Complex Antenna System and FM/AM Receiver

The present invention relates to an FM and/or AM receiver to be mounted on a vehicle such as an automobile.

As is well known in the art, it has been a problem that an FM receiver mounted on a vehicle such as an automobile is subject to so-called "multi-path" noises during cruising of the vehicle. The multi-path noises are produced by interference between FM waves reaching a point through a plurality of different propagation paths from a broad casting station.

In a prior-art FM receiver, a plurality of rod antennas are provided the directivity characteristics of which are alternately changed by supplying thereto with frequencies differing in phase from each other. Another FM receiver is provided with a plurality of antennas spaced from each other and each having a single directivity characteristic.

However, such antenna systems as mentioned above all have so large dimensions that they are not suitable for antennas of an FM receiver on a vehicle. If, furthermore, the FM receiver is combined with an AM receiver, an antenna for AM wave must be provided in addition to the FM receiver whereby the overall antenna system occupies inappropreately wide space around the vehicle.

The present invention contemplates elimination of such a drawback inherent in the prior art antenna and receiver systems and it is, accordingly, a primary object of the present invention to provide an improved complex antenna system which is compact and robust in construction even though it has a plurality of directivity characteristics which can be selected so as to avoid "multi-path" noises.

Another object of the present invention is to provide a complex antenna system which is suitable for an antenna system for FM/AM receiver installed in a vehicle such as an automobile.

A further object of the present invention is to provide an FM/AM receiver which is compact in its electromagnetic wave receiving portion and can readily avoid multi-path noises.

In accordance with one aspect of the present invention, such objects as mentioned above are accomplished basically by a complex antenna system which comprises at least two loop antennas crossing each other, connected with each other at the neutral points thereof and having the output terminals thereof positioned closely to each other, and a rod antenna extending from a point close to the connection point of the loop antennas to a point close to the output terminals of the loop antennas.

In accordance with another aspect of the present invention, it is provided an FM/AM receiver to be mounted on a vehicle which comprises at least two loop antennas crossing each other, connected with each other at the neutral points thereof and having the output terminals thereof positioned closely to each other, a rod antenna extending from a point close to the connection points of the loop antennas to a point close to the output terminals of the loop antennas, at least two impedance matching circuit respectively connected to the output terminals of the loop antennas, and FM turner, a switching circuit for selectively connecting the impedance matching circuits to the FM tuner, and an AM tuner connected to the rod antenna.

The features and advantages of the present invention will be more clearly understood from the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a perspective view of a complex antenna system according to the present invention;

FIG. 2 is a plan view of the antennas shown in FIG. 1 for explaining the directivity characteristics of the antennas;

FIG. 3 is a block diagram of an FM/AM receiver combined with the complex antenna system of FIG. 1;

FIG. 4 is a circuit diagram of a switching circuit used in an FM/AM receiver shown in FIG. 1;

FIG. 5 is a block diagram showing another embodiment of a change-over switching circuit shown in FIG. 3;

FIG. 6 is a block diagram showing still another embodiment of a change-over switching circuit shown in FIG. 3;

FIG. 7 is a perspective view of another embodiment of a complex antenna system according to the present invention;

FIG. 8 is a perspective view showing a portion of the complex antenna system of FIG. 7;

FIG. 9 is a block diagram showing an FM/AM receiver combined with the complex antenna system shown in FIGS. 7 and 8; and

FIGS. 10 and 11 is a perspective views of modifications of the complex antenna system of FIG. 7.

In FIG. 1, there is shown a complex antenna system according to the present invention, which comprises a complex antenna portion 1 including two small loop antennas 1a and 1b crossing each other and connected with each other at their electric neutral points by means of an electroconductive connector 1d. The output terminals of the loop antennas 1a, 1b are mounted on a non-conductive connector 1e so that the loop antennas 1a and 1b are rigidly supported by the connector 1e and the output terminals of the loop antennas are positioned closely to each other. A rod antenna 1c is provided, and upper end of which is mechanically connected to but preferably electrically isolated from the connector 1d. The upper end of the rod antenna 1c may be connected through an inductance element (not shown) to the neutral points of the loop antennas 1a and 1b, if desired. The lower end of the rod antenna 1c is connected to the connector 1e so that the lower end of the rod antenna is positioned in the vicinity of the output terminals of the loop antennas 1a and 1b. The rod antenna 1c can act as a reinforcement for strengthening the overall antenna portion 1. The antenna portion 1 is mounted by means of the connector 1e on a case element 2 which contains therein various circuit elements (not shown) electrically connected and cooperating with the antennas 1a, 1b and 1c. The circuit elements are connected by means of a cable 4 to FM and AM tuners which will be described below in conjunction with FIGS. 3 and 4. The case element 2 is mounted on a supporting disc 3 by means of a support column 5. The supporting disc 3 is to be mounted on a suitable upper portion of a vehicle such as a roof of an automobile and may be provided with a thin and flat magnet on its lower surface so that the supporting disc can readily fasten on the vehicle.

The loop antennas 1a and 1b cross each other rectangularly as clearly seen from FIG. 2.

In FIG. 2, there are shown in broken lines the radiation patterns 1a.sub.1, 1a.sub.2 and 1b.sub.1, 1b.sub.2 of the respective loop antennas 1a and 1b in a place parallel to the supporting disc 3.

It is clear that the maximum amplitude directions of the respective radiation patterns 1a.sub.1, 1a.sub.2 and 1b.sub.1, 1b.sub.2 cross each other rectangularly, that is, equiangularly. The antenna portion 1 according to the present invention may be provided with loop antennas more than three, if desired, in which those loop antennas cross each other substantially equiangularly so as to distribute the radiation patterns of the loop antennas uniformly.

The height of the complex antenna system shown in FIG. 1 is preferably lower than about 30 cm so that the antenna system will not adversely affect the appearance or aspect of an automobile on which the antenna system is mounted.

As will be well understood from the above, the complex antenna system according to the present invention has a plurality of radiation patterns, that is, directivity characteristics which can be selected so as to avoid multi-path noises even though the overall antenna system is compact in construction. Furthermore, the rod antenna is provided within the loop antennas and a central portion or space of the loop antennas is effectively exploited for the purpose of receiving AM waves.

In FIG. 3, there is shown an FM/AM receiver which utilizes the complex antenna system shown in FIG. 1. The FM/AM receiver comprises a couple of impedance matching circuits 10a and 10b which are respectively connected with output terminals 9a.sub.1, 9a.sub.2 and 9b.sub.1, 9b.sub.2 of the loop antennas 1a and 1b. Output terminals of the impedance matching circuits 10a and 10b are respectively connected to input terminals of a change-over switching circuit 12. As is well known in the art, the impedance matching circuit 10a or 10b may include an impedance matching transformer and capacitors so that the input impedance of the impedance matching circuit 10a or 10b is equal to the output impedance of the impedance matching circuit 10a or 10b is equal to the input impedance of the next stage. Furthermore, the impedance matching circuit 10a or 10b may further include an amplifying element such as a transistor so as to obtain a sufficiently high magnitude of the received signal on the output signal thereof. The change-over switching circuit 12 interconnects a selected one of the input terminals thereof between the output terminal thereof in response to one or more control signals supplied from a noise detecting circuit 17. The output terminal of the change-over switching circuit 12 is connected to a front-end circuit 14 of an FM tuner 13. As is well known in the art, the front-end circuit 14 is comprised of a radio frequency amplifier for amplifying the received radio signal passing through the switching circuit 12, a local oscillator for producing a selected local frequency, and a mixer for mixing the received radio frequency signal with the local frequency so as to produce an IF frequency. The front-end circuit 14, in this case, produces a tuning information signal representative of the selected local frequency. The tuning information signal is supplied through a line 14a to the impedance matching circuits 10a and 10b which respectively change their impedances in response to the tuning information signal. The FM tuner 13 is a usual type and further includes an IF amplifier 15 connected to an output terminal of the front-end circuit 14 for amplifying the IF frequency fed from the front-end circuit 14. An output terminal of the IF amplifier 15 is connected to an input terminal of a multiplex demodulator 16 having two output terminals 16a and 16b on which L and R channel signals appear. The noise detecting circuit 17 includes a high pass filter 18 having the input terminal thereof connected to the output terminal of the IF amplifier 15. The high pass filter 18 is adapted to pass only high frequency components, that is, noise components contained in the output signal from the IF amplifier 15. The high frequency components passed through the high pass filter 18 are amplified by an amplifier 19 and to a detector 20. An output terminal of the detector 20 is connected to a schmitt trigger circuit 21 having the output terminal thereof connected to a trigger input terminal of an R-F flip-flop circuit 23. An output terminal of the flip-flop circuit 23 is connected to the change-over switching circuit 12 thereby supplying the control signal to the change-over switching circuit 12. The control signal may be a logic "1" or "0" appearing on the Q and Q terminals of the flip-flop circuit 23. The change-over switching circuit 12 may, for example, act to connect the line 11a with the output terminal thereof when a logic "1" is supplied from the flip-flop circuit 23 and to connect the line 11b with the output terminal thereof when a logic "0" is supplied from the flip-flop circuit 23.

An output terminal 9c of the rod antenna 1c is connected to an input terminal of an AM tuner 30. Since the AM tuner 30 has a usual construction and function, no explanation is given in this instance.

When, in operation, multi-path noises appears on the output terminal of the IF amplifier 15, the multi-path noises pass through the high pass filter 18 and is supplied via the amplifier 19 to the detector 20 so that the detector 20 produces one or more pulses which are supplied to a schmitt trigger circuit 21. The schmitt trigger circuit 21 then triggers the flip-flop circuit 22 which turns its condition and changes its output signals on Q and Q terminals from logic "0" to "1" or vice versa.

It is to be noted that the construction of the noise detecting circuit 17 is not limited to such circuit arrangement as shown in FIG. 3. The essential feature of the noise detecting circuit 17 is to detect the multi-path noises from the signal line in the FM tuner 13 and to produce the control signal for controlling the change-over switching circuit 12 so as to select one loop antenna which can catch an FM wave in a higher level than the others.

It is, in this instance, to be noted that the impedance matching circuits 10a and 10b, the change-over switching circuit 12 and the noise detecting circuit 17 may be accommodated within the case element 2 shown in FIG. 1, if preferred, so that the antenna unit shown in FIG. 1 can be manufactured separately from the FM or AM tuner and combined with an existing FM and/or AM tuners by users, if desired.

In FIG. 4, there is shown a circuit arrangement of the change-over switching circuit 12 of FIG. 3, which comprises a couple of transformer T.sub.1 and T.sub.2 respectively having the primary coils thereof connected via input terminals 41 and 42 to the impedance matching circuits 10a and 10b. The secondary coils of the transformer T.sub.1 and T.sub.2 are respectively by-passed by means of capacitors C.sub.1 and C.sub.2 and connected to first gates of dual-gate FETs Q.sub.1 and Q.sub.2. A second gate of the dual-gate FET Q.sub.1 is connected to a joint A between a resistor R.sub.1 and a capacitor C.sub.3 which are serially connected to each other. A voltage source V.sub.0 is provided for impressing a voltage V.sub.0 across the series connetion of the resistor R.sub.1 and the capacitor C.sub.3. Both terminals of the capacitor C.sub.3 are connected to emitter and collector of an NPN transistor Q.sub.3 the base of which is connected to an input terminal 43. Similarly, a second gate of the dual-gate FET Q.sub.2 is connected to a joint B between a resistor R.sub.2 and a capacitor C.sub.4 which are serially connected to each other. The voltage V.sub.0 is also supplied across the series connection of the resistor R.sub.2 and the capacitor C.sub.3. Both terminals of the capacitor C.sub.4 are connected to emitter and collector of an NPN transistor Q.sub.4 the base of which is connected to an input terminal 44. The input terminals 43 and 44 may be connected to the Q and Q terminals, respectively. The source terminals of the dual-gate FETs Q.sub.1 and Q.sub.2 are respectively connected to the ground through a resistors R.sub.3 and R.sub.4. The drain terminals of the dual-gate FETs Q.sub.1 and Q.sub.2 are respectively connected to terminals of a primary coil of an output transformer T.sub.3. To neutral point of the primary coil of the output transformer T.sub.3 is supplied a voltage source +V. A capacitor C.sub.5 is connected across output terminals of the secondary coil of the output transformer T.sub.3. One terminal of the secondary coil of the output transformer T.sub.3 is connected to an output terminal 45 and the other terminal of the secondary coil is grounded. The output terminal 45 may be connected to the input terminal of the front-end circuit 14 of the FM tuner 13.

When, in operation, the transistors Q.sub.3 and Q.sub.4 are alternately triggered and made conductive by control signals supplied from the noise detecting circuit 17 and differring in phase by 180.degree., so that the second gates of the FETs Q.sub.1 and Q.sub.2 are alternately grounded. In this instance, the potentials of the source terminal of the FETs Q.sub.1 and Q.sub.2 are maintained at a constant level by means of the resistors R.sub.3 and R.sub.4 and therefore either one of the FETs has a high impedance between the source and drain terminals. Accordingly, either one of the received signals passed through the impedance matching circuits 10a and 10b is applied to the output transformer T.sub.3 and transferred to the FM tuner 13.

It is to be understood that the switching duration or period of the change-over switching circuit 12 should be as short as possible and the switching duration is governed by time constants of the resistors R.sub.1 C.sub.3 and R.sub.2 C.sub.4 in the case of the circuit of FIG. 4.

In FIG. 5, there is shown another circuit arrangement of the change-over switching circuit 12, which includes a couple of amplifiers 50 and 51 having the input terminals thereof respectively connected to the input terminals 41 and 42. The output terminals of the amplifiers 50 and 51 are connected to input terminals of a summing circuit 52 the output terminal of which is connected to the output terminal 45. The voltage source terminals of the amplifier 50 and 51 are connected to output terminals of a switch circuit 53 an input terminal of which is connected to voltage source +B. The switch circuit 53 is adapted to selectively supply the voltage source +B to the output terminals thereof in response to the control signal deliverred through the terminal 43 or 44 from the noise detecting circuit 17, whereby either one of the amplifier 50 and 51 becomes active to pass one of the received signals from the impedance matching circuits 10a and 10b up to the output terminal 45 connected to the FM tuner 13.

In FIG. 6, there is shown still another circuit arrangement of the change-over switching circuit 12, which includes a variable gain amplifiers 54 and 55 having the input terminals thereof respectively connected to the input terminals 41 and 42. The output terminals of the variable gain amplifier 54 and 55 are connected to input terminals of a summing circuit 52 the output terminal of which is connected to the output terminal 45. The gains of the amplifiers 54 and 55 are regulated by a controller 56 which in turn triggerred by the control signal from the noise detecting circuit 17.

In operation, one of the variable gain amplifiers 54 and 55 is controlled by the controller 56 to have a high gain to pass therethrough the received signal and the other amplifier is controlled to have a low gain to prohibit the received signal to pass therethrough.

In FIGS. 7 and 8, there is shown another embodiment of a complex antenna system according to the present invention, which has substantially the same construction as the complex antenna system shown in FIG. 1 except of a portion indicated by a circle A in FIG. 7. FIG. 8 clearly shows in an enlarged scale partly in section that portion A in which the connector 1d has a projection 1dd extending toward the upper end of the rod antenna 1c. The projection 1 dd is connected with the upper end of the rod antenna 1c by means of a non-conductive sleeve connector 60. The sleeve connector 60 holds therein a discharging or arrestor element 61 and a conductive coil spring 62 both of which are arranged in series between the end of the rod antenna 1c and the projection 1dd.

In FIG. 9, there is shown a receiver system which is combined with the complex antenna system shown in FIG. 7 and 8 and explained above. The receiver system of FIG. 9 is substantially the same as the receiver system shown in FIG. 3 except that the complex antenna system contains the discharging element between the rod antenna 1c and the loop antennas 1a, 1b and that the lower and i.e. the output terminal 9c of the rod antenna 1c is connected to the input terminal of the AM tuner 30 by way of a protecting circuit 70. The protecting circuit 70 includes inductance elements L.sub.1 and L.sub.2, and diodes D.sub.1 and D.sub.2. The protecting circuit 70 suppress peack voltages of discharged electric energy through the discharging element from the loop antennas 1a and 1b by means of the inductance elements L.sub.1 and L.sub.2 and then drain the electric energy to the ground by means of the diodes D.sub.1 and D.sub.2, thereby to prohibit dangerour high voltage power to be impressed on the input terminal of the AM turner 30.

The complex antenna system and the receiver system shown in FIGS. 7, 8 and 9 and described above are effectively protected from a high level of noises or voltages appearing on the loop antennas which will be caused by ignitions in an engine of the vehicle or a thunderbolt onto the antennas.

It is now to be noted that various types of discharging elements may be used as the discharging or arrestor element 61.

FIG. 10 shows another form of complex antenna system according to the present invention which is generally the same as the complex antenna system of FIGS. 7 and 8 except that the discharging element 61 and the coil spring 62 are omitted. Thus, the upper end of the rod antenna and the projection 1dd of the connector 1d are maintained close to each other by means of the sleeve connector 60. By the provision of such gap between the upper end of the rod antenna 1c and the connector 1d, the rod antenna 1c is isolated or protected from high frequency noises caught by the loop antenna 1a or 1b.

FIG. 11 shows a still other form of complex antenna system according to the present invention which is generally the same as the complex antenna system of FIG. 10 except that an induction element L.sub.3 is provided within the sleeve connector 60. The induction element L.sub.3 is connected at the terminals thereof with the upper end of the rod antenna 1c and the projection 1dd. With this arrangement, the rod antenna 1c is dc-coupled with the loop antenas 1a and 1b but isolated from or protected from high frequency noises on the loop antenna 1a or 1b.

When the complex antenna system shown in FIG. 10 or 11 is combined with FM and AM tuners shown in FIG. 9, the protection circuit may be omitted.

As being apparent from the foregoing description. the complex antenna system according to the present invention has a plurality of directivity characteristics differently distribute from each other and therefore the complex antenna system of the invention has the following features and advantages:

(1) To reduce multi-path noises by selectively using one antenna which has its maximum sensitivity in a direction in which a strongest or desired FM wave propagates;

(2) To suppress intermodulation since an unwanted FM wave is omitted due to the selection of the antennas;

(3) To further reduce the multi-path noises by increasing the number of loop antennas;

(4) To make small the loop antennas by combining impedance matching circuits, amplifiers, etc. with the loop antennas;

(5) To dispense with cables for interconnecting antennas since all the antennas are positioned closely to each other; and

(6) To exploit the center portion of the loop antennas by placing a rod antenna, which results in robust construction and attractive shape or contour of the overall antenna system.

It will be understood that the invention is not to be limited to the exact construction shown and described and that various changes and modifications may be made without departing from the spirit and scope of the invention, as defined in the appended claims.