Circularly polarized wave microstrip antenna and frequency adjusting method therefor

Abstract: In a circularly polarized wave microstrip antenna 1, a ground conductor 3 and a radiation conductor 2 are provided respectively on one surface and the other surface of a dielectric substrate 4 for feeding electric power to a feeding point P eccentrically provided on the radiation conductor 2. The radiation conductor 2 is provided with at least one of projections 21a through 21d for adjusting the axial ratio at a position of an angle of 45 × (2N + 1)° (N: Integer) with respect to a reference line passing through a center point O and the feeding point P on the periphery thereof and is provided with at least one of frequency adjusting projections 22a through 22d at a position of an angle of 90N° (N: Integer) with respect to the reference line.

Description: Background of the Invention Field of the Invention The present invention relates to a circularly polarized wave microstrip antenna where a dielectric substrate has a ground conductor on its one surface and a radiation conductor on its other surface, and to a frequency adjustment method therefor. Description of the Prior Art Conventionally, there has been known a circularly polarized wave microstrip antenna where a projection or a notch for generating a circularly polarized wave is formed at a specified position on the periphery of a radiation conductor for feeding electric power to a power feeding point eccentrically located on the radiation conductor as disclosed in the Japanese Patent Laid-open Publication (unexamined) 3-80603. Fig. 12 shows a conventional circularly polarized wave microstrip antenna. In the conventional circularly polarized wave microstrip antenna 7 shown in Fig. 12, a ground conductor (not shown) is provided on the entire part of one surface of a circular dielectric substrate 4, and a radiation conductor 8 is provided at a center position on the other surface of the substrate 4. With the above construction, there is fed an electric power from the ground conductor to a feeding point P located on the radiation conductor 8 by way of a coaxial cable (not shown), wherein the feeding point P is provided radially eccentrically to the center point O. The radiation conductor 8 is in a circular form and provided with rectangular projections 8a through 8d for radiating a circularly polarized wave at four peripheral portions where the radiation conductor 8 intersects two straight lines m and n which are at an angle of ± 45° with respect to a straight line M passing through the center point O and the feeding point P. It is conventionally known that, when the above-mentioned projections 8a through 8d are reduced in length, the axial ratio between the major axis and the minor axis of a circularly polarized wave microstrip antenna varies and the resonance frequency at which the axial ratio is minimum is made higher, and by taking advantage of the above-mentioned characteristics, adjustment of the axial ratio and the resonance frequency of the circularly polarized wave microstrip antenna 7 have been effected. In more detail, the resonance frequency of the circularly polarized wave microstrip antenna 7 is generally determined depending on the diameter R of the radiation conductor 8, the dielectric constant ε of the dielectric substrate 4, and the thickness t of the dielectric substrate 4. Therefore, by setting the above-mentioned three parameters so that the initial frequency (unadjusted resonance frequency) of the circularly polarized wave microstrip antenna 7 is made slightly lower than an intended frequency, and by abrading the aforesaid four projections 8a through 8d by the same amount to reduce the length Lt of each projection, the axial ratio is adjusted minimum and the resonance frequency at which the axial ratio is minimum is made gradually higher to achieve the intended resonance frequency. Although the above-mentioned conventional circularly polarized wave microstrip antenna 7 is capable of adjusting the resonance frequency to the intended frequency by gradually making higher the resonance frequency through abrading the projections 8a through 8d for generating a circularly polarized wave, since there is no adjustment section for making lower the resonance frequency, it is very difficult to adjust the resonance frequency by gradually making lower the resonance frequency. Therefore, when the projections 8a through 8d are excessively abraded to make the resonance frequency to be adjusted exceeding the intended frequency, the antenna cannot be adjusted any more to result in reducing the yield in the manufacturing process. Furthermore, since the axial ratio and the resonance frequency of the circularly polarized wave microstrip antenna are adjusted at the same time by abrading the aforesaid projections 8a through 8d, it is difficult to achieve a balanced adjustment between both the factors. Fig. 13 shows another conventional circularly polarized wave microstrip antenna which is similar to that of Fig. 12, and therefore, similar parts of Fig. 13 are designated by the same reference numerals as those of Fig. 12. In Fig. 13, a rectangular dielectric substrate 9 is used instead of using a circular one. The radiation conductor 8 is in a circular form having a radius R and provided with rectangular projections 81a and 81b on the periphery of the radiation conductor on a line M2 inclined at an angle of 45° with respect to a straight line M1 passing through the center point O and the power feeding point P and notches 82a and 82b formed on the periphery of the radiation conductor 8 on a line M3 inclined at an angle of -45° with respect to the straight line M1. The projections 81a and 81b as well as the notches 82a and 82b serve as mode degeneration separation elements for generating a circularly polarized wave, and by changing the length of each of the projections 81a and 81b and the depth of the notches 82a and 82b, the axial ratio between the major axis and the minor axis of the circularly polarized wave microstrip antenna is varied, also varying the resonance frequency at which the axial ratio is minimum. In more detail, when the length L1 of each of the projections 81a and 81b is reduced, the resonance frequency is made higher, or when the depth L2 of each of the notches 82a and 82b is increased, the resonance frequency is made lower. In view of the above fact, there has been conventionally proposed a method of adjusting the resonance frequency of the circularly polarized wave microstrip antenna by adjusting the length L1 of the projections 81a and 81b and the depth L2 of the notches 82a and 82b through abrading the projections 81a and 81b and the notches 82a and 82b. The above-mentioned conventional circularly polarized wave microstrip antenna 7 is required to adjust both the axial ratio and the resonance frequency of the circularly polarized wave at the same time by abrading the projections 81a and 81b and the notches 82a and 82b for generating a circularly polarized wave, and therefore it is difficult to adjust both the above-mentioned factors keeping a balance between the two. When the length L1 of each of the projections 81a and 81b and the length L2 of each of the notches 82a and 82b are changed, an influence is exerted, for example, on such characteristics as the input impedance and the directivity of the antenna, and therefore it is difficult to adjust only the frequency. Summary of the Invention The present invention was made in view of the problems mentioned above, and accordingly it is an essential object of the present invention to provide a circularly polarized wave microstrip antenna capable of adjusting the frequency without exerting any influence on the other characteristics such as the axial ratio and to provide a frequency adjustment method therefor. In order to give solution to the above-mentioned problems, according to a feature of the present invention, a circularly polarized wave microstrip antenna comprises a dielectric substrate which is provided with a ground conductor on one surface thereof and a radiation conductor on the other surface thereof, and the radiation conductor is further provided with an electric power feeding point located eccentrically on the radiation conductor, and is further provided with at least one projection or notch each for adjusting the axial ratio of the antenna at a position of an angle of 45 × (2N + 1)° (N: Integer) with respect to a reference line passing through the center point of the radiation conductor and the power feeding point on the periphery of the radiation conductor, and at least one frequency adjusting projection or notch at a position of an angle of 90N° (N: Integer) with respect to the above-mentioned reference line on the periphery of the radiation conductor. It is noted that a second power feeding point may be provided on the radiation conductor at a position locating on the line at an angle of 90° and 270° with respect to the reference line. It is noted that each frequency adjusting projection or notch at a position of an angle of 90N° may consists of a plurality of projection members and conductor-blank portions formed in proximity to the root portions of the frequency adjusting projections thereby to form a slit-like notch. According to the present invention, at least one projection or notch is formed at each of the above-mentioned specified positions on the periphery of the radiation conductor for adjusting the resonance frequency, and when the length of each projection or notch is changed, the resonance frequency can be varied without exerting any influence on the other characteristics such as the directivity and the input impedance. In other words, when the length of each projection is reduced, the resonance frequency is made higher, or when the length of each projection is increased, the resonance frequency is made lower. Therefore, in the circularly polarized wave microstrip antenna of the present invention, it is possible to gradually make higher the resonance frequency in adjustment by abrading each of the projections provided on the periphery of the radiation conductor portions by the same amount to reduce the length of each projection without exerting any influence on the other characteristics. Moreover, in the case where the notches are formed in place of the projections for varying the resonance frequency, when the notch length is reduced, the resonance frequency is made higher, and when the notch length is increased, the resonance frequency is made lower. Therefore, in the circularly polarized wave microstrip antenna of the present invention, it is possible to adjust the resonance frequency without exerting any influence on the other characteristics by abrading each of the notches formed on the periphery of the radiation conductor by the same amount to adjust the notch length. Therefore, the circularly polarized wave microstrip antenna is capable of gradually making higher or lower the resonance frequency in adjustment by abrading each projection or notch provided on the periphery of the radiation conductor by the same amount to thereby reduce the length of each projection or increase the length of each notch without exerting any influence on the other characteristics. On the other hand, in the case where the conductor-blank portion is provided for reducing the resonance frequency in adjustment, by abrading the radiation conductor circumferentially with the conductor-blank portion serving as a guide to form the same number of slits on the periphery of the four radiation conductor portions, the resonance frequency of the circularly polarized wave microstrip antenna can be reduced. Therefore, in the circularly polarized wave microstrip antenna of the present invention, the resonance frequency of the circularly polarized wave microstrip antenna is gradually made higher to achieve adjustment by abrading each of the projections provided on the four peripheral portions of the radiation conductor by the same amount to reduce the length of each projection, while the resonance frequency of the circularly polarized wave microstrip antenna is gradually made lower to achieve adjustment by abrading the radiation conductor circumferentially to form slit-like notches on the periphery of the four radiation conductor portions. Brief Description of the Drawings These and other objects and features of the present invention will become apparent from the following description taken in conjunction with the preferred embodiments thereof with reference to the accompanying drawings, in which: Fig. 1 is a plan view of a circularly polarized wave microstrip antenna in accordance with a first embodiment of the present invention; Fig. 2 is a section view of the antenna taken along the line A - A in Fig. 1; Fig. 3 is a view of the form of a radiation conductor of a circularly polarized wave microstrip antenna in accordance with a second embodiment of the present invention; Fig. 4 is a graph of the variation amount of the resonance frequency with respect to the length of each of the projections or notches; Fig. 5 is a view of the form of a radiation conductor of a circularly polarized wave microstrip antenna in accordance with a third embodiment of the present invention; Fig. 6 is a view of the form of a radiation conductor of a circularly polarized wave microstrip antenna in accordance with a fourth embodiment of the present invention; Fig. 7 is a plan view of a circularly polarized wave microstrip antenna in accordance with a fifth embodiment of the present invention; Fig. 8 is an enlarged view of a projection, a conductor-blank portion, both for frequency adjustment, and a projection for adjusting the axial ratio formed on the periphery of the radiation conductor in Fig. 7; Fig. 9 is a graph of a variation amount in frequency with respect to an abrading amount of a projection for frequency adjustment of Fig. 8; Fig. 10 is a graph of a variation amount in frequency with respect to notch length of Fig. 8; Fig. 11 is a view of a radiation conductor of a circularly polarized wave microstrip antenna in accordance with a sixth embodiment of the present invention; Fig. 12 is a plan view of an exemplified conventional circularly polarized wave microstrip antenna; and Fig. 13 is a plan view of another exemplified conventional circularly polarized wave microstrip antenna. Detailed Description of the Preferred Embodiments Before the description proceeds, it is noted that, since the basic structure of the preferred embodiments of a circularly polarized wave microstrip antenna is similar to those of the conventional ones, like parts are designated by the same reference numerals throughout the drawings. Figs. 1 and 2 show a circularly polarized wave microstrip antenna in accordance with a first embodiment of the present invention. In a circularly polarized wave microstrip antenna 1 shown in Figs. 1 and 2, a circular dielectric substrate 4 is provided with a ground conductor 3 on its entire lower surface and a circular radiation conductor 2 having a diameter R sufficiently shorter than the diameter D of the dielectric substrate 4 centrally on its upper surface, with the construction of which electric power feeding is effected by way of a coaxial cable 5 from the ground conductor 3 to a power feeding point P of the radiation conductor 2. The power feeding point P is located radially eccentrically to the center point O. The coaxial cable 5 has its outer conductor 5a connected to the ground conductor 3 and its inner conductor 5b connected to the radiation conductor 2 passing through the dielectric substrate 4. Rectangular projections 21a through 21d each having a width Wt and a length Lt are formed on the periphery of the radiation conductor 2 in a direction at an angle of 45 × (2N + 1)° (N: Integer) with respect to a radial direction passing through the center point O of the radiation conductor 2 and the power feeding point P, i.e., in the directions at angles of 45°, 135°, 225°, and 315°. It is noted that each of the projections 21a and 21c in the direction of 45° and 225° has a length Lt longer than the length of each of the projections 21b and 21d in the direction of 135° and 315°. The projections 21a through 21d are mode degeneration separation elements for radiating a circularly polarized wave. So long as at least one of the four peripheral portions of the radiation conductor 2 is provided with a projection, a circularly polarized wave can be generated. By varying the length Lt of the projections 21a through 21d, it is possible to vary the axial ratio (which is the ratio of the major axis to the minor axis of the circularly polarized wave) as well as to vary the resonance frequency at which the axial ratio is minimum. When the length Lt of each of the projections 21a through 21d is reduced, the resonance frequency at which the axial ratio is minimum is made higher. When the projection length Lt is increased, the resonance frequency is made lower. Therefore, by adjusting the length Lt of each of the projections 21a through 21d in a manner as described hereinafter, the ratio of the major axis to the minor axis of the circularly polarized wave microstrip antenna can be adjusted. It is noted that the projections 21a through 21d may be replaced with notches, and the axial ratio may be adjusted by adjusting the length of each of the notches in order to radiate a circularly polarized radio wave. In the case where notches are provided, contrary to the case of providing projections, the resonance frequency at which the axial ratio is minimum is made lower when the notch length is reduced, or the resonance frequency is made higher when the notch length is increased. Rectangular projections 22a through 22d each having a width W and a length L are provided in a direction at an angle of 90N° (N:Integer), i.e., in the directions at angles of 0°, 90°, 180°, and 270° on the periphery of the radiation conductor 2. The projections 22a through 22d serve as frequency adjusting sections for adjusting the resonance frequency of the circularly polarized wave microstrip antenna 1. When the length L of each of the projections 22a through 22d is increased, the resonance frequency can be made lower, or when the projection length L is reduced, the resonance frequency can be made higher. Therefore, by abrading the projections 22a through 22d provided at the four peripheral portions of the radiation conductor 2 by the same amount as described hereinafter to reduce the projection length L, the resonance frequency can be gradually made higher in adjustment without exerting any influence on such characteristics as the directivity, the input impedance, and the axial ratio of the circularly polarized wave of the circularly polarized wave microstrip antenna 1. It is noted in the present embodiment that, although the projections 22a through 22d for frequency adjustment are provided at the four peripheral portions of the radiation conductor 2, the projections 22a through 22d may be replaced with notches 23a through 23d each having a width d and a length (depth) S as shown in Fig. 3 of a second embodiment. Also, it is noted that each of the projections 22a through 22d may be replaced with slit-shaped projection groups as shown in Fig. 7 of a fifth embodiment. Referring to Fig. 3, in the case where the notches 23a through 23d are formed, the resonance frequency can be made lower when the length (depth) S of each of the notches 23a through 23d is increased, or made higher when the notch length S is reduced. Therefore, by abrading the notches 23a through 23d formed at the four peripheral portions of the radiation conductor 2 by the same amount to increase the notch depth S, the resonance frequency can be gradually made lower in adjustment without exerting any influence on the other characteristics of the circularly polarized wave microstrip antenna 1. Fig. 4 shows an experimental example of the variation amount of the resonance frequency with respect to the length L of each of the projections 22a through 22d and to the length S of each of the notches 23a through 23d. Referring to Fig. 4, a circularly polarized wave microstrip antenna 1 having a resonance frequency of about 1.575 GHz was subjected to an experiment, where the variation of the resonance frequency was examined by changing in length the projections 22a through 22d in the case of Fig. 1 (or changing in depth the notches 23a through 23d in the case of Fig. 3) formed at the four peripheral portions of the radiation conductor 2 by the same amount at the same time. In Fig. 4, the condition that each projection (or notch) has a length of O mm means the condition that any of the projections 22a through 22d (or notches 23a through 23d) is not formed, where the resonance frequency is represented by a reference value of O (MHz). The curve in Fig. 4 indicates the variation amount of the resonance frequency obtained by changing the length L of each of the projections 22a through 22d or the length S of each of the notches 23a through 23d with regard to the above-mentioned reference condition of the resonance frequency. The circularly polarized wave microstrip antenna 1 subjected to the experiment has a resonance frequency fo = 1.575 GHz and the following dimensions: Dielectric substrate 4 having: Dielectric constant ε = 21.4, Diameter D = 37 mm, Thickness t = 6 mm Circular radiation conductor 2 having: a diameter R = 20.6 mm Axial ratio adjusting projections 21a and 21c having: Width Wt = 1 mm, Length Lt = 2 mm Axial ratio adjusting projections 21b and 21d having: Width Wt = 1 mm, Length Lt = 1 mm Projections 22a through 22d having: Width W = 0.7 mm, Length L = 0 to 1 mm Notches 23a through 23d having: Width d = 0.7 mm, Length S = 0 to 1 mm. As obvious from Fig. 4, the resonance frequency varies in proportion to the length L of each of the projections 22a through 22d or in proportion to the length (depth) S of each of the notches 23a through 23d, and the rate of variation of the resonance frequency is about + 10 MHz/mm when the length L of each of the projections 22a through 22d is reduced, or about - 10 MHz/mm when the length S of each of the notches 23a through 23d is increased. Therefore, by abrading the projections 22a through 22d little by little to reduce the length L of each of the projections 22a through 22d or by abrading the notches 23a through 23d little by little to increase the depth S of each of the notches 23a through 23d, the resonance frequency can be made higher or lower in a unit of several megahertz to enable achieving a fine tuning of the frequency. The following describes frequency adjustment procedures of the circularly polarized wave microstrip antenna 1 provided with the radiation conductor 2 having the projections 22a through 22d mentioned above. The resonance frequency of the circularly polarized wave microstrip antenna 1 is determined depending principally on the parameters of the thickness t of the dielectric substrate 4, the dielectric constant ε of the dielectric substrate 4, and the diameter R of the radiation conductor 2. Therefore, the above-mentioned three parameters are designed to have appropriate values, and the initial value of the resonance frequency (unadjusted resonance frequency at which the dielectric substrate 4 provided with the radiation conductor 2 and the ground conductor 3 respectively on its upper and lower surfaces and the antenna has a minimum axial ratio) of the circularly polarized wave microstrip antenna 1 is made slightly lower than the intended value. For example, in the case shown in Fig. 4, the initial frequency is set at about 1.57 GHz. When the axial ratio of the circularly polarized wave microstrip antenna is out of the standard range, the axial ratio adjusting projections 21a through 21d are abraded by the same amount once or several times to adjust the axial ratio of the circularly polarized wave microstrip antenna within the standard range. Then, by abrading the frequency adjusting projections 22a through 22d by the same amount once or several times, the resonance frequency fo is gradually made higher to be adjusted to the intended frequency. For example, in the case shown in Fig. 4, the resonance frequency is adjusted to the intended frequency of 1.575 GHz. It is noted that, in the second embodiment, when the radiation conductor 2 is provided with the notches 23a through 23d as shown in Fig. 3 instead of providing the projections 22a through 22d shown in Fig. 1, by making the initial frequency slightly higher than the intended frequency on the contrary to the case of the first embodiment and by abrading the notches 23a through 23d to increase the depth S thereof by the same amount once or several times, the resonance frequency fo is gradually made lower to be adjusted to the intended frequency. Although, in the above-mentioned first and second embodiments, the radiation conductor 2 is provided with a single power feeding point P thereon in the circularly polarized wave microstrip antenna 1, the same effect can be obtained by providing a double power feeding points P1 and P2 on the radiation conductor 2 of a circularly polarized wave microstrip antenna 1. Fig. 5 shows a third embodiment of a radiation conductor 2 which is provided with two power feeding points P1 and P2 in the circularly polarized wave microstrip antenna 1, and which is provided with the frequency adjusting projections 22a through 22d. The first and second power feeding points P1 and P2 are eccentrically provided at appropriate portions of the radiation conductor 2 as located respectively on straight lines m and n which intersect each other at the center point O of the radiation conductor 2 having a circular form. The frequency adjusting projections 22a through 22c are provided at positions in the direction of angles of 0° and 180° with respect to a direction passing through the center point O and the first feeding point P1, while the projections 22b and 22d are provided at positions of angles of 0° and 180° with respect to the direction passing through the center point O and the second feeding point P2. Fig. 6 shows a fourth embodiment of a radiation conductor 2 of such a double-point feeding type circularly polarized wave microstrip antenna having the frequency adjusting notches 23a through 23d formed instead of providing the projections 22a through 22d on the radiation conductor 2 shown in Fig. 5. It is noted that, although each of the projections 22a through 22d or each of the notches 23a through 23d has one constituent member at the aforesaid specific positions on the periphery of the radiation conductor 2 in the third and fourth embodiments shown in Figs. 5 and 6, each of the projections 22a through 22d or the notches 23a through 23d may have two or more constituent members. The frequency adjusting projections 22a through 22d or notches 23a through 23d may be formed at the specific peripheral portions of a radiation conductor 2 having a rectangular or another arbitrary form other than the circular form. As described above, according to the first embodiment of the present invention, in a one-point feeding type circularly polarized wave microstrip antenna where a ground conductor and a radiation conductor are provided respectively on a lower surface and an upper surface of a dielectric substrate, since one or not fewer than two projections for frequency adjustment are formed in the direction at an angle of 90N° (N: Integer) with respect to the line passing through the center of the radiation conductor and the feeding point, by abrading the projections to reduce the projection length, the resonance frequency can be made higher in adjustment without exerting any influence on the other characteristics. According to the second embodiment of the present invention, since notches are formed instead of the projections for frequency adjustment, by abrading the notches to increase the notch length, the resonance frequency can be made lower in adjustment without exerting any influence on the other characteristics. According to the third and fourth embodiments of the present invention, there is provided a double-point feeding type circularly polarized wave microstrip antenna in which a ground conductor and a radiation conductor are disposed respectively on a lower surface and an upper surface of a dielectric substrate, and since one or not fewer than two frequency adjusting projections or notches are provided at positions of angles of 0° and 180° with respect to the direction passing through the center point and the first feeding point P1 and at positions of angles of 0° and 180° with respect to the direction passing through the center point and the second feeding point P2, the resonance frequency can be made higher or lower in adjustment without exerting any influence on the other characteristics in the same manner as described above. Figs. 7 and 8 show a fifth embodiment of a circularly polarized wave microstrip antenna in accordance with the present invention, which is similar to the first embodiment except providing projection groups 121a through 121d each consisting of, for example, five projection members for frequency adjustment instead of providing the projections 22a through 22d in Fig. 1 in a direction at an angle of 90N° (N:Integer), i.e., in the directions at angles of 0°, 90°, 180°, and 270° on the periphery of the radiation conductor 2. Moreover, in this fifth embodiment, in proximity to root portions of the projection groups 121a through 121d in the periphery of the radiation conductor 2, there are formed conductor-blank portions 122a through 122d each consisting of, for example, four holes for frequency adjustment. It is noted that each of the projection groups 121a through 121d may have at least one member or not fewer than five members, while each of the conductor-blank portions 122a through 122d may also have at least one hole or not fewer than four holes. Fig. 8 shows an enlarged view of the projection group 121a and conductor-blank portion 122a both for frequency adjustment formed in the direction at an angle of 0°, and the projection 123a for axial ratio adjustment formed in the direction at an angle of 315° on the periphery of the radiation conductor 2. Each of the five members of the projection group 121a has an appropriate width W' and length L' while radially projecting on the periphery of the radiation conductor 2 with appropriate intervals therebetween. Each of the four holes of the conductor-blank portion 122a is a circular hole having an appropriate diameter d' formed in the vicinity apart from the edge of the periphery of the radiation conductor 2 by a prescribed distance S' on a line passing through the interval portions of the projection 121a and the center point O. The four circular holes of the conductor-blank portion 122a are formed in the dielectric substrate 4 before the radiation conductor 2 is formed on the dielectric substrate 4, or after the radiation conductor 2 is formed on the dielectric substrate 4. It is noted that the conductor-blank portions 122a through 122d are made to serve as guides for forming a notched portion 124, and therefore they may have an arbitrary form such as circle, ellipse, or rectangle. The projection groups 121a through 121d are formed for making higher the resonance frequency in adjustment. Practically, by abrading the projection groups 121a through 121d (refer to the dotted portion of the projection group 121a in Fig. 8) to reduce the length L', the resonance frequency fo of the circularly polarized wave microstrip antenna 1 is made higher according to reduction of the length L'. Particularly when the projection groups 121a through 121d provided at the four peripheral portions of the radiation conductor 2 are abraded by the same amount, the resonance frequency fo can be made gradually higher without exerting any influence on the characteristics such as input impedance and axial ratio of the circularly polarized wave microstrip antenna 1. The conductor-blank portions 122a through 122d are formed for making lower the resonance frequency. Practically, by radially abrading the radiation conductor 2 with the conductor-blank portion 122a serving as a guide as shown in Fig. 8 to form a slit-like notched portion 124 on the periphery of the radiation conductor 2, the resonance frequency fo can be made lower according to increment of the number of the notches 124. Particularly by forming the same amount of the notches 124 at the four peripheral portions 122a through 122d of the radiation conductor 2, the resonance frequency can be made lower without exerting any influence on the characteristics such as input impedance and axial ratio of the circularly polarized wave microstrip antenna 1. Fig. 9 shows a variation amount (increase amount) of the resonance frequency with respect to an abrading amount of the projection member 121a obtained through an experiment. Fig. 10 shows a variation amount (decrease amount) of the resonance frequency with respect to the length (depth) S' of the notch 124 obtained through an experiment. It is noted that the abrading amount of the projection shown in Fig. 9 indicates the abrading amount of each of the projection groups 121a through 121d provided at the four peripheral portions of the radiation conductor 2. In Fig. 10, the length S' indicates the length of the notch 124 in the case where one notch 124 is formed at each of the four peripheral portions of the radiation conductor 2. The circularly polarized wave microstrip antenna 1 subjected to an experiment has a resonance frequency fo = 1.575 GHz and the following dimensions: Dielectric substrate 4 having: Dielectric constant ε = 21.4, Diameter D = 37 mm, Thickness t = 6 mm Circular radiation conductor 2 having: a diameter R = 20.6 mm; Frequency adjusting projection groups 121a through 121d having: Width W' = 0.4 mm, Length L' = 1 mm; Conductor-blank portions 122a through 122d: Circular hole having a diameter d' = 0.7 mm, Formable notch (124) having length S' = 0.25 mm to 0.75 mm; Axial ratio adjusting projections 123a and 123c having: Width Wt' = 1 mm, Length Lt' = 1 mm; Axial ratio adjusting projections 123b and 123d having: Width Wt' = 1 mm, Length Lt' = 2 mm As shown in Fig. 9, it was found that the resonance frequency fo is made higher at steps of 0.7 MHz every time reducing each of the projection groups 121a through 121d at the four peripheral portions of the radiation conductor 2 by 0.1 mm. Therefore, by abrading each of the projection groups 121a through 121d at the four peripheral portions of the radiation conductor 2 by an appropriate amount, the resonance frequency fo of the circularly polarized wave microstrip antenna 1 is gradually made higher thereby to effect fine adjustment of the resonance frequency. As shown in Fig. 10, it was found that the resonance frequency fo is made lower by about 2.5 MHz when a notch 124 having a length S' = 0.25 mm is formed on each of the four peripheral portions of the radiation conductor 2, and that the resonance frequency is made lower by about 1 MHz every time increasing the length S' of the notch 124 by 0.1 mm. Therefore, with the conductor-blank portions 122a through 122d so as to enable forming a notch 124 having an appropriate length formed, by increasing the amount of the notch 124 formed at the four peripheral portions of the radiation conductor 2, the resonance frequency is made lower step by step to enable finely tuning the resonance frequency fo of the circularly polarized wave microstrip antenna 1. The following describes the frequency adjustment procedure of the circularly polarized wave microstrip antenna 1 mentioned above. The resonance frequency fo of the circularly polarized wave microstrip antenna 1 is determined depending principally on the parameters of the thickness t of the dielectric substrate 4, the dielectric constant ε of the dielectric substrate 4, and the diameter R of the radiation conductor 2. Therefore, the three parameters t, ε and R are designed to have appropriate values, and the initial frequency (unadjusted resonance frequency at which the axial ratio of the circularly polarized wave microstrip antenna is minimum with the dielectric substrate 4 provided with the radiation conductor 2 and the ground conductor 3 respectively on its upper and lower surfaces) of the resonance frequency fo of the circularly polarized wave microstrip antenna 1 is made slightly lower than the intended value. For example, in the case shown in Fig. 9, the initial frequency is set at about 1.57 GHz. When the axial ratio of the circularly polarized wave microstrip antenna is not within the standard range, the projections 123a through 123d are abraded by the same amount once or several times for adjusting the axial ratio of the circularly polarized wave within the standard range. When the resonance frequency fo at which the axial ratio, after being adjusted, is smaller than the intended value, the projections 121a through 121d are abraded by the same amount once or several times, whereby the resonance frequency fo is gradually made higher to be adjusted to the intended frequency. For example, in the case shown in Fig. 9, the resonance frequency is adjusted to the intended frequency of 1.575 GHz. It is noted that, in the above-mentioned abrading procedure, the members of the projection groups 121a through 121d may be abraded off one by one in one processing time, or abraded in such a manner that a part of each member of the projections 121a through 121d is abraded in one processing time and, after completely abrading off the entire member in several processing times, the abrading process of the next member of each of the projection groups 121a through 121d is started. When the projections 121a through 121d are excessively abraded to make the resonance frequency fo higher than the intended frequency, a notch 124 is formed at each of the four peripheral portions of the radiation conductor 2 with the conductor-blank portions 122a through 122d serving as guides, the work of which is repeated once or several times so that the resonance frequency fo is gradually made lower to be adjusted to the intended frequency. When the resonance frequency fo after undergoing the axial ratio adjustment procedure is higher than the intended frequency, the resonance frequency fo is gradually made lower to be adjusted to the intended frequency by forming a notch 124 with the conductor-blank portions 122a through 122d serving as guides. When the resonance frequency fo is made lower than the intended frequency in the process, the projections 121a through 121d are further abraded, whereby the resonance frequency fo is gradually made higher to be adjusted to the intended frequency. It is noted that, although the above first embodiment describes a circularly polarized wave microstrip antenna 1 having a circular radiation conductor 2, the shape of the radiation conductor 2 is not limited to circular one and the present invention may have a rectangular radiation conductor 2' as shown in Fig. 11 or may be applied to a circularly polarized wave microstrip antenna 1 having an arbitrarily-shaped radiation conductor. As described above, according to the fifth embodiment of the present invention, in a circularly polarized wave microstrip antenna where a ground conductor and a radiation conductor are provided on a dielectric substrate, since the projections for making higher the resonance frequency and the conductor-blank portions for making lower the resonance frequency in adjustment are formed in the direction at an angle of 90N° (N: Integer) with respect to the line passing through the center O of the radiation conductor and the power feeding point P, by adjusting the length of the above-mentioned projections or the length of the notches formed with the conductor-blank portions serving as guides, the resonance frequency can be adjusted without exerting any influence on the other characteristics. Moreover, since the resonance frequency is adjusted by abrading the projections for making higher the resonance frequency preformed at the specific peripheral portions of the radiation conductors of the circularly polarized wave microstrip antenna or by forming a notch for making lower the resonance frequency with the conductor-blank portions serving as guides, the resonance frequency can be easily adjusted without exerting any influence on the other characteristics. When frequency adjustment is effected so excessively as to exceed the intended frequency, the frequency can be readjusted reversely, which enables reducing the possibility of the resulting unadjustable frequency of the antenna.

Claims: A circularly polarized wave microstrip antenna (1) comprising: a dielectric substrate (4) provided with a ground conductor (3) on one surface thereof and a radiation conductor (2) on the other surface thereof, said radiation conductor (2) being a plane and being shaped such that a center point (O) of said radiation conductor (2) can be defined, wherein electrical power is fed to an electric feeding point (P) located eccentrically on said radiation conductor (2), wherein a reference line is defined by the center (O) and the electric feeding point (P) on said radiation conductor (2); said radiation conductor (2) being further provided with: at least one axial ratio adjusting projection (21a,21b,21c, 21d) arranged in the plane of said radiation conductor (2) for adjusting the axial ratio of the antenna (1) located at each position of an angle of 45 x (2N + 1)° (N: Integer) with respect to the reference line on the periphery of said radiation conductor (2), wherein each of said axial ratio adjusting projections (21a,21b,21c,21d) are abraded for adjustment of the axial ratio of the antenna (1); and at least one frequency adjusting projection (22a,22b,22c, 22d) arranged in the plane of said radiation conductor (2) for adjusting the resonance frequency of said antenna (1) located at each position of an angle of 90N° (N: Integer) with respect to the reference line on the periphery of said radiation conductor (2), wherein each of said frequency adjusting projections (22a,22b,22c,22d) are abraded for adjustment of the frequency. A circularly polarized wave microstrip antenna as claimed in the claim 1, wherein the radiation conductor is provided with a notch in place of each of said frequency adjusting projection. A circularly polarized wave microstrip antenna as claimed in the claim 1, wherein the radiation conductor is provided with a notch in place of each of said axial ratio adjusting projection. A method of adjusting the resonance frequency of the circularly polarized wave microstrip antenna as claimed in claim 1 comprising the steps of: abrading each of said frequency adjusting projections; abrading each of said axial ratio adjusting projections; and adjusting the resonance frequency of said circularly polarized wave microstrip antenna.

Inventors: SONODA YOSHIYUKI, SONODA, YOSHIYUKI
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1992
Main classification: H01Q9
IPC codes: H01Q9