Antenna matching device

Abstract: An antenna apparatus (10; 20; 30) having a predetermined impedance is provided. The apparatus has a chip antenna (11) including a conductor (16) and a mounting board (12) on which the antenna is mounted. A ground pattern (13) and a microstrip line (14) are formed on the board (12). A feeding terminal (17) for applying a voltage to the conductor (16) is deposited on the surface of the antenna. Solder may be used to connect the feeding terminal (17) of the antenna to the microstrip line (14) formed on the board (12). A capacitance-generating device (19; 21; 31), e. g., a dielectric resin (19) or capacitor (21) is disposed between the microstrip line (14) connected to the feeding terminal (17) of the antenna and the ground pattern (13). A capacitance is thus generated between the microstrip line (14) and the ground pattern (13) thereby enabling regulation of the antenna apparatus impedance and bandwidth.

Description: The present invention relates to an antenna apparatus for use in mobile communications and local area networks (LAN). Referring to a perspective view of Fig. 11, a conventional antenna apparatus generally designated by 50 includes a chip antenna 51, a mounting board 52 on which the chip antenna 51 is mounted, a ground pattern 53 formed on the board 52, and a microstrip line 54 also formed on the board 52. This chip antenna 51 is formed, as shown in the longitudinal cross-sectional view of Fig. 12, of an insulator 55, a coil-like conductor 56, a magnetic substance 57, and external connecting terminals 58a and 58b. However, the antenna apparatus of the above conventional type encounters the following problem. Namely, when the chip antenna is mounted on the board, the resonant frequency and the impedance of the chip antenna disadvantageously deviate from the determined values depending on the material of the board, the configuration of the ground pattern, the material of a casing for accommodating the apparatus therein, and other conditions. The resonant frequency of the chip antenna is adjustable in advance allowing for deviations, while the impedance is unadjustable. EP-B-0616383 relates to an antenna arrangement wherein an antenna having two antenna arms is provided on a surface of an circuit board. In addition, a terminal line having a resistance of 50 Ohm is arranged on the same surface of the circuit board. A matching circuit is connected between the terminal line and a feeding point of the antenna. The matching circuit comprises a high-impedance meander-shaped micro-strip line and capacitors connected between the micro-strip line and a ground electrode of the circuit board. It is the object of the present invention to provide an antenna apparatus having an improved characteristic by providing an enhanced matching. This object is achieved by an antenna apparatus according to claim 1. With this arrangement, according to the antenna apparatus of the present invention, the provision of the capacitance-generating device makes it possible to adjust the impedance of the apparatus. Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings. Fig. 1 is a perspective view of an antenna apparatus according to a first embodiment of the present invention; Fig. 2 is a perspective phantom view of the chip antenna of the apparatus shown in Fig. 1; Fig. 3 is a perspective view of an antenna apparatus; Fig. 4 is a perspective view of an antenna apparatus; Fig. 5 illustrates impedance characteristics of an antenna apparatus without having a capacitance-generating device; Fig. 6 illustrates impedance characteristics of an antenna apparatus provided with a capacitor-generating device having a capacitance of 1 pF; Fig. 7 illustrates impedance characteristics of an antenna apparatus provided with a capacitor-generating device having a capacitance of 2 pF; Fig. 8 illustrates reflection-loss characteristics of an antenna without having a capacitance-generating device; Fig. 9 illustrates reflection-loss characteristics of an antenna apparatus provided with a capacitor-generating device having a capacitance of 1 pF; Fig. 10 illustrates reflection-loss characteristics of an antenna apparatus provided with a capacitor-generating device having a capacitance of 2 pF; Fig. 11 is a perspective view of a conventional antenna apparatus; and Fig. 12 is a sectional view of the chip antenna of the apparatus shown in Fig. 11. Referring to the perspective view shown in Fig. 1, an antenna apparatus generally designated by 10 includes a chip antenna 11, a mounting board 12 on which the chip antenna 11 is mounted, a ground pattern 13, and a microstrip line 14, both the pattern 13 and the line 14 being formed on the board 12. The chip antenna 11 is packaged within a rectangular prism-shaped dielectric substrate 15 essentially consisting of titanium oxide and barium oxide and comprises, as illustrated in Fig. 2, a conductor 16 that is spirally wound in or along the substrate 15 in its longitudinal direction, a feeding terminal 17 for applying a voltage to the surface of the substrate 15, and a mounting terminal 18 for securing the chip antenna 11 on the board 12. One end of the conductor 16 serves as a feeding end 16a connected to the feeding terminal 17, while the other end is free to form a free end 16b within the dielectric substrate 15. The feeding terminal 17 of the chip antenna 11 is connected to the microstrip line 14 formed on the board 12 by means of soldering (not shown), thereby forming the antenna apparatus 10. Further, a capacitance-generating device, i.e., a dielectric resin 19, is disposed between the microstrip line 14 connected to the terminal 17 of the antenna 11 and the ground pattern 13 so as to generate a capacitance between the microstrip line 14 and the ground pattern 13. Referring to the perspective view illustrated in Fig. 3, there is shown an antenna apparatus. The antenna apparatus generally designated by 20 differs from the antenna apparatus 10 of the first embodiment in that a capacitor 21 is used as the capacitance-generating device disposed between the micro-strip line 14 connected to the feeding terminal 17 of the chip antenna 11 and the ground pattern 13. This capacitor 21 generates a capacitance between the microstrip line 14 and the ground pattern 13. Fig. 4 is a perspective view of an antenna apparatus. The antenna apparatus generally indicated by 30 differs from the antenna apparatus 10 of the first embodiment in that an open stub 31 serves as the capacitance-generating device formed between a portion extending from the microstrip line 14 connected to the feeding terminal 17 of the chip antenna 11 and a grounding electrode (not shown) on the reverse surface of the ground pattern 13. This open stub 31 produces a capacitance between the microstrip line 14 and the ground pattern 13. Impedance characteristics of the antenna apparatus are shown in Figs. 5, 6 and 7, while insertion-loss characteristics of the apparatus are illustrated in Figs. 8, 9 and 10. Figs. 5 and 8 illustrate impedance characteristics and insertion-loss characteristics resulting from the apparatus without disposing capacitance-generating device; Figs. 6 and 9 illustrate the above-described characteristics obtained from the apparatus provided with a capacitance-generating device having a capacitance of 1 pF; and Figs. 7 and 10 show the above-described characteristics obtained from the apparatus provided with a capacitance-generating device having a capacitance of 2 pF. Table 1 shows the impedances at the center frequency (1.9 GHz: indicated by the arrow 1 in Figs. 5 through 7) resulting from the characteristics shown in Figs. 5 through 7 and also indicates the bandwidths resulting from the characteristics shown in Figs. 8 through 10. Impedance at the center frequency (Ω) Bandwidth of the antenna apparatuses (MHz) No capacitance-generating device 12.99 123.5 Capacitance-generating device of 1 pF 32.72 91.6 Capacitance-generating device of 2 pF 49.59 57.3 As is clearly seen from Table 1, it has been validated that if a dielectric resin is disposed as the capacitance-generating device, the impedance at the center frequency can be adjusted, and accordingly, the bandwidth can also be regulated. Alternatively, if a capacitor is used as the capacitance-generating device, only a small area is required for mounting the capacitor on the board, which makes it possible to select a desired capacitance. The costs of the apparatus can also be reduced. On the other hand, if an open stub is employed as the capacitance-generating device, the capacitance of the open stub is adjustable by varying the area of the extending portion from the microstrip line, thereby obtaining a desired impedance. In this manner, according to the above-described embodiments, the provision of the capacitance-generating device makes it possible to obtain a desired impedance at the center frequency, whereby a desired bandwidth can also be acquired. The foregoing embodiments have been explained in which the substrate of the chip antenna is formed of a dielectric material comprising mainly titanium oxide and barium oxide. However, this is not exclusive, and the substrate of the chip antenna may be formed of a dielectric material comprising mainly aluminum oxide or silica, a magnetic material essentially comprising nickel, cobalt and iron, or a combination of a dielectric material and a magnetic material. Moreover, although in the above-described embodiments a spiral conductor is formed within the substrate of the chip antenna, it may be formed on at least one of the surface portion and the inside portion of the substrate. Alternatively, a meander-like conductor may be formed on at least one of the surface portion and the inside portion of the substrate. Additionally, the position of the feeding terminal defined in the above-described embodiments is not essential to carry out the present invention. As is clearly understood from the foregoing description, the present invention offers the following advantages. The capacitance-generating device is disposed between the feeding terminal of the chip antenna and the microstrip line formed on the mounting board so as to obtain a desired impedance at the center frequency, thereby acquiring a desired bandwidth.

Claims: An antenna apparatus (10) comprising: a chip antenna (11) having a substrate (15), at least one conductor (16) formed on at least one of a surface portion and an inside portion of said substrate (15), and at least one feeding terminal (17) disposed on the surface of said substrate (15) so as to apply a voltage to said conductor (16); and a mounting board (12) including a ground pattern (13), said chip antenna (11) being mounted on said board (12); characterized by a capacitance-generating device (19) comprising a dielectric resin and being disposed between the feeding terminal (17) of said chip antenna (11) and the ground pattern (13) of said mounting board (12). An antenna apparatus (10) according to claim 1, wherein the capacitance-generating device (19) has a capacitance for adjusting an impedance of the antenna apparatus. An antenna apparatus (10) according to claim 1or 2, wherein the capacitance-generating device (19) has a capacitance for adjusting a desired bandwidth of the antenna apparatus. An antenna apparatus (10) according to one of claims 1 to 3, wherein the conductor (16) is spiral shaped. An antenna apparatus (10) according to one of claims 1 to 3, wherein the conductor (16) is a meander-like conductor. An antenna apparatus (10) according to one of claims 1 to 5, wherein the substrate (15) comprises one of a dielectric material, a magnetic material and a combination of a dielectric material and a magnetic material.

Inventors: ASAKURA KENJI, KANBA SEIJI, MANDAI HARUFUMI, TSURU TERUHISA, ASAKURA, KENJI, KANBA, SEIJI, MANDAI, HARUFUMI, TSURU, TERUHISA, Asakura, Kenji, Murata Manufact. Co., Ltd., Kanba, Seiji, Murata Manufact. Co., Ltd., Mandai, Harufumi, Murata Manufact. Co., Ltd., Tsuru, Teruhisa, Murata Manufact. Co., Ltd.
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1996
Main classification: H01Q1
IPC codes: H01Q1, H05K1, H01P5, H05K3