Antenna apparatus with tuned loop
This application is a continuation, of application Ser. No. 382,660, filed May 27, 1982, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to an antenna apparatus and more particularly, to a compact antenna apparatus. 
     BACKGROUND OF THE INVENTION 
     It has been hoped to obtain an antenna apparatus which is compact in size but has high gain. 
     Conventional antenna apparatus used for FM (frequency modulation) radio receivers, in the past, must be large in size for obtaining high gain. For example, dipole antennas for FM radio receivers ordinarily have lengths of around two meters. Therefore, such large size antenna apparatus is inconvenient for setting at a place near FM radio receivers in living areas. On the other hand, a conventional large size antenna apparatus located outdoors, such as on roofs, is inconvenient for matching frequencies to those of individually desired broadcasting stations. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to improve antenna apparatus so as to obtain both relatively small size and capacity for matching frequencies to those of individually desired broadcasting stations. 
     Another object of the present invention is to improve the ease of adjustment of operating frequencies of antenna apparatus. 
     A further object of the present invention is to obtain high gain in small size antenna apparatus. 
     According to the present invention, the antenna apparatus is provided with: (a) an insulating base plate, (b) a loop antenna element secured on the base plate, (c) capacitor means centrally located within the loop antenna element both for tuning the antenna element and for counteracting possible capacitance between the loop antenna element and an outside foreign object, and (d) means interconnecting the capacitor means and the loop antenna element to form a loop. 
     Additional objects and advantages of the present invention will become apparent to persons skilled in the art from a study of the following description of the accompanying drawings, in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of the antenna apparatus according to the present invention; 
     FIG. 2 is an exploded view of the antenna apparatus of FIG. 1; 
     FIG. 3 is a plan view of one side of the circuit assembly shown in FIG. 2; 
     FIG. 4 is a plan view of other side of the circuit assembly of FIG. 3; 
     FIG. 5 is a circuit diagram for the circuit assembly of FIG. 2; 
     FIG. 6 is a side view of another embodiment of the circuit assembly of the invention; and 
     FIG. 7 is a plan view of further embodiment of the circuit assembly of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will be described in detail with reference to the accompanying drawings FIG. 1 to FIG. 7. Throughout the drawings like reference numerals and letters will be used to designate like or equivalent elements for the sake of simplicity of explanation. 
     Referring now to FIG. 1, there is shown an antenna apparatus of one embodiment, according to this invention. The antenna apparatus has a disc-shaped upper portion 10 and a conical lower portion 12 for supporting upper portion 10 on its smaller end. 
     Details of the antenna apparatus as to both upper and lower portions are shown in FIGS. 2-4. Upper portion 10 has a main chassis 14, an annular cover 16, a circuit assembly 18 and a dial 20. Circuit assembly 18 is supported on chassis 14 and covered by cover 16. Dial 20 is located in the open center area of annular cover 16 and is connected by shaft 22 to variable capacitors 50 and 52 (FIG. 4) of circuit assembly 18. Chassis 14, cover 16 and dial 20 are made of dielectric materials, like plastics, and therefore do not shield circuit assembly 18 from broadcasting waves. 
     Lower portion 12, supporting upper portion 10, is composed primarily of conical case 24, holder 26, switch assembly 28 and bottom plate 30. Holder 26, located in case 24, contains signal output terminal 32, auxiliary source terminal 34 and batteries (not shown). Switch assembly 28, located in case 24, includes power switch 36 and antenna selector switch 38. 
     Referring now to FIGS. 3 and 4, there is shown in detail circuit assembly 18. In circuit assembly 18, two open-ended loop-shaped antenna elements 40 and 42 are fabricated on base plate 44 by any appropriate printed circuit method. The antenna elements 40 and 42 are arranged concentrically to one another and each includes a small open end portion, 40(a) and 42(a), respectively, as is known in the art. Two pairs of coupling wirings 46 and 48 are connected between respective open ends of antenna elements 40 and 42 and terminals 51 of variable capacitors 50 and 52 (shown in FIG. 4) respectively. Specifically, as shown in FIG. 5, variable capacitor 50 is connected across the open end of antenna element 40 to form a first closed loop and variable capacitor 52 is connected across the open end of antenna element 42 to form a second closed loop. 
     A pair of coupling wirings 54 is connected between antenna element 40 and amplifier section 58. The connecting points on antenna element 40 for coupling wirings 54 thereto are separated by a given distance to define a segment 54&#39; of antenna element 40. The length of segment 54&#39; is selected to permit maximum gain of antenna output to be fed out between the connecting points which define the ends of segment 54&#39;. Coupling wirings 54 from antenna element 40 are connected to a two-port input of amplifier section 58 as shown in FIG. 5. Amplifier section 58 has a conventional RF (radio frequency) amplifier structure like one cited in the Jitsuyo Denshi-kairo Handbook Vol. 1, p. 199, FIGS. 3-51, published by CQ Publishing Co., on Sept. 30, 1979. 
     A pair of coupling terminals 56 on antenna element 42 defines a segment 56&#39; of antenna element 42. Coupling terminals 56 are directly connected to one input of antenna selector switch 38 through lead wires 57 shown in FIG. 5. The other input of antenna selector switch 38 is connected to the output of amplifier section 58. The output of antenna selector switch 38 is connected to output terminal 32. Accordingly, selector switch 38 may be operated to connect either the output of amplifier section 58 or segment 56&#39; of antenna element 42 to output terminal 32. In case of excessively strong broadcasting waves, segment 56&#39; of antenna element 42 is selected for connection to output terminal 32. 
     As shown in FIGS. 3, 4 and 6, variable capacitors 50 and 52 and amplifier section 58 are all located on base plate 44 within the loops of antenna elements 40 and 42. In FIG. 4, capacitors 50 and 52 are both rotatably mounted on base plate 44 on the side opposite to the side on which antenna elements 40 and 42, are mounted. Capacitors 50 and 52 are further shown in FIGS. 4 and 6 to have a common capacitance control shaft 22. Shaft 22 penetrates base plate 44 at its center portion and the end of the shaft is coupled to rotatable dial 20 (shown in FIG. 2). 
     As is further shown in FIGS. 3, 4 and 6, amplifier section 58 includes a plurality of circuit parts such as transistor 60, resistors 62, capacitors 64 and coils 66 which are interconnected by the printed circuit patterns of amplifier section 58 shown on the side of base plate 44 which is illustrated in FIG. 3. 
     Parts 45 of the base plate 44 which are unnecessary for antenna elements 40 and 42, amplifier section 58 and the like, are cut out for increasing the electrical insulation between antenna elements 40 and 42 and amplifier section 58. 
     The front side of base plate 44 may be covered by solder-resist coating 68 (shown by hatching on FIG. 3) except for the soldering points of the circuit parts. Solder-resist coating 68 also increases the electrical insulation between antenna elements 40 and 42 and amplifier section 58, and further prevents changes in the electrical characteristics of antenna elements 40 and 42 due to solder attaching to the antenna elements. 
     Referring now particularly to FIG. 6, there is shown a side view of the circuit assembly 18 of another embodiment of the present invention. In FIG. 6, two antenna elements 40 and 42 are arranged on opposite sides of base plate 44. Otherwise the elements are the same as the first embodiment shown in FIGS. 3 and 4. In this embodiment, it is easy to make the electrical characteristics of both antenna elements 40 and 42 have the same conditions. 
     Referring now to FIG. 7, there is shown a plan view of the circuit assembly 18 of a further embodiment of the present invention. In the embodiment of FIG. 7, a common coupling wire 70 and two independent coupling wirings 72, 74 give flexibility of use to the single antenna element 40. The common coupling wiring 70 is connected between an intermediate point of loop antenna element 40 and ground (not shown). Independent coupling wiring 72, interconnects a first segment 72&#39; of antenna element 40 across the two-port input of amplifier section 58. Independent coupling wiring 74 couples second segment 74&#39; of antenna element 40 to antenna selector 38. Segments 72&#39; and 74&#39; are juxtapositioned to one another and both share the intermediate point of loop antenna element 40 where coupling wire 70 is connected. According to the embodiment of FIG. 7, either a non-amplified output from coupling wire 74 or an amplified output from amplifier section 58 may be obtained in spite of the fact that the antenna apparatus has only a single loop antenna element 40.