Antenna device

Abstract: An antenna device includes radiation elements, feed means for feeding the radiation elements, a ground plate provided in electrically-spaced relation to the radiation elements, and a cover member covering the radiation elements and the feed means. In use, the ground plate, mounted on a box-like metal body, and the box-like metal body are short-circuited together. The value, obtained by dividing the distance (H) between the radiation elements and the ground plate by a wavelength (Λ), is 1 ÷ 250 ≦ H ÷ Λ ≦ 1 ÷ 80, and preferably 1 ÷ 200 ≦ H ÷ Λ ≦ 1 ÷ 100. With this construction, an overall loss of the antenna can be suppressed while suppressing the decrease of the antenna impedance, and therefore the antenna device of high reliability can be provided which can positively operate under a wide range of conditions of use.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to an antenna device of a compact, thin design having wideband frequency characteristics. Description of the Related Art Recently, mobile satellite communications, using a mobile means (e.g. an airplane, a ship or a car) and a communication satellite, have come into wide use. With the spread of the mobile satellite communications, there has now been a stronger demand for a compact, high-performance antenna. As is well known, in order to achieve a compact antenna design, there has been proposed a meandering-type antenna in which a radiation element of the antenna comprises a wire-like conductor formed or bent into a meandering configuration. One example of such antenna is disclosed in JP-A-6-90108. Generally, the frequency bandwidth of a conventional antenna is about several % in terms of the specific band, and when the length of a radiation element was reduced so as to achieve a compact design, a problem was occurred that the band was further narrowed. When the transmitting band and the receiving band were larger than the specific band thereof, a problem was occurred that a plurality of antennas for transmitting and receiving purposes were required. Figs. 13A to 13B are a plan view showing the construction of a conventional antenna, Figs. 13B and 13C are views showing the installation of the conventional antenna, and Fig. 13D is a graph showing resonance characteristics of the conventional antenna. The resonance frequency of the conventional antenna is a single resonance, and therefore the conventional antenna can not deal with a plurality of frequency bands (that is, the frequency band between 137.0 MHz and 138.0 MHz for a down-line and the frequency band between 148.0 MHz and 150.05 MHz for an up-line) assigned to a mobile satellite communication system which effects a ground-satellite-ground data communication using a satellite orbiting in a low orbit. Namely, the resonance frequency fr is determined by the length L of a radiation element, and this has resulted in a problem that the resonance occurred only for the single frequency. In the installation of the antenna on a mobile means such as a vehicle, it is desirable that the antenna should have a low posture (reduced antenna height) in order to reduce the wind pressure, acting on the antenna open surface, end also to prevent damage to the antenna upon contact with other object. Particularly when the antenna is installed on a container, the antenna height is about 0.5 m as a result of the stacking of the containers even if the above 1/4-wavelength grounded-type antenna is used, and therefore the installation of the antenna is impossible. If the conventional antenna, shown in Fig. 13A, is mounted vertically on the vehicle body as shown in Fig. 13B, in addition to the above bandwidth problem, further problems concerning the reduction of the wind pressure and the damage to the antenna upon contact with other object will be occurred. If the antenna is mounted horizontally on the vehicle body as shown in Fig. 13C, the impedance is lowered as the antenna approaches an electrically-conductive panel or plate of the vehicle body, and also the resonance frequency is shifted, so that the impedance matching between the antenna and the feeder line is adversely affected, which has resulted in a problem that the transmitting and receiving operation can not be operated. SUMMARY OF INVENTION It is an object of this invention to provide an antenna device which can deal with more frequencies, and has a compact design and a low posture, and is hardly influenced by a box-like metal body such a vehicle body. In order to achieve the object of present invention there is provided an antenna device comprising a first radiation element, a second radiation element different in electrical length from said first radiation element, a first feed point for feeding electric power to said first radiation element, and a ground plate provided in spaced relationship to said first radiation element and said second radiation element, said first radiation element being grounded to said ground plate at a first ground point and said second radiation element being grounded to said ground plate at a second ground point, said antenna device being characterised by a second feed point for feeding electric power to said second radiation element, the distance between said first ground point and said first feed point being different from the distance between said second ground point and said second feed point. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of an antenna with radiation elements of unequal line length; Fig 2 is a perspective view of the antenna with radiation elements of unequal line length; Fig 3 is a plan view showing the construction of radiation elements formed on an antenna board; Fig 4 is a plan view showing the construction of radiation elements formed on an antenna board, Fig 5 is a plan view showing the construction of radiation elements formed on the antenna board; Fig 6A and 6B are plan views showing the construction of radiation elements formed on the antenna board; Fig 7 deleted Fig. 8 deleted Fig. 9 deleted Fig. 10 deleted. Fig. 11A is a plan view of an antenna and Fig. 11B is a graph showing resonance characteristics of an antenna according to the first embodiment of the invention; Figs. 12A and 12B are graphs showing a current distribution in the antenna according to the first embodiment of the invention; Fig. 13A is a plan view of the construction of a conventional antenna; Fig. 13B is a view showing the installation of the conventional antenna; Fig. 13C is a view showing the installation of the conventional antenna; Fig. 13D is a graph showing resonance characteristics of the conventional antenna; Figs. 14A and 14B are plan views showing the construction of radiation elements formed in an antenna board in the invention; Figs. 15A and 15B are plan views showing the construction of radiation elements formed in an antenna board in the invention; Figs. 16A and 16B are plan views showing the construction of radiation elements formed in an antenna board in the invention; Figs. 17A and 17B are plan views showing the construction of radiation elements formed in an antenna board in the invention; Figs. 18A and 18B are plan views showing the construction of radiation elements formed in an antenna board in the invention; Figs. 19A and 19B are plan views showing the construction of radiation elements formed in an antenna board in the invention; Figs. 20A and 20B are plan views showing the construction of radiation elements formed in an antenna board in the invention; Figs. 21A and 21B are plan views showing the construction of radiation elements formed in an antenna board in the invention; Figs. 22A and 22B are plan views showing the construction of radiation elements formed in an antenna board in the invention; Figs. 23A and 23B are plan views showing the construction of radiation elements formed in an antenna board in the invention; Figs. 24A and 24B are plan views showing the construction of radiation elements formed in an antenna board in the invention; Figs. 25A and 25B are plan views showing the construction of radiation elements formed in an antenna board in the invention; Figs. 26A and 26B are plan view showing the construction of radiation elements formed in an antenna board in the invention; Fig. 27 is a perspective view showing the construction of an antenna according to a fourth embodiment of the invention; Fig. 28 is a perspective view showing the construction of radiation elements in the first embodiment; Fig. 29 is a perspective view showing the construction of radiation elements in the first embodiment; Fig. 30 is a diagram showing characteristics of the radiation elements in the first embodiment; Figs. 31A and 31B are illustrations showing a current distribution in the antenna of the first embodiment; Fig. 32 is a perspective view showing the construction of an antenna according to the first embodiment; Figs. 33A and 33B are plan views showing the construction of radiation elements in a second embodiment of the invention; Figs. 34A and 34B are plan views showing the construction of radiation elements in the second embodiment; Fig. 35 is a diagram showing the relation between radiation power and frequencies in the second embodiment; Fig. 36 is a perspective view showing the construction of a feed portion in the first embodiment of the invention; Fig. 37 is an enlarged view showing the feed portion in the first embodiment; Fig. 38 is a perspective view showing the grounding of the radiation elements in the first embodiment; and Fig. 39 is a perspective view showing the construction of the antenna device of the first embodiment. DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below. (First Example) An antenna device with radiation elements of unequal line length will first be described with reference to the drawings, for example only. Such a device may be used in combination with the first and second embodiments of the invention. Fig. 1 is a perspective view of such an antenna and Fig. 2 is a perspective view of the same antenna of the antenna of the invention. Fig 3 is a plan view showing the construction of a radiation element formed on an antenna board. In Fig 1, the antenna board 1 comprises of a dielectric material, and has a thickness t . The antenna board 1 usually comprises a printed circuit board or a PET film board. Various elements are formed and mounted on the antenna board 1, and these elements will be described below. Radiation elements 2a, 2b, 2c and 2d (Fig 2) are formed on one or both sides of the antenna board 1 by etching, photolithography, sputtering or other method. In this embodiment, in order that a plurality of wavelengths can be transmitted and received, one pair of radiation elements corresponds to one wavelength, and the radiation element pair 2e, constituted by the radiation elements 2a and 2b corresponds to a short wavelength (λg1), and the radiation element pair 2f, constituted by the radiation elements 2c and 2d, corresponds to a long wavelength (λg2). The radiation elements 2a and 2b are formed substantially symmetrically in a longitudinal direction a of the antenna board 1, and similarly the radiation elements 2c and 2d are formed substantially symmetrically in the longitudinal direction a of the antenna board 1. The radiation element pair 2e and the radiation element pair 2f are arranged generally symmetrically each other in a transverse direction b of the antenna board 1. The radiation element 2a and the radiation element 2c are connected together in the vicinity of the center of the antenna board 1, and the radiation element 2b and the radiation element 2d are connected together in the vicinity of the center of the antenna board 1 being independent of the radiation elements 2a and 2c. Each of the radiation elements 2a, 2b, 2c and 2d is formed by a meandering line being bent regularly into such a meandering configuration wherein a line width w ≒ λ/200∼λ/400, an element interval d ≒ λ/100∼λ/200, and an element width l ≒ λ/10∼λ/20 The radiation element 2a and the radiation element 2b, constituting the radiation element part 2e, have substantially the same line length, and also the radiation element 2c and the radiation element 2d, constituting the radiation element pair 2f, have substantially the same line length. On the other hand, the radiation elements, constituting one of the radiation element pairs, are different in line length from the radiation elements constituting the other radiation element pair (for example, the radiation element 2a is different in line length from the radiation element 2c). More specifically, each of the radiation elements 2a and 2b, constituting the radiation element pair 2e, has the length L1 (L1 ≒ (λg1)/4), and each of the radiation elements 2c and 2d, constituting the radiation element pair 2f, has the length L2 (L2 ≒ (λg2)/4). The radiation elements, corresponding to different wavelengths, are thus formed on the common board, and therefore as compared with the case where such radiation elements are formed on separate boards, the antenna construction can be simplified, and the productivity can be enhanced, besides the compact, thin design of the antenna can be achieved. Furthermore, since the different line lengths are provided, a plurality of wavelengths can be transmitted and received. Particularly, by setting the line length to about 25% of the corresponding wavelength, the optimum transmitting and receiving operation can be achieved for each corresponding wavelength, and also the polarization characteristics of the antenna can be enhanced. The arrangement of the radiation elements 2a 2b, 2c and 2d are not limited to that of this example. Various arrangements of radiation elements 2a, 2b, 2c and 2d are shown in Figs. 14A to 26B. Figs. 14A to 26B are plan views showing the constructions of the radiation elements that may be formed on the antenna board of the invention. In the arrangements shown in Figs. 14A and 15B, the antenna width is constant, and the element widths are the same. In the arrangement shown in Fig. 14A, the configuration of the radiation elements is symmetrical in a right-left direction, and is asymmetrical in a upward-downward direction, and power is fed right and left. Thus, the lengths of the elements are different at those ends which are the opposite sides of the feed portion, and merely with this construction, radio waves with different wavelengths can be transmitted and received, and therefore the design of the radiation elements is easy. Besides, since the power is fed in the right and left directions, the radiation elements with different lengths can be brought into the same power-fed condition, and therefore the difference in antenna characteristics due to the difference of the power-fed condition can be suppressed to a minimum. In the arrangement shown in Fig. 14B, the configuration of the radiation elements is symmetrical in the center, and is asymmetrical in an upward-downward direction, and power is fed right and left. In the arrangement shown in Fig. 15A, the configuration of the radiation elements is symmetrical in a right-left direction, and is asymmetrical in an upward-downward direction, and power is fed upward and downward. In this case, the power is fed upward and downward, and therefore the radiation elements, which correspond to the same wavelength, can be brought into the same power-fed condition, and therefore a variation in antenna characteristics on the antenna board, which would be caused by the radiation elements corresponding to the same frequency, can be suppressed to a minimum. In the arrangement shown in Fig. 15A, the configuration of the radiation elements is symmetrical in the center, and is asymmetrical in an upward-downward direction, and power is fed upward and downward. With this arrangement, the substantial element lengths can be increased, and therefore the width of the antenna device in its longitudinal direction can be shortened. In the arrangements shown in Figs. 16A, 16B, 17A and 17B, the antenna width is constant, and the element widths are different. In Fig. 16A, the configuration of the radiation elements is symmetrical in a right-left direction, and is asymmetrical in an upward-downward direction, and power is fed right and left. In Fig. 16B, the configuration of the radiation elements is symmetrical in the center, and is asymmetrical in an upward-downward direction, and power is fed right and left. In Fig. 17A. the configuration of the radiation elements is symmetrical in a right-left direction, and is asymmetrical in an upward-downward direction, and power is fed upward and downward. In Fig. 17B, the configuration of the radiation elements is symmetrical in the center, and is asymmetrical in an upward-downward direction, and power is fed upward and downward. In the arrangements shown in Figs. 18A, 18B, 19A and 19B, the antenna width is different, and the element widths are different. In Fig. 18A, the configuration of the radiation elements is symmetrical in a right-left direction, and is asymmetrical in an upward-downward direction, and power is fed right and left. In Fig. 18B, the configuration of the radiation elements is symmetrical in the center, and is asymmetrical in an upward-downward direction, and power is fed right and left. In Fig. 19A, the configuration of the radiation elements is symmetrical in a right-left direction, and is asymmetrical in an upward-downward direction, and power is fed upward and downward. In Fig. 19B, the configuration of the radiation elements is symmetrical in the center, and is asymmetrical in an upward-downward direction, and power is fed upward and downward. The arrangements, shown in Figs. 20A, 20B, 21A, and 21B, show the arrangements of Figs. 14A to 15B with a ground plate being provided. The arrangements, shown in Figs. 22A, 22B, 23A and 23B, show the arrangements of Figs. 16A to 17B with a ground plate being provided. The arrangements, shown in Figs. 24A, 24B, 25A and 25B, show the arrangements of Figs. 18A and 19B with a ground plate being provided. In the arrangement shown in Fig. 26A, the antenna width is constant, and the element widths are the same, and four pairs of radiation elements (four radiation element pairs) are used, and these are arranged in a rotation symmetry manner to form an array. With this arrangement, the antenna device, capable of dealing with three or more frequencies, can be achieved with the simple construction. Also, the plurality of radiation element pairs can correspond to the same frequency, and therefore the antenna characteristics are enhanced, and if the directions of the radiation elements differ from one another, the antenna device having the wider receiving range can be realized. In the arrangement shown in Fig. 26B, the antenna width is constant, and the element widths are the same, and each radiation element pair is arranged in a rotation asymmetrical manner, and further a ground plate is provided. The feed portion 3 serve to feed high-frequency power to the radiation elements 2, and is formed on that side (surface) of the antenna board 1, having the radiation elements formed thereon, or the back side thereof, the feed portion 3 being disposed in the vicinity of the center of the antenna board 1. For this, the feed portion 3a of the antenna board 1 is formed in the vicinity of the point of junction between the radiation elements 2a and 2c connected together, and feeds power to the radiation elements 2a and 2c. The feed portion 3b is formed in the vicinity of the point of junction between the radiation elements 2b and 2d connected together, and feeds power to the radiation elements 2b and 2d. A matching circuit 4 and a transmission line 5 both serve to efficiently feed the predetermined high-frequency power to the feed portion 3. A coaxial cable or a micro-strip line can be used as the transmission line 5. The feed equipment is constituted by the transmission line 5, the matching circuit 4 and the feed portion 3. Thus, the power is fed to the plurality of radiation elements 2a, 2b, 2c and 2d through the single feed equipment comprising the transmission line 5, the matching circuit 4 and the feed portion 3. With this construction, the power can be fed to the plurality of radiation elements 2 through the single feed equipment, and therefore the area of the antenna board 1, occupied by the feed equipment, can be reduced, and therefore the size of the antenna board 1 size can be reduced. Besides, since the power is fed both to the radiation element pair 2e and the radiation element pair 2f through the common feed equipment, the power can be fed to the radiation element pairs 2e and 2f under the same condition, and therefore the antenna of high reliability having excellent antenna characteristics can be provided both for the frequencies f1 and f2 to which the radiation element pairs 2e and 2f correspond, respectively. Besides, since the single transmission line 5 connected to the antenna board 1 is provided , the construction of the antenna can be simplified compared with the case where one transmission line is provided for each radiation element pair, and therefore the productivity of the antenna can be enhanced. Furthermore, the number of lead-in through holes, extending to the exterior of the antenna, can be reduced, and therefore the intrusion of water and foreign substances through the lead-in holes can be suppressed to a minimum, and therefore the malfunction of the antenna due to these factors can be suppressed, and the antenna with high reliability can be achieved . The radiation elements 2, the feed portion 3 and the matching circuit 4 may be formed on the same side (surface) of the antenna board 1, or the radiation elements 2 may be formed on one side of the antenna board 1 while the feed portion 3 and the matching circuit 4 may be formed on the other side thereof. If the radiation elements 2, the feed portion 3 and the matching circuit 4 are formed on the same side of the antenna board, the construction of the antenna board 1 can be simplified, and therefore the steps of processing and forming the antenna board 1 can be shortened, so that the productivity of the antenna board 1 can be enhanced. Besides, various circuits to be formed on the antenna board 1 can be simultaneously formed thereon, and therefore the productivity can be enhanced as well. Furthermore, one radiation element pair 2e may be formed on one side of the antenna board while the other radiation element pair 2f may be formed on the other side thereof. With this construction, the distance between the radiation element pair 2e and the ground plate 6 and the distance between the radiation element pair 2f and the ground plate 6 are different from each other by an amount corresponding to the thickness t of the antenna board 1, and therefore in the case where the antenna characteristics required respectively for these radiation element pairs are different, the characteristics required respectively for the radiation element pairs can be optimized. If the radiation elements 2, the feed portion 3 and the matching circuit 4 are formed on the same side of the antenna board 1, it is preferred that the radiation elements 2 and so on formed on the antenna board, should face the ground plate 6, that is, should be directed to the inner side of the antenna device. With this construction, the overall thickness of the antenna device can be reduced. The ground plate 6 is made of a metal conductor such as aluminum, stainless steel or a plated copper. A gap 9, with the height h , is formed between the antenna board 1 and the ground plate 6. A dielectric plate may be inserted into the gap 9 over the entire area thereof, or several support members or means (not shown) may be provided in the gap 9 to support between them. If the gap 9 is formed by the use of the support means, the gap 9 is filled with the air. The operation of the antenna of the above construction will now be described. High-frequency power, fed through the transmission line 5, is supplied to the radiation elements 2a, 2b, 2c and 2d via the matching circuit 4. In this case, the dimensions of various portions of the radiation elements 2a and 2b, as well as the dimensions of various portions of the radiation elements 2c and 2d, are suitably selected, and by doing so, the radiation elements can radiate radio waves into the air with desired resonance frequency. Here, if the length of the radiation elements 2a and 2b is represented by L1, the length of the radiation elements 2c and 2d is represented by L2, the dielectric constant of the dielectric material, constituting the antenna board 1, is represented by ε1, the thickness of the antenna board 1 is represented by t , the dielectric constant of the gap 9 between the antenna board 1 and the ground plate 6 is represented by ε2, and the velocity of light is represented by C, the resonance frequencies f1 and f2 of the antenna are expressed by the following formulas: f1 ≒ C/2L1√ε f2 ≒ C/2L1√ε However, the following formula is established, and a plurality of resonance characteristics (two cycles in the case), as shown in Figs. 11A and 11B, are obtained: ε ≒ (ε1 x ε2(t + h))/(ε1 x h + ε2 x t) Figs. 11A and 11B are graphs showing the resonance characteristics of such an antenna. The current distribution in the antenna, obtained in this case, will be described with reference to Figs. 12A and 12B. Figs. 12A and 12B are views showing the current distribution in such an antenna. For the sake of simplicity, only the radiation element pair 2f is shown in Figs. 12A and 12B. As shown in the drawings, the element line length L1 of each of the radiation elements 2c and 2d is set to about 1/4 length of a respective one of the two line wavelengths λg1 corresponding to the desired frequency f1, and by doing so, the current is distributed generally sinusoidally in the antenna in such a manner that the amplitude becomes maximum in the vicinity of the feed portion 3 at the desired frequency f1 while the amplitude becomes minimum at the distal end portions of the radiation elements 2a and 2b. As indicated by arrows (→) in the drawings, the two parallel lines, extending obliquely in an upward-downward direction on the sheet of the drawings, are opposite to each other with respect to the direction of flow of the current, and vertically-polarized waves due to this current cancel each other, and therefore the radiation of the vertically-polarized waves in a vertical direction on the drawing sheet can be almost eliminated. The two parallel lines, extending in the right-left direction on the sheet of the drawings, are the same with respect to the direction of flow of the current, and horizontally-polarized waves due to this current are radiated in a horizontal direction on the drawing sheet. Therefore, the antenna which has excellent polarization-identifying characteristics, and has a compact size can be provided. As shown in Fig. 5, an inductance device, comprising an air-core coil or a micro-strip line, can be provided at the distal end of each of the radiation elements 2. Fig. 5 is a plan view showing the construction of the radiation elements formed on the antenna board in the first embodiment of the invention. With this construction, the effective length of the antenna is increased in an equivalent manner, and therefore the current amplitude (which contributes to radiation of the waves), flowing through the two parallel lines of the radiation element, extending in the main polarization direction, that is, in the right-left direction on the sheet is increased, and therefore the efficiency of the antenna is enhanced by the increased amplitude of the current. As shown in Figs. 6A and 6B, a construction in which a plurality of antennas are formed on one antenna board, using a plurality of radiation element pairs 2e, a plurality of radiation element pairs 2f and one feed equipment is possible. Figs. 6A and 6B are plan views showing the construction of the radiation elements formed on the antenna board in the first embodiment of the invention. In this embodiment, the ground plate 6 is provided beneath the antenna. However, if there is no need to provide the ground plate 6 in the vicinity of the antenna board 1, the antenna can be formed by the antenna board 1, having the radiation elements 2, the feed portion 3 and the matching circuit 4 formed thereon, and the transmission line 5, as shown in Fig. 2. In this case, the antenna can be further reduced in thickness, and can be further simplified in construction. Therefore, the antenna can be installed in a narrow space, and the productivity of the antenna device can be enhanced. A further arrangement of radiation elements 2 will be described. This construction is the same construction as the above example except for the arrangement of the radiation elements. Fig. 4 is a plan view showing the construction of the radiation elements formed on an antenna board. As shown in Fig. 4, the radiation element width W1 of two parallel lines of radiation elements 2, extending in a main polarization direction, that is, in a right-left direction on the drawing sheet, is larger than the radiation element width W2 of those portions of the radiation elements extending in a polarization direction perpendicular to the main polarization direction, that is, in a vertical direction on the drawing sheet. Generally, when a transmission line, such as a micro-strip line, is provided on an upper surface of a ground plate, the larger the line width of the radiation element 2 is, the larger the amount of radio waves radiated from this transmission line, is, if the distance h between the ground plate 6 and the radiation element 2 is constant. and in contrast, the smaller the line width of the radiation element 2 is, the smaller the amount of the radiated radio waves is. Therefore, the radiation element width W1 of the two parallel lines, extending in the main polarization direction, that is, in the right-left direction on the drawing sheet, is increased, and the radiation line width W2 of those portions of the radiation elements, extending in the polarization direction perpendicular to the main polarization direction, that is, in the vertical direction on the drawing sheet, is made smaller than the width W1. By doing so, the radiation of the horizontally-polarized waves in the direction, parallel to the drawing sheet, can be further increased while more efficiently suppressing the radiation of the vertically-polarized waves (which cancel each other) in the vertical direction on the drawing sheet. Therefore, the gain of the necessary horizontally-polarized waves can be increased while reducing a radiation loss due to the unnecessary vertically-polarized waves, thereby greatly enhancing the efficiency of the antenna can be realized. (First Embodiment) A first embodiment of the invention will be described with reference to the drawings. Fig. 27 is a perspective view showing the construction of an antenna of the first embodiment, and Figs. 28 and 29 are perspective views showing the construction of radiation elements in the first embodiment. In Fig. 27, an antenna board 20 is usually made of a dielectric material, and comprises a printed circuit board, a PET film board or the like having a conductor layer formed on one or both sides thereof, and radiation elements 21 and 22 with different electrical lengths are formed on these boards by etching method. With respect to the electrical length of the radiation elements 21 and 22, the line lengths L21 and L22 of the radiation elements 21 and 22 may be different from each other while their line widths W are the same, as shown in Fig. 28. Alternatively, the line widths W1 and W2 of the radiation elements may be different from each other while their line lengths are the same, as shown in Fig. 29. In either case, preferably, the element line length of each of the radiation elements is set to about 1/4 of a respective one of a plurality of line wavelengths λg1, λg2, ... λgn corresponding to a plurality of desired frequencies f1, f2, ... fn. A transmission line 24 is connected to an end 21a of the radiation element 21, and a transmission line 25 is connected to an end 22a of the radiation element 22. The transmission line 24 connects the radiation element 21 to a ground point 26 grounded to a ground plate 23 disposed substantially parallel to the radiation elements 21and 22, and the transmission line 25 connects the radiation element 22 to a ground point 27 grounded to the ground plate 23. Preferably, the transmission lines 24 and 25 are connected respectively to central portions of the ends 21a and 22a since this enhances antenna characteristics. The ground plate 23 may be provided on that side of the antenna board 20 facing the opposite side thereof having the radiation elements 21 and 22 formed, or the ground plate 23 may be provided separately in substantially parallel relation to the antenna board 20. A transmission line 28 is connected at a feed point F1 to the transmission line 24, and a transmission line 29 is connected at a feed point F2 to the transmission line 25. The transmission lines 28 and 29 are respectively connected to a common transmission line 30 (extending from the ground plate 23) through a feed portion 31, and feed high-frequency power in a matched manner to the radiation elements 21 and 22 via the feed points F1 and F2 through transmission lines 24 and 25. Although the transmission line 30 is disposed substantially vertically relative to both of the ground plate 23 and the antenna board 20, the transmission line 30 and the feed portion 31 may be both formed on the antenna board 20. The distance L1 between the ground point 26 and the feed point F1 is different from the distance L2 between the ground point 27 and the feed point F2, and by doing so, the impedance matching can be accurately achieved, and therefore the antenna device can be provided in which the resonance frequency is not shifted in the radiation elements 21 and 22, and which can be shared between two wavelengths. If the lengths of the transmission lines 28 and 29 are different from each other, the feed phase can be adjusted more easily. Coaxial cables, micro-strip transmission lines or the like can be used as the transmission lines 24, 25, 28, 29 and 30. In this embodiment, although only one feed portion 31 is provided, two separate feed portions may also be possible. In this embodiment, although only two radiation elements are provided, more than two radiation elements can of course be provided. The radiation elements 21 and 22 may be formed into a meandering line as shown in Fig. 32, or may be formed into any other suitable configuration. With the construction of the radiation elements 21 and 22 shown in Fig. 27, the dimension of each of the radiation elements 21 and 22 in the transverse direction can be reduced, and therefore the size of the antenna can be reduced, and therefore the antenna device, having a narrow, elongate configuration, can be achieved. With the construction of the radiation elements 21 and 22 shown in Fig 32, the length of the radiation elements 21 and 22 in the longitudinal direction can be reduced, and therefore there can be achieved the antenna device of a compact design having a smaller projected area. As shown in Fig. 36 (which is a perspective view showing the construction of a feed portion of the invention), a construction may be possible in which a coaxial feeder cable 36, connected to the transmission line 31, is provided between the ground plate 23 and the antenna board 20 in substantially parallel relation thereto. With this construction, the antenna can have a thinner design as compared with the case where a connector is exposed at the reverse surface of the ground plate 23. As shown in Fig. 37 (which is an enlarged, perspective view showing a feed portion of the invention), a metal member of an integral construction may be provided which includes braid-clamping portions 37 for clamping a braid of a coaxial feeder cable 36, and ground line portions 38 for grounding the radiation elements 21 and 22 via transmission lines 24 and 25. This metal member may be electrically connected to the ground plate by soldering or welding. In this construction, the structural portion, having the braid-clamping portions 37 (which receive the braid of the coaxial cable 36 therebetween, and are deformed by a tool (e.g. pliers) to hold this cable therebetween), is formed integrally with the ground line portions 38 for grounding the radiation elements 21 and 22 to the ground board 23 each of the ground line portions 38 having a soldering projection formed at its upper end. Therefore, the number of the component parts is reduced, and the assembling process is simplified, thus enhancing the productivity, besides the reliability is enhanced since the number of the connected and processed portions is reduced. As shown in Fig. 38 (which is a perspective view showing a grounded form of radiation elements in the invention), a construction may be provided in which a plurality of ground points 26 and 27 are grounded to the ground plate 23 by a wall 39 made of electrically-conductive metal. With this construction, the area of grounding is increased, and the electrical operation which is stable for noises can be effected, besides the mechanical strength of the structure increases. As shown in Fig. 39 (which is a perspective view showing the construction of an antenna device of the invention), a construction may be provided in which the plurality of radiation elements 21 and 22 are formed on the common antenna board 20, and a gap between the antenna board 20 and the ground plate 23 is maintained by spacers 40 made of a resin or the like. Owing to the provision of the spacers 40, the gap between the antenna board 20 and the ground plate 23 can be kept to a predetermined height, and the antenna device having stable antenna characteristics can be provided . In this construction, although the spacers 40 are provided in spaced relation to one another, a single board, made of a dielectric material, may be inserted in the gap. The operation of the antenna of the above construction will be described. The high-frequency power, supplied via the transmission line 31, is supplied to the radiation elements 21 and 22 via the feed portion 30, the transmission lines 28 and 29, the feed points F1 and F2, the transmission lines 24 and 25 and the ends 21a and 22a of the radiation elements 21 and 22. By setting the distance L1 between the ground point 26 and the feed point F1 and the distance L2 between the ground point 27 and the feed point F2 to different desired lengths, respectively, the power can be fed in a matched manner so that the desired impedance can be obtained. By suitably selecting the respective dimension of each portion of the radiation elements 21 and 22, the radiation elements 21 and 22 radiate radio waves into the air at the desired reasonance frequencies f1 and f2. In this case, as shown in Fig. 27, if the electrical length of the radiation element 21 is represented by Le1, and the electrical length of the radiation element 22 is represented by Le2, the following formulas are provided: Le1 ≒ C/4f1 Le2 ≒ C/4f2 A plurality of resonance characteristics are obtained (two cycles in the case) as shown in Fig. 30 (which is a diagram showing the characteristics of the radiation elements in the fourth embodiment of the invention). The current distribution I in the antenna of the above construction will be described with reference to Figs. 31A and 31B. Figs. 31A and 31B are views showing the current distribution in the antenna of this embodiment of the invention. As shown in Fig. 31A, the electrical length Le1, Le2 of each of the radiation elements 21 and 22 is set to about 1/4 of a respective one of the two line wavelengths λg1 and λg2 corresponding to the desired frequencies f1 and f2, and by doing so, the current is distributed generally half-sinusoidally in the antenna in such a manner that the amplitude is the maximum in the vicinity of the central poriton of the feed portion at the desired frequencies f1 and f2 while the amplitude is the minimum at the distal end portions of the radiation elements. As indicated by arrows (→) in the drawings, the direction of the current, flowing in a horizontal direction on the drawing sheet, is horizontal, and horizontally-polarized radio waves due to this current are radiated. In this case, the potential of the voltage distribution V is the minimum at the ground point, and is the maximum at the free end of the radiation element, as shown in Fig. 31(b). Therefore, the impedance Z at the feed point F is expressed by the following formula: Z = V ÷ I Here, the gap between the ground plate and the radiation elements is represented by h . As shown in Fig. 31B, by adjusting the distance between the ground point G and the feed point F, that is, by varying L, the values of the voltage distribution V and current distribution I are varied at that point (position) in accordance with the change of L. Namely, the impedance Z, representing the ratio of the voltage distribution V to the current distribution I, can also be adjusted by varying the distance L between the ground point G and the feed point F. As shown in Fig. 27, in the case where the resonance occurs at the two frequencies f1 and f2, the impedance Z1, Z2 can be adjusted for each of the frequencies f1 and f2 by adjusting the distance L1 between the ground point G and the feed point F1 and the distance L2 between the ground point G and the feed point F2 for each of the frequencies f1 and f2. Therefore, the matching with the impedance ZO of the feeder, such as a coaxial cable, can be achieved. (Second Embodiment) Next, a second embodiment of the invention will be described with reference to the drawings. Those portions of the antenna device of this embodiment, which are not described, are almost similar to those of the first embodiment. Figs. 33A and 33B are plan view showing the construction of radiation elements of the second embodiment, and the radiation elements in these Figures are different only in configuration. As shown in Figs. 33A and 33B, impedance devices 34 and 35 are provided at free end portions 32 and 33 of the radiation elements 21 and 22, respectively, and by setting these impedance values to desired values, respectively, the voltage standing wave ratio (VSWR) in a wide-band pattern can be obtained as indicated by a line b in Fig. 30. The desired results can be obtained if the impedance device is provided at one of the radiation elements. With this construction, even if slight variations are developed in the mass-produced antennas, the antenna characteristics can be kept within the predetermined range, and the antenna device, which is less liable to be defective during production, can be achieved. Besides, the antenna device of high reliability can be achieved in which the degradation of the antenna characteristics is extremely low even when snow, rain, dirt and so on deposits on the surface of the antenna device. As shown in Figs. 34A and 34B (which are plan views showing the construction of radiation elements of the invention), the impedance devices 34 and 35 are provided in the vicinity of feed points F1 and F2 at one or both sides, and by doing so, similar effects can be obtained. More specifically, generally, in the vicinity of the feed points F1 and F2, the value of the voltage distribution V is small, and the value of the current distribution I is large, and the impedance Z, representing the ratio of the voltage distribution V and the current distribution I, decreases (usually, several tens Ω, from Z = 5 to 75Ω). Therefore, the impedance devices 34 and 35 are provided in the vicinity of the feed points F1 and F2, and their impedance values are set to desired values, respectively, and by doing so, the VSWR in a wide-band pattern can be obtained. Besides, by setting the values of the impedance devices 34 and 35 to desired values, respectively, the radiation power can be varied arbitrarily. For example, a solid line a in Fig. 35 (which shows the relation between the radiation power and the frequency in the invention) represents the radiation power obtained when the impedance devices 34 and 35 are substantially the same in impedance device ratio, and a line b represents the radiation power obtained when the impedance devices 34 and 35 are not equal in impedance device ratio. Thus, in accordance with the demand of the radio system, the radiation power at the frequencies f1 and f2, that is, the antenna gain, can be adjusted. Besides, using the above constructions in combination, the antenna device can be provided which has a wider band and adjustable in antenna gain

Claims: An antenna device comprising a first radiation element (21), a second radiation element (22) different in electrical length from said first radiation element (21), a first feed point (F1) for feeding electric power to said first radiation element (21), and a ground plate (23) provided in spaced relation to said first radiation element (21) and said second radiation element (22), said first radiation element(21) being grounded to said ground plate (23) at a first ground point (26) and said second radiation element (22) being grounded to said ground plate (23) at a second ground point (27), said antenna device being characterised by a second feed point (F2) for feeding electric power to said second radiation element (22), the distance (L1) between said first ground point (26) and said first feed point (F1) being different from the distance (L2) between said second ground point (27) and said second feed point (27). An antenna device according to claim 1, in which widths (W1, W2) of said first and second radiation elements (21, 22) are equal to each other while lengths (L21, L22) thereof are different from each other, so that an electrical length of said first radiation element (21) is different from an electrical length of said second radiation element (22). An antenna device according to claim 1 in which lengths (L21, L22) of said first and second radiation elements (21, 22) are equal to each other while widths (W1, W2) thereof are different from each other , so that an electrical length of said first radiation element (21) is different from an electrical length of said second radiation element (22). An antenna device according to any preceding claim, in which an impedance device (34, 35) is provided in the vicinity of a free end portion of the radiation element (21, 22). An antenna device according to any preceding claim, in which an impedance device (34, 35) is provided in the vicinity of the feed point (F1, F2). An antenna device according to any preceding claim, in which a feeder, connected to feed means for feeding the electric power both to said first and second feed points (F1, F2), comprises a coaxial cable (36), and said coaxial cable (36) is provided between said ground plate (23) and said first and second radiation elements (21, 22), and is disposed generally parallel to said ground plate (23) and said first and second radiation elements (21, 22). An antenna device according to claim 6, in which a single metal member of an integral construction is provided including a clamp portion (37) for clamping said coaxial cable (36), a first ground line portion (24) extending from said first ground point (26) of said first radiation element (21) to said first feed point (F1), and a second ground line portion (25) extending from said second ground point (27) of said second radiation element (22) to said second feed point (F2),and said metal member is electrically connected to said ground plate (23). An antenna device according to any preceding claim, in which said first ground point (26) and said second ground point (27) are grounded to said ground plate (23) through a metal wall (38). An antenna device according to any preceding claim, in which said first radiation element (21) and said second radiation element (22) are formed on a common printed circuit board (20). An antenna device according to any preceding claim, in which each of said first radiation element (21) and said second radiation element (22) is formed by a meandering line and an element length of said meandering line is set to about ¼ of a corresponding line wavelength.

Inventors: DEGUCHI FUTOSHI, NAKASHIMA KAZAYUKI, TATE SUMIO, DEGUCHI, FUTOSHI, NAKASHIMA, KAZAYUKI, TATE, SUMIO
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1999
Main classification: H01Q1
IPC codes: H01Q1, H01Q9