Antenna

Abstract: An antenna (10; 20; 30; 50) includes a spirally wound conductor (11; 21; 31; 51) composed of a copper wire or a covered copper wire, and a covering material (12; 23; 32; 52) consisting essentially of a resin or a resin mixture with 1 < ε ≤ 10. At least part of the conductor is covered with the covering material. One end of the conductor leads to the outside of the covering material to form an external terminal (33; 53). Another end of the conductor forms a free end (14; 54) in the covering material. In addition, a method for adjusting the resonant frequency of the antenna includes at least the step of changing a winding interval of a part of the conductor which is not covered with the covering material or the step of covering with the covering material a part of the conductor which is not covered with the covering material, or both. Furthermore, the antenna preferably satisfies the following numerical expression: 1.3 ≤ l/a·n ≤ 4 where l represents the coil length of said conductor; a represents the diameter of said conductor; and n represents the number of turns of said conductor.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to antennas and resonant-frequency-adjustment methods therefor, and in particular, to an antenna and a resonant-frequency-adjustment method therefor which are for use in a portable radio. 2. Description of the Related Art Whip antennas are conventionally used for portable radio devices such as liquid-crystal televisions (90 to 800 MHz) and FM radios (75 to 90 MHz) (88 to 108 MHz in the U.S.) (hereinafter referred to generally as "portable radios"). Loop antennas are used for pagers. A conventional whip antenna must be extended for use. In a frequency band equal to or less than 1 GHz, the length of the whip antenna needs to be 7.5 cm or longer, which makes it unstable when setting up a portable radio, and presents a problem when the radio falls. JP-A-59017705 discloses a multi-layer antenna having a coil pattern consisting of a plurality of mutually overlapping coil portions printed on the respective layers of a multilayered laminate. WO-A1-9300721 discloses a planar zig-zag antenna comprising a strip conductor arrangement on the front side and on the back side of a dielectric substrate. The dielectric constant of the dielectric substrate is greater than 10, preferably about 25. The present invention is based on the object of providing a small-sized antenna having a relatively broader relative bandwidth when referred to the resonant frequency thereof. This object is achieved by an antenna in accordance with claim 1. The antennas satisfy the following numerical expression: 1.3 ≤ l/a•n ≤ 4 where 1 represents the coil length of the conductor; a: the diameter of the conductor; and n: the number of turns of the conductor. In each antenna one end of the conductor may be connected to an input/output terminal formed on the surface of the covering material. According to a further aspect of the present invention, the foregoing object may be achieved through provision of a method for adjusting the resonant frequency of the antenna, in which the method comprises either the step of changing a winding interval for a part of the conductor which is not covered with the covering material, or the step of covering with a mixture of a resin and a filler a part of the conductor which is not covered with the covering material. According to the foregoing aspects of the present invention, a coiled conductor is covered with a covering material composed of a resin or a mixture of the resin and a filler, the covering material having a dielectric constant ε expressed as 1 < ε ≤ 10. The covering material has a wavelength shortening effect which can electrically shorten the coil length of the conductor. Accordingly, the desired characteristics of an antenna are satisfied, and compared with a conventional whip antenna, the antenna can be reduced in size to 1/9 or less of volume of a conventional whip antenna in a frequency band at or below 1 GHz, and can be built into the casing of a portable radio. By satisfying the relation 1.3 ≤ l/a•n ≤ 4, the characteristics of an antenna can be improved without enlarging the size of the antenna. Since a coiled conductor is wound on the surface of a base member, changes in the cross-sectional shape of the winding, taken perpendicular to the winding axis, and changes in its winding pitch, can be avoided. Therewith, undesirable changes in the antenna characteristics can be reduced. If the surface of a covering material is provided with an input/output terminal, the antenna can be easily surface-mounted. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view illustrating an antenna according to a first embodiment of the present invention. Fig. 2 is a perspective view illustrating an antenna according to a second embodiment of the present invention. Fig. 3 is a perspective view illustrating an antenna according to a third embodiment of the present invention. Figs. 4A, 4B and 4C are perspective views illustrating respective modifications of a base member included in the antenna shown in Fig. 2. Fig. 5 is a perspective view of an antenna according to a fourth embodiment of the present invention. DESCRIPTION OF EMBODIMENTS OF THE INVENTION Fig. 1 shows a perspective view of an antenna according to a first embodiment of the present invention. The antenna 10 includes a spirally wound conductor 11 composed of a copper wire or a covered copper wire, and a covering material 12 composed of a resin or a mixture of the resin and a filler. The whole conductor 11 is covered with the covering material 12. One end of the conductor 11 leads to the outside of the covering material 12 to form an external terminal 13. Another end of the conductor 11 forms a free end 14 inside the covering material 12. The following Table 1 shows the resonant frequency (f0) and relative bandwidth (BW/f0: bandwidth/resonant frequency) of the antenna 10 obtained when materials with a dielectric constant (ε) of 1 to 14 are used as the covering material 12. The materials used as the covering material 12 are a fluororesin (ε:2), an epoxy resin (ε:4), and a mixture (ε:6 to 14) of the epoxy resin and a filler chiefly composed of titanium. ε f0(MHz) BW/f0(%) 1 800 6.0 2 710 5.9 4 630 5.8 6 555 5.7 8 500 5.6 10 480 5.4 12 470 5.0 14 460 4.6 From the results of Table 1 it is understood that an antenna having the materials with dielectric constants of 1 to 10 exhibits a small change in the ratio between the relative bandwidth and the resonant frequency in accordance with a change in the resonant frequency. However, the materials having dielectric constants larger than 10 exhibit a large change in the ratio between the relative bandwidth and the resonant frequency in accordance with a change in the resonant frequency. Consequently, it is found that a resin or a mixture of the resin and a filler with 1 < dielectric constant ≤ 10 is suitable for the covering material 12. The dielectric constant = 1 represents a condition without the covering material 12, and is accordingly omitted. The reason why an increase in the dielectric constant increases a change in the ratio between relative bandwidth and the resonant frequency with respect to a change in the resonant frequency is that a capacitive component is added in parallel to the coiled conductor 11 included in the antenna 10, and the capacitive component and the inductive component of the conductor 11 constitute an antiresonant point narrowing the bandwidth. Accordingly, a suitable bandwidth for the antenna can be provided by adjusting the dielectric constant of the covering material 12. The following Table 2 shows the resonant frequency (f0) and relative bandwidth (BW/f0:bandwidth/resonant frequency) of the antenna 10 obtained when l/a•n (where l: the coil length of a conductor; a: the diameter of the conductor; and n; the number of turns of the conductor) is set from 1.1 to 6.0. Constant values are a: 0.3(mm), n: 22(turns) in this example. l l/a•n f0(MHz) BW/f0(%) 6.6 1.1 200 1.2 7.2 1.2 210 2.0 7.8 1.3 220 3.8 12 2.0 250 5.5 18 3.0 300 6.0 24 4.0 350 6.5 30 5.0 400 6.8 36 6.0 450 7.0 From the results in Table 2 it is understood that, when l/a•n decreases to less than 1.3, the relative bandwidth (BW/f0) decreases to sharply narrow the bandwidth. This reason is that an increase in the floating capacitance of the coiled conductor 12 causes the antiresonant point to approach the resonant point. In addition, when l/a•n increases to more than 4, there is little additional change in the ratio between the relative bandwidth and the resonant frequency. In other words, it is understood that, even if the coil length is increased by enlarging the shape of the antenna, it is difficult to improve the antenna characteristics any further. Specifically, when a comparison in size is made between a whip antenna having a resonant frequency of 47.2 MHz and the antenna 10 having the same frequency in which the conductor 11 is covered with the covering material 12 consisting essentially of a mixture of an epoxy resin having a dielectric constant of 6 and a filler chiefly composed of titanium, it is found that the whip antenna is approximately 158 cm long, while the antenna 10 is 5 mm wide, 8 mm deep, and 2.5 mm high, which is approximately 1/200 of volume of a conventional whip antenna. Also in a frequency bandwidth at or below 1 GHz in which the length of the whip antenna needs to be 7.5 cm or more, the antenna 10 is 1/9 or less in size. According to the first embodiment, by using a material with 1 < dielectric constant ≤ 10, the desired characteristics of an antenna are satisfied, and when compared with a conventional whip antenna, the size of the antenna can be reduced to 1/9 in a frequency band equal to or less than 1 GHz. Accordingly, the antenna can be built into the casing of a portable radio. In addition, by setting l/a•n in the range of 1.3 to 4, the characteristics of the antenna can be improved without enlarging the size of the antenna. For example, its bandwidth can be broadened. The mentioned advantages can be sufficiently obtained when the number n of turns is 5 to 100. Fig. 2 shows a perspective view of an antenna according to a second embodiment of the present invention. The antenna 20 includes: a base member 22 comprising a dielectric material chiefly composed of barium oxide, aluminum oxide and silica and having a wound conductor 21 composed of copper or a copper alloy on its surface; and a covering material 23 comprising a mixture of an epoxy resin and a filler chiefly composed of titanium. The entire conductor 21 and base member 22 are covered with the covering material 23. One end of the conductor 21 leads to the outside of the covering material 23 to form an external terminal 24. Another end of the conductor 21 forms a free end 25 inside the covering material 23. According to the second embodiment, spirally winding a conductor on the surface of a base member avoids inadvertent changes in the cross-sectional shape of the wound conductor, taken perpendicular to the winding axis, and in the winding pitch. Therewith, inadvertent changes in the antenna characteristics are avoided. Fig. 3 shows a perspective view of an antenna according to a third embodiment of the present invention. Compared with the antenna 10 according to the first embodiment, the antenna 30 differs in that one end of a conductor 31 leads to the surface of a covering material 32, and is connected to a signal input/output terminal 33 for connecting the conductor 31 with an external transmitter and/or receiver circuit. According to the third embodiment, the input/output terminal is formed on the surface of the covering material 32. Thus, the surface mounting of the antenna can be easily performed. In the first to third embodiments, a coiled conductor, or a conductor and a base member on which this conductor is spirally wound, is entirely covered with a covering material having a dielectric constant ε expressed as 1 < ε ≤ 10. However, the covering material may only partially cover the coiled conductor or the conductor and the base member on which the conductor is spirally wound. In such an arrangement, the resonant frequency of an antenna can be adjusted by changing a winding interval of a part of the conductor which is not covered with the covering material, or by covering with a resin, or a mixture of a resin and filler a part of the conductor which is not covered with the covering material, or both. Such an adjustment can be performed with the antenna mounted on a mounting board. Also, the first to third embodiments have described cases in which a filler chiefly composed of titanium is used as a filler included in a covering material. However, fillers which are chiefly composed of alumina, barium titanate and so forth may be used. In addition, cases in which one conductor is used have been described. However, a plurality of conductors arranged in parallel may be included. This enables an antenna to have a plurality of resonant frequencies in accordance with the number of conductors. Thus, one antenna can be used in a plurality of bands. The second embodiment has described a case in which a base member on which a conductor is wound is plate-shaped. However, in modified embodiments as shown in Fig. 4A and Fig. 4B, spaces 43 are formed in base members 41 and 42. When the space is formed in the base member, the dielectric constant of the inside of the base member decreases. Thus, an antiresonant point can be moved away from a resonant point, thereby reducing a corresponding decrease in the bandwidth. In addition, as shown in Fig. 4B and Fig. 4C, the surfaces of the base member 42 and a base member 44 may be provided with grooves 45. When grooves for winding the conductor are formed on the surface of a base member, the precision of the position where the conductor is wound can be improved. Thus, a change in antenna characteristics can be suppressed. Fig. 5 shows a perspective view of an antenna according to a fourth embodiment of the present invention. Compared with the antenna 10, 20 and 30 according to the first, second and third embodiments, the antenna 50 comprises a plurality of dielectric ceramic layers. Conductor 51 having a free end 54 and another end connected to a signal input/output terminal 53 is directly covered with a resin 52 or a mixture of a resin and a filler which has a dielectric constant ε where 1 < ε ≤ 10. It is desired that each connecting portion (not shown) of each dielectric ceramic layer is not coated with the resin or the mixture of the resin and the filler in order not to prevent adjacent the layers from electrically connecting each other.

Claims: An antenna (10; 30; 50) in which a coiled conductor (11; 31; 51) composed of a metallic wire is at least partially covered with a covering material (12; 32; 52) comprising a resin or a mixture of a resin and a filler having a dielectric constant ε where 1 < ε ≤ 10, wherein said antenna satisfies the following numerical expression: 1.3 ≤ l/a·n ≤ 4 where 1 represents the coil length of said conductor; a represents the diameter of said conductor; and n represents the number of turns of said conductor. An antenna (20) in which a base member (22; 41; 42; 44) composed of a dielectric material, having a conductor (21) wound on the surface thereof, is at least partially covered with a covering material (23) composed of a resin or a mixture of a resin and a filler having a dielectric constant ε where 1 ε ≤ 10. An antenna (10; 20; 30; 50) according to Claim 1 or 2, wherein the number n of said turns of said conductor is substantially 5 to 100. An antenna (30) according to one of Claims 1 to 3, wherein one end of said conductor (31) is connected to a terminal (33) formed on the surface of said covering material (32). An antenna according to Claim 2, wherein said base member has an aperture (43) which extends within said conductor (41; 42). An antenna according to Claim 2 or 5, wherein said base member (42; 44) has a groove (45) therein for defining a winding path of said member. An antenna (50) according to Claim 1, .wherein said antenna comprises a base having a plurality of dielectric ceramic layers.

Inventors: MANDAI HARUFUMI, TSURU TERUHISA, MANDAI, HARUFUMI, TSURU, TERUHISA
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1997
Main classification: H01Q1
IPC codes: H01Q9, H01Q1, H01Q11