Antenna unit for portable wireless apparatus

Abstract: An antenna for portable wireless apparatus as shown in Figure 5 is capable of operating satisfactorily when an antenna element (8) is extended out of a case (7) or stowed in the case (7). A portion (8 a ) of the antenna element (8) not stowed in an antenna stowage pipe (12) when the antenna is pushed into the plastic case, functions as a radiation element. A portion of the antenna element and the antenna stowage (12) pipe produced of a metal for stowing this portion constitute an impedance stub circuit. Antenna impedances of the antenna element in the extended and stowage positions as well can be made approximately the same by properly setting the length of the antenna element which is stowed in the antenna stowage pipe and the length of the portion not stowed in the stowage pipe.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an antenna unit for portable wireless apparatus to be used primarily within the UHF band. Description of the Prior Art Fig. 1 is a sectional side view showing a prior-art antenna unit for portable wireless apparatus disclosed in for example Japanese Utility Model Laid-Open Publication No. 42148/86. In this drawing, a numeral 1 refers to a wireless apparatus case; a numeral 2 denotes a telescopic whip antenna mounted in the wireless apparatus case 1; a numeral 3 represents a small antenna built in the wireless apparatus case 1; a numeral 4 is a high-frequency switch for switching between the whip antenna 2 and the small antenna 3; a numeral 5 refers to a contact changeover lever of the high-frequency switch 4, which is opened outward by the force of a spring when the whip antenna is pulled out, connecting the switch contact 4a to a receiving-transmitting circuit 6 as shown in Fig. 2, and also connecting the switch contact 4a to the receiving-transmitting circuit 6 when the whip antenna 2 is pushed into the wireless apparatus case 1. A numeral 6 refers to a receiving-transmitting circuit which is changed over from the whip antenna 1 to the small antenna 3 or vice versa by means of the high-frequency switch 4. Next, the operation of this wireless apparatus will be explained. For communication with a remote station or in such a place where the condition of reception is poor, the whip antenna 2 is pulled out of the wireless apparatus case 1, being in the state shown in Fig. 2(a). Then the contact changeover lever 5 becomes free to connect the whip antenna 2 to the receiving-transmitting circuit 6 via the contact 4a of the high-frequency switch 4, thereby enabling good communication through the whip antenna 2. For short-range communication, or during waiting after completion of communication, the whip antenna 2 is pushed into the wireless apparatus case 1, being stowed in the position indicated by a dotted line in Fig. 1. In this case, as shown in Fig. 2(b), the contact changeover lever 5 is pressed by the whip antenna 2 thus stowed, connecting the built-in small antenna to the receiving-transmitting circuit 6 via the contact 4b of the high-frequency switch 4. Thus communication is made through the built-in small antenna, enabling reception of an emergency call signal and sufficient communication with a short-range counterpart wireless station in a place where high radio field intensity is present. Since the prior-art antenna unit for portable wireless apparatus has the above-described constitution, it is necessary to provide a space for mounting the small antenna 3 in the wireless apparatus case 1. The provision of this space, however, will become an obstacle to the miniaturization of wireless apparatus. In addition, the prior art has such a problem that the adoption of the small antenna 3 and high-frequency switch 4 is costly and its reliability will decrease with an increase in the number of use of the high-frequency switch. JP-A-60107903 discloses an antenna unit in which an antenna element is retractable into an equipment casing. A short, grounded cylinder surrounds a part of the antenna element when retracted. However the relationship between the lengths of the extended portion of the antenna element, and of the portions of the antenna which are within the cylinder and not within the cylinder when the antenna element is retracted is not such that the impedance is the same in both positions. According to the invention there is provided an antenna unit comprising: a case housing a wireless apparatus body including a receiving-transmitting circuit; an antenna element extendibly mounted in said case, and connected with said wireless apparatus body through an impedance matching circuit; a conductive antenna stowage pipe which provides stowage for said antenna element, partly insulated, when said antenna element is stowed in said case; and a connecting means connecting a specific position on said antenna stowage pipe with a reference potential; characterised in that the length of a portion of said antenna element extended out of said case and functioning as a radiation element, the length of a portion of said antenna element stowed in said case and functioning as a radiation element, and the length of a portion of said antenna element stowed in said conductive antenna stowage pipe are so related that the impedance of the radiation element, when viewed from an antenna feeding section in the state when said antenna element is extended out of said case is equal to the impedance of the radiation element when viewed from said antenna feeding section in the state when said antenna element is stowed in said case. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention and wherein: Fig. 1 is a sectional side view of a prior-art antenna unit; Fig. 2 is an explanatory view for explaining the function of a high-frequency switch; Fig. 3 is a sectional side view showing a first embodiment of an antenna unit according to the present invention with the antenna extended; Fig. 4 is a sectional side view showing the first embodiment of the antenna unit according to the present invention with the antenna stowed in the case; Fig. 5 is a sectional side view showing in detail the first embodiment of the antenna unit according to the present invention; Fig. 6 is a Smith chart showing antenna impedance characteristics of the first embodiment of the antenna unit according to the present invention; Fig. 7 is a sectional side view showing a second embodiment of the antenna unit according to the present invention; Fig. 8 is a sectional side view showing a third embodiment of the antenna unit according to the present invention; and Fig. 9 is a sectional side view showing a fourth embodiment of the antenna unit according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Figs. 3 to 5 show a first embodiment of the antenna unit according to the present invention. In Fig. 3, a numeral 7 denotes a plastic case of the wireless apparatus; a numeral 8 refers to a rod antenna element used as a whip antenna of the length L 1 which can be extended out of, and pushed in, the plastic case; a numeral 9 refers to an antenna feeding section in contact with the antenna element 8; a numeral 10 refers to a wireless apparatus body including a receiving-transmitting circuit housed in the plastic case, and enclosed with a metal case as described later. A numeral 11 is an impedance matching circuit provided between the wireless apparatus body 10 and the antenna feeding section 9; and a numeral 12 represents an antenna stowage pipe produced of metal for stowing the antenna element 8, and grounded through the metal case of the wireless apparatus body 10. Fig. 4 is a sectional side view of the antenna unit with the antenna element 8 of Fig. 3 stowed in the case 7. Fig. 5 is a sectional side view showing the antenna unit embodied in the present invention in Fig. 4, in which a numeral 8a is an electrically conductive part located in the upper part of the antenna element 8; a numeral 8b denotes a plastic cap covering the electrically conductive part 8a; a numeral 8c refers to a plastic stopper covering the lower part of the antenna element 8; a numeral 9a refers to an antenna feeding section provided in the plastic case 7; a numeral 9b is an elastic-member connecting the antenna feeding section 9a with the antenna element 8; a numeral 11a is a matching circuit feeding terminal; a numeral 11b expresses a capacitor; a numeral 11c is a coil; a numeral 10a is a metal case covering the wireless apparatus body 10; and a numeral 10b represents a bent plate-like stub tuning spring as a means for connecting the metal case 10a with the antenna stowage pipe 12. In the following the operation of this antenna unit will be explained. In the state shown in Fig. 3, with a change in the length L 1 of the antenna element 8, the impedance viewed from the antenna feeding section 11 varies as roughly indicated by a dotted line a in Fig. 6. For example, when L 1 of a value from (1/4)λ to (1/2)λ (λ: Radio frequency wavelength) is selected, the impedance will come near the point A (the area near the point A is represented by the point A for brevity). In the meantime, in the state shown in Fig. 4, if the portion of the length L 2 is taken out, with the length L 3 of the antenna element 8 covered with the antenna stowage pipe 12 disregarded, the value of impedance as viewed from the antenna feeding section 11 comes to the vicinity of the point B in Fig. 6 when the length L 2 of about (1/10)λ is selected. Here, there is established a relationship between L 1 and L 2 which equalizes the resistance component of impedance in the state shown in Fig. 3 to the resistance component of impedance with the portion of the length L 3 disregarded in the state shown in Fig. 4. That is, both the resistance components of these impedances can be made equal when L 1 and L 2 are properly selected. In the meantime, when the upper part of the antenna stowage pipe 12 contacts the metal case 10a through the stub tuning spring 10b, the stub circuit is formed by the portion of the length L 3 of the antenna element 8 and the antenna stowage pipe 12 when the antenna is stowed in the case. When the antenna element 8 and the antenna stowage pipe 12 are insulated by the plastic stopper 8c, an open stub is formed. In this state, when L 3 is varied, the impedance at only the length L 2 of the antenna element 8 indicated at the point B varies on the line b indicating the resistance component of Fig. 6. That is, when the length of L 3 is selected properly, the impedance is changed from the value at the point B to that at the point A. Theoretically, as described above, it is possible to set, to the value of the point A of Fig. 6, both the antenna impedance as viewed from the antenna feeding section 9 with the antenna element 8 pulled out as shown in Fig. 3 and the impedance as viewed from the antenna feeding section 9 with the antenna element 8 stowed as shown in Fig. 4. In this state, the impedance of the wireless apparatus feeding section of the wireless apparatus body 10 and the antenna impedance are matched by the impedance matching circuit 11. Accordingly, the power can efficiently be fed, without a loss caused by mismatching, by means of one impedance matching circuit 11 when the antenna is in an extended position and in a stowed position as well. In Fig. 5, the elastic member 9b is used to always maintain electrical continuity between the telescoping antenna element 8 and the antenna feeding section 9. The impedance matching circuit 11 consists of the matching circuit feeding terminal 11a, the capacitor 11b and the coil 11c. The antenna stowage pipe 12 is made of a tubular metal pipe. At the bottom end of the antenna element 8 is provided the plastic stopper 8c composed of an insulator for holding the antenna element 8 at the center of the pipe 12 as well as for providing insulation between the antenna stowage pipe 12 and the antenna element 8 when the antenna element 8 is in the stowage position. In the example described above, the metal pipe is used for the antenna stowage pipe 12, but a plastic pipe 12a coated on the outside surface with a conductive coating 12b as shown in the second embodiment in Fig. 7 may be used. In this case, the device is light in weight as compared with that using the metal pipe, and besides it is unnecessary to use the plastic stopper 8c for the provision of insulation between-the antenna element 8 and the antenna stowage pipe 12 shown in Fig. 5. Further, the metal antenna stowage pipe 12 may be coated on the inner wall with an insulating paint. In this case also, the use of the plastic stopper 8c becomes unnecessary. In the first and second embodiments the stub tuning spring 10b is installed in contact with the top end section of the antenna stowage pipe 12, but in the third embodiment, as shown in Fig. 8, another state of contact of these parts is shown. That is, the stub tuning spring 10c may be installed in a position a little below the top end as shown in Fig. 8 for the purpose of insuring optimum impedance matching when the antenna is in the stowage position. Further a slidable stub tuning spring 10c as indicated by an arrow may be adopted. And further, where cost reduction takes precedence, the pipe 12 may be soldered directly to the wireless apparatus body 10, not using an elastic sheet such as the spring 10c. In each of the embodiments described above, the impedance matching circuit 11 using the capacitor 11b and the coil 11c was shown, but the impedance matching circuit 11 of any optional circuit formation may be adopted in accordance with the impedance of the feeding section of the wireless apparatus. Further the impedance matching circuit 11 may be installed inside the metal case 10a of the wireless apparatus body. Fig. 9 is a sectional side view showing the fourth embodiment of the antenna unit according to the present invention. In this case, the antenna feeding section 9a is not used, but a bent sheet-like elastic member 9c is fixed directly on the plastic case 7, and is in elastic contact with the antenna element 8, thereby enabling the reduction of cost and weight of the apparatus. According to the above-described antenna unit in which the impedance stub is formed by a part of the antenna element in the stowage position and the antenna stowage pipe and the other part of the antenna element works as a radiation element, it is unnecessary to provide an independent built-in antenna, and only a single antenna element functions satisfactorily as an antenna regardless of its position, that is, when stowed as well as when extended.

Claims: An antenna unit, comprising: a case (7) housing a wireless apparatus body (10) including a receiving-transmitting circuit; an antenna element (8) extendibly mounted in said case (7), and connected with said wireless apparatus body (10) through an impedance matching circuit (11); a conductive antenna stowage pipe (12) which provides stowage for said antenna element (8), partly insulated, when said antenna element (8) is stowed in said case (7); and a connecting means (10b, 10c) connecting a specific position on said antenna stowage pipe (12) with a reference potential; characterised in that the length (L 1 ) of a portion of said antenna element (8) extended out of said case (7) and functioning as a radiation element, the length (L 2 ) of a portion of said antenna element (8) stowed in said case (7) and functioning as a radiation element, and the length (L 3 ) of a portion of said antenna element (8) stowed in said conductive antenna stowage pipe (12) are so related that the impedance of the radiation element, when viewed from an antenna feeding section (9a) in the state when said antenna element (8) is extended out of said case (7) is equal to the impedance of the radiation element when viewed from said antenna feeding section (9a) in the state when said antenna element (8) is stowed in said case (7). An antenna unit as claimed in Claim 1, wherein said antenna element (8) is connected with said wireless apparatus body section (10) through said impedance matching circuit (11) and said antenna feeding section (9a). An antenna unit as claimed in Claim 1 or 2, wherein, when said antenna element (8) is stowed in said case (7), a part of said antenna element stowed in said antenna stowage pipe (12) and said antenna stowage pipe (12) constitute a stub circuit; the other part of said antenna element not stowed in said antenna stowage pipe is fed with power through said antenna feeding section (9a), functioning as a radiation element; and said antenna element, when extended from said case, is fed with power through said antenna feeding section (9a), functioning as a radiation element as a whole. An antenna unit as claimed in Claim 1, wherein said antenna element (8) is connected with said matching circuit by a bent sheet-like elastic member (9c) having elasticity on the side of said antenna element (8). An antenna unit as claimed in Claim 1, wherein a nonconductive stopper (8c) covering the bottom end of said antenna element (8) insures insulation between said antenna element (8) and said antenna stowage pipe (12). An antenna unit as claimed in Claim 1, wherein said connecting means (10b, 10c) is provided with a bent sheet-like elastic member interposed between an outside wall of said antenna stowage pipe (12) and an outside wall of said wireless apparatus body and is in contact with both said outside walls. An antenna unit as claimed in Claim 6, wherein said sheet-like elastic member is provided movable in the same direction of movement of said antenna element between said outside wall of said antenna stowage pipe and said outside wall of said wireless apparatus body. An antenna unit as claimed in Claim 1, wherein said conductive antenna stowage pipe has a conductive outside wall and an insulated inside wall. An antenna unit as claimed in Claim 8, wherein said antenna stowage pipe is a plastic pipe coated on the outside surface with a conductive paint.

Inventors: TOKO YOSHIO, TOKO, YOSHIO, TOKO, YOSHIO, MITSUBISHI DENKI K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1992
Main classification: H01Q1
IPC codes: H01Q1