Circulary-polarized-wave flat antenna

Abstract: A plurality of insulators (18) are disposed in holes, respectively, defined in a metal plate (10) of a waveguide of a circularly-polarized-wave flat antenna. Each insulator (18) has a through-hole (18b; 31) and a protrusion (18d). The through-hole extends from the outside of the metal plate to the inside of the waveguide. The protrusion (18d) protrudes outward beyond the metal plate (10). The protrusion (18d) has a groove (18c) which is open to the outside. Curl antenna elements (22) each have a shaft portion (22a), an arm portion (22b), and a curl portion (22c). The shaft portion (22a) is fitted in the through-hole (18b; 31) of the insulator and partly protrudes outward from the metal plate (10). The arm portion (22b) extends from the protruded end of the shaft portion. The curl portion (22c) is in a substantially helical shape and connected to the top of the arm portion (22b). When the arm portion is engaged with the groove (18c) of the insulator, provided that the position of the groove of the insulator has been set in accordance with a desired orientation of the curl portion (22c) of the curl antenna element, the orientation of the curl portion is automatically set to a predetermined direction.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a circularly-polarized-wave flat antenna having a large number of circularly-polarized-wave antenna elements protruding on the outer surface of a metal plate of a waveguide power distributor, e.g. as disclosed in EP-A1-0 132 945. 2. Description of the Related Art In recent years, a variety of communication systems and broadcasting systems using circularly-polarized microwaves have been proposed. Accordingly, a variety of circularly-polarized-wave antennas for transmitting and receiving circularly polarized microwaves have been studied and developed. One of the circularly-polarized-wave antennas, a circularly-polarized-wave flat antenna using curl antenna elements has been proposed. More specifically, the circularly-polarized-wave flat antenna has a hollow disc-shaped waveguide constructed of two disc-shaped metal plates equally spaced by a predetermined distance. The waveguide and a power feeding member disposed at the middle of a first metal plate construct a waveguide power distributor. On the outer surface of a second metal plate, a curl antenna array consisting of a large number of curl antenna elements is disposed. Each curl antenna element comprises a shaft portion and a helical curl portion. One end of the shaft portion extends almost vertically from the second metal plate. The other end of the shaft portion is inserted into the waveguide. The other end of the shaft portion is insulated from the waveguide. The curl portion is curled for 1 to 1.5 turns and connected to the protruded end of the shaft portion. The curl antenna element array is separated into a plurality of rows disposed in a ring shape about the center of the second metal plate. Microwaves are received by the curl portion and the protruded end of the shaft portion of each curl antenna element. The received microwaves are sent to the power feeding member through the inserted portion of the shaft portion and the waveguide. In the case of transmission, signals received from a transmitter are sent to the curl portion and the protruded portion of the shaft portion through the waveguide and the inserted portion of the shaft portion. Thus, circularly polarized microwaves are radiated from the curl portion and the protruded portion of each curl antenna element. With respect to the circularly-polarized-wave flat antenna, the phases of circularly polarized waves radiated from all the curl antenna elements should match each other. Thus, the orientation of the curl portion of each curl antenna element, that is, the direction about the shaft portion of the curl antenna element, depends on its position disposed on the metal plate. Therefore, the curl portion of each curl antenna element should be oriented in a predetermined direction. As a result, it is desired to simplify the operation for mounting a large number of curl antenna elements on the metal plate in different directions according to predetermined positions. SUMMARY OF THE INVENTION An object of the present invention is to provide a circularly-polarized-wave flat antenna where the orientation of the curl portion of each curl antenna element, that is, the direction about the shaft portion of the curl antenna element, can be precisely set in a predetermined direction and the operation for mounting a large number of curl antenna elements in different directions according to predetermined positions is facilitated. The circulary-polarized-wave flat antenna according to the present invention comprises a waveguide having a metal plate, the metal plate having a plurality of holes, at least one insulator, the insulator being mounted in a hole of the metal plate and having a through-hole and a protrusion, the through-hole extending from the outside of the metal plate to the inside of the waveguide, the protrusion protruding to the outside of the metal plate and having a groove which is open to the outside, and at least one circular wave antenna element having a shaft portion, an arm portion, and a curl portion, the shaft portion being fitted in the through-hole of the insulator and having a top thereof protruding outward beyond the metal plate, the arm portion protruding from the top of the shaft portion to terminate at an end, the curl portion being in a substantially helical shape and connected to the end of the arm portion, the arm portion being engaged with the groove of the insulator, whereby if the position of the groove has been set in accordance with a desired orientation of the curl portion of the circularly polarized wave antenna element, the engagement of the arm portion with the groove will automatically set the orientation of the curl portion in a predetermined direction. According to the present invention, the groove which is open to the outside is formed on the protruded portion of the insulator. Provided that the position of the groove has been set to a desired orientation of the curl portion of each curl antenna element about the shaft portion, the orientation of the curl portion can be automatically set to a predetermined direction when the arm portion of the curl antenna element is engaged with the groove of the arm portion. Thus, the orientation of the curl portion can be precisely set to a predetermined direction. As a result, the operation for mounting a large number of curl antenna elements in desired different directions according to predetermined positions can be easily performed. These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of best mode embodiments thereof, as illustrated in the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1A is a plan view showing a curl antenna element; Fig. 1B is a side view of the curl antenna element of Fig. 1A; Fig. 2A is a perspective view showing the curl antenna element; Fig. 2B is a perspective view showing a modifications of the curl antenna element; Fig. 2C is a perspective view showing another modification of the curl antenna element; Fig. 2D is a perspective view showing a further modification of the curl antenna element; Fig. 3A is a plan view showing a circularly-polarized-wave flat antenna having curl antenna elements mounted on a waveguide power distributor; Fig. 3B is a sectional view taken along line IIIB-IIIB of Fig. 3A; Fig. 4A is a schematic diagram showing a modification of the waveguide power distributor; Fig. 4B is a schematic diagram showing another modification of the waveguide power distributor; Fig. 4C is a schematic diagram showing a further modification of the waveguide power distributor; Fig. 4D is a schematic diagram showing a still further modification of the waveguide power distributor; Fig. 5 is an exploded perspective view of a curl antenna element and an insulator in accordance with a first embodiment of the present invention; Fig. 6 is a sectional view of the insulator and so forth of Fig. 5; Fig. 7 is a plan view of the insulator and so forth of Fig. 5; Fig. 8 is a side view of the insulator and so forth of Fig. 5; Fig. 9 is a partially exploded perspective view of a circularly-polarized-wave flat antenna in accordance with a first embodiment of the present invention; Fig. 10 is a sectional view of a curl antenna element, an insulator, and so forth in accordance with a second embodiment of the present invention; Fig. 11 is an exploded perspective view of the curl antenna element, the insulator, and so forth of Fig. 10; Fig. 12 is a sectional view taken along line XII-XII of Fig. 11; Fig. 13 is a side view of a first modification of the curl antenna element in accordance with the second embodiment; Fig. 14 is a sectional view showing a shaft portion and an insulator of a second modification of the curl antenna element in accordance with the second embodiment; Fig. 15 is an exploded perspective view showing a circularly-polarized-wave flat antenna in accordance with a third embodiment of the present invention; Fig. 16 is a sectional view showing the circularly-polarized-wave flat antenna of Fig. 15; Fig. 17 is a partially exploded plan view of the circularly-polarized-wave flat antenna of Fig. 15; Fig. 18 is a sectional view of a modification of the circularly-polarized-wave flat antenna in accordance with the third embodiment; Fig. 19 is a sectional view showing a circularly-polarized-wave flat antenna in accordance with a fourth embodiment of the present invention; Fig. 20 is a plan view showing the circularly-polarized-wave flat antenna of Fig. 19; Fig. 21 is a sectional view showing a circularly-polarized-wave flat antenna in accordance with a fifth embodiment of the present invention; Fig. 22 is a sectional view showing a converter, a housing container thereof, and so forth of the circularly-polarized-wave flat antenna of Fig. 21; Fig. 23 is a sectional view of the housing container of the converter of the circularly-polarized-wave flat antenna in accordance with the fifth embodiment of the present invention; Fig. 24 is a bottom view showing the housing container of Fig. 23; Fig. 25 is a side view showing the housing container of Fig. 23; and Fig. 26 is an enlarged sectional view of an air penetration film and an air ventilation cover of Fig. 23. DESCRIPTION OF THE PREFERRED EMBODIMENTS First, with reference to Figs. 1 and 2, a curl antenna element will be described. As shown in Figs. 1A, 1B, and 2A, the curl antenna element 2 comprises a curl portion 2a and a shaft portion 2b. The curl portion 2a is curled circumferentially from a point S to a point e (see Figs. 1A and 2A). The shaft portion 2b has a line segment fq and a line segment qs. The line segment fq will be referred to as an upstanding portion of the shaft portion 2b, while the line segment qs will be referred to as a branch or arm portion of the shaft portion 2b. A point f of the shaft portion 2b will be referred to as a rear terminal. The portion adjacent to the point f will be referred to as a rear terminal portion. As shown in Figs. 1A and 2A, the curl portion 2a is made of a wire material and formed in a curl shape. The curl shape means a helical shape, where semi-circles with different diameters are connected, or a similar helical shape. More specifically, in the case where the center of a helix is O, the start point thereof is S, the end point thereof is e, and the distance between the center O and the end point e of the helix is r, the outer circumference C (2πr) should satisfy the following relation. (3/4)λ < C < (3/2)λ where λ is the propagation wavelength of the antenna. The number of turns of the curl is in the range from 1 to 1.5. Thus, the length of the curl portion 2b is a half or less than that of a conventional helical antenna (which normally has five or more turns of curl). As shown in Figs. 1A, 1B, and 2A, the shaft portion 2b is made of a wire material. The upstanding portion extends vertically, whereas the branch portion 2c extends to the start point S of the curl portion 2a with an angle to the upstanding portion. When a current flows in the shaft portion 2b (which has the upstanding portion and the branch portion 2c) and the curl portion 2a, power radiated from the curl portion 2a is superimposed to power radiated from the shaft portion 2b, thereby forming a desired radiation beam. The curl antenna element 2 may be constructed as shown in Figs. 2B, 2C, and 2D. More specifically, as shown in Fig. 2B, the branch portion 2c may be shorter than that shown in Figs. 1A, 1B, and 2A. In Fig. 2B, the point O of Figs. 1A, 1B, and 2A is inwardly shifted to point O'. As shown in Fig. 2C, the shaft portion 2b may be constructed of only an upstanding portion. Thus, the upstanding portion 2b is connected directly to the start point S of the curl portion 2a. In addition, as shown in Fig. 2D, the upstanding portion 2b may be connected directly to the end point e of the curl portion 2a. Moreover, the curl portion 2a and the shaft portion 2b can be integrally formed. Furthermore, after the curl portion 2a and the shaft portion 2b are separately formed, they may be soldered or welded. In Figs. 1A, 1B, and 2A, the branch portion 2c is connected to the start point S of the curl portion 2a. Rather, the branch portion 2c may be connected to the end point e of the curl portion 2a. In addition, as shown by an imaginary line L in Figs. 2A and 2B, the upstanding portion may vertically extend to the point O (or point O'), whereas the branch portion 2c may extend horizontally. Next, with reference to Figs. 3A and 3B, a circularly-polarized-wave flat antenna having curl antenna elements will be described. The circularly-polarized-wave flat antenna comprises a waveguide power distributor 1 and at least one curl antenna element 2. The power is fed to the curl antenna element 2 from the waveguide power distributor 1. This construction is referred to as a waveguide-power-feeding construction. The waveguide power distributor 1 comprises a pair of an upper metal plate 1a and a lower metal plate 1b which are opposed to each other. The upper metal plate 1a has at least one through-hole 1f. Between the metal plates 1a and 1b, a power propagation space 1c is formed. The distance between the metal plates 1a and 1b is smaller than the wavelengths of microwaves being transmitted and received. A shortcircuit metal ring 1d is disposed between the outer circumference of the metal plate 1a and that of the metal plate 1b. It should be noted that the shape of the metal plates 1a and 1b is not limited to a circle shown in the figure. The shape of the metal plates 1a and 1b may be a polygon. As shown in Fig. 3B, a center hole 1e is formed at the center of the lower metal plate 1b. At the center hole 1e, a coaxial feeder 4 is mounted. An outer conductor 4a of the coaxial feeder 4 is connected to the metal plate 1b. An inner conductor 4b of the coaxial feeder 4 is inserted into the power propagation space 1c. When necessary, a radio wave absorber 1g for absorbing a residual power may be disposed inside the metal ring 1d. The curl antenna elements 2 are disposed in accordance with the through-holes 1f on the upper metal plate 1a. In other words, each through-holes 1f holds an insulator 5. The shaft portion 2b of each curl antenna element 2 is rotatably supported by the insulator 5. The lower end portion of the shaft portion 2b protrudes from the insulator 5 to the inside of the power propagation space 1c. On the other hand, the upper end portion of the shaft portion 2b protrudes upwardly from the upper metal plate 1a so that power radiated from the shaft portion 2b is superimposed to that from the curl portion 2a. The distance h from the connection point between the curl portion 2a and the shaft portion 2b to the upper metal plate 1a is limited to approximately (1/4)λ or less (where λ is the propagation wavelength of the antenna). Since the circularly-polarized-wave flat antenna is constructed as described above, power is fed from the coaxial feeder 4 to each curl antenna element 2 through the waveguide power distributor 1. In other words, the power in the waveguide power distributor 1 is sent to the lower end portion of the shaft portion 2b. The power radiated from the protruded upstanding portion 2b and the branch portion 2c above the metal plate 1a is superimposed to the power radiated from the curl portion 2a. Thus, a radiation beam is formed. At this point, since a large number of curl antenna elements 2 are adjacently disposed, because of the array effect, a sharp radiation beam can be formed. Thus, the gain of the antenna can be improved. Moreover, by rotating the curl portion 2a about the upstanding portion of the shaft portion 2b, the phase of the radiation field can be adjusted. It should be appreciated that the metal plates 1a and 1b may be constructed as shown in Figs. 4A, 4B, 4C, and 4D. More specifically, as shown in Fig. 4A, the lower metal plate 1b may be formed in a cone shape where the center thereof dents. In addition, as shown in Fig. 4B, the lower metal plate 1b may be formed in a reverse cone shape where the center thereof protrudes upwardly. Moreover, as shown in Fig. 4C, the upper metal plate 1a may be formed in a cone shape where the center thereof dents. Furthermore, the lower metal plate 1b (not shown) may be formed in a reverse cone shape where the center thereof protrudes upwardly. The protruded surface may be in a curved surface shape. The upper metal plate 1a and the lower metal plate 1b may be formed in a shape of a combination of a plane and a curved surface. Fig. 4D shows an example of the lower metal plate 1b which is in a curved surface shape. Next, with reference to Figs. 5 to 9, a circularly-polarized-wave flat antenna in accordance with a first embodiment of the present invention will be described. As shown in Fig. 9, a hollow disc-shaped waveguide power distributor is constructed of two metal plates 10 and 12 opposed to and equally spaced from each other. At the center of the lower metal plate 12, a power feeding member 14 is disposed. On the upper metal plate 10, a large number of holes are formed. In the respective holes 16 of the metal plate 10, a large number of insulators 18 which are made of an insulating resin are formed by an outsert forming process. As shown in Fig. 6, each insulator 18 has two enlarged-diameter portions 18a formed on both the surfaces of the metal plate 10. The enlarged-diameter portions 18a prevent the insulator 18 from being detached from the metal plate 10. In addition, each insulator 18 has a shaft insertion hole 18b extending in the vertical direction of the metal plate 10. A lower portion of the shaft insertion hole 18b has a greater diameter than the other portion thereof. In addition, at the top of the insulator 18 is formed a protrusion 18d in the form of an upwardly protruding cylinder, and a groove 18c which is open to the outside is provided at the top of the protrusion 18d. The groove 18c extends in a substantially radial direction of the protrusion. The mold for use in the outsert forming process is constructed so that each groove 18c is oriented in a predetermined direction according to the position of an antenna element disposed. Moreover, a connection member 20 for connecting two adjacent insulators 18 is integrally formed therewith, as shown in Figs. 5 and 9. As described above, each curl antenna element 22 mounted to the insulator 18 comprises a shaft portion 22a, a branch or arm portion 22b, and a curl portion 22c. In the example of Fig. 5, the arm portion 22b is disposed nearly in parallel with the metal plate 10. In addition, at the middle of the shaft portion 22a, bumps 22d are disposed. The bumps 22d are formed by flattening parts of the shaft portion 22a. When assembled, the shaft portion 22a is inserted into the shaft insertion hole 18b of the insulator 18. In addition, the branch or arm portion 22b is engaged with the groove 18c. Thus, as shown in Fig. 6, the bumps 22d are positioned at the enlarged-diameter portions of the shaft insertion hole 18b, thereby preventing the curl antenna element 22 from slipping outward. Moreover, provided that the position of the groove 18c has been set in accordance with the orientation of the curl portion 22c, when the arm portion 22b is engaged with the groove 18c, the orientation of the curl portion 22c is automatically set to a predetermined direction. As described above, since the two insulators 18 are connected by the connection member 20, they are not rotated about the hole 16. The position of the groove 18c has been set in accordance with the orientation of the curl portion 22c disposed. Thus, when the arm portion 22b is engaged with the groove 18c, the direction of the curl portion 22c is automatically set to the predetermined direction. Since the two insulators 18 are connected by the connection member 20, they are not rotated about the hole 16. Thus, since the hole 16 may be formed in a perfect circular shape, and therefore the shape of the mold for forming holes can be simplified. Thus, a large number of holes 16 can be easily formed on the metal plate 10. It should be noted that the number of insulators 18 connected together by the connection member 20 can be three or more. In addition, rather than using the connection member 20, the holes 16 formed on the metal plate 10 may be in a non-circular shape, for example, a square shape, an elliptic shape, or a gourd shape in cross section, thereby preventing the insulators from rotating. At this point, the insulators 18 may be formed by the outsert forming process. In addition, the insulators 18 may be formed by another forming process. Furthermore, after the insulators 18 are formed by the insert forming process or the like rather than the outsert forming process, they can be mounted in the holes 16 of the metal plate 10. In this case, the insulators 18 can be formed in a non-circular shape such as a gourd shape in cross section and each hole 16 may be formed in the corresponding shape thereof so that they are not rotated. At this point, each hole 16 should be formed so that the groove 18 is oriented in the predetermined direction in accordance with the position of each curl antenna element 22. Thus, in inserting the curl antenna element 22 into the insulator 18, when the arm portion 22b is engaged with the groove 18c, the curl portion 22c of the curl antenna element 22 is oriented in the predetermined direction in accordance with the position of the curl antenna element 22. Thus, the orientation of the curl portion 22c can be precisely set and the operation for mounting a large number of curl antenna elements 22 on insulators in different directions in accordance with the positions thereof can be easily performed. In addition, when at least two insulators 18 are connected by a connection member, they are not rotated about their axes of the holes 16. Thus, the holes formed on the metal plate can be in a perfect circular shape, and therefore the shape of the mold or the like for forming them can be simplified whereby the cost becomes cheap. As a result, the circularly-polarized-wave flat antenna can be produced at a low cost. Moreover, when the insulators 18 have been formed on the metal plate by the outsert forming process, the groove 18c of each insulator 18 may be precisely formed by a forming mold so that the orientation of the groove 18c is set in accordance with the position of each curl antenna element 22. Thus, in comparison with the case where the insulators are independently formed and mounted on the metal plate, the number of assembling steps can be decreased, so that this antenna is suitable for mass-production. Next, with reference to Figs. 10 to 12, a circularly-polarized-wave flat antenna in accordance with a second embodiment of the present invention will be described. As shown in Fig. 10, an insulator 18 has a through-hole 31 in which a shaft portion 22a of a curl antenna element 22 is inserted. The through-hole 31 has a reduced-diameter portion 31a and an enlarged-diameter portion 31b. The reduced-diameter portion 31a is formed in the upper side of the insulator 18, while the enlarged-diameter portion 31b is formed in the lower side of the insulator 18. At the middle of the shaft portion 22a of the curl antenna element 22, a pair of lugs or bumps 22d are provided. The bumps 22d are formed by flattening parts of the shaft portion 22a. As shown in Fig. 12, the inner diameter of the reduced-diameter portion 31a of the through-hole 31 is larger than the outer diameter dl of the shaft portion 22a and is smaller than the outer diameter d2 of the shaft portion 22a and the bumps 22d. In other words, the inner diameter of the reduced-diameter portion 31a is larger than the outer diameter dl. When the shaft portion is inserted into the through-hole, the through-hole is resiliently deformed so as to allow the bumps to pass therethrough. Thus, when the shaft portion 22a of the curl antenna element 22 is inserted into the through-hole 31 of the insulator 18, the reduced-diameter portion 31 is elastically deformed and thereby widened so that the bumps 22d can be inserted thereinto. When the bumps 22d come to the enlarged-diameter portion 31b, the force necessary for the insertion thereof immediately decreases. Thus, the worker can know that the shaft portion 22a has been completely inserted into the through-hole 31 for a predetermined length. To remove the shaft portion 22a from the through-hole 31, the reduced-diameter portion 31b should be resiliently deformed and thereby widened by the bumps 22d. Therefore, the curl antenna element 22 which is relatively light in weight does not slip out by vibration or the like. In addition, since the required work for assembling the curl antenna element 2 is only to insert the shaft portion 22a into the through-hole 31, it can be easily performed. Fig. 13 shows a modification of the second embodiment. A shaft portion 22a of a circular-polarized-wave antenna element 22 has a bent portion 32 formed at the middle thereof. The bent portion 32 may be in for example an arc shape or a non-straight line shape. The outer diameter d3 of the shaft portion 22a and the bent portion 32 is larger than a reduced-diameter portion 31a and smaller than an enlarged-diameter portion 31b. Thus, according to this modification, when the shaft portion 22a is inserted into a through-hole 31, the reduced-diameter portion 31a is resiliently deformed and thereby widened by the shaft portion 22a so that the bumps 22d can be inserted thereinto. Thus, the antenna element 22 can be prevented from slipping out from the through-hole 31 and easily assembled. Fig. 14 shows a second modification of the second embodiment. At the middle of a shaft portion 22a, a groove 33 is circumferentially formed. At a predetermined position of a through-hole 31, a radially inward protrusion which is engaged with the groove 33 is circumferentially formed. The inner diameter of the inward protrusion 34 is smaller than the outer diameter of the shaft portion 22a and larger than the diameter of the bottom of the groove 33. The inward protrusion 34 has for example a taper and a step on the side where the shaft portion 22a is inserted and on the side where the shaft portion 22a is removed, respectively. Thus, according to the second modification of the second embodiment, when the shaft portion 22a is inserted into the through-hole 31, the inward protrusion 34 is resiliently deformed and thereby widened so that the shaft portion 22a can be inserted thereinto. When the shaft portion 22a is inserted by the predetermined length, the inward protrusion 34 is engaged with the groove 33. Thus, the shaft portion 22a is prevented from slipping out from the through-hole 31. In addition, the antenna element 22 can be easily assembled. Next, with reference to Figs. 15 to 17, a circularly-polarized-wave flat antenna in accordance with a third embodiment of the present invention will be described. As shown in Fig. 15, outside the metal plate 10, a radome 42 is disposed. The radome 42 is used to protect a circularly-polarized-wave flat antenna installed outdoors from being exposed to rain and snow, and from being attacked by birds and so forth. The radome 42 is made of a dielectric material. The radome 42 should be designed neither to increase the reflection loss and transmission loss of the circularly-polarized-wave flat antenna, nor to affect the directivity of the antenna. Thus, the radome 42 is thinly formed. In addition, to prevent curl antenna elements 22 from being damaged by strong wind pressure and outer substances, the radome 42 must have a rigidity of a predetermined level. According to this embodiment, on the outer surface of the metal plate 10, a protection plate 40 is disposed. The protection plate 40 is made of insulating styrofoam. On the outer surface of the protection plate 40, the radome 42 is disposed. The radome 42 is made of polypropylene or the like in a thin plate shape. The protection plate 40 has through-holes 41 for holding the curl antenna elements 22. The height of the protection plate 40 is larger than that of the curl antenna elements 22. The specific inductive capacity of the protection plate 40 and that of the radome 42 are low. The thickness of the radome 42 is relatively small. Thus, the reflection loss and transmission loss are very low. In addition, since a force applied to the radome 42 is supported by a large surface of the protection plate 40, the radome 42 can be prevented from being deformed by forces applied thereto. Thus, since the curl antenna elements 22 are not deformed, the performance of the antenna is not deteriorated. Moreover, since forces applied to the radome 42 are supported by the large surface of the protection plate 40, the radome 42 can be formed of a deformable resin such as polypropylene. Thus, as opposed to a conventional radome made of a hard resin, the radome 42 has a resistance to damages such as cracks. Furthermore, since the protection plate 40 can be formed of styrofoam, the weight thereof is very light. As a result, without a remarkable weight change, the circularly-polarized-wave flat antenna can be improved. When the circularly-polarized-wave flat antenna is fixed to a pole or the like, because of its light weight, the antenna can be easily fixed. Fig. 18 shows a modification of the third embodiment. In this modification, a protection plate is not provided. Instead, a radome 43 is relatively thickly formed. In addition, bottomed-holes 44 for holding curl antenna elements 22 are provided. The inner diameter of the bottomed-hole 44 is larger than the outer diameter of the curl antenna element 22. The depth of the bottomed-hole 44 is larger than the height of the curl antenna element 22. Thus, the bottomed-hole 44 can encase the curl antenna element 22. The bottom of the bottomed-hole 44 is relatively thinly formed so as to decrease the transmission loss of the curl antenna element 22. As described above, the thin bottom of the bottomed-hole 44 operates as a conventional radome, while the other portion operates as a protection plate for protecting the radome from being deformed. Thus, the radome 43 also operates as the protection plate, so that the number of constructional parts of the circularly-polarized-wave flat antenna according to this modification is small. Thus, this antenna can be easily produced and suitable for mass-production. Although the protection plate 20 in accordance with the above embodiment is formed of styrofoam, the material thereof is not limited thereto. The protection material 20 may be formed of any insulating material with a small specific inductive capacity and which does not affect radiation from the circularly-polarized-antenna elements 22. On the other hand, the material of the radomes 42 and 43 is not limited to polypropylene. Rather, they may be formed of any material with a low transmission loss and a low reflection loss. As described above, since an external force applied to the radome can be supported by the large surface of the protection plate, the radome itself does not require a large rigidity. Thus, the radome can be thinly formed so as to decrease the transmission loss of the antenna elements. In addition, since the radome does not require a large rigidity, it can be formed of a deformable material, thereby preventing it from being damaged by cracking or the like. Moreover, when the protection plate is formed of styrofoam whose specific inductive capacity is low, it does not affect radiation from the circularly-polarized-wave antenna elements at all. In addition, since the weight of the protection plate is very light, without a remarkable weight change, the circularly-polarized-wave flat antenna can be improved. When the circularly-polarized-wave antenna is fixed to a pole or the like, because of its light weight, the antenna can be easily fixed. Furthermore, since the radome also operates as the protection plate, the circularly-polarized-wave flat antenna in accordance with this modification can be easily assembled because of a small number of constructional parts thereof. Thus, this antenna is suitable for mass-production. Next, with reference to Figs. 19 and 20, a circularly-polarized-wave flat antenna in accordance with a fourth embodiment of the present invention will be described. With respect to a circularly-polarized-wave flat antenna, a large number of curl antenna elements should be disposed on a metal plate of a waveguide so as to obtain a predetermined antenna gain. Thus, the more the number of curl antenna elements increases, the more must the metal plate be enlarged. As a result, the metal plate tends to be deformed by outer forces or the like. In addition, the inside of the waveguide is airtight. Thus, a large temperature drop may cause the air pressure in the waveguide to become lower than the atmospheric pressure. As a result, a force which deforms the metal plate inwardly may work. When the metal plate is inwardly deformed and thereby the distance g between the two metal plates of the waveguide becomes short, the frequency band of signals which can be transmitted varies and the characteristics of the waveguide as the waveguide power distributor may fluctuate. As shown in Fig. 19, according to this embodiment, insulators 18 are formed in a large number of holes in the metal plate of the waveguide by an outsert forming process. In a through-hole for the insulator 18, a shaft portion 22a of a curl antenna element 22 is inserted. At the center of a lower metal plate 12, a power feeding member 14 is disposed. Two adjacent insulators are connected by a connection member 46 disposed below the metal plate 10. Below the center portion of the connection member 46, a brace member 48 extending between the two metal plates 10 and 12 is provided integrally with the connection member 46. As shown in Fig. 20, the number of the connection members 46 and that of the brace members 48 are, for example, six. These connection members 46 and the brace members 48 are radially and circumferentially equidistantly disposed and integrally formed along with the insulators 18. The inside of the waveguide constructed of the two metal plates 10 and 12 is airtight, thereby preventing moisture from condensing on the inner wall of the waveguide. Since the brace member 48 is disposed and connected between the two metal plates 10 and 12, even if an outer force is applied to the metal plate 10 and associated members or even if a temperature change results in a force which causes the metal plate 10 to inwardly dent, the brace member 48 prevents the metal plate 10 from being deformed. Thus, even if the metal plates 10 and 12 are thin, they are not deformed and thereby the characteristics of the waveguide do not vary. In addition, since the rigidity of the waveguide becomes large, the metal plates 10 and 12 can be thinly formed. Thus, the entire weight of the circularly-polarized-wave antenna can be reduced. In addition, the brace member 48 is integrally formed along with the insulator 18 and so forth by the outsert forming process. Thus, the step for placing the brace member 48 is not necessary, so that the number of constructional parts of the antenna is not increased. It should be noted that the brace member 48 can be formed by other than the outsert forming process. Rather, the brace member 48 may be assembled so that it is held between the two metal plates 10 and 12. In this case, unless these metal plates 10 and 12 are not shortcircuited, the material of the brace member may be a metal. In addition, the shape of the brace member 48 is not limited to a columnar shape, but a variety of shapes such as a plate shape. As described above, since the brace member prevents the two metal plates from being deformed, the distance therebetween does not vary. Thus, the characteristics of the waveguide can be maintained by a simple construction. In addition, since the metal plates are thin, the entire weight of the circularly-polarized-wave antenna can be reduced. Moreover, when the inside of the waveguide is airtight, since a temperature change causes the inner pressure to become lower than the atmospheric pressure, a force which causes the metal plates to inwardly dent acts. However, the brace member can stand against this force. Furthermore, when the brace member is integrally formed along with the insulator and so forth by the outsert forming process, the work for placing the brace member can be omitted. Thus, the number of constructional parts of the antenna is not increased. Next, with reference to Figs. 21 and 22, a circularly-polarized-wave flat antenna in accordance with a fifth embodiment of the present invention will be described. Microwaves received by a circularly-polarized-wave flat antenna are frequency-converted by a converter and then sent to a receiver through a power feeding probe disposed at the center of a hollowed disc-shaped waveguide. Signals from a transmitter are frequency-converted by a converter. Thereafter, the resultant signals are sent from the probe to the hollowed disc-shaped waveguide. Thus, curl antenna elements are excited and thereby circularly polarized microwaves are radiated. Conventionally, the converter is disposed at a predetermined position outside the circularly-polarized-wave flat antenna. The input/output signals of the converter are received/sent from/to the hollowed disc-shaped waveguide through an adequate square waveguide. As described above, since the input/output signals of the converter are received/sent from/to the probe of the hollowed disc-shaped waveguide through the square waveguide, whenever the electromagnetic wave mode is converted, signals are attenuated. Thus, the gain of the entire equipment decreases. In addition, since the square waveguide for transmission is used, the construction of the equipment becomes complicated and heavy. Moreover, since the construction of the equipment is complicated, moisture easily enters the hollowed disc-shaped waveguide and the square waveguide. Thus, condensation or the like takes place, thereby varying the frequency characteristics. As shown in Fig. 21, outside a metal plate 12 constructing a hollowed disc-shaped waveguide, a housing 50 is provided. As shown in Fig. 22, in the housing 50, a converter comprising a micro-strip-line 51 is accommodated. From the micro-strip-line 51, a probe 52 extends to the outside of the housing 50. The probe 52 extends through the metal plate 12, where the probe 52 is insulated from the metal plate 12. In addition, the probe 52 extends into the hollowed disc-shaped waveguide. Thus, the probe 52 constructs a power feeding probe. Moreover, this probe 52 is watertightly covered by a cap 53 which is made of an insulating resin. The cap 53 and the housing 50 are water-sealed by an O ring 54. In addition, the housing 50 and an outer surface of the metal plate 12 are covered by a case member 55. The case member 55 is made of a resin. Outside the metal plate 10, a radome 42 for covering curl antenna elements is disposed. The circumference of the radome 42 and that of the case member 55 are watertightly connected by a mole 56. A connector 57 protrudes from the rear surface of the housing 50 and outwardly extends through the case member 55. The connector 57 and the case member 55 are water-sealed by a rubber bush 58. As described above, since the probe 52 protrudes into the hollowed disc-shaped waveguide as an input/output terminal of the converter, the square waveguide for transmission can be omitted. Thus, the number of times electromagnetic wave mode is converted decreases. As a result, a high gain can be correspondingly obtained. In addition, since the square waveguide for transmission can be omitted, the construction of the equipment is simplified and the weight is reduced. Moreover, since the housing 50 and the outer surface of the metal plate 12 are covered by the case member 55 which is made of a resin, the connection surface between the housing 50 and the metal plate 12 is free of moisture and so forth. Thus, moisture does not enter the inside of the waveguide from the hole of the metal plate 12 passing through the probe 52. In addition, the probe 52 is covered by the cap 53, even if condensation takes place in the hollowed disc-shaped waveguide, an electric shortcircuit does not occur between the probe 52 and the metal plate 12. Moreover, since the cap 53 and the housing 50 are water-sealed by the O ring 54, moisture does not enter the inside of the housing 50. Thus, the characteristics of the converter does not vary. Next, with reference to Figs. 23 to 26, a circularly-polarized-wave flat antenna in accordance with a sixth embodiment of the present invention will be described. As shown in Fig. 23, a converter constructed of a micro-strip-line and so forth is housed in a metal housing member 60. The housing member 60 is closed by a metal cover 61. On the connection surface 62 between the housing member 60 and the cover 61, a groove 63 is circumferentially formed. In this groove 63, a packing 64 is inserted. The housing member 60 and the cover 61 are airtightly connected by machine screws 65. Thus, a housing container is constructed. A probe 52 which operates as an input/output terminal of a converter protrudes through the housing member 60. This probe 52 is watertightly covered by a cap 53. The cap 53 is made of an insulating resin. Between the cap 53 and the housing 50, an O ring 54 is watertightly disposed. A connector 57 of the converter protrudes through a hole of the housing member 60. On the outer circumference of the connector 57, an O ring 66 is airtightly disposed. As shown in Fig. 26, on the wall of the housing member 60, an air hole 67 connecting the inside of the housing to the outside is formed. The outer surface of the air hole 67 is closed by an air penetration film 68. The air penetration film 68 has a large number of minute holes whose diameter is smaller than that of water molecules and larger than that of air molecules. The outside of the air penetration film 68 is covered by an air ventilation cover 69. The air ventilation cover 69 has an outwardly convex surface. Thus, an air chamber 70 is formed between the air penetration film 68 and the air ventilation cover 69. The air ventilation cover 69 has small through-holes 71 for connecting the air chamber 70 and the outside (the diameter of each small through-hole 71 is for example 0.8 mm). As described above, even if air in the housing container expands or contracts according to a temperature change, air can enter and exit through the air penetration film 68. Thus, the change of pressure which acts on the wall of the container due to expansion or contraction of air decreases, and the rigidity of the housing container can be reduced in comparison with that of the related art. Therefore, the wall of the housing container can be thinly formed. In addition, ribs and so forth for reinforcement can be omitted. Moreover, the housing container can be small in size and light in weight. At this point, the air penetration film 68 prevents moisture from entering the inside of the housing container. Thus, condensation does not take place in the inside of the housing container. As a result, the electrical characteristics of the antenna can be stabilized. Since the air penetration film 68 is covered by the air ventilation cover 69, it is not damaged by an outer force. In addition, since the area of the air penetration film 68 is large, the inside of the housing container can be effectively ventilated through the air chamber 70 and small through-holes 71. The air penetration film 68 is made, for example, by sticking polyester-textured clothes and performing water-repellent treatment therefor. However, it should be noted that the air penetration film 68 is not limited to it. Rather, any material which can penetrate air and prevent moisture from penetrating may be used for the air penetration film 68. In addition, the probe 52 need not necessarily be covered by the cap 53. Differently put, the requirement is that the probe 52 should be airtight against the housing member 60. Moreover, it should be noted that the present invention is not limited to each of the above-mentioned six embodiments. For example, the waveguide power distributor is not limited to the hollowed disc-shaped waveguide according to the embodiments. Rather, the waveguide power distributor can be a square waveguide which propagates microwave signals. According to the embodiments, the first metal plate 12 constructing the hollowed disc-shaped waveguide is a shortcircuit plate whose outer circumference is bent. Instead, the shortcircuit plate may be constructed of another member. The metal plates 10 and 12 may be a metal-coated thin film where a metal is deposited or plated on a resin member. The curl antenna elements 22 are not limited to those in accordance with the embodiments. Instead of these curl antenna elements 22, any construction where circularly-polarized-wave antenna elements are each constructed of a shaft portion and a helical portion connected to the top end thereof may be used.

Claims: A circularly-polarized-wave flat antenna, having: a waveguide (1) having a metal plate (1a; 10), said metal plate having a plurality of holes (1f); at least one insulator (5; 18), said insulator being mounted in a hole (1f) of said metal plate and having a through-hole (18b; 31) and a protrusion (18d), said through-hole extending from the outside of said metal plate to the inside of said waveguide (1), said protrusion (18d) protruding to the outside of said metal plate and having a groove (18c) which is open to the outside; and at least one circularly-polarized-wave antenna element (22) having a shaft portion (22a), an arm portion (22b), and a curl portion (22c), said shaft portion being fitted in said through-hole (18b; 31) of said insulator and having a top protruding outward beyond said metal plate (la; 10), said arm portion (22b) protruding from said top of said shaft portion to terminate at an end, said curl portion (22c) being in a substantially helical shape and connected to said end of the arm portion (22b), said arm portion being engaged with said groove (18c) of said insulator, whereby if the position of said groove has been set in accordance with a desired orientation of said curl portion of said circular wave antenna element, the engagement of said arm portion (22b) with said groove (18c) will automatically set the orientation of said curl portion in a predetermined direction. The circularly-polarized-wave flat antenna as set forth in claim 1, characterised in that adjacent two insulators (18) have a common connection member (20; 46) for connecting the two insulators, said connection member being integrally formed therewith. The circularly-polarized-wave flat antenna as set forth in claim 1, characterised in tha said insulator (18) is formed in said hole in said metal plate (1a; 10) by an outsert forming process. The circularly-polarized-wave flat antenna as set forth in claim 1, characterised in that said shaft portion (22a) of said circularly-polarized-wave antenna element (22) has bumps (22d) formed at the middle thereof, said through-hole (18b; 31) of said insulator having an enlarged-diameter portion (31b) and a reduced-diameter portion (31a), said enlarged-diameter portion (31b) being disposed on the side of said metal plate (10), said reduced-diameter portion (31a) being disposed on the opposite side thereof, an inner diameter of said reduced-diameter portion (31a) being larger than an outer diameter of said shaft portion (22a) so that the insertion of said shaft portion into said through-hole causes said through-hole to be resiliently deformed and thereby allowing said bumps (22d) to pass through said reduced-diameter portion, the inner diameter of said enlarged-diameter portion being larger than the outer diameter of said shaft portion (22a) and said bumps. The circularly-polarized-wave flat antenna as set forth in claim 1, characterised in that said shaft portion (22a) of said circularly-polarized-wave antenna element (22) has a bent portion (32) formed at the middle thereof, said through-hole (31) of said insulator (18) having an enlarged-diameter portion (31b) and a reduced-diameter portion (31a), said enlarged-diameter portion being disposed on the side of said metal plate (10), said reduced-diameter portion being disposed on the opposite side thereof, an inner diameter of said reduced-diameter portion (31a) being larger than an outer diameter of said shaft portion so that the insertion of said shaft portion into said through-hole (31) causes said through-hole to be resiliently deformed and thereby allowing said bent portion (32) to pass through said reduced diameter portion, an inner diameter of said enlarged-diameter portion being larger than the outer diameter of said shaft portion and said bent portion. The circularly-polarized-wave flat antenna as set forth in claim 1, characterised in that said shaft portion (22a) of said circularly-polarized-wave antenna element (22) has a groove (33) circumferentially formed at the middle thereof, said through-hole (31) of said insulator having an inward protrusion (34), said protrusion being resiliently deformed so as to allow said shaft portion to pass through said through-hole when said shaft portion is inserted into said through-hole, said inward protrusion (34) being engaged with said groove (33) so as to prevent said shaft portion from slipping out of said through-hole when a force for removing said shaft portion is applied. The circularly-polarized-wave flat antenna as set forth in claim 1, characterised in that it further comprises: a protection plate (40) disposed on an outer surface of said metal plate (10), having holes (41) for encasing said circularly-polarized-wave antenna elements (22); and a radome (42) being in a thin plate shape and disposed on an outer surface of said protection plate (40). The circularly-polarized-wave flat antenna as set forth in claim 7, characterised in that said protection plate (40) is formed of styrofoam. The circularly-polarized-wave flat antenna as set forth in claim 1, characterised in that it further comprises: a radome disposed on an outer surface of said metal plate, having bottomed holes for encasing said circularly-polarized-wave antenna elements. The circularly-polarized-wave flat antenna as set forth in claim 1, characterised in that said waveguide comprises: another metal plate (12) opposed to said metal plate (10); and a brace member (48) disposed between the two metal plates (10, 12), for preventing the distance between the two metal plates from becoming shorter than a predetermined distance. The circularly-polarized-wave flat antenna as set forth in claim 10, characterised in that said waveguide has an airtightly formed interior. The circularly-polarized-wave flat antenna as set forth in claim 10, characterised in that said insulator (18) and said brace member (48) are integrally formed. The circularly-polarized-wave flat antenna as set forth in claim 1, characterised in that said waveguide has a second metal plate (12) opposed to said metal plate (10), said circularly-polarized-wave antenna further comprising: a housing (50) disposed outside said second metal plate (12); and a converter housed in said housing; and that said converter has a probe (52) as an input/output terminal, said probe protruding from said housing, said probe extending through said second metal plate (12) and protruding in said waveguide, said probe being insulated from said second metal plate. The circularly-polarized-wave flat antenna as set forth in claim 13, characterised in that said housing (50) and an outer surface of said second metal plate (12) are covered by a case member (55). The circularly-polarized-wave flat antenna as set forth in claim 13, characterised in that said probe (52) is watertightly covered by an insulating resin, said housing (50) being watertightly formed. The circularly-polarized-wave flat antenna as set forth in claim 13, characterised in that it further comprises: a housing container having a housing member (60), for housing a converter, and that said housing container has: a packing (64) for airtightly connecting the connection surfaces (62) of said housing member (60); a seal member (54) for keeping said input/output terminal (52) of said converter protruding from said housing member (60) airtight; and an air penetration film (68) having a large number of holes whose diameter is smaller than the diameter of water molecules and larger than the diameter of air molecules, for closing air holes defined on a wall of said housing member (60). The circularly-polarized-wave flat antenna as set forth in claim 16, characterised in that said housing container has an air ventilation cover (69) for covering said air penetration film (68) from the outside thereof and for forming an air chamber (70) along with said air penetration film, said air ventilation cover having at least one through-hole (71) for connecting said air chamber to the outside.

Inventors: KASAMA MASAAKI, KAWAHARA TAKAO, NAKAGAWA YUKIO, NAKANO HISAMATSU, NAKAYAMA ITSUO, SEKINE MOTOSHIGE, TANAKA EIICHI, YOKOYAMA KAZUHIRO, YOSHIDA SHIGERU, KASAMA, MASAAKI, KAWAHARA, TAKAO, NAKAGAWA, YUKIO, NAKANO, HISAMATSU, NAKAYAMA, ITSUO, SEKINE, MOTOSHIGE, TANAKA, EIICHI, YOKOYAMA, KAZUHIRO, YOSHIDA, SHIGERU
Applicants: NAKANO HISAMATSU, YOKOWO SEISAKUSHO KK, NAKANO, HISAMATSU, YOKOWO CO., LTD.
Application year: 1993
Main classification: H01Q21
IPC codes: H01Q21, H01Q11