Wide-band antenna

Technical Field

0001The present invention relates to an antenna used in radio communication including wireless LAN (Local Area Network). More particularly, it relates to a wideband antenna comprising a radiation electrode provided in a substantially conical concavity formed in one end face of a dielectric; and a ground conductor provided on the other end face of the dielectric.

0002Further particularly, the present invention relates to a wideband antenna wherein its inherent quality of wideband characteristics is sufficiently maintained and further size reduction is accomplished by dielectric loading. Especially, it relates to a wideband antenna wherein reduction in profile and width is accomplished regardless of the selection of dielectric.

Background Art

0003With the enhancement of speed of and the reduction in the price of wireless LAN systems, recently, the demand for them has significantly grown. Especially these days, the introduction of personal area network (PAN) has been widely considered to build a small-scale wireless network among a plurality of pieces of electronic equipment common around the house for information communication. For example, different radio communication systems have been defined using frequency bands, such as 2.4-GHz band and 5-GHz band, for which licenses from competent authorities are unnecessary.

0004In radio communication including wireless LAN, information is transmitted through antennas. For example, a monoconical antenna comprises a radiation electrode formed in a substantially conical concavity in a dielectric, and a ground electrode formed on the bottom face of the dielectric. Thus, a small antenna having relatively wideband characteristics can be constituted by the wavelength shortening effect from the dielectric positioned between the radiation electrode and the ground electrode.

0005An antenna having wideband characteristics can be used inUWB (Ultra-WideBand) communication wherein, for example, data is spread in as ultra-wide a frequency band as 3GHz to 10GHz for transmission and reception. A small antenna contributes to reduction in the size and weight of radio equipment.

0006For example, Japanese Unexamined Patent Publication No. <patcit id="pcit0001" dnum="JPHEI8139515B"><text>Hei 8 -139515 (1996</text></patcit>) discloses a small dielectric vertical polarization antenna for wireless LAN. This dielectric vertical polarization antenna is constituted as follows: one end face of a cylindrical dielectric is conically hollowed out, forming a concavity in the dielectric, a radiation electrode is formed therein, and an earth electrode is formed on the opposite side, at the other end face of the dielectric. The radiation electrode is drawn out to the earth electrode side through a conductor in a through hole. (Refer to <figref idref="f0001">FIG. 1</figref> in the Unexamined Patent Publication.)

0007<figref idref="f0005">FIG. 5</figref> in the Unexamined Patent Publication illustrates the antenna characteristics of this dielectric vertical polarization antenna. According to the figure, its operating band is approximately 100MHz. (The center frequency is approximately 2.5GHz; therefore, the relative bandwidth is approximately 4%.) The monoconical antenna has inherently an operating band not less than one octave; therefore, it cannot be said that the above antenna sufficiently delivers expected wideband characteristics.

0008The miniaturization of an antenna means reduction in, for example, its profile or width. For example, Japanese Unexamined Patent Publication No. <patcit id="pcit0002" dnum="JPHEI9153727B"><text>Hei 9 -153727 (1997</text></patcit>) presents a proposal with respect to reduction in the width of a monoconical antenna. However, the proposal is such that a radiation conductor should be simply formed in the shape of semi-elliptic solid of revolution, and whether it is applicable to the structure of an antenna whose side face is covered with dielectric without any modification is unknown.

0009<figref idref="f0025">FIG. 31</figref> schematically illustrates the general constitution of a monoconical antenna having a single conical radiation electrode. The monoconical antenna illustrated in the figure comprises a radiation conductor formed in substantially conical shape, and a ground conductor formed with a gap provided between it and the radiation conductor. Electrical signals are fed to the gap.

0010<figref idref="f0025">FIG. 32</figref> illustrates an example of the VSWR (Voltage Standing Wave Ratio) characteristics of a monoconical antenna. A VSWR not more than 2 is attained over a wide range from 4GHz to 9GHz, and this indicates that the antenna has a wide relative bandwidth.

0011One of known methods for further widening the band of this monoconical antenna is loading resistance on the radiation conductor. <figref idref="f0026">FIG. 33 and FIG. 34</figref> illustrate examples of the constitutions of monoconical antennas whose radiation conductor is formed of a low-conductivity member containing a resistance component, instead of high-conductivity metal. With this constitution, reflective power to a feeding portion is diminished, and this results in expanded matching band. Especially, since the lower limit frequency of the matching band is expanded (downward), the above constitutions are also utilized as means for the reduction of antenna size. As illustrated in <figref idref="f0026">FIG. 33</figref>, the radiation electrode may be formed of a material having a constant low conductivity. However, if the conductivity is distributed as illustrated in <figref idref="f0026">FIG. 34</figref> (lower conductivity on the upper base side), the effect is produced better.

0012Various methods are known for loading resistance on the radiation conductor of a monoconical antenna. Concrete examples include a method of sticking a low-conductivity member formed in sheet shape to a conical insulator, and a method of applying a low- conduc tivi ty member prepared as coating material. (Refer to "<nplcit id="ncit0001" npl-type="s"><text>Optimization of a Conical Antenna for Pulse Radiation: An Efficient Design Using Resistive Loading, written by James G. Maloney, et al. (IEEE Transactions on Antennas and Propagation, Vol. 41, No. 7, July, 1993, pp. 940-947</text></nplcit>), for example.)

0013However, if mass production is considered, the method of sticking a sheet is indeed inferior in productivity, and is not realistic. With the method of applying coating, it is difficult tomake the thickness of coating uniform to control conductivity, and this method is also unrealistic.

0014<nplcit id="ncit0002" npl-type="s"><text>NUSSEIBEH F ET AL: "TRANSIENT RESPONSE OF A WIDE-ANGLE CONE WTH DIELECTRIC LOADING" RADIO SCIENCE, AMERICAN GEOPHYSICAL UNION, WASHINGTON, DC., US, vol. 31, no. 5, September 1996 (1996-09), pages 1047-1052</text></nplcit>, discloses an antenna according to the preamble of claim 1.

0015Following two documents also disclose an antenna according to the preamble of claim 1: "<nplcit id="ncit0003" npl-type="s"><text>Dielectrically Loaded Biconical Antennas", Nusseibeh, Fouad Ahmed, Dissertation University of Connecticut, 1995, pages 1-93</text></nplcit> "<nplcit id="ncit0004" npl-type="s"><text>Biconical antennas with elliptical cross-sections", C. Ruan, 'Electronics Letters', Vol. 36, No. 16, 3rd August 2000, pages 1339-1340</text></nplcit>,

0016An object of the present invention is to provide an excellent monoconical antenna comprising a radiation electrode provided in a substantially conical concavity formed in one end face of a dielectric, and a ground conductor provided on the other end face of the dielectric.

0017Another object of the present invention to provide an excellent monoconical antenna wherein its inherent quality of wideband characteristics is sufficiently maintained and further size reduction is accomplished by dielectric loading.

0018A further object of the present invention is to provide an excellent monoconical antenna wherein reduction in profile and width is accomplished regardless of the selection of dielectric.

0019The present invention has been made with the above problems taken into account.

0020According to the present invention, the quality of wideband characteristics a monoconical antenna inherently has is sufficiently maintained and further size reduction is accomplished by dielectric loading.

0021From the result of several simulations, the present inventors found that: the half-cone angle value which optimizes the matching of a circular cone formed in one end face of a dielectric depends on the relative dielectric constant ε_r of the dielectric covered. An approximate expression is obtained by appropriately formulating an approximate expression and adjusting its coefficients.

0022The half-cone angle α of the substantially conical concavity is defined case by case as follows : in case of a circular cone, the angle is that formed between the central axis of the circular cone and its side face. In case of an elliptic cone or a pyramid, the angle is the average of the minimum angle and the maximum angle formed between the central axis and the side face.

0023The radiation electrode may be formed so that the substantially conical concavity is filled with it.

0024An aspect of the present invention is a monoconical antenna according to claim 1, comprising a.o : a substantially conical concavity formed in one end face of a dielectric; a radiation electrode provided on the surface of the concavity or a radiation electrode provided so that the concavity is filled with it; and a ground conductor provided in proximity to and substantially in parallel with the other end face of the dielectric opposite the one end face. The monoconical antenna is so constituted that electrical signals are fed to the part between the near vertex region of the radiation electrode and the region of the ground conductor.

0025The monoconical antenna is characterized in that: <ul id="ul0001" list-style="none"><li>the half-cone angle of the concavity varies stepwise or continuously that the half-cone angle of said concavity decreases from the base portion to the vertex portion, and the ratio of the height h of the concavity to the effective radius r of the base of the concavity is determined by the following expression which describes its relation relative dielectric constant ε_r of the dielectric: <maths id="math0001"><math display="block"><msup><mi mathvariant="bold-italic">tan</mi><mrow><mo mathvariant="bold-italic">-</mo><mn mathvariant="bold-italic">1</mn></mrow></msup><mfenced><mi mathvariant="bold-italic">r</mi><mo mathvariant="bold-italic">/</mo><mi mathvariant="bold-italic">h</mi></mfenced><mo mathvariant="bold-italic">></mo><mn mathvariant="bold-italic">0.8</mn><mo mathvariant="bold-italic">⋅</mo><msup><mi mathvariant="bold-italic">tan</mi><mrow><mo mathvariant="bold-italic">-</mo><mn mathvariant="bold-italic">1</mn></mrow></msup><mfenced><mn mathvariant="bold-italic">1.7</mn><mo mathvariant="bold-italic">/</mo><msub><mi mathvariant="bold-italic">ε</mi><mi mathvariant="bold-italic">r</mi></msub></mfenced><mo mathvariant="bold-italic">+</mo><mn mathvariant="bold-italic">13</mn><mfenced><mi>Unit of angle</mi><mo>:</mo><mspace width="1em" /><mi>degree</mi></mfenced><mn>.</mn></math><img file="EP1555719B1_D0001.tif" /></maths></li></ul>

0026However, "height of concavity" herein referred to is defined as the length of the segment of a perpendicular drawn from the vertex of the concavity to the base of the concavity. "Effective radius of base of concavity" is defined as the average distance between the center point, for which the point of intersection of the base of the concavity and the perpendicular is taken, and the outer envelope of the base. "Half-cone angle of concavity" is defined as the angle formed between a tangent of the side face of the concavity and the perpendicular.

0027The present inventors found that a setting of the half-cone angle of a monoconical antenna has great influence on impedance matching band. Then, the present inventors derived the following: the impedance matching band can be maximized by determining the half-cone angle α (angle formed between the central axis and the side face of a cone) of a conical concavity formed in one end face of a dielectric with a constant half-angle, by the following expression which describes its relation with relative dielectric constant ε_r: <maths id="math0002"><math display="block"><mi mathvariant="bold-italic">α</mi><mo mathvariant="bold-italic">=</mo><mn mathvariant="bold-italic">0.8</mn><mo mathvariant="bold-italic">⋅</mo><msup><mi mathvariant="bold-italic">tan</mi><mrow><mo mathvariant="bold-italic">-</mo><mn mathvariant="bold-italic">1</mn></mrow></msup><mfenced><mn mathvariant="bold-italic">1.7</mn><mo mathvariant="bold-italic">/</mo><msub><mi mathvariant="bold-italic">ε</mi><mi mathvariant="bold-italic">r</mi></msub></mfenced><mo mathvariant="bold-italic">+</mo><mn mathvariant="bold-italic">13</mn><mfenced><mi>Unit of angle</mi><mo>:</mo><mspace width="1em" /><mi>degree</mi></mfenced></math><img file="EP1555719B1_D0002.tif" /></maths>

0028That is, the optimum half-cone angle of a circular cone depends on the relative dielectric constant of the dielectric. In a monoconical antenna constituted based on the above expression, its side face is covered with a dielectric; therefore, the effect of miniaturization is inevitably produced. (This is caused by that the wavelength of the electromagnetic field produced between the radiation electrode and the ground conductor is shortened.) In packaging, therefore, a relative dielectric constant, that is, a dielectric is appropriately selected to meet requests for miniaturization, and then a half-cone angle of the circular cone is determined.

0029If a monoconical antenna is formed based only on such a constituting method, reduction in the size of the antenna can be accomplished by enhancing the relative dielectric constant ε_r of the dielectric. However, in conjunction with this, the half-cone angle α is also reduced (that is, the antenna becomes longer than is wide). Therefore, the height of the antenna is not extremely reduced. If it is desired that an antenna is extremely slenderly formed, the relative dielectric constant ε_r can be enhanced according to the above expression. As a matter of fact, however, dielectrics of various relative dielectric constants do not infinitely exist.

0030In short, the half-cone angle of a circular cone whose profile or width is reduced deviates from an optimum value which brings favorable impedance matching. To cope with this, the present invention is so constituted that it is compensated by stepping the half-cone angle.

0031A low-profile constitution is adopted according to claim 1. Therein, the half-cone angle of the concavity is varied stepwise or continuously so that it is reduced as it goes from the base portion to the vertex portion in accordance with the following expression. This expression describes the relation between the ratio of the height h of the concavity to the effective radius r of the base of the concavity and relative dielectric constant ε_r. <maths id="math0003"><math display="block"><msup><mi mathvariant="bold-italic">tan</mi><mrow><mo mathvariant="bold-italic">-</mo><mn mathvariant="bold-italic">1</mn></mrow></msup><mfenced><mi mathvariant="bold-italic">r</mi><mo mathvariant="bold-italic">/</mo><mi mathvariant="bold-italic">h</mi></mfenced><mo mathvariant="bold-italic">></mo><mn mathvariant="bold-italic">0.8</mn><mo mathvariant="bold-italic">⋅</mo><msup><mi mathvariant="bold-italic">tan</mi><mrow><mo mathvariant="bold-italic">-</mo><mn mathvariant="bold-italic">1</mn></mrow></msup><mfenced><mn mathvariant="bold-italic">1.7</mn><mo mathvariant="bold-italic">/</mo><msub><mi mathvariant="bold-italic">ε</mi><mi mathvariant="bold-italic">r</mi></msub></mfenced><mo>+</mo><mn mathvariant="bold-italic">13</mn><mfenced><mi>Unit of angle</mi><mo>:</mo><mspace width="1em" /><mi>degree</mi></mfenced></math><img file="EP1555719B1_D0003.tif" /></maths>

0032In a low-profile constitution two steps of half-cone angle are basically sufficient. Needles to add, the number of steps maybe increased to three or more.

0033However, the half-cone angle at the vertex portion of a radiation electrode must be less than 90 degrees. Further, it is preferable that variation in half-cone angle should be gentle in proximity to the vertex portion of a radiation electrode. It follows that an effort shouldbe made to maintain an equiangular circular cone in proximity to the vertex portion, that is, the feeding portion in accordance with Rumsey's Equiangular Theory. (<nplcit id="ncit0005" npl-type="b"><text>For Rumsey's Equiangular Theory, refer to "Frequency Independent Antenna," written by V. Rumsey (Academic Press, 1966</text></nplcit>)) . Care must be taken not to depart from the above principle. Otherwise, the ultra-wideband characteristics inherent in the monoconical antenna can be lost.

0034Here, the following constitution may be adopted: an electrode for feeding is formed over the above other end face, and the dielectric is penetrated. Thus, the radiation electrode and one end of the feeding electrode are electrically connected together in the near vertex region. Further, the other end of the feeding electrode may be formed so that it reaches the side face of the dielectric. In this case, electrical signals are fed to between the other end of the feeding electrode and the ground conductor. Therefore, a feeding portion structure suitable for mass production is obtained.

Brief Description of Drawings

0035<ul id="ul0002" list-style="none" compact="compact"><li><figref idref="f0001">FIG. 1</figref> is a drawing illustrating the appearance and constitution of an example of a monoconical antenna.</li><li><figref idref="f0002">FIG. 2</figref> is a drawing illustrating an example of computation (result of electromagnetic field simulation) of the frequency characteristics of the monoconical antenna based on the constitution of the example.</li><li><figref idref="f0003">FIG. 3</figref> is a drawing illustrating another example of computation (result of electromagnetic field simulation) of the frequency characteristics of the monoconical antenna based on the constitution according to the above example.</li><li><figref idref="f0004">FIG. 4</figref> is a drawing including charts and graphs illustrating half-cone angle versus frequency characteristics (right) and a graph plotted by an expression for setting half-cone angle according to the example (left). The figure illustrates the relation between them when the relative dielectric constant ε_r of the dielectric 10 is 1.</li><li><figref idref="f0005">FIG. 5</figref> is another drawing including charts and graphs illustrating half-cone angle versus frequency characteristics (right) and a graph plotted by the expression for setting half-cone angle according to the example (left). The figure illustrates the relation between them when the relative dielectric constant ε_r of the dielectric 10 is 3.</li><li><figref idref="f0006">FIG. 6</figref> is a further drawing including charts and graphs illustrating half-cone angle versus frequency characteristics (right) and a graph plotted by the expression for setting half-cone angle according to the example (left) The figure illustrates the relation between them when the relative dielectric constant ε_r of the dielectric 10 is 5.</li><li><figref idref="f0007">FIG. 7</figref> is a further drawing including charts and graphs illustrating half-cone angle versus frequency characteristics (right) and a graph plotted by the expression for setting half-cone angle according to the example (left). The figure illustrates the relation between them when the relative dielectric constant ε_r of the dielectric 10 is 8.</li><li><figref idref="f0008">FIG. 8</figref> is a drawing illustrating the constitutions of monoconical antennas so constituted that the half-cone angle α of the substantially conical concavity formed in one end face of a dielectric is in accordance with a predetermined rule corresponding to relative dielectric constant ε_r.</li><li><figref idref="f0009">FIG. 9</figref> is a drawing illustrating the antenna characteristics of a monoconical antenna with the optimum half-cone angle for the relative dielectric constant ε_r of 2 and 4, respectively.</li><li><figref idref="f0010">FIG. 10</figref> is a drawing illustrating an embodiment of a monoconical antenna whose profile is reduced as compared with the optimum half-cone angle constitution according to the present invention.</li><li><figref idref="f0010">FIG. 11</figref> is a drawing illustrating the VSWR characteristics of a monoconical antenna having the constitution illustrated in <figref idref="f0010">FIG. 10</figref>.</li><li><figref idref="f0011">FIG. 12</figref> is a drawing illustrating an example of a monoconical antenna whose width is reduced as compared with the optimum half-cone angle constitution.</li><li><figref idref="f0011">FIG. 13</figref> is a drawing illustrating the VSWR characteristics of a monoconical antenna having the constitution illustrated in <figref idref="f0011">FIG. 12</figref>.</li><li><figref idref="f0012">FIG. 14</figref> is a drawing illustrating an example of the constitution of a feeding portion structure suitable for mass production in a monoconical antenna according to the present invention.</li><li><figref idref="f0013">FIG. 15</figref> is a drawing illustrating how a monoconical antenna having the constitution illustrated in PIG. 14 is mounted on a circuit board.</li><li><figref idref="f0013">FIG. 16</figref> is a drawing illustrating the cross-sectional structure of a monoconical antenna using low-profile constitution.</li><li><figref idref="f0014">FIG. 17</figref> is the impedance characteristic diagram and VSWR characteristic diagram of the low-profile monoconical antenna illustrated in <figref idref="f0013">FIG. 16</figref>.</li><li>PIG. 18 is a drawing illustrating the cross-sectional structure of a low-profilemonoconical antenna wherein the vertex of the conical radiation electrode is set off the center by 25% with respect to radius.</li><li><figref idref="f0016">FIG. 19</figref> is the impedance characteristic diagram and VSWR characteristic diagram of the low-profile monoconical antenna illustrated in <figref idref="f0015">FIG. 18</figref>.</li><li><figref idref="f0017">FIG. 20</figref> is a drawing illustrating the constitution of a further monoconical antenna.</li><li><figref idref="f0018">FIG. 21</figref> is a drawing illustrating an example of computation for demonstrating the electrical effect of the monoconical antenna of <figref idref="f0017">Fig 20</figref>,</li><li><figref idref="f0019">FIG. 22</figref> is a drawing illustrating the constitutions of antennas wherein two electrode stripped portions are formed in the direction of the depth of the concavity formed in an insulator.</li><li><figref idref="f0020">FIG. 23</figref> is a drawing illustrating examples with the formation of the ground conductor on the other end face of the insulator. In these examples, resistive loading is applied to biconical antennas constituted by disposing radiation electrodes on the internal surfaces of substantially conical concavities symmetrically formed in both the end faces.</li><li><figref idref="f0021">FIG. 24</figref> is a drawing illustrating the cross-sectional structure of a further antenna.</li><li><figref idref="f0021">FIG. 25</figref> is a drawing illustrating the constitution of a conical antenna wherein two stripped and cut portions are formed in the direction of the depth of the substantially conical radiation electrode formed on an insulator.</li><li><figref idref="f0022">FIG. 26</figref> is a drawing illustrating examples of the constitutions of biconical antennas constituted using conical antennas which are formed by providing circumferential stripped and cut portions in the radiation electrodes formed on the surfaces of conical insulators.</li><li><figref idref="f0022">FIG. 27</figref> is a drawing illustrating the cross-sectional structure of a further conical antenna.</li><li><figref idref="f0023">FIG. 28</figref> is a drawing illustrating the cross-sectional structure of a modification to the conical antenna illustrated in <figref idref="f0022">FIG. 27</figref>.</li><li><figref idref="f0023">FIG. 29</figref> is a drawing illustrating the constitution of a biconical antenna constituted using a conical antenna which is formed by filling a low-conductivity member in the feeding electrode formed on the surfaces of the conical concavities in an insulator.</li><li><figref idref="f0024">FIG. 30</figref> is a drawing illustrating the cross-sectional structure of a modification to the conical antenna illustrated in <figref idref="f0023">FIG. 29</figref>.</li><li><figref idref="f0025">FIG. 31</figref> is a drawing illustrating the constitution (conventional example) of a monoconical antenna having a single conical radiation electrode.</li><li><figref idref="f0025">FIG. 32</figref> is a drawing illustrating an example (conventional example) of the VSWR (Voltage Standing Wave Ratio) characteristics of a monoconical antenna.</li><li><figref idref="f0026">FIG. 33</figref> is a drawing illustrating the constitution (conventional example) of a monoconical antenna wherein a radiation conductor is constituted of a low-conductivitymember containing a resistance component in place of high-conductivity metal.</li><li><figref idref="f0026">FIG. 34</figref> is a drawing illustrating the constitution (conventional example) of a monoconical antenna wherein a radiation conductor is constituted of a non-uniform low-conductivity member containing a resistance component in place of high-conductivity metal.</li></ul>

0036Referring to the drawings, examples and an embodiment of the present invention will be described in detail below.

0037<figref idref="f0001">FIG. 1</figref> illustrates the appearance and constitution of an example of a monoconical antenna 1.

0038As illustrated in the figure, the monoconical antenna 1 comprises: a substantially conical concavity 11 with a base (the upper part of the concavity) and a vertex, formed in one end face of a dielectric cylinder 10; a radiation electrode 12 provided on the surface of the concavity; and a ground conductor 13 which is provided in proximity to and substantially in parallel with the other end face opposite the one end face of the dielectric 10. The monoconical antenna 1 is so constituted that electrical signals are fed to between the near vertex region 14 of the radiation electrode 12 and the region of the ground conductor 13.

0039With respect to the half-cone angle α (angle between the central axis and the side face of the cone) of the substantially conical concavity 11 formed in the one end face of the dielectric 10, the monoconical antenna 1 is constituted as follows: the half-cone angle α is determined by a predetermined rule according to relative dielectric constant ε_r. The rule is, for example, as follows: <ol id="ol0001" compact="compact"><li>(1) If the monoconical antenna 1 is covered with a dielectric with the relative dielectric constant ε_r = 2, the monoconical antenna 1 is so constituted that the half-cone angle is approximately 45 degrees.</li><li>(2) If the monoconical antenna 1 is covered with a dielectric with the relative dielectric constant ε_r = 3, the monoconical antenna 1 is so constituted that the half-cone angle is approximately 37 degrees.</li><li>(3) If the monoconical antenna 1 is covered with a dielectric with the relative dielectric constant ε_r = 5, the monoconical antenna 1 is so constituted that the half-cone angle is approximately 28 degrees.</li><li>(4) If the monoconical antenna 1 is covered with a dielectric with the relative dielectric constant ε_r = 8, the monoconical antenna 1 is so constituted that the half-cone angle is approximately 23 degrees.</li></ol>

0040The rule on which the above constitution of the monoconical antenna 1 is based is Expression (1) below. Expression (1) describes the relation between the half-cone angle α of the conical concavity 11 formed in one end face of the dielectric 10 and relative dielectric constant ε_r. <maths id="math0004" num="(1)"><math display="block"><mi mathvariant="bold-italic">α</mi><mo mathvariant="bold-italic">=</mo><mn mathvariant="bold-italic">0.8</mn><mo mathvariant="bold-italic">⋅</mo><msup><mi mathvariant="bold-italic">tan</mi><mrow><mo mathvariant="bold-italic">-</mo><mn mathvariant="bold-italic">1</mn></mrow></msup><mfenced><mn mathvariant="bold-italic">1.7</mn><mo mathvariant="bold-italic">/</mo><msub><mi mathvariant="bold-italic">ε</mi><mi mathvariant="bold-italic">r</mi></msub></mfenced><mo mathvariant="bold-italic">+</mo><mn mathvariant="bold-italic">13</mn><mfenced><mi>Unit of angle</mi><mo>:</mo><mspace width="1em" /><mi>degree</mi></mfenced></math><img file="EP1555719B1_D0004.tif" /></maths>

0041The effective range of half-cone angle setting is between the value given by Expression (1) above plus several degrees and minus several degrees. Any value within this range does not pose a problem in practical use.

0042With the above-mentioned constitution of monoconical antenna, the bandwidth of an antenna is dramatically enhanced.

0043- <figref idref="f0002">FIG. 2</figref> and <figref idref="f0003">FIG. 3</figref> illustrate examples of computations of the frequency characteristics of a monoconical antenna according to this example (the results of electromagnetic field simulations). <figref idref="f0002">FIG.2</figref> illustrates the frequency characteristics in the form of Smith chart (center: 50Ω) and VSWR characteristic diagram which frequency characteristics are measured when the relative dielectric constant ε_r is 3 and the half-cone angle is 40 degrees. <figref idref="f0003">FIG. 3</figref> illustrates themmeasuredwhen the relative dielectric constant ε_r is 8 and the half-cone angle is 22 degrees.

0044In either example of constitution, the antenna has spiral characteristics in proximity to the center of the Smith chart, and obtains favorable frequency characteristics. It is said that an antenna has favorable antenna characteristics in the frequency domain if the VSWR is not more than 2. In either example of constitution, the relative bandwidth with VSWR ≤ 2 accounts for nearly 100%. It is apparent that the bandwidth is dramatically enhanced as compared with examples of characteristics presented in Japanese Unexamined Patent Publication No. <patcit id="pcit0003" dnum="JPHEI8139515B"><text>Hei 8 -139515 (1996</text></patcit>).

0045With respect to the method for constituting this monoconical antenna, the shape of the concavity 11 formed in one end face of the dielectric 10 is not limited to circular cone. Even if it is formed in the shape of elliptic cone or pyramid, the effect of the present invention is equally produced. If pyramidal concavity is used, the definition of its half-cone angle α is as follows: the average of the minimum angle and the maximum angle among angles formed between the central axis and the side face.

0046There is no special limitation on the outside shape of the dielectric cylinder 10 as well. Basically, any shape, including circular cylinder and prism, is acceptable as long as the radiation electrode is covered with it. The radiation electrode may be formed by filling it in the conical concavity 11, instead of forming it on the surface of the concavity 11.

0047The effective range of the relative dielectric constant ε_r of the dielectric 10 is up to 10 or so.

0048The present inventors carried out electromagnetic field simulations and approximately derived Expression (1) above, on which a setting of the half-cone angle α of the circular cone formed in the one end face of the dielectric is based. From the results of several simulations, the present inventors found the following: as illustrated in <figref idref="f0004 f0005 f0006 f0007">FIG. 4 to FIG. 7</figref>, the half-cone angle value which brings optimum matching of the circular cone formed in one end face of a dielectric depends on the relative dielectric constant ε_r of the dielectric covered. An approximated curve significant from the viewpoint of design is obtained by approximately formulating an approximate expression and adjusting its coefficients. With respect to <figref idref="f0004 f0005 f0006 f0007">FIG. 4 to FIG. 7</figref>, additional description will be given below.

0049<figref idref="f0004">FIG. 4</figref> includes charts and graphs illustrating half-cone angle versus frequency characteristics (right) and a graph plotting the half-cone angle based on the expression for setting according to the present example (left). (The right charts and graphs illustrate three cases: case where the half-cone angle is 58 degrees, case where the half-cone angle is 40 degrees, and case where the half-cone angle is 24 degrees, from above.) The figure illustrates the relation between them when the relative dielectric constant ε_r of the dielectric 10 is 1. The frequency characteristic diagrams comprise Smith chart and VSWR characteristic diagram.

0050From the frequency characteristic diagrams on the right of the figure, the following is evident: when the half-cone angle is approximately 58 degrees, the Smith chart has a spiral in proximity to the center, and the relative bandwidth with VSWR ≤ 2 is maximized. That is, the following is evident: the half-cone angle which brings optimum matching is 58 degrees, and further that half-cone angle value is very close to the line plotted by the expression for setting half-cone angle according to the present example.

0051<figref idref="f0005">FIG. 5</figref> includes charts and graphs illustrating half-cone angle versus frequency characteristics (right) and a graph plotting the half-cone angle based on the expression for setting according to the present example (left). (The right charts and graphs illustrate three cases: case where the half-cone angle is 58 degrees, case where the half-cone angle is 40 degrees, and case where the half-cone angle is 24 degrees, from above.) The figure illustrates the relation between them when the relative dielectric constant ε_r of the dielectric 10 is 3. The frequency characteristic diagrams comprise Smith chart and VSWR characteristic diagram.

0052From the frequency characteristic diagrams on the right of the figure, the following is evident: when the half-cone angle is approximately 40 degrees, the Smith chart has a spiral in proximity to the center, and the relative bandwidth with VSWR ≤ 2 is maximized. That is, the following is evident: the half-cone angle which brings optimum matching is 40 degrees, and further that half-cone angle value is very close to the line plotted by the expression for setting half-cone angle according to this example.

0053<figref idref="f0006">FIG. 6</figref> includes charts and graphs illustrating half-cone angle versus frequency characteristics (right) and a graph plotting the half-cone angle based on the expression for setting according to the present example (left). (The right charts and graphs illustrate three cases: case where the half-cone angle is 40 degrees, case where the half-cone angle is 26 degrees, and case where the half-cone angle is 15 degrees, from above.) The figure illustrates the relation between them when the relative dielectric constant ε_r of the dielectric 10 is 5. The frequency characteristic diagrams comprise Smith chart and VSWR characteristic diagram.

0054From the frequency characteristic diagrams on the right of the figure, the following is evident: when the half-cone angle is approximately 26 degrees, the Smith chart has a spiral in proximity to the center, and the relative bandwidth with VSWR ≤ 2 is maximized. That is, the following is evident: the half-cone angle which brings optimum matching is 26 degrees, and further that half-cone angle value is very close to the line plotted by the expression for setting half-cone angle according to the present example.

0055<figref idref="f0007">FIG. 7</figref> includes charts and graphs illustrating half-cone angle versus frequency characteristics (right) and a graph plotting the half-cone angle based on the expression for setting according to the present example (left). (The right charts and graphs illustrate three cases: case where the half-cone angle is 36 degrees, case where the half-cone angle is 22 degrees, and case where the half-cone angle is 10 degrees, from above.) The figure illustrates the relation between them when the relative dielectric constant ε_r of the dielectric 10 is 8. The frequency characteristic diagrams comprise Smith chart and VSWR characteristic diagram.

0056From the frequency characteristic diagrams on the right of the figure, the following is evident: when the half-cone angle is approximately 22 degrees, the Smith chart has a spiral in proximity to the center, and the relative bandwidth with VSWR ≤ 2 is maximized. That is, the following is evident: the half-cone angle which brings optimum matching is 22 degrees, and further that half-cone angle value is very close to the line plotted by the expression for setting half-cone angle according to this example.

0057The monoconical antenna comprises a substantially conical concavity formed in one end face of a dielectric cylinder; a radiation electrode provided on the surface of the concavity (or provided so that the concavity is filled with it); and a ground conductor provided in proximity to and substantially in parallel with the other end face opposite the one end face of the dielectric. The monoconical antenna is so constituted that electrical signals are fed to between the near vertex region of the radiation electrode and the region of the ground conductor. The monoconical antenna can be constituted as a small antenna having relatively wideband characteristics because of the wavelength shorting effect from the dielectric positioned between the radiation electrode and the ground electrode.

0058The present inventors found that a setting of the half-cone angle of a monoconical antenna has great influence on impedance matching band. Then, the present inventors derived the following: the impedance matching band can be maximized by determining the half-cone angle α (angle formed between the central axis and the side face of a cone) of a conical concavity formed in one end face of a dielectric by said expression (1) which describes its relation with relative dielectric constant ε_r: <maths id="math0005" num="(1)"><math display="block"><mi mathvariant="bold-italic">α</mi><mo mathvariant="bold-italic">=</mo><mn mathvariant="bold-italic">0.8</mn><mo mathvariant="bold-italic">⋅</mo><msup><mi mathvariant="bold-italic">tan</mi><mrow><mo mathvariant="bold-italic">-</mo><mn mathvariant="bold-italic">1</mn></mrow></msup><mfenced><mn mathvariant="bold-italic">1.7</mn><mo mathvariant="bold-italic">/</mo><msub><mi mathvariant="bold-italic">ε</mi><mi mathvariant="bold-italic">r</mi></msub></mfenced><mo mathvariant="bold-italic">+</mo><mn mathvariant="bold-italic">13</mn><mfenced><mi>Unit of angle</mi><mo>:</mo><mspace width="1em" /><mi>degree</mi></mfenced></math><img file="EP1555719B1_D0005.tif" /></maths>

0059That is, the optimum half-cone angle of a circular cone As illustrated in the examples of <figref idref="f0008">FIG. 8</figref>, showing a slightly different shape of the dielectric, the optimum half-cone angle is 48 degrees when the relative dielectric constant ε_r is 2, and 31 degrees when the relative dielectric constant ε_r is 4. <figref idref="f0009">FIG. 9</figref> illustrates the antenna characteristics of a monoconical antenna with an optimum half-cone angle for the relative dielectric constant ε_r of 2 and 4, respectively. However, the figure represents the antenna characteristics by VSWR characteristics. From <figref idref="f0009">FIG. 9</figref>, the following is evident: favorable impedance matching is obtained over an ultra-wide band by designing the monoconical antenna based on Expression (1) above which describes the relation between the relative dielectric constant ε_r and the optimum half-cone angle α of the concavity.

0060In the monoconical antenna constitutedbased on Expression (1) above, its side face is covered with a dielectric; therefore, the effect of miniaturization is inevitably produced. (This is caused by that the wavelength of the electromagnetic field produced between the radiation electrode and the ground conductor is shortened.) In packaging, therefore, a relative dielectric constant, that is, a dielectric is appropriately selected to meet requests for miniaturization, and then a half-cone angle of the circular cone is determined.

0061With the constitution of the monoconical antenna based on Expression (1) above, reduction in the size of the antenna can be accomplished by enhancing the relative dielectric constant ε_r of the dielectric. However, in conjunction with this, the half-cone angle α is also reduced (that is, the antenna becomes longer than is wide). Therefore, the height of the antenna is not extremely reduced. As a matter of fact, low profile is often requested.

0062Extremely slender constitution may be conversely desired sometimes. If a monoconical antenna is constituted according to Expression (1) above, this is accomplished by enhancing the relative dielectric constant ε_r. As a matter of fact, however, dielectrics of various relative dielectric constants do not infinitelyexist. Further, available dielectrics are naturally limited in terms of workability in electrode formation and cutting and heat resistance. Therefore, a desired slender constitution is quite likely to be difficult to implement.

0063The half-cone angle of a circular cone whose profile or width is reduced deviates from an optimum value which brings favorable impedance matching. To cope with this, it is compensated by stepping or continuously altering the half-cone angle.

0064More specific description will be given. If low-profile constitution is adopted, as in this embodiment, the half-cone angle is varied stepwise or continuously so that it is reduced as it goes from the base portion to the vertex portion. However, the ratio of the height h of the concavity to the effective radius r of the base of the concavity is set in accordance with the following expression (2) which describes its relation with relative dielectric constant ε_r. <maths id="math0006" num="(2)"><math display="block"><msup><mi mathvariant="bold-italic">tan</mi><mrow><mo mathvariant="bold-italic">-</mo><mn mathvariant="bold-italic">1</mn></mrow></msup><mfenced><mi mathvariant="italic">r</mi><mo mathvariant="italic">/</mo><mi mathvariant="italic">h</mi></mfenced><mo>></mo><mn mathvariant="bold-italic">0.8</mn><mo mathvariant="bold-italic">⋅</mo><msup><mi mathvariant="bold-italic">tan</mi><mrow><mo mathvariant="bold-italic">-</mo><mn mathvariant="bold-italic">1</mn></mrow></msup><mfenced><mn mathvariant="italic">1.7</mn><mo mathvariant="bold-italic">/</mo><msub><mi mathvariant="bold-italic">ε</mi><mi mathvariant="bold-italic">r</mi></msub></mfenced><mo mathvariant="bold-italic">+</mo><mn mathvariant="bold-italic">13</mn><mfenced><mi>Unit of angle</mi><mo>:</mo><mspace width="1em" /><mi>degree</mi></mfenced></math><img file="EP1555719B1_D0006.tif" /></maths>

0065If slender constitution is adopted, as an example, the half-cone angle is varied so that it is increased as it goes from the base portion to the vertex portion. However, the ratio of the height h of the concavity to the effective radius r of the base of the concavity is set in accordance with the following expression (3) which describes its relation with relative dielectric constant ε_r. <maths id="math0007" num="(3)"><math display="block"><msup><mi mathvariant="bold-italic">tan</mi><mrow><mo mathvariant="bold-italic">-</mo><mn mathvariant="bold-italic">1</mn></mrow></msup><mfenced><mi mathvariant="italic">r</mi><mo mathvariant="italic">/</mo><mi mathvariant="italic">h</mi></mfenced><mo>&lt;</mo><mn mathvariant="bold-italic">0.8</mn><mo mathvariant="bold-italic">⋅</mo><msup><mi mathvariant="bold-italic">tan</mi><mrow><mo mathvariant="bold-italic">-</mo><mn mathvariant="bold-italic">1</mn></mrow></msup><mfenced><mn mathvariant="italic">1.7</mn><mo mathvariant="bold-italic">/</mo><msub><mi mathvariant="bold-italic">ε</mi><mi mathvariant="bold-italic">r</mi></msub></mfenced><mo mathvariant="bold-italic">+</mo><mn mathvariant="bold-italic">13</mn><mfenced><mi>Unit of angle</mi><mo>:</mo><mspace width="1em" /><mi>degree</mi></mfenced></math><img file="EP1555719B1_D0007.tif" /></maths>

0066In either case of low-profile constitution as in this embodiment or slender constitution as in the example above, two steps of half-cone angle are basically sufficient. Needless to add, thenumberof steps maybe increased to three or more, or a portion where the half-cone angle is continuously varied may be present. However, the half-cone angle at the vertex portion of a radiation electrode must be less than 90 degrees. Further, it is preferable that variation in half-cone angle should be gentle in proximity to the vertex portion of a radiation electrode. It follows that an effort should be made to maintain an equiangular circular cone in proximity to the vertex portion, that is, the feeding portion in accordance with <nplcit id="ncit0006" npl-type="b"><text>Rumsey's Equiangular Theory. (For Rumsey's Equiangular Theory, refer to "Frequency Independent Antenna," written by V. Rumsey (Academic Press, 1966</text></nplcit>)) . Care must be taken not to depart from the above principle. Otherwise, the ultra-wideband characteristics inherent in the monoconical antenna can be lost.

0067<figref idref="f0010">FIG. 10</figref> illustrates an embodiment of a monoconical antenna whose profile is reduced as compared with optimum half-cone angle constitution according to the present invention. In the emb. illustrated in the figure, the profile is lower than in the optimum half-cone angle constitution. In this emb, a dielectric with a relative dielectric constant ε_r of 4 is selected; the height h of the circular cone is set to 6 mm; and the radius r of the base of the circular cone is set to 12.6 mm. Thus, as a natural consequence, the relation expressed by Expression (2) above holds.

0068As illustrated in the figure, further, two step constitution is adopted. With this constitution, the half-cone angle is stepped at a midpoint, and the half-cone angle value α₀ on the base side is set to 70 degrees with the half-cone angle value α₁ on the vertex side set to 45 degrees. Thus, the half-cone angle value on the vertex side is made smaller than that on the base side.

0069<figref idref="f0010">FIG. 11</figref> illustrates the result of a simulation conducted with respect to the VSWR characteristics of amonoconical antenna having the constitution illustrated in <figref idref="f0010">FIG. 10</figref>. As illustrated in the figure,favorable impedance matching is generally obtained, and a state in which the impedance matching is greatly lost and thus wideband characteristics are lost is avoided. If the combination of half-cone angle values is more finely adjusted, more favorable characteristics would be obtained.

0070<figref idref="f0011">FIG. 12</figref> illustrates an example of a monoconical antenna whose width is reduced as compared with optimum half-cone angle constitution. In the example illustrated in the figure, the width is smaller than the optimum half-cone angle constitution. In this example, a dielectric with a relative dielectric constant ε_r of 2 is selected; the height h of the circular cone is set to 17.4 mm; and the radius r of the base of the circular cone is set to 9 mm. Thus, as a natural consequence, the relation expressed by Expression (3) above holds.

0071As illustrated in the figure, further, two step constitution is adopted. With this constitution, the half-cone angle is stepped at a midpoint, and the half-cone angle value α₀ on the base side is set to 11 degrees with the half-cone angle value α₁ on the vertex side is set to 41 degrees. Thus, the half-cone angle value on the vertex side is made larger than that on the base side.

0072<figref idref="f0011">FIG. 13</figref> illustrates the result of a simulation conducted with respect to the VSWR characteristics of a monoconical antenna having the constitution illustrated in <figref idref="f0011">FIG. 12</figref>. As illustrated in the figure, favorable impedance matching is generally obtained.

0073<figref idref="f0012">FIG. 14</figref> illustrates an example of the constitution of a monoconical antenna provided with a feeding portion structure suitable for mass production.

0074In the example illustrated in the figure, a track-like feeding electrode is provided on the base of a dielectric, and the feeding electrode and a radiation electrode are electrically connected with each other through a hole made in the center of the bottom of the dielectric. As illustrated in the figure, this feeding electrode is so formed that its one end reaches the dielectric side face.

0075A ground conductor is also formed on the dielectric base. As illustrated in the figure, the ground conductor is so formed that it averts and encircles the feeding electrode. Further, the ground conductor is also so formed that it is extended to the dielectric side face.

0076The feeding electrode and ground conductor illustrated in <figref idref="f0012">FIG. 14</figref> can be easily formed on the surface of a dielectric by plating, for example. Therefore, use of such a monoconical antenna as illustrated in the figure makes it possible to follow a technique for so-called surface mounting when the antenna is mounted on a circuit board in mass production, and thus the manufacturing process is simplified.

0077As illustrated in <figref idref="f0013">FIG. 15</figref>, the body of the monoconical antenna can be fixed on and electrically connected with a circuit board only by soldering the electrodes on the dielectric side face to the electrodes on the circuit board from the surface side.

0078The ground conductor need not necessarily be formed on the base of a dielectric, and alternatively, a ground conductor may be formed on the circuit board on which the body of the antenna is to be mounted. In this case, for example, adhesive may be used to fix the body of the antenna.

0079The monoconical antenna according to the embodiment illustrated in <figref idref="f0010">FIG. 10</figref> is so constituted that: when an antenna is reduced in profile based on the optimum values of half-cone angle obtained by Expression (2) above, deviation of its half-cone angle from the optimum values is compensated. This compensation is carried out by stepping the half-cone angle, and this results in favorable impedance matching.

0080If the profile of an antenna is reduced, a problem arises. The half-cone angle of the cone deviates from the optimum value which brings favorable impedance matching. To cope with this, the vertex of the circular cone of the monoconical antenna is set off the center in this further example, and impedance matching is thereby compensated. In this case, the straight line connecting the vertex of the substantially conical radiation electrode and the center of the base of the cone is not perpendicular to the base of the cone.

0081An example will be taken. <figref idref="f0013">FIG. 16</figref> illustrates the cross-sectional structure of a monoconical antenna using low-profile constitution. In the example illustrated in the figure, the half-cone angle of the circular cone is 64.5 degrees, which differs from 31 degrees, the optimum value with ε_r = 4. As dielectric to be filled in the area between the radiation electrode and the ground conductor, a material with a relative dielectric constant ε_r of 4 is used. <figref idref="f0014">FIG. 17</figref> includes the impedance characteristic diagram and VSWR characteristic diagram of the low-profile monoconical antenna illustrated in <figref idref="f0013">FIG. 16</figref>. As is evident from the figure, the impedance greatly differs from 50 ohm, and the VSWR characteristics are impaired, especially, in high frequency domain.

0082Meanwhile, <figref idref="f0015">FIG. 18</figref> illustrates the cross-sectional structure of a low-profile monoconical antenna wherein the vertex of the conical radiation electrode is set off the center by 25% with respect to radius. In this case, as illustrated in the figure, the straight line connecting the vertex of the substantially conical radiation electrode and the base of the cone is not perpendicular to the base of the cone.

0083<figref idref="f0016">FIG. 19</figref> includes the impedance characteristic diagram and VSWR characteristic diagram of the low-profile monoconical antenna illustrated in <figref idref="f0015">FIG. 18</figref>. As is evident from the figure, the impedance characteristics are close to 50 ohm, and the VSWR characteristics are enhanced as well. Especially, it is important that the lower limit frequency of the matching band is lowered.

0084As mentioned above, it is apparent that if the impedance cannot matched in a monoconical antenna due to profile reduction or the like, setting the vertex of the cone off the center is effective as a means for enhancing its characteristics.

0085With respect to the method for constituting the monoconical antenna the shape of the concavity formed in one end face of the dielectric is not limited to circular cone. Even if it is formed in the shape of elliptic cone or pyramid, the effect of the present invention is equally produced.

0086If pyramidal concavity is used, the definition of its half-cone angle α is as follows: the average of the minimum angle and the maximum angle among angles formed between the central axis and the side face.

0087There is no special limitation on the outside shape of the dielectric cylinder as well. Basically, any shape, including circular cylinder and prism, is acceptable as long as the radiation electrode is covered with it. The radiation electrode may be formed by filling it in the conical concavity 11, instead of forming it on the surface of the concavity.

0088<figref idref="f0017">FIG. 20</figref> illustrates the constitution of an example of a monoconical antenna. The monoconical antenna comprises: an insulator; a substantially conical concavity provided in one end face of the insulator; a radiation electrode formed on the internal surface of the concavity; a stripped portion obtained by circumferentially stripping part of the radiation electrode; a low-conductivity member filled in the concavity to the level at which at least the stripped portion is buried; and a ground conductor provided in proximity to and substantially in parallel with the other end face of the insulator.

0089First, the substantially conical concavity is provided in the one end face of the insulator. The radiation electrode is formed on the internal surface of the concavity by plating or the like. Subsequently, part of the radiation electrode is circumferentially stripped by cutting or the like. Then, the low-conductivity member is filled to the level at which the stripped portion is buried. For the low-conductivity member, rubber or elastomer containing conductor is suitable. A desired conductivity is obtained with comparative ease by adjusting the conductor content. Further, the ground conductor is provided in proximity to and substantially in parallel with the other end face of the insulator. Needless to add, an electrode may be formed as ground conductor directly on the other end face of the insulator.

0090As in conventional monoconical antennas, electrical signals are fed to the gap between the radiation electrode and the ground conductor. If electrical signals are fed from the back face side of the ground conductor, the same constitution as conventional antennas may adopted. That is, a hole is made in the ground conductor, and the vertex region of the radiation electrode is extended to the back face side.

0091The antenna illustrated in <figref idref="f0017">FIG. 20</figref> basically functions as a monoconical antenna. By the way, no conductor is present on the upper base of the concavity; however, this does not become a cause of preventing the proper operation of the monoconical antenna. In addition, since the low-conductivity member exists between the two divided radiation electrodes, the electrical effect equivalent to resistive loading is produced. (<figref idref="f0017">FIG. 20</figref> is depicted so that the concavity is formed on the upper side of the insulator. However, there are not the conceptions of top and bottom because of the structure of conical antenna. In this specification, the end face provided with the concavity is designated as upper base for convenience in description. However, that does not limit the scope of the present example (The is the same with the following.))

0092<figref idref="f0018">FIG. 21</figref> illustrates an example of computation for demonstrating the electrical effect of the monoconical antenna according to this example. On the left of the figure is a VSWR characteristic diagram obtained when the electrode stripped portion is not formed, and on the right is that obtained when the stripped portion is formed. (The other conditions are completely identical.) The conditions for the computation will be briefly described below. As is evident from the figure, the formation of the electrode stripped portion brings the following advantages: the band wherein VSWR is not more than 2 is expanded to the low-frequency band; the matching property is improved; and band widening of the conical antenna is accomplished. <ol id="ol0002" compact="compact"><li>(1) Radiation electrode portion: it is assumed that a metal with a conductivity of 1×10⁷S/m is used. Upper base diameter: 12.6 mm, height: 12.6 mm.</li><li>(2) Low-conductivity member: it is assumed that a material with a conductivity of 2S/m is used.</li><li>(3) Insulator: it is assumed that a dielectric with a relative dielectric constant of 4 is used.</li></ol>

0093In the example of the constitution of conical antenna illustrated in <figref idref="f0017">FIG. 20</figref>, one circumferential stripped portion is formed in the radiation electrode formed on the internal surface of the concavity in the insulator. The number of the circumferential stripped portions is not limited to one. More specific description will be given. As mentioned above, the presence of the low-conductivity member between the radiation electrodes divided by the stripped portion produces the electrical effect equivalent to resistive loading. For this purpose, two or more circumferential stripped portions may be provided as required.

0094<figref idref="f0019">FIG. 22</figref> illustrates the constitutions of conical antennas wherein two electrode stripped portions are formed in the direction of the depth of the concavity formed in an insulator. In this case, the low-conductivity member in the concavity may be provided with multilayer structure as illustrated on the right side of the figure. The multilayer structure is such that low-conductivity members different in conductivity are filled level by level at which each electrode strippedportion is buried. At this time, the low-conductivity members are so distributed that the conductivity is lower on the upper base side. Thus, the effect of diminishing reflective power to the feeding portion is enhanced, and this results in expanded matching band.

0095The example is not limited to monoconical antenna, and is effective as a resistive loading method for biconical antenna. <figref idref="f0020">FIG. 23</figref> illustrates examples wherein the formation of the ground conductor on the other end face of the insulator. In these examples, the resistive loading is applied to biconical antennas formed by disposing radiation electrodes on the internal surfaces of substantially conical concavities symmetrically formed in both the end faces.

0096Each of the biconical antennas illustrated in the figure comprises: an insulator; a first substantially conical concavity formed in one end face of the insulator; a first radiation electrode formed on the internal surface of the first concavity; a first stripped portion obtained by circumferentially stripping part of the first radiation electrode; a first low-conductivity member filled in the concavity to the level at which at least the first stripped portion is buried; a second substantially conical concavity formed in the other end face of the insulator; a second radiation electrode formed on the internal surface of the second concavity; a second stripped portion obtained by circumferentially stripping part of the second radiation electrode; and a second low-conductivity member filled in the concavity to the level at which at least the second stripped portion is buried.

0097In the examples illustrated in <figref idref="f0020">FIG. 23</figref>, electrical signals are fed to the gap between both the radiation electrodes. For thispurpose, various methods canbeused. Forexample, parallel lines can be extended from the insulator side face and connected to the vertex regions of both the radiation electrodes. (This method is not shown in the figure.)

0098As described in connection with <figref idref="f0019">FIG. 22</figref>, the presence of the low-conductivity member between the radiation electrodes divided by the stripped portion produces the electrical effect equivalent to resistive loading. If the resistive loading is applied to a biconical antenna, this constitution can be similarly adopted. That is, for the above-mentioned purpose, two or more circumferential stripped portions may be provided in each of the upper and lower radiation electrodes as required. (Refer to the center of <figref idref="f0020">FIG. 23</figref>.)

0099As illustrated on the right side of <figref idref="f0020">FIG. 23</figref>, the low-conductivity members in the concavities may be provided with multilayer structure. The multilayer structure is such that the low-conductivity members different in conductivity are respectively filled to the level at which each electrode stripped portion is buried. At this time, the low-conductivity members are so distributed that the conductivity is lower on the base side. Thus, the effect of diminishing reflective power to the feeding portion is enhanced, and this results in expanded matching band.

0100<figref idref="f0021">FIG. 24</figref> illustrates the cross-sectional structure of a monoconical antenna which is a modification to the example of <figref idref="f0017">Fig. 20</figref>. The monoconical antenna illustrated in the figure comprises: an insulator formed in substantially conical shape; a radiation electrode formed on the surface of the substantially conical insulator; a circumferential slit portion which circumferentially divides part of the radiation electrode together with the insulator thereunder; a low-conductivity member filled in the circumferential slit portion; and a ground conductor provided in proximity to the near vertex region of the radiation electrode.

0101In the example illustrated in <figref idref="f0021">FIG. 24</figref>, the radiation electrode is first formed on the surface of the insulator formed in conical shape. The radiation electrode can be formed by plating or the like. Subsequently, part of the radiation electrode is circumferentially stripped and cut together with the insulator thereunder by cutting or the like. The thus obtained stripped and cut portion is filled with the low-conductivity member. For the low-conductivity member, rubber or elastomer containing conductor is suitable. A desired conductivity is obtained with comparative ease by adjusting the conductor content. Further, the ground conductor is provided in proximity to the vertex region of the radiation electrode.

0102With the constitution of monoconical antenna illustrated in <figref idref="f0021">FIG. 24</figref>, the presence of the low-conductivity member between the two divided radiation electrodes produces the electrical effect equivalent to resistive loading. (This is the same as the foregoing.)

0103Needless to add, a support for fixing the disposition of the ground conductor and the insulator is separately required though it is not shown in <figref idref="f0021">FIG. 24</figref>.

0104In the example of the constitution of a conical antenna illustrated in <figref idref="f0021">FIG. 24</figref>, the radiation electrode formed on the surface of the insulator is provided with only one circumferential stripped and cut portion. The example does not limit the number of the circumferential stripped and cut portions to one. More specific description will be given. As mentioned above, the presence of the low-conductivity member between the radiation electrodes divided by the stripped portion produces the electrical effect equivalent to resistive loading. For this purpose, two or more circumferential stripped and cut portions may be provide as required.

0105<figref idref="f0021">FIG. 25</figref> illustrates the constitution of a conical antenna wherein two stripped and cut portions are formed in the direction of the depth of the substantially conical radiation electrode formed on an insulator. In this case, low-conductivity members different in conductivity may be filled in the individual stripped and cut portions. At this time, the low-conductivity members are so distributed that the conductivity is lower on the base side of the insulator. Thus, the effect of diminishing reflective power to the feeding portion is enhanced, and this results in expanded matching band.

0106The example illustrated in <figref idref="f0021">FIG. 24</figref> is not limited to monoconical antenna, and is effective as a resistive loading method for biconical antenna. <figref idref="f0022">FIG. 26</figref> illustrates examples of the constitutions ofbiconical antennas using conical antennas which are formed by providing circumferential stripped and cut portions in the radiation electrodes formed on the surfaces of conical insulators.

0107Biconical antenna illustrated or the left of <figref idref="f0022">FIG. 26</figref> comprises a first insulator formed in substantially conical shape; a first radiation electrode formed on the surface of the substantially conical insulator; a first circumferential slit portion which circumferentially divides part of the first radiation electrode together with the insulator thereunder; a first low-conductivity member filled in the first circumferential slit portion; a second insulator formed in substantially conical shape whose vertex is opposed to that of the first insulator and whose base is symmetrical with that of the first insulator; a second radiation electrode formed on the surface of the substantially conical insulator; a second circumferential slit portion which circumferentially divides part of the second radiation electrode together with the insulator thereunder; and a second low-conductivity member filled in the second circumferential slit portion.

0108As illustrated in <figref idref="f0022">FIG. 26</figref>, the formation of the ground conductor on the other end face of each insulator in proximity to the near vertex region of the radiation electrode is omitted. The conical insulators are so disposed that their respective vertexes are opposed to each other and their respective bases are symmetrical with each other, and the radiation electrode is formed on the surface of each conical insulator. Part of each radiation electrode is circumferentially stripped and cut together with the insulator thereunder, and these stripped and cut portions are filled with the low-conductivity member. Needless to add, a support for fixing the disposition of the two conical antennas is required though it is not shown in the figure.

0109In the example illustrated in <figref idref="f0022">FIG. 26</figref>, electrical signals are fed to the gap between both the radiation electrodes. For this purpose, variousmethodscanbeused. For example, parallel lines can be extended from the insulator side face and connected to the vertex regions of both the radiation electrodes. (This method is not shown in the figure.)

0110As mentioned above, the presence of the low-conductivity member between the radiation electrodes divided by the stripped and cut portion produces the electrical effect equivalent to resistive loading. If the resistive loading according to the example illustrated in <figref idref="f0021">FIG. 24</figref> is applied to a biconical antenna, this constitution can be similarly adopted. For this purpose, as described in connection with <figref idref="f0021">FIG. 25</figref>, two or more circumferential stripped and cut portions may be provided in each of the upper and lower radiation electrode as required. (Refer to the right side of <figref idref="f0022">FIG. 26</figref>.)

0111As illustrated on the right side of <figref idref="f0022">FIG. 26</figref>, low-conductivity members different in conductivity may be filled in the two stripped and cut portions formed in the direction of the depth of the substantially conical radiation electrode formed on each of the upper and lower insulators. At this time, the low-conductivity members are so distributed that the conductivity is lower on the upper base side. Thus, the effect of diminishing reflective power to the feeding portion is enhanced, and this results in expanded matching band.

0112<figref idref="f0022">FIG. 27</figref> illustrates the cross-sectional structure of a monoconical antenna which is another modification to the example of <figref idref="f0017">Fig. 20</figref>. The monoconical antenna illustrated in the figure comprises: an insulator; a substantially conical concavity provided in one end face of the insulator; a feeding electrode formed on the surface of the near vertex region in the concavity; a low-conductivity member filled in the concavity; and a ground conductor provided in proximity to and substantially in parallel with the other end face of the insulator or formed directly on the other end face of the insulator.

0113In the example illustrated in the figure, the conical concavity is first formed in the surface of the insulator, and then the feeding electrode is formed on the internal surface of the concavity in proximity to its vertex. The feeding electrode can be formed by plating or the like. Subsequently, the concavity is filled with the low-conductivity member. For the low-conductivity member, rubber or elastomer containing conductor is suitable. A desired conductivity is obtained with comparative ease by adjusting the conductor content. Then, the ground conductor is provided in proximity to and substantially in parallel with the other end face of the insulator. Alternatively, the ground conductor may be formed directly on the other end face of the insulator.

0114With the constitution of monoconical antenna illustrated in <figref idref="f0022">FIG. 27</figref>, the low-conductivity member functions as a radiation conductor, and further the electrical effect equivalent to resistive loading is obtained. As illustrated in the figure, the area of the electrode is significantly reduced, and the cost can be accordingly reduced. Unlike the above-mentioned examples, the electrode stripping process is omitted, and the cost can be accordingly reduced.

0115Electrical signals are fed to the gap between the feeding electrode and the ground conductor. If electric signals are fed from the back face side of the ground conductor, such a constitution that a hole is made in the ground conductor and the vertex region of the concavity is extended to the back face side may be adopted.

0116<figref idref="f0023">FIG. 28</figref> illustrates a modification to the monoconical antenna illustrated in <figref idref="f0022">FIG. 27</figref>. As illustrated in <figref idref="f0023">FIG. 28</figref>, the low-conductivity member filled in the concavity may be provided with multilayer structure wherein members different in conductivity are respectively filled to individual predetermined levels. At this time, the low-conductivity members are so distributed that the conductivity is lower on the upper base side. Thus, the effect of diminishing reflective power to the feeding portion is enhanced, and this results in expanded matching band.

0117The example illustrated in <figref idref="f0022">FIG. 27</figref> is not limited to monoconical antenna, and is effective as a resistive loading method for biconical antenna. <figref idref="f0023">FIG. 29</figref> illustrates the cross-sectional structure of a biconical antenna constituted using conical antennas which are formed by filling a low-conductivity member in feeding electrodes formed on the surfaces of the conical concavities in an insulator.

0118In the biconical antenna illustrated in <figref idref="f0023">FIG. 29</figref>, the formation of the ground conductor on both the end faces of the insulator is omitted. The biconical antenna comprises: a first conical concavity and a second conical concavity symmetrically formed in both the end faces; a first feeding electrode formed on the surface of the near vertex region in the first concavity; a first low-conductivity member filled in the first concavity; a second feeding electrode formed on the surface of the near vertex region in the second concavity; and a second low-conductivity member filled in the second concavity.

0119With the constitution of biconical antenna illustrated in <figref idref="f0023">FIG. 29</figref>, the low-conductivity members function as radiation conductors, and further the electrical effect equivalent to resistive loading is obtained. As illustrated in the figure, the area of the electrodes is significantly reduced, and the cost can be accordingly reduced. Unlike the above-mentioned examples, the electrode stripping process is omitted, and the cost can be accordingly reduced.

0120In the example illustrated in <figref idref="f0023">FIG. 29</figref>, electrical signals are fed to the gap between the first and second feeding electrodes. For this purpose, various methods can be used. For example, parallel lines can be extended from the insulator side face and connected to the vertex regions of both the radiation electrodes. (This method is not shown in the figure.)

0121<figref idref="f0024">FIG. 30</figref> illustrates an modification to the biconical antenna illustrated in <figref idref="f0023">FIG. 29</figref>. As illustrated in <figref idref="f0024">FIG. 30</figref>, the low-conductivity member filled in each concavity may be provided with multilayer structure wherein members different in conductivity are respectively filled to individual predetermined levels. At this time, the low-conductivity members are so distributed that the conductivity is lower on the upper base side. Thus, the effect of diminishing reflective power to the feeding portion is enhanced, and this results in expanded matching band.

0122In the embodiment and examples mentioned above referring to the figures, the radiation electrode of the conical antenna is formed in conical shape. Even if the shape of the radiation electrode is substantially conical, as an elliptic cone or pyramid, the effect of the present invention is equally produced. There is no special limitation on the outside shape of the insulator cylinder, either and basically, any shape, including circular cylinder and prism, easy to handle may be adopted.

Industrial Applicability

0123According to the present invention, an excellent monoconical antenna wherein its inherent quality of wideband characteristics is sufficiently maintained and further size reduction is accomplished by dielectric loading can be provided.

0124Further, according to the present invention, the scope of application of a dielectric loading monoconical antenna can be dramatically expanded and thus the antenna can be brought into practical use, for example, as a small antenna forultra-wide band communication system.

0125Further, according to the present invention, an excellent monoconical antenna wherein reduction in profile is accomplished regardless of the selection of dielectric can be provided.

0126If the constituting methods according to the present invention are used when a monoconical antenna is reduced in size by dielectric loading, the quality of wideband characteristics inherent in the monoconical antenna can be sufficiently maintained. At the same time, the low-profile constitution can be adopted. The thus obtained antenna is useful, for example, as a small, low-profile antenna for ultra-wide band communication system.