved portion 46 is formed on the periphery of the core 44, as shown in Figure 12, and the loop antenna 42 is wound around the grooved portion 46 in a plurality of turns. The grooved portion 46 serves as a guide for winding the loop antenna 42, as described above. It is also useful for improving the degree of accuracy in positioning the loop antenna 42 and the marginal edge portion by allowing the high-frequency pickup 38 to be mounted in such a manner that the grooved portion 46 may fit over the marginal edge portion of the vehicle body, as is shown in Figure 13. In addition, a closed magnetic circuit is formed between the marginal edge portion of the vehicle body and the core 44, so that any leakage of magnetic flux induced by the high-frequency surface current is prevented. Accordingly, the magnetic flux is safely caught by the loop antenna 42 and further the casing 40 shields the magnetic flux from undesirable external electromagnetic flux, so that the current induced on the vehicle body can be detected with good sensitivity by the high-frequency pickup 38. In order to locate and fix the casing 40 of the high-frequency pickup 38 in relation to the rearwindow frame 34, L-shaped brackets 54 and 56 are provided on both side surfaces of the casing 40. These brackets 54 and 56 are screwed to the rearwindow frame 34. The casing 40 of the high-frequency pickup 38 includes a circuitry 58 contained therein which is connected with the loop antenna 42. The circuitry 58 includes its internal components such as a pre-amplifier and others for processing detected signals. The resulting high-frequency detection signals are externally taken through a coaxial cable 60 and then processed by the same circuit as that used in measuring the distribution of surface currents. The circuitry 58 receives power and control signals through a coaxial cable 62. The loop antenna 42 is in the form of a wound coil which is covered with an insulation such that the coil can be arranged in an electrically insulated relationship with and in close contact with the open marginal portion of the vehicle body. Thus, the magnetic flux induced by the surface currents can intersect the loop antenna 42 with an increased intensity. In this embodiment, the loop antenna 42 is disposed within a distance of 4.5 cm from the edge portion of the rearwindow frame 34, whereby the FM broadcast waves of the frequency of 80 MHz can be positively detected from the surface currents flowing in the marginal edge portion of the rearwindow frame 34. Since the surface currents on the vehicle flow along its marginal portions, as is clear from Figure 6, the loop antenna 42 is disposed longitudinally along the marginal edge portion of the rearwindow frame 34. As described above, in this embodiment, the surface currents flowing along the marginal portions of the vehicle, especially along the marginal portion of the roof panel are electromagnetically detected by the high-frequency pickup, and leakage of magnetic flux is prevented by clamping with the core 44 the marginal edge portion to which the pickup is attached. Thus the pickup in this embodiment enables secure reception in a high-frequency band and provides a very useful pickup for an automobile antenna.

Claims: An automobile antenna system comprising a pick-up (38) mounted on a portion (34) of an automobile body to detect radio frequency surface currents induced in said body portion by broadcast radio frequency signals; said pick-up comprising an elongate loop antenna (42) extending around a core (44) of material of high permeability; characterized in that: said pick-up is adapted to receive signals at a frequency above 50 MHz; said pick-up (38) includes a casing (40) of electrically conductive material having an opening; said loop antenna (42) is disposed within said casing (40) with one side thereof externally exposed through said opening and the remainder of said loop antenna is shielded from external electromagnetic fields by said casing; said loop antenna (42) is a wound antenna having a plurality of turns, said externally exposed side thereof being wound to lie within the depth of a groove (46) extending along at least one side surface of said core (44); and mounting means (54,56) mounting said casing to said automobile body portion so that said groove in said core fits over a marginal edge portion (34) of the automobile body to locate said externally exposed side of the loop antenna in a predetermined position extending lengthwise of and closely adjacent to an end surface of said marginal edge portion.

Inventors: KONDO, HIROSHI, OHE, JUNZO
Applicants: TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1985
Main classification: H01Q1
IPC codes: H01Q7, H01Q1