ANTENNA AND SMALL PORTABLE RADIO DEVICE

Abstract: An antenna which can provide a radio device stable and highly sensitive irrespective of its attitude or its carried direction and of the effect of the proximity to a human body when it is carried with the human body. When the radio device having this antenna is not carried with a human body, regardless of the plane of polarization of an electromagnetic wave, i.e., even in a place where the plane of polarization under multipath distorsion and fading is always changed, the radio device has a stable sensitivity characteristic. Also, the antenna circuit achieving these characteristics requires no additional element for matching, and has a remarkable effect on reducing the size of a radio device.

Description: [Technical Field] The present invention relates to a miniature portable wireless transmitter/receiver (referred to as transceiver, hereinafter) and, more particularly, to an antenna and an antenna circuit system incorporated in such a transceiver. [Background Art] Loop antennas have been widely used as antenna of miniature wireless portable transceivers and advantages of this type of antenna is reported in an article entitled LOOP ANTENNA FOR MINIATURE WIRELESS TRANSCEIVER: National Technical Report Vol. 19 No. 2, April 1973. An antenna system also has been known in which a reactance element having a specific reactance value is fixed to an intermediate portion of a loop antenna. When signals transmitted and received are of comparatively high frequencies, an antenna known as inverse-F antenna is used besides a loop antenna. [Problems to be Solved by the Invention] The transmission/receiving characteristics of a miniature portable wireless transceiver is largely affected by the body of the user who carries the transceiver. A typical example of the miniature portable wireless transceivers is a device called selective-call receiver (pager). One of the circuit components which affects the receiving sensitivity is the antenna. A loop antenna is generally used as the antenna of the pager. In general, a loop antenna efficiently detects magnetic field components so as to provide 6 dB improvement in the gain when the loop antenna is held at a certain azimuth with respect to the front side of the human body. This phenomenon is described in the aforementioned article and is well known. The loop antenna, however, is ineffective and reduces the gain when it is oriented in an azimuth different from that mentioned above. The loop antenna, even when it is not carried by the human body, efficiently receives waves having polarization planes of a specific azimuth but extremely deteriorates its receiving characteristics for waves having different directions of polarization planes. This means that the receiving sensitivity is inconveniently varied according to the posture or azimuth of the transceiver. The aforementioned article teaches that, by arranging the loop antenna element and the earth surface presented by the substrate of the transceiver orthogonally to each other, it is possible to detect both magnetic field component and electric field component, thereby reducing directivity variation caused by change in the posture of the transceiver. This type of transceiver, however, causes 7 dB reduction in the sensitivity as an average when it is carried by a human body in such an azimuth as to detect the electric field component. It is said that this type of transceiver exhibits 5 dB increase as an average when carried by a human body in such an azimuth as to detect the magnetic field component. Actually, however, the electric field also is detected slightly so that the increase in the sensitivity is reduced to 3 dB or so due to approach to the human body. Thus, the known transceivers of the type described does not make effective use of the effect produced by the human body. A loop antenna arrangement is also disclosed in European Patent Publication No. 0122485A. This describes a double loop antenna comprising two conductive metal loops of different sizes electrically coupled together to provide a wide band frequency response. The two loop antennas can be formed parallel or perpendicular to one another by a single conductor comprising two parallel elements connected together at appropriate points to define the two loops. The two loop antennas are tuned by a single variable capacitor connected between the two parallel conductive elements. When the two loop antennas are parallel, a part of each of the two parallel conductive elements forms a part of both of the two loop antennas. It is suggested that the double loop antenna arrangement can be used in a miniature radio receiver. The known transceivers also suffer from a disadvantage in that, when the transceiver is not carried by human body, the characteristics of the transceiver are periodically deteriorated in terms of time or distance due to variation of the polarization plane under influence of multi-path or fading. Furthermore, when the loop antenna itself has a length which is not negligible with respect to the wavelength of the signal used, impedance of the antenna is locally increased and lowered and, when a portion having large impedance is approached by the human body or a substance having a large dielectric constant, the impedance is deviated to cause a mismatch, with the result that the transmission/receiving sensitivities are lowered. An object of the present invention is to provide an antenna which can improve transmission/receiving sensitivities of a transceiver, thereby overcoming the above-described problems of the prior art. [Disclosure of Invention] According to the present invention, there is provided a loop antenna device suitable for use in portable apparatus including a wireless circuit, the device comprising a first portion and a second portion which in combination form a loop antenna, the first portion having one terminal connected to a first variable capacitance means and the second portion having one terminal connected to a high-frequency grounded surface, and characterised by the other terminal of the first portion being connected to one of the terminals of a second variable capacitance means, and the second portion having another terminal connected to the other terminal of the second variable capacitance means. The high-frequency grounded surface may have a rectangular form the shorter sides of which extend along the longer sides of the loop antenna device while the longer sides of the rectangular form extend perpendicularly to the longer sides of the loop antenna device. Each of the first and second variable capacitance means may be adapted to change its capacitance as a result of application of a voltage across the variable capacitance means. The value of the ratio of the capacitance of the second variable capacitance means to that of the first variable capacitance means is nearly equal to the ratio of the length of the first loop antenna to the length of the second loop antenna. The voltage may be varied in accordance with the posture of the portable transceiver. The loop antenna device may further comprise a switch which changes over the voltage in accordance with the posture of the portable transceiver. The invention also provides a portable transceiver incorporating the loop antenna device set forth hereinbefore. The invention also provides a loop antenna device for use in a miniature portable transceiver having the loop antenna device, a wireless transmission/receiving circuit, a data decoder, a CPU, a data display means and an informing circuit which are encased in a casing, the loop antenna device comprising a board mounting a circuit for operating the portable transceiver, the board having a length ranging between 1/10 and 1/6 the wavelength of the received transmitted wave and is provided with an open area in the direction perpendicular to the longitudinal direction, thus forming a loop antenna having a length ranging between 1/10 and 1/6 the wavelength of the wave received transmitted by the transceiver. Preferably, the loop antenna has a length nearly the same as the length of the board. The invention also provides a portable transmitter incorporating the loop antenna device of the type described. The antenna device may be divided at a position which is between 6: 1 and 8 : 1 of the overall length thereof into a first loop antenna having a greater length and a second loop antenna having a smaller length, the first loop antenna being connected at its one terminal to a common potential pattern of the transceiver and at its other terminal to one terminal of the second loop antenna through a capacitive reactance means. Preferably, the length of the circuit board is nearly equal to the sum of the lengths of the first loop antenna and the second loop antenna. The invention also provides a loop antenna device for use in a portable transceiver having the loop antenna device, a wireless transmission/receiving circuit, a data decoder, a CPU, a data display means and an informing circuit which are encased in a casing, wherein the loop antenna device has a slit. The loop antenna device may further comprise a capacitive reactance means which interconnects certain points on two conductor plates which oppose to each other across the slit. One of two conductor plates facing each other across the slit may be connected at a certain point thereof to a common potential pattern of the wireless transmitting/receiving circuit. The other of the two conductor plates may be connected at a certain point thereof to a high-frequency input terminal of the wireless transmitting/receiving circuit. The loop antenna device may further has another capacitive reactance so as to be formed in two pairs across the another capacitive reactance means. The capacitive reactance means may be disposed on the loop antenna device at positions opposite to each other with respect to the center of the slit. The invention further provides a portable transceiver incorporating a loop antenna device of the type set forth above. [Brief Description of the Drawings] Fig. 1 is a sectional view of an antenna device of the present invention as viewed from the open side thereof; Fig. 2 is an illustration of an antenna circuit including the antenna device of the present invention; Fig. 3 is a perspective view of the arrangement of the loop antenna device in accordance with the present invention; Figs. 4(a) and 4(b) are diagrams showing changes of capacitance values of variable capacitance diodes 14, 15 shown in Fig. 2 in relation to voltage applied to these sides; Figs. 5(a) and 5(b) are illustrations of directivity characteristic of the antenna device as observed when the antenna is carried by a human body; Fig. 6 shows appearance of a transceiver equipped with an antenna device of the present invention; Fig. 7 is an illustration of the transceiver of Fig. 6 held in a holster; Figs. 8(a) and 8(b) are illustrations of a transceiver of the present invention equipped with an antenna device of the present invention and incorporating a mercury switch for detecting the posture of the transceiver; Figs. 9(a) and 9(b) are illustrations of a transceiver of the present invention equipped with an antenna device of the present invention and incorporating a load cell for detecting the posture of the transceiver; Fig. 10 is a circuit diagram showing the circuitory of the transceiver shown in Fig. 9; Figs. 11(a) and 11(b) are illustration of a transceiver equipped with an antenna device of the present invention and carried by a human body; Fig. 12 is an illustration of an antenna in accordance with the present invention applied to a wrist type transceiver; Figs. 13(a) and 13(b) are illustrations of the transceiver of Fig. 12 carried by a human body; Fig. 14 is a top plan view of a circuit board on which an antenna device of the present invention is mounted; Fig. 15 is a sectional view of the antenna device shown in Fig. 14 as viewed in the direction of an arrow X; Fig. 16 is a view similar to that of Fig. 15 but showing a loop antenna device with a capacitor inserted to an intermediate portion thereof; Fig. 17 is an illustration of the electrical operation of the antenna device shown in Fig. 16; Fig. 18 is an illustration of output impedance locus of an antenna in accordance with the present invention; Fig. 19 is an illustration of directivity characteristic of an antenna device in accordance with the present invention; Fig. 20 is an illustration of directivity characteristic of an antenna device in accordance with the present invention; Fig. 21 shows appearance of a miniature portable wireless transceiver incorporating an antenna device in accordance with the present invention; Figs. 22(a) and 22(b) are illustrations of a miniature portable wireless transceiver of the present invention in a state mounted on a human body; Fig. 23 is an illustration of antenna device in accordance with the present invention having a slit formed in a conductor plate of the antenna; Fig. 24 is a simple illustration of magnetic field components which can be detected by an embodiment of the present invention; Fig. 25 is an illustration of an antenna device of the present invention mounted on a circuit board; Fig. 26 is an illustration of a different embodiment of the antenna in accordance with the present invention; Fig. 27 is an illustration of an antenna device of the present invention applied to a wrist type transceiver; Fig. 28 is a sectional view illustrative of the electric characteristics of a connector for attaching and detaching wrist band of the transceiver shown in Fig. 27; and Fig. 29 is an illustration of an antenna device of the present invention which does not use a connector a an electric circuit but utilizes an arm as a capacitor. [The Best Mode for Carrying Out the Invention] The invention will be described in detail with reference to the accompanying drawings. Fig. 1 is a sectional view of an antenna device of the present invention as viewed from an open side thereof. A first loop antenna 1 and a second loop antenna 2 are connected to each other through a second variable capacitance element 4 at points 6 and 7. The second loop antenna 2 is connected at the connecting point 8 to an inner printed pattern 10 which spreads over the entire area of the circuit board 3. The other terminal of the first loop antenna 1 is connected to a first variable capacitance element 5 via a connecting point 9.. Although not shown in this Figure, a high-frequency amplifier circuit is connected to the other terminal of the first variable capacitance element 5. A single turn of the loop antenna is thus formed. Fig. 2 is a circuit diagram showing a circuit including the antenna device of the invention. Variable capacitance diodes 14 and 15 respectively correspond to the variable capacitance elements 4 and 5. A manual variable capacit 20 is connected in parallel with the first variable capacitance diode 15 and a capacitor 22 is connected between the ground or earth 18 and the point of connection between the diode 15 and the variable capacitor 20. The earth 18 provides the D.C. potential of the inner printed pattern 10 shown in Fig. 1. With this arrangement, an LC resonance circuit is formed by the loop antennas 11, 12, variable capacitance diodes 14, 15, variable capacitor 20 and the capacitor 22. The LC resonance circuit resonates at a target frequency. Numeral 23 denotes a terminal for the connection to the high-frequency amplifier circuit. The variable capacitor 20 is used to provide a impedence matching so as to maximize the sensitivity of the transceiver. A D.C. current is applied to the connection point 19 through the resistor 21. The resistor 21 has a resistance value which is high and which does not cause reduction of Q value of the antenna. The resistor 21 is connected to a change-over switch 24 which changes conducting terminals according to the posture of the transceiver. Two D.C. voltages applied to the switch 24 are determined by resistors 25a, 26, 27 and 28. The power supply terminal 13 is connected to a power supply for driving the transceiver circuit. Voltages are so set that the highest potential appears at the terminal 23 and the ground level appearing at the earth 18, with the connecting point 19 set to an intermediate level. Consequently, inverse voltages are applied to the variable capacitance diodes 14, 15 and, when the voltage at the connecting point 19 is varied within the range which does not exceed the voltage of the terminal 23, two types of combination of the values of the diodes 14 and 15 become available. Both combinations provide an equal impedence characteristics of the lop antennas 11,12 as viewed from the terminal 23, so that the impedance matching at the received frequency does not change and, hence, reduction in the sensitivity due to impedance mismatching is avoided. According to the invention, two different values are set as the ratio of the capacitance between the variable capacitance diodes 14 and 15, so as to enable the current distribution on the loop antenna to be varied to control the detection amount of the electric field component, thereby preventing variation in the sensitivity attributable to the posture of the transceiver when the latter is mounted on the human body. Fig. 3 is a perspective view of a loop antenna device of the present invention, illustrating how the loop antenna is arranged. The loop antenna 1 is disposed along the shorter side of a circuit board 3. With this arrangement, the loop antennas 1 and 2 serve as an antenna for detecting magnetic field, while an inner printed pattern 10 of the circuit board 3 in cooperation with the loop antennas 1, 2 serves as an antenna for detecting electric field. It is assumed that a wave received has a magnetic field component 101 and an electric field 102 which are related to each other as illustrated in Fig. 3. In such a case, the loop antenna device operates as a magnetic field antenna alone when it takes such a posture that the shorter side of the circuit board 3 extends along the electric field component 102 (this posture will be referred to as "laid-down position", hereafter) while the change-over switch 24 is located at a predetermined position, whereas, when the antenna device is in such a posture that the longer side of the circuit board 3 extends in the direction of the electric field component 102 (this posture will be referred to as "upright" position, hereafter), it serves as an antenna sensitive both to electric and magnetic fields. The antenna device also serves as electric/magnetic field antenna when it is set in the posture as illustrated in Fig. 3. Figs. 4(a) and 4(b) respectively are graphs showing changes in the capacitance values of the first and second variable capacitance diodes 15 and 14 in relation to voltages applied thereto. When the loop antenna device is in the laid-down position, the diodes 15 and 14 are respectively set at points 32 and 37. In this state, the capacitances of the first and second variable diodes, together with capacitances of the variable capacitor 20 and the capacitor 22, form a composite capacitor which resonates with the loop antennas 11 and 12 at the frequency of the received signal. The loop antenna device operates as a magnetic field antenna and does not operate as an electric field antenna when such a capacitance ratio is attained. When the antenna device is set in the upright position, the switch 24 is changed over so that the diodes 15 and the diode 14 are respectively set to points 31 and 38. Resonance takes place also in this state at the same frequency of the received signal. With this capacitance ratio, the antenna device serves as an electric field antenna and also as a magnetic field antenna. Thus, in this embodiment, the antenna device operates as a magnetic field antenna alone when it is set in the laid-down position. This is intended for obtaining a greater increase in the sensitivity when mounted on the human body. In addition, the antenna device of this embodiment serves also as an electric field antenna when it is set in the upright position. This is intended for compensating for any reduction in the sensitivity which may occur when the antenna device is carried by a human body while suppressing variation in the sensitivity caused by a change in the posture of the transceiver. When the antenna device is in the laid-down position, the capacitance ratio between the first and second variable capacitance diode 15 and 14 are expressed by C12 : C22. This value is determined to coincide with the ratio of the length between the second loop antenna 12 and the first loop antenna 11. In this state, the internal pattern 10 of the substrate and the loop antenna 12 function as a magnetic field antenna alone, without working as an electric field antenna. When the antenna device is set in the upright position, the capacitance ratio is set to C11 : C21 in the illustrated case. By varying this ratio, it is possible to vary the efficiency of the antenna device as an electric field antenna, thus attaining an operation characteristic suitable for use on the human body. Figs. 5(a) and 5(b) illustrate directivity characteristic of the antenna device obtained when the device is carried by a human body in the laid-down position and upright position, respectively. More specifically, in Fig. 5(a), a graph 80 shows the directivity characteristic as obtained when the antenna device serves as a magnetic/electric field antenna, a graph 81 shows the directivity characteristic as obtained when the antenna device functions as a magnetic field antenna alone, and a graph 86 shows the directivity characteristic as obtained when the antenna device serving as the magnetic field antenna alone is placed in a space. The graph 86 also shows characteristics as obtained when the antenna device serving electric/magnetic field antenna is placed in the space. There is a large difference in the front sensitivity between the graph 81 and the graph 86. It will be seen that the effect of the human body can be utilized more effectively so that the front sensitivity characteristic of the antenna device used on human body can be improved when the antenna device is used as the magnetic field antenna. Numeral 88 denotes a reference sensitivity level which indicates the front sensitivity of the graph 86. Referring now to Fig. 5, a graph 83 shows the directivity characteristic as observed when the antenna device is used as a magnetic/electric field antenna, a graph 82 shows the sensitivity characteristic as obtained when the antenna device operates as a magnetic field antenna alone, and a graph 87 shows the sensitivity characteristic as observed when the antenna device serving as a magnetic/electric field antenna is placed in a space. The antenna device operating as an electric field antenna shows a reduction in the front sensitivity characteristic when mounted on a human body but exhibits front sensitivity characteristic exceeding the reference sesitivity level 88 in Fig. 5(a). From these facts, it is understood that the antenna device of this embodiment serves as a magnetic field antenna alone when set in the laid-down posture but operates, when set in the upright position, as a magnetic/electric field antenna so as to provide a stable characteristic in whatever posture it may be mounted on the human body. In addition, the antenna device of this embodiment provides a high sensitivity regardless of any variation in the polarization plane, even if it is placed alone in a pane, provided that it is allowed to operate as a magnetic/electric field antenna. Fig. 6 shows the appearance of a transceiver having the antenna device of the present invention. A display panel 41 provided on the upper central portion of the main body 40 displays the content of the signal receive, time and so forth. A switch 47 is provided for the purpose of switching the content of the display on the display panel 41. A switch 42 provided on the bottom of a recess 43 formed in the lower portion of the main body detects the posture of the transceiver. Fig. 7 shows the transceiver of Fig. 6 held in a holster. The holster denoted by 46 is a case which is adapted to be suspended from a waist belt to enable the user to carry the transceiver. The holster 46 is provided at its bottom with a projection 45 which engages with the switch 42 of the transceiver when the latter is inserted into the holster. This switch 42 corresponds to the change-over switch 24 shown in Fig. 2, so that the loop antenna device operates as a magnetic field antenna alone when the switch 42 is pushed as a result of insertion of the transceiver into the holster 46. When the transceiver is taken out of the holster, however, the switch 24 is released so that the loop antenna device operates as a magnetic/electric field antenna. The transceiver also may be held in a breast pocket. In such a case, the loop antenna device serves as an electric field antenna so that no reduction in the sensitivity is caused. As will be clear also from Fig. 5, the switch 24 is opened when the transceiver alone is placed in a space, so that the loop antenna device functions as a magnetic/electric field antenna, whereby stable sensitivity characteristic is obtained regardless of the posture. Figs. 8(a) and 8(b) show an embodiment in which a transceiver incorporating the antenna device of the present invention is provided with a mercury switch for detecting the posture of the transceiver. According to this method,the mercury switch serves as the change-over switch 24 shown in Fig. 2 so that the mode of the antenna device is determined depending on whether the mercury switch is conductive or not conductive. More specifically, referring to Fig. 8(a), the transceiver 54 is held in laid-down state so that the mercury switch 51 stands upright. In this state, a mercury column 50 which occupies 50% the internal volume of a cylinder 52 does not provide electrical connection between electrodes 53 and 55 which are provided on both ends of the cylinder 52. In this state, the antenna device serves only as a magnetic field antenna. Referring now to Fig. 8(b), when the transceiver is set in the upright position, the mercury switch 51 is laid horizontally so that the mercury 50 interconnects the electrodes 53 and 55. In this case, therefore, the antenna device operates as a magnetic/electric field antenna. The characteristics of the antenna during the operation are the same as those explained before in connection with Fig. 7. Figs. 9(a) and 9(b) illustrate an embodiment ion which a transceiver incorporating the antenna device of the present invention employs a load cell for detecting the posture of the transceiver. Thus, the load cell functions as the change-over switch 24 shown in Fig. 2. Various types of load cells 61 are available, among which a load cell of the type which changes its resistance value is used in this embodiment. Referring to Fig. 9(a), when the transceiver 60 is set in laid-down position, a spring 63 incorporated in the cylinder 64 is contracted by the load of a weight 52, so that the load cell 61 is flexed as its one end is pulled by a wire 68 connected to the weight 62. Electrodes 66 and 67 provided on both ends of the load cell 61 are connected to an antenna circuit. The resistance value between the electrodes 66 and 67 is so determined as to enable the antenna device to function only as a magnetic field antenna. Referring now to Fig. 9(b), when the transceiver 60 is set in upright position, the load of the weight 62 is not transmitted to the wire 68 so that the load cell 61 is never flexed. In this state, a resistance value which enable the antenna device to operate as a magnetic/electric field antenna is developed between the electrodes 66 and 67. The spring 63 preferably has a large spring constant so as to suppress oscillation of the weight due to vibration. Fig. 10 illustrates the circuit of the embodiment shown in Figs. 9(a) and 9(b). The variable resistor 25b corresponds to the load cell 61. The electrodes 66 and 67 correspond to the terminals 29a and 29b. Figs. 11(a) and 11(b) illustrate an embodiment in which a transceiver incorporating the antenna device of the present invention is carried by a human body. More specifically, in Fig. 11(a), a holster 71 suspended from a waist belt 72 is used when the transceiver is to be carried in laid-down posture, whereas, in Fig. 11(b), the transceiver 74 is held in a breast pocket in upright position. High sensitivity characteristic is stably obtained in both cases. Fig. 12 illustrates an embodiment in which the loop antenna device of the present invention is incorporated in a wrist type transceiver. The loop antenna device is encased in a wrist belt 90, and a display panel 92 and a switch 93 are disposed on the front side of the main body 91. Figs. 13(a) and 13(b) illustrate the embodiment shown in Fig. 12 carried by a human body. Fig. 13(a) shows the posture of the transceiver 95 obtained when the user who carries the transceiver 95 is standing up or walking, whereas Fig. 13(b) illustrates the state in which the user is seated. It will be seen that there is a distinctive difference in the posture of the transceiver 95 between the states shown in Figs. 13(a) and 13(b). The loop antenna device of the present invention, however, can provide stable characteristic regardless of the posture of the transceiver. Various embodiments of the present invention have been described. It is to be understood, however, the described embodiments are only illustrative and the invention can be applied to transceivers which are adapted to be carried by human bodies in various postures. In addition, various forms of sensors for sensing the posture of the transceivers can be obtained by using switches sensitive to the force of gravity. The present invention also can be applied to various other types of apparatuses than transceivers, such as, for example, a measuring device for detecting polarization plane of electric wave, and a circuit for automatically removing polarized wave component of disturbance wave for improving anti-disturbance characteristic of transceivers. Figs. 14 onwards illustrate embodiments of the antenna device of the present invention designed on the basis of the length of the circuit board. Fig. 14 is a top plan view of an antenna device of the present invention mounted on a circuit board. A loop antenna 201, wireless transmission/receiving circuit elements 204, 205, 206, and a display panel 203 are mounted on the circuit board 202. A digital circuit portion, although not shown in the Figure, is spaced apart from both blocks or provided beneath the display panel 203 because it is liable to generate noise which may be caught by the wireless portion or the antenna. The loop antenna 201 is formed by bending a flat plate into the form of a loop antenna. This loop antenna is disposed such that its longitudinal axis orthogonally intersects the longitudinal axis of the circuit board 202 expressed by X-Y. The length of the circuit board 207 is determined to be about λ/8, representing the wavelength of the transmission wave by λ. Fig. 15 is a sectional view of the antenna portion as viewed in the direction of an X in Fig. 14. As illustrated, the loop antenna 201 surrounds the circuit board 202 and has a length approximating λ/8. The point 208 of connection between the loop antenna 201 and the circuit board 202 is connected to the same pattern as the pattern which is held at a potential, e.g., ground level, common to circuits mounted on the circuit board 202. The other point 209 of connection is connected to a high-frequency amplifier circuit which constitutes a stage next to the antenna. The circuit board 202 has a multi-layered structure having an internal layer entirely covered by a print pattern which is held at at least the ground potential. Fig. 16 shows a modification of the embodiment shown in Fig. 15. In this modification, the loop antenna 201 is divided into two sections: namely, a first loop antenna 211 and a second loop antenna 212, and a capacitor 213 is connected between the terminals 210a and 210b of both loop antennas. The capacitor 213 is inserted such that the ratio of the length between the first loop antenna 211 and the second loop antenna 212 is 7 : 1. When this ratio is large, i.e., when the length of the first loop antenna 211 is comparatively large, the antenna gain is increased when this antenna performs a dipole operation. At the same time, however, such a large ratio causes a greater deviation of the impedance when the transceiver is used in the vicinity of the human body. The influence of the impedance deviation is more dominant than the increase in the gain. This undesirable effect is produced during the operation of the loop antenna 211, 212 so that the sensitivity of the transceiver is generally lowered when the transceiver is carried by the human body. For these reasons, it is not preferred to increase the above-mentioned ratio unlimitedly. When the ratio mentioned above is comparatively small, i.e., when the length of the first loop antenna 211 is comparatively small, the deviation of impedance which is caused when the transceiver has approached human body is also small, but the antenna gain during dipole operation also is largely decreased. Consequently, the gain of the loop antenna during operation is substantially unchanged. An antenna having such a small ratio does not meet the object of the present invention because it exhibits unacceptably large difference in the sensitivity according to the azimuth of the transceiver. According to the invention, the ratio between the first loop antenna 211 and the second loop antenna 212 is determined to be about 7 : 1, so as to ensure a sufficiently large gain during dipole operation while minimizing undesirable effect which is produced when he transceiver is used in the vicinity of the human body. Fig. 17 is an illustration of electrical operation of the antenna shown in Fig. 16. When the whole antenna device operates as a single loop antenna, a closed loop is formed by the first loop antenna 211, the capacitor 213, the second loop antenna 212, capacitors 214, 215 and points of connection 208, 209, 210a, 210b. 218 and 219a, and this closed loop functions as a loop antenna. An electromotive force generated in this loop antenna, appearing as a potential difference between the connecting points 219a and 219b, is input to a high-frequency amplifier circuit 216, and the amplified voltage between point 218 and 219b appears at an output terminal 217. Thus, the circuit board 202 has a potential pattern which is common both to the circuit 216 and the loop antenna. When the antenna device operates as a dipole antenna, the above-mentioned common potential pattern of the circuit board 2 serves as one of two elements of the dipole arrangement, while the other element is constituted by the first loop antenna 211. Connection points 219a, 219b are located substantially midst between these two elements. The connection point 210b is connected to the capacitor 213 which is then connected to the second loop antenna 212. However, since the capacitor 213 has a large impedence valve the elements such as the capacitor 213, second loop antenna 212 and so forth, connected to the connection point 210b, can be materially neglected when the whole antenna device functions as a dipole antenna. The overall length of the dipole antenna is then expressed by λ/8 (length of circuit board) + λ/8 * 7/8 (length of first loop antenna 21). The dipole antenna having such a length cannot be regarded as being so-called small dipole, and an appreciable current distribution is formed on the antenna. The pattern of the current distribution is similar to that of λ/2 dipole antenna. In such a case, the impedance of the connecting points connected to the circuit 216 is low and very closely approximate the impedance matching condition of the circuit 216. Fig. 18 is a graph showing the locus of the output impedance with respect to the frequency as observed in the antenna device of the present invention. More specifically, the impedance locus denoted by 226 is drawn on a Smith chart 220. Numeral 221 designates an impedance point obtained when the antenna of Fig. 16 is adjusted for best matching by adjustment of the capacitor 214, while 222 denotes an impedance point which minimises the noise figure in the circuit 216. Numeral 223 denotes an equi-noise index circle which interconnects impedance points of the same noise figure. The impedance point 222 has an impedance value approximating 50 + 50 j (Ω). Higher matching of the transceiver is obtained when the impedance value of the point 222 approaches the above-mentioned value. When the antenna device is used as a dipole antenna, since the impedance at the power supply point is low, it is possible to approximate the impedance point without substantially necessitating impedance conversion. This means that it is not necessary to add any specific electronic element for the purpose of conducting impedance conversion. Consequently, a compact antenna circuit which can operate with reduced loss can be obtained. Numeral 225 designates an impedance point which is obtained when the antenna circuit of Fig. 16 has been brought close to the human body. It will be seen that this impedance point 225 is substantially the same as the impedance point 222. Numeral 224 designates an impedance point which is obtained when the loop antenna device has been brought close to a human body with the capacitance 213 of Fig. 16 having been shifted near to the connection point 209 while omitting the second loop antenna 212. This corresponds to the case where the antenna of Fig. 15 is used. In this case, the impedance is largely deviated from the impedance point 221 so that the sensitivity is inevitably reduced. As will be understood from these facts, the antenna device of the type shown in Fig. 16 can reduce any influence of the human body which is caused when the antenna device is used in the vicinity of the human body. Figs. 19 and 20 illustrate directivity characteristics of the antenna device in accordance with the present invention. More specifically, Fig. 19 shows characteristics as observed when the antenna device is disposed such that the X-Y axis of Fig. 14 is set perpendicular to the electric field polarization plane which is vertical and rotated about Z-axis. This state is referred to as laid-down state. The term "front direction" is used to mean the orientation of the loop antenna device of Fig. 14 with respect to an electric wave impinging upon the antenna device in the direction normal to the plane of Fig. 14. In Fig. 19, a graph 231 shows the characteristic as observed when the transceiver is placed alone, while a graph 232 indicates the characteristic as observed when the transceiver is carried by the human body. The antenna device, when carried by the human body with its front side facing the wave, exhibits a rise of the antenna gain by several dB, by virtue of the fact that the antenna of the invention operates as a loop antenna. Fig. 20 shows the directivity characteristic as observed when the loop antenna device is disposed such that the X-Y axis in Fig. 14 is placed in parallel with the electric field polarisation plane which is vertical and rotated about the X-Y axis. this posture will be referred to as "upright state", hereinunder. The advantage of the present invention will be best seen from Fig. 20. In this Figure, the graphs 241 and 243 show gains of the antenna disposed in a space: more specifically, the graph 241 shows the gain of the antenna of the invention as shown in Fig. 15, while the graph 243 shows the gain as obtained when the length of the loop antenna 201 in Fig. 15 is reduced to 1/2 or when the length of the circuit board 202 is reduced to 1/2 without changing the length of the loop antenna 201. The difference between the values indicated by the graphs 241 and 243 is as large as more than 10 dB. From this fact, it is understood that, when the circuit board 202 and the loop antenna 201 are made to have lengths substantially equal to each other as in the present invention, the circuit board 202 is enabled to efficiently pick up the electric field component of the wave and to efficiently pass the received signal to the circuit 216. When the antenna of the invention has a construction of the type shown in Fig. 15, the gain of the circuit board 202 as an dipole antenna is somewhat reduced, but the undesirable effect produced by the human body can be remarkably suppressed as shown in Fig. 18 when the antenna device is positioned in the vicinity of such human body. In Fig. 20, a graph 242, which should be contrasted to the graph 241, shows the antenna gain as obtained when the antenna is carried by the human body. In this case, the antenna device operates in a mode for picking up the electric field component, so that the sensitivity is inevitably lowered when the antenna device is held at the front side of the human body. This fact is shown also in the article which is mentioned before in the description of the related arts. It is to be pointed out that the absolute level of the graph 242 is almost the same as the antenna gain as shown in Fig. 19. This means that the gain is not substantially changed regardless of the direction or orientation of the antenna device on the human body. This feature provides an advantage specifically when the antenna device of the invention is incorporated in a portable transceiver, because in such an application the receiving sensitivity is not changed substantially regardless of the direction or orientation of the transceiver on the human body. The embodiment shown in Fig. 16 also can provide sensitivity characteristic having no substantial directivity, although the gain of the antenna body is slightly decreased. Fig. 21 illustrates a miniature portable transceiver incorporating the antenna device in accordance with the present invention. A display panel 251 is disposed horizontally. Manipulations required for confirming the display are executed while laying the transceiver in the horizontal posture as illustrated. Numerals 252 and 253 denote push-button type input switches used for the above-mentioned manipulations. The main body 250 is usually disposed in laid-down position as illustrated. For instance, the main body 250 of the transceiver is attached to a holder 262 which is positioned on a suitable portion, e.g., waist, of the user's body, as illustrated in Fig. 22(a) . In this case, the main body 250 is held horizontally. Preferably, the holder 262 is mounted such that it can be tilted as desired, so as to enable easy confirmation of the content of the display without requiring demounting of the main body 250 from the holder 262, as well as easy demounting of the main body 250 from the same. The holder 262 is integral with a belt 260 so that it can easily be carried by the user's body as the belt 260 is wound around the user's waist. When the transceiver 250 is placed in a pocket 263 near the breast, the main body 250 is held in the upright posture as shown in Fig. 22(b). Thus, the main body 250 is held substantially vertically. Thus, the transceiver can be carried in any desired posture on the user's body. The main body 250 incorporating the antenna of the present invention exhibits, as explained before, a substantially constant antenna gain regardless of its posture. This feature is advantageous particularly when the transceiver is a portable transceiver which is to be carried by the user's body. Figs. 23 onwards show different embodiments of the antenna device of the present invention having specific forms of the antenna body. More specifically, Fig. 23 shows an embodiment in which a slit of formed in a portion of the conductor plate. The conductor plate 301 cooperates with a capacitor 303 informing a loop antenna. The conductor plate 301 is partly slitted as at 302. A capacitor 304 is disposed at any desired position within the slit 302 so as to interconnect connection points 310 and 311. The connection point 310 is connected to the ground 305. A power supply point 309 is disposed at any desired position on the conductor plate 312 which extends in parallel with the slit 302. The conductor plate 312 is connected to a high-frequency amplifier circuit 307 through the power supply point 309 and a capacitor 306 for obtaining matching of the antenna circuit. The output of the circuit 307 is connected through a terminal 308 to a component of a subsequent stage such as an intermediate-frequency circuit. The capacitor 303 is positioned so as to confront the slit 302. This type of antenna,having a conductor plate provided with a slit across which the power is supplied is generally referred to as "slot antenna". The antenna of this embodiment is characterized in that a part of a conductor plate which forms a loop antenna functions also as a slot antenna. The characteristic feature of the slot antenna resides in that it can efficiently detect the magnetic field component of the direction in which the slit extends. The antenna shown in Fig. 23, therefore, can detect also a magnetic field component of a direction orthogonal to the direction of the magnetic field detected by the loop antenna. Fig. 24 schematically illustrates the directions of the magnetic field components which can be detected by the embodiment shown in Fig. 23. The loop antenna constituted by the conductor plates 321, 332 and the capacitor 323 detects a magnetic field component 335. A slot antenna constituted by the slit 322 and the capacitor 324 detects a magnetic field component 337. Furthermore, a magnetic field component 336 is detected by a loop antenna which is constituted by the conductor plate 332 surrounding the slit 322 and the capacitor 324 when the positions of the connection points 330 and 331 are suitably selected along the slit 322. The magnetic field component 335, 336 and 337 are orthogonal to one another, so that they can be detected by the antenna device of the present invention regardless of the posture of the antenna device. This feature is quite advantageous for transceivers which are intended to be carried on the user's body during the use. Fig. 25 illustrates the antenna device of this embodiment mounted on a circuit board. A conductor plate 341 is mounted on the circuit board which is denoted by 352. Capacitors and a circuit 347 are indicated by symbols which are used in ordinary electric circuit diagrams. In this arrangement, the capacitor 343 is disposed so as not to oppose the slit 342. A conductor plate 355 and a circuit board 352 are connected to each other through conductor plates 353,354. The conductor plate 353 is for the supply of electrical power, while the conductor plate 354 is for grounding. Thus, the conductor plate 353 and the conductor plate 354 respectively correspond to the connection points 329, 330 and the connection point 331, respectively. Fig. 26 shows a different embodiment which also is of the type having a slitted conductor plate. In this embodiment, however, the slit 366 is opened at its one end and a capacitor 362 is provided. In this embodiment, therefore, a loop antenna is formed by a pair of conductor plates 360, 361, capacitors 363, 364 and the capacitor 362. This antenna device may be incorporated in a transceiver of the type shown in Fig. 21 or in a wrist type transceiver as shown in Fig. 27. The transceiver shown in Fig. 27 has a conductor plate 371 which extends through a wrist band 370. A slit 374 is formed so as to extend in parallel with the longitudinal axis of the slit 374 substantially over the entire length of the conductor plate 370. The opposing ends of the internal conductor plate 371 are connected to each other through a connector 372 having a capacitor 378, so that the antenna has the form of a loop both in appearance and electrical function. The main body 373 of the transceiver has a circuit board 375 onto which is extended a pattern leading from the conductor plate 371. The pattern is composed of a pair of parallel conductor strips or plate separated from each other by a slit. Capacitors 376, 377 and a high-frequency amplifier circuit 304 are mounted on the circuit board 375. Electrical connections are materially the same as those in Fig. 23. Directivity characteristics somewhat different from those obtained from the embodiment of Fig.23 are obtained when the embodiment of Fig. 26 is used in a wrist type transceiver shown in Fig. 27. Namely, the transceiver of Fig. 27 is different from that shown in Fig. 23 in that the slot antenna is curved in the form of a loop. This specific form of the slot antenna enables detection of all the magnetic field components of the directions extending long the slit, as indicated by an arrow 381. Numeral 382 designates a magnetic field component which corresponds to the magnetic field component 336 of Fig. 24. These magnetic field component are detectable because the antenna device seemingly has the form of an elongated loop antenna when viewed in all these directions. These bidirectional component provides a uniform directivity characteristic having no null point, which is quite convenient for portable transceivers. Fig. 29 illustrates a modification of the embodiment of Fig. 27, in which the electric circuit omits the connector and utilizes the user's arm as an antenna. More specifically, this modification utilizes a buckle 383 in place of the connector 372 used in the embodiment of Fig. 27. Thus, in the modification shown in Fig. 29, the connector portion is required only to provide a mechanical connection of the wrist band, without providing any electrical connection. The wrist band 370 is disposed near an arm 384 of the user so that a capacitance 385 is formed between the conductor plate 371 and the arm 384. The capacitance 385, arm 384 and the conductor plate 371 form a loop antenna. The antenna device of this modification can detect magnetic field components 380, 381 and 382, so that antenna gain can be obtained regardless of the direction of the arm 384. The magnetic field distribution on a human arm has not been clarified yet but it is clear that the antenna device shown in Fig. 29 makes an effective use of the magnetic field components on the human arm so as to exhibit an improvement in the gain. Thus, the transceivers incorporating the antenna device of the present invention always exhibit stable transmission/receiving characteristics.

Claims: A loop antenna device suitable for use in portable apparatus including a wireless circuit, the device comprising a first portion (1) and a second portion (2) which in combination form a loop antenna, the first portion (1) having one terminal (9) connected to a first variable capacitance means (5) and the second portion having one terminal (8) connected to a high-frequency grounded surface (10), and characterised by the other terminal (6) of the first portion being connected to one of the terminals of a second variable capacitance means (4), and the second portion (2) having another terminal (7) connected to the other terminal of the second variable capacitance means (4). A device according to claim 1, wherein the loop antenna (1,2) is capable of detecting a magnetic field component of an electromagnetic wave, and the grounded surface (10) is perpendicular to a plane of the loop antenna and is capable of detecting the electric field of the electromagnetic wave. A device according to claim 1 or 2, wherein the high-frequency grounded surface (10) has a rectangular form, the shorter sides of which extend along the longer sides of the loop antenna, while the longer sides of the rectangular form extend perpendicularly to a plane of the loop antenna. A device according to any preceding claim, wherein the value of the ratio of the capacitance of the second variable capacitance means (4) to the that of the first variable capacitance means (5) is substantially equal to the ratio of the length of the first loop antenna portion (1) to the length of the second loop antenna portion (2). A device according to any preceding claim, wherein each of the first and second variable capacitance means (5, 4) is adapted to change its capacitance as a result of application of a voltage across the variable capacitance means. A device according to claim 5, wherein the voltage is varied in accordance with the posture of the portable apparatus. A device according to claim 6, wherein a means for applying voltage increases a voltage applied to the second capacitance means (4) and decreases a voltage applied to the first capacitance means (5) when the antenna device is change from a horizontal to a vertical position. A device according to claim 6 or 7, including a switch (24; 51) which changes over the voltage in accordance with the posture of the portable apparatus. A portable apparatus characterised by having a loop antenna device according to any preceding claim. Apparatus according to claim 9, including a wireless circuit. Apparatus according to claim 10, wherein the apparatus is a receiver and the circuit is a receiving circuit. Apparatus according to claim 10, wherein the apparatus is a transmitter and the circuit is a transmitting circuit. Apparatus according to claim 10, wherein the apparatus is a transceiver and the circuit is a transmitting/receiving circuit. Apparatus according to any of claims 10 to 13, wherein the apparatus includes a data decoder, a CPU, a data display means and/or an informing circuit.

Inventors: HAMA NORIO, HAMA, NORIO
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1992
Main classification: H01Q1
IPC codes: H01Q7, H01Q1