Printed Circuit Inductive Loop Antenna

Abstract

A relatively high-impedance FM radio antenna is provided consisting essentially of a circular loop of conductive foil having a discontinuity therein and deposited on one side of a substantially planar, nonconductive substrate. Attached to one end of the loop is a conductive foil impedance element, such as an inductor, for providing impedance matching in order to efficiently transfer the received electrical signals from the high-impedance antenna to a relatively low-impedance radio input circuit. A conductive foil capacitance element is connected between the impedance-matching element and the other end of the loop in order to parallel tune the antenna to the FM radio signal band. The capacitance is formed by depositing spaced interdigitated elongated strips of conductive foil on the substrate, with successive strips alternately attached to the loop end and to the impedance-matching element.

Description

BACKGROUND OF THE INVENTION

It has long been a practice of radio manufacturers to incorporate a suitable and effective receiving antenna within the confines of the radio cabinet. The operational and appearance advantages afforded by the use of an inductive loop antenna for intercepting electromagnetic radio waves and impressing the intercepted waves on the input terminals of a radio receiver are well known in the radio art. In the home-entertainment radio field, such an antenna provides reasonable signal reception for the AM portion of the frequency spectrum; that is, from approximately 535 to 1,620 kilohertz. For FM radio receivers, however, the loop antenna has not been found to be very desirable because of its low sensitivity in the FM portion of the frequency spectrum (approximately 88 to 108 megahertz). Moreover, it has been subject to signal strength variations resulting from the proximity of various objects including the human body. The loop antenna suitable for AM reception has too many turns (i.e., too much inductance and interwinding distributed capacitance) for the FM frequency band. Merely decreasing the number of turns does not solve the problem, however, because although this may tune the antenna to the FM band, there is not enough signal coupling for satisfactory FM reception. Of course, for an antenna having a given number of turns, the amount of signal coupling may be increased by increasing the loop area; however, the size of the loop thereby becomes too large to fit inside a typical radio cabinet.

Size is a limitation which is especially critical for relatively small, transistorized portable FM radios which are presently becoming quite popular. To overcome this limitation, radio manufacturers have adopted a monopole antenna consisting of a single telescoping rod and sometimes referred to as a "whip" antenna. The rod is extended to a length of several feet (i.e., approximately one-quarter wavelength of the desired carrier signal) when the radio is in use, yet it may be telescoped into the radio cabinet when the radio is not in use so that the antenna is substantially out of sight. Although the monopole antenna is somewhat susceptible to signal strength variations with changes in antenna orientation (which is especially bothersome to a person carrying and listening to a portable receiver while walking, for example), it does provide reasonable FM signal reception and, when in the retracted state, it provides for a relatively attractive, compact, and lightweight cabinet design. When extended during radio operation, however, it is not very attractive and sometimes even subject to breakage.

It is therefore an important object of the invention to provide a new and improved loop antenna.

It is a more specific object of the invention to provide a new and improved inductive loop antenna which is more economically and aesthetically suitable for operation in a relatively small, portable FM radio.

SUMMARY OF THE INVENTION

A high-impedance antenna constructed in accordance with the invention for receiving radio signals within a predetermined frequency band and converting them into corresponding electrical signals for application to a relatively low-impedance radio input circuit, comprises means including a substantially planar nonconductive substrate for supporting the antenna. It further comprises a coplanar deposition of conductive material on the substrate, with the deposition comprising a series combination of a single-turn inductive loop for receiving the electrical signals, a capacitance element for tuning the antenna to the frequency band, and an impedance-matching element for transferring the electrical signals from the high-impedance antenna to the relatively low-impedance input circuit.

BRIEF DESCRIPTION OF THE DRAWING

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in the several figures of which like reference numerals identify like elements, and in which:

FIG. 1 is a plan view of a printed circuit inductive loop antenna constructed in accordance with the invention;

FIG. 2 is a schematic diagram of the equivalent circuit for the antenna shown in FIG. 1;

FIG. 3 is a fragmentary plan view of an alternative embodiment of the antenna shown in FIG. 1;

FIG. 4 is a schematic diagram of the equivalent circuit of the antenna shown in FIG. 3; and

FIG. 5 is a graphical representation of the amplitude response of the antenna shown in FIG. 1 with the antenna in a horizontal plane.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 1, there is shown a high-impedance printed circuit antenna 10 for receiving radio signals within a predetermined frequency band, such as the FM band of 88-108 megahertz, and converting them into corresponding electrical signals for application to a relatively low-impedance input circuit of radio 20. In accordance with the invention, inductance means in the form of a thin, wide single-turn loop of conductive foil 11 deposited on one side of substrate 12 and having a discontinuity therein in the general area designated "A" and defined by loop ends 11a and 11b are provided for receiving the radio signals and converting them into electrical signals. The diameter and foil width of loop 11 are proportioned according to the type of antenna application being considered. For an FM radio application, the foil may be constructed by a photoetching process using copper which may have a thickness of 0.0015 inch and a width of approximately 0.5 inch in order to minimize the resistance of the loop and thereby maximize the efficiency of the antenna; the substrate may be bakelite or formica having a thickness of approximately one thirty-second inch and of a shape conforming to that of the loop.

In most applications, the input circuit with which the high-impedance antenna is designed to operate has a relatively low impedance. Accordingly, the invention also comprises impedance matching means including a conductive foil impedance element 15 deposited on substrate 12 and connected to inductance 11 for efficiently transferring the received electrical signals from antenna 10 to radio 20. The precise configuration of element 15 may be fashioned to suit the impedance-transformation requirements of the particular application. As shown in FIG. 1, element 15 is substantially in the shape of a 1.75-inch square having its side member 15b connected to loop end 11b and having a slot 15c to thereby provide an inductive reactance having a low resistance. In this form, element 15 functions as a small single-turn inductance having a low reactance and connected out of phase with loop 11. It has been found that this configuration is preferable to an in-phase configuration (see FIG. 3) with respect to stability and bandwidth.

At the loop discontinuity generally indicated by reference character "A," a further aspect of the invention is shown comprising capacitance means 14 connected to inductance loop 11 at end 11a and to element 15 at side member 15a for tuning the antenna to the center of the particular frequency band of interest. In the embodiment shown in FIG. 1, capacitance 14 is designed to so tune antenna 10 by resonating the loop at 98 megahertz, the center of the 88 to 108 megahertz FM frequency band. Capacitance 14 includes a plurality of spaced interdigitated elongated strips 14a of conductive foil deposited on substrate 12 with successive strips alternately connected to inductance loop 11 at end 11a and to matching element 15 at 15a. The amount of capacitance thus provided is determined primarily by the number and length of the interdigitated strips, and to a lesser degree by the dielectric constant of the substrate. This construction provides a substantially noninductive capacitance which thereby requires a minimum amount of capacitance (and therefore a minimum antenna substrate area) to tune the inductive loop. Of course, for other applications such as a paging receiver operating on a 150-megahertz FM carrier signal, the illustrated construction may be slightly modified in order to resonate at 150 megahertz (i.e., the capacitance may be reduced by decreasing the number of strips, or shortening the strips, or a combination of both techniques in order to properly tune the antenna to the frequency of interest).

As is true of most antennas, the optimum design of antenna 10 for a given application is empirically determined. However, there are a few techniques to optimize performance which are especially applicable to the loop of the present invention. At the top of the antenna in FIG. 1, for example, an inwardly extending essentially rectangular portion 13 is added to the foil loop 11 in order to reduce the value of the inductance of the loop while substantially maintaining the same loop area which is important for maximum signal reception. The "Q" of the antenna is thereby reduced in order to broaden its frequency response and thus provide more uniform signal reception across the entire frequency band of interest. For the antenna shown in FIG. 1, the loaded "Q" is less than 10. Of course, the particular configuration of portion 13 may be tailored to suit specific design requirements. Another technique is to add a small foil stub 14c to capacitance 14 as shown in order to provide means for compensating for variations in the dielectric material of substrate 12. In other words, since different batches of substrate material may vary slightly in their physical characteristics, the area of tuning stub 14c may be reduced or increased as required (e.g., by removing some of the foil or by adding some solder or low-value capacitance to the unattached end, respectively) after the antenna is constructed in order to provide a fine tuning adjustment. Furthermore, although not shown in the drawing, a metallic ground plane consisting of a thin layer of metal may be deposited on the rear side of the antenna substrate in order to minimize the effects on signal reception caused by various objects being brought in proximity to the antenna (i.e., a "proximity effect"). As an alternative to depositing this metallic ground plane on the rear of the substrate, the layer of metal may be attached to the cabinet wall next to which the antenna is closely mounted; that is, a piece of metallic ornamentation may be affixed to the outside of a cabinet wall and the antenna parallelly mounted adjacent thereto on the inside.

Still another technique for optimizing the performance of the antenna in some applications is to further include in the impedance matching means a transmission line 16 having an impedance which, for the frequency band of interest, is substantially equal to the input impedance of radio 20. Line 16 is connected from junctions B and G of element 15 to the input circuit of radio 20, with junction G providing a ground reference potential for the antenna. This technique allows the antenna to be mounted several inches away from the input circuit (such as on a side of the radio cabinet), yet it permits energy transfer therebetween with minimum signal loss and antenna detuning. Moreover, the reception of extraneous signals, such as harmonic signals originating from the receiver's local oscillator may be minimized by employing a balanced, shielded transmission line and having the shield connected to ground. For an FM radio application, a shielded transmission line having a characteristic impedance of 50 ohms and a length of 9 inches has been found to be quite satisfactory.

In FIG. 2, the equivalent circuit of the antenna depicted in FIG. 1 is shown in schematic diagram form. The circuit is substantially conventional and it should be noted that it incorporates only one of several possible known methods of impedance transformation. The method of impedance transformation shown in FIG. 2 is sometimes referred to as "bucking" because inductance 15 is partially in phase opposition to the main loop inductance 11 and therefore slightly reduces the overall inductance. It has been found that, for an antenna constructed in accordance with the invention for an FM radio application, this type of circuit is preferable in terms of stability and efficiency. Alternative circuits include one with "aiding" impedances in which inductance 15' is in phase coincidence with the loop inductance 11' and therefore tends to increase the overall inductance. This circuit may include two coils or simply one coil having a tap, such as shown in FIGS. 3 and 4. Of course, it is also possible to effect impedance transformation by substituting a matched capacitance for inductance 15 or 15', provided that it is properly terminated.

In FIG. 3, there is shown an alternative embodiment of the antenna shown in FIG. 1. The structure of the antenna shown in FIG. 3 is identical to that of FIG. 1 except for the configuration of matching impedance element 15'. In FIG. 3, element 15' is deposited on the substrate in substantially the reverse configuration as compared with element 15 in FIG. 1; that is, the inductance of element 15' is in phase coincidence with the loop inductance and is therefore said to aid. The "aiding" configuration is acceptable but is not quite as stable and efficient as the "bucking" configuration. The equivalent circuit of this antenna configuration is illustrated by the schematic diagram of FIG. 4.

The antenna structures shown in FIGS. 1 and 3 have been empirically found to best suit the requirements of a portable FM radio receiver. Quite obviously, the advantages of the invention may be had with various modifications in the overall antenna structure. For example, the shape of loop 11 may be noncircular such as an oval, square, or rectangle. It has been found that right angles in such configurations tend to cause interference and loss of signal strength in the FM band; nevertheless, where a particular cabinet configuration is controlling, a rectangular shape may still be employed to provide satisfactory results. As discussed above, impedance element 15 may also take on various configurations depending on particular design objectives. Moreover, the exact size and shape of capacitance 14 may be altered to suit the particular design objective. The two antenna configurations shown have been empirically developed and found to produce excellent results for portable FM radio receivers. In addition, the relatively nondirectional characteristic of an antenna constructed in accordance with the invention is graphically illustrated by the polar diagram in FIG. 5. This diagram depicts the relative response (R) of the antenna (at center point C), positioned in a horizontal plane, as a function of the polar coordinate of the signal source. The 0.degree. reference axis corresponds to the top portion of the antenna shown in FIG. 1. In all other antenna orientations, the antenna constructed in accordance with the invention shows no perceptible change in response. This feature of the invention is highly desirable in small portable receivers which are frequently carried while a person is walking and therefore subject to various antenna orientations. It is not presently known why this antenna has such an outstanding nondirectional characteristic, although it may be due in part to the particular shape of the loop.

It is important to note that an antenna constructed in accordance with the invention may be manufactured quite economically and efficiently because of its coplanar structure and uniform composition. That is to say, since the single-turn inductance loop, the capacitance element, and the impedance-matching element are composed of the same conductive material, they may be formed on the substrate in a single manufacturing operation and thereby reduce production costs and complexities.

Thus there has been shown a new and improved loop antenna economically and aesthetically suitable for operation in a relatively small, portable FM radio. The advantages of the antenna are many. With such an antenna it is possible to confine the antenna to the radio receiver cabinet itself and thereby eliminate protruding devices, such as "whip" or monopole antennas, which are subject to breakage and detract from the overall appearance of the radio. Another advantage is the relatively nondirectional characteristic, as shown in FIG. 5, of the printed circuit loop antenna; that is, the antenna may be oriented in any direction without a noticeable change in reception. This is especially advantageous for a portable receiver, such as an FM radio or a paging receiver, which is subject to operation in various orientations. Furthermore, the antenna shown may be very economically constructed using printed circuit processes providing an antenna that is also quite durable.

While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

Claims

I claim:

1. A high-impedance printed circuit antenna for receiving radio signals within a predetermined frequency band and converting them into corresponding electrical signals for application to a relatively low-impedance radio input circuit, comprising:

means including a substantially planar nonconductive substrate for supporting said antenna;

inductance means, including a loop of conductive foil having a discontinuity therein and deposited on said substrate, for receiving said radio signals and converting them into said electrical signals;

impedance-matching means, including a conductive foil impedance element deposited on said substrate and connected to said inductance means, for transferring said electrical signals from said high-impedance antenna to said relatively low-impedance input circuit with maximum efficiency; and

capacitance means connected to said inductance means and said matching means for broadly tuning said antenna to said frequency band, said capacitance means including a plurality of spaced interdigitated elongated strips of conductive foil deposited on said substrate and having successive strips alternately connected to said inductance means and to said matching means.

2. An antenna according to claim 1, in which said impedance-matching means further includes a balanced transmission line having an impedance substantially equal to said input impedance and connected between said impedance element and said input circuit.

3. An antenna according to claim 1, in which said impedance element comprises a printed circuit inductance connected in phase opposition to said inductance means.

4. An antenna according to claim 1, which further comprises an additional portion of conductive foil connected to said loop for reducing the value of said inductance means and reducing the Q of the antenna.

5. An antenna according to claim 1, in which said inductance means is a substantially circular loop, having an outside diameter of approximately 5 inches and a foil width of approximately one-half inch.