Antenna device

An antenna device includes radiation elements formed on a common antenna board, a feeding unit for feeding the radiation elements, and a ground plate provided in electrically-spaced relation to the radiation elements. The ground plate is mounted on a box-like metal body to form a short-circuit therewith. The radiation elements may have different line lengths for transmitting and receiving a plurality of wavelengths. The feeding unit may include a first radiation element having a different electrical length than a second radiation element, a first feed point for feeding electric power to the first radiation element, and a second feed point for feeding radiation elements are grounded to the ground plate at respective first and second ground points. The difference from the first and second feed points to their respective ground points are different from one another. The antenna device may have a value equal to (H).div.(.LAMBDA.), where (H) is the distance between the radiation elements and the ground plate, and (.LAMBDA.) is a wavelength of at least one frequency transmitted or received by the antenna device, and the distance H satisfies a relationship of 1.div.250.ltoreq.(H).div.(.LAMBDA.).ltoreq.1.div.100.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 This invention relates to an antenna device of a compact, thin design
 having wideband frequency characteristics.
 2. 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, and also to prevent damage to the antenna upon contact with other
 objects. 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 THE 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 as a vehicle body.
 In order to achieve the object of present invention an antenna device of
 the present invention comprising radiation elements, feed means for
 feeding electric power to the radiation elements, a ground plate, and a
 cover member covering the radiation elements and the feed means, which is
 mounted on metal cubic and using short circuit between ground plate and
 metal cubic, wherein a value, obtained by dividing the distance (H)
 between the radiation elements and the ground plate by a wavelength
 (.LAMBDA.), is 1.div.250.ltoreq.H.LAMBDA..ltoreq.1.div.80, and preferably
 1.div.200.ltoreq.H.div..LAMBDA.1.div.100.
 With this construction, an overall loss of the antenna can be suppressed
 while suppressing the decrease of the antenna impedance, and therefore
 there can be provided the antenna device of high reliability which can
 positively operate under a wide range conditions of use.
 The plurality of radiation elements and the feed equipment are formed on a
 common antenna board, and therefore as compared with the case where such
 elements are formed on separate boards, the antenna construction can be
 simplified, and the productivity can be enhanced, and 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 becomes
 possible corresponding to wavelength, and also the polarization
 characteristics of the antenna can be enhanced.
 Electric power is fed to the plurality of radiation elements through the
 single feed equipment, and therefore the area of the antenna board, which
 is occupied by the feed equipment can be reduced, then the antenna board
 size can be reduced. Besides, since the power is fed to the plurality of
 radiation elements through the common feed equipment, the power can be fed
 to the plurality of radiation elements under the same condition, and
 therefore there can be provided the antenna of high reliability having
 excellent antenna characteristics each frequency to which the plurality of
 radiation elements correspond, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Preferred embodiments of the present invention will be described below.
 First Embodiment
 A first embodiment of an antenna device of the present invention will now
 be described with reference to the drawings.
 FIG. 1 is a perspective view of the first embodiment of the antenna of the
 invention, and FIG. 2 is a perspective view of the antenna of the first
 embodiment 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 the
 first embodiment of the invention.
 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 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
 (.lambda.g1), and the radiation element pair 2f, constituted by the
 radiation elements 2c and 2d, corresponds to a long wavelength
 (.lambda.g2).
 The radiation elements 2a and 2b are formed substantially symmetrically in
 a longitudinal direction a of teh 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
 lind being bent regularly into such a meandering configuration wherein a
 line width w.apprxeq..lambda./100.about..lambda./400, an element interval
 d.apprxeq..lambda./100.about..lambda./200, and an element width
 l.apprxeq./10.about..lambda./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.apprxeq.(.lambda.g1)/4), and each of the radiation elements 2c and 2d,
 constituting the radiation element pair 2f, has the length L2
 (L2.apprxeq.(.lambda.g2)/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 embodiment. 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 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, differ from those
 of FIGS. 14A to 15B in a ground plate being provided.
 The arrangements, shown in FIGS. 22A, 22B, 23A and 23B, differ from those
 of FIGS. 16A to 17B in a ground plate being provided.
 The arrangements, shown in FIGS. 24A, 24B, 25A and 25B, differ from those
 of FIGS. 18A and 19B in 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. In this embodiment, 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 element s 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 use d 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 5 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 .di-elect cons.1, the
 thickness of the antenna board 1 is represented by t, the dielectric
 constant of the gap 9 between the antenna board I and the ground plate 6
 is represented by .di-elect cons.2, and the velocity of light is
 represented by C, the resonance frequencies f1 and f2 of the antenna are
 expressed by the following formulas:
EQU f1.apprxeq.C/2L1.di-elect cons. (Formula 3)
EQU f2.apprxeq.C/2L1.di-elect cons. (Formula 4)
 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:
EQU .di-elect cons..apprxeq.(.di-elect cons.1.times..di-elect
 cons.2(t+h))/(.di-elect cons.1.times.h+.di-elect cons.2.times.t) (5)
 FIGS. 11A and 11B are graphs showing the resonance characteristics of the
 antenna of the first embodiment of the invention.
 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 the antenna of the first embodiment of
 the invention. 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 .lambda.g1 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 (.fwdarw.) 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.
 Second Embodiment
 A further preferred arrangement of radiation elements 2 will be described.
 This construction is the same construction as the first embodiment 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 in the second embodiment of the invention. As shown in FIG. 4, in
 this embodiment, 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. 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.
 Third Embodiment
 Next, a third embodiment of the present invention will be described with
 reference to the drawings. FIG. 7 is a perspective view showing the
 construction of an antenna of the third embodiment of the invention, and
 those members identical to those of the first embodiment will be
 designated by identical reference numbers, respectively.
 In the antenna of this embodiment, an antenna board 1, on which radiation
 elements 2, a feed portion 3 and a matching circuit 4 are formed, and a
 ground plate 6 are basically similar in construction to those of the first
 embodiment, respectively.
 Spacers 10 are made of an elastic material such as rubber or a resin, and
 are interposed, as support ,members, between the antenna board 1 and the
 ground plate 6 to keep the gap between the antenna board 1 and the ground
 plate 6 accurately to a height h. Preferably, the spacers 10 are mounted
 respectively on those portions of the antenna board 1 on which the
 radiation elements 2 are not formed. With this arrangement, a change in
 dielectric constant of the gap between the ground plate 6 and the antenna
 board 1, which affects the antenna characteristics, can be kept to the
 minimum.
 Although not shown in the drawing, in order that the mounting positions can
 be easily recognized, mounting recesses are preferably formed in at least
 one of the ground plate 6 and the antenna board 1, and the spacers 10 are
 fitted in these recesses, respectively. With this construction, the
 spacers 10, when mounted, will not be disposed out of position, and
 therefore a change of the antenna characteristics, resulting from the
 mounting of the spacer 10 on that portion of the antenna board 1 on which
 the radiation element 2 is formed, hardly occurs. As a result, the antenna
 of high reliability can be achieved.
 An arrangement may be used in which projections are formed on at least one
 of the ground plate 6 and the antenna board 1 while recesses for fitting
 respectively on these projections are formed in the spacers 10,
 respectively. Alternatively, an arrangement may be used in which through
 holes are formed through at least one of the ground plate 6 and the
 antenna board 1, and the spacers 10 are fixed by screws passing
 respectively through these through holes.
 In this embodiment, in addition to the spacers 10 provided only between the
 antenna board 1 and the ground plate 6, spacers may be further provided
 between the ground plate 6 and a radome 11. In this case, preferably, the
 spacers, provided between the antenna board 1 and the ground plate 6, are
 larger in height than the spacers provided between the ground plate 6 and
 the radome 11, and with this construction the antenna board 1 is spaced
 farther from the ground plate 6, and this enhances the antenna
 characteristics.
 If the additional spacers are not used, it is preferred for the same reason
 that the antenna board 1 should be disposed closer to the radome 11 than
 to the ground plate 6.
 Preferably, the elasticity of the spacer is different depending on whether
 the spacer is held in contact with the obverse surface or the reverse
 surface of the antenna board 1. More specifically, the spacer, held in
 contact with that surface of the antenna board 1 having the radiation
 elements 2 formed thereon, has higher elasticity, and the spacer, held in
 contact with that surface of the antenna board 1 having no radiation
 element 2 formed thereon, has lower elasticity. With this construction,
 when the antenna board 1 is displaced out of position by vibrations or the
 like, the possibility of damaging the radiation elements 2 by the spacers
 is lowered.
 In the case where the antenna board 1 is held in direct contact with the
 radome 11, it is preferred for the same reason that that surface of the
 antenna board 1, having the radiation elements 2 formed thereon, should
 face the ground plate 6.
 The radome 11 is provided to cover the antenna board 1 having various
 circuits and so on formed thereon, and preferably the radome 11 is made of
 a material (e.g. a resin) having weather resistance. The antenna board 1,
 the spacers 10 and so on are covered with the radome 11 and the ground
 plate 6, and the radome 11 and the ground plate 6 are bonded together by a
 bonding material, or fixed together by bolts or the like. Preferably, the
 boundary portion between the radome 11 and the ground plate 6 is sealed
 against water or moisture by a waterproof seal member or an O-ring, and by
 doing so, the intrusion of water into the inside of the antenna is
 prevented, thereby preventing the degradation of the antenna
 characteristics and the malfunction of the antenna.
 More preferably, the inside of the antenna is completely sealed, and inert
 gas, such as dry air or nitrogen gas, is sealed in the inside of the
 antenna. With this construction, dew condensation, developing within the
 antenna which may be used in a weather-beaten condition, can be
 suppressed, and therefore the degradation of the antenna characteristics
 and the -malfunction of the antenna due to such condensation can be
 prevented.
 Mounting holes 12 are formed through end portions of the ground plate 6,
 and the ground plate 6 of the antenna can be mounted directly on a
 box-like metal body of a vehicle or a container through the mounting holes
 12. With this construction, the antenna device of this embodiment can be
 mounted or installed directly on the mounting body or structure, with the
 ground plate 6 serving as the bottom surface. Therefore, the height of the
 antenna device from the installation surface is smaller as compared with
 the case where the antenna device is installed with using an
 antenna-mounting member.
 In this embodiment, although the mounting holes are arranged only in the
 transverse direction of the antenna, the mounting holes may be formed and
 arranged only in the longitudinal direction, or may be formed and arranged
 in the transverse and longitudinal directions in surrounding of the
 antenna device.
 If the ground plate 6 of the antenna and the box-like metal box (i.e., the
 mounting object) are bonded together by an electrically-conductive bonding
 material, the mounting holes 12 do not need to be provided.
 Next, the mounting of the antenna board 1 will a be described. In this
 embodiment, the antenna board 1 is pressed against the inner surface of
 the radome 11 by the spacers 10, provided between the antenna board 1 and
 the ground plate 6, and therefore is fixed. In this case, the spacers 10
 have a certain degree of elasticity, and the inner surface of the radome
 11 is disposed substantially parallel to the ground plate 6 after the
 assembling of the antenna is completed. The antenna board 1 and the
 spacers 10 are beforehand located at their respective predetermined
 positions relative to the ground plate 6, and in this condition the radome
 11 is mounted on the ground plate 6. By doing so, the mounting and fixing
 of the spacers 10 relative to the antenna board 1 and the ground plate 6,
 as well as the fixing of the antenna board 1 relative to the ground plate
 6, can be effected, and therefore the antenna of high productivity can be
 provided in which the number of the antenna-assembling and mounting steps
 can be reduced. Since the antenna board 1 is pressed against the radome
 11, the antenna board 1 can have the good flatness with the simple
 construction, and therefore the distance between the antenna board 1 and
 the ground plate 6, which particularly exerts a great influence on the
 antenna characteristics, can be made substantially constant. Therefore,
 there can be achieved the antenna which has the excellent antenna
 characteristics and productivity, and can deal with a plurality of
 frequencies.
 For fixing the radome 11 and the ground plate 6 together, mounting holes
 are formed in those portions of the radome 11 aligned respectively with
 the mounting holes 12, and common mounting members are used.
 Alternatively, mounting holes are formed in the radome 11, and mounting
 portions are formed on those portions of the ground plate 6 aligned
 respectively with these mounting holes in the radome 11, and the fixing is
 effected using fixing members such as screws. In the case where the
 mounting portions are formed on the ground plate 6, it is preferred that
 projections or convex portions, projecting toward the outer surface of the
 antenna, should not be formed. With this construction, the height of the
 antenna device can be lowered, and also the flatness of the ground plate
 6, serving as the mounting surface for mounting on the antenna-mounting
 object, can be secured. Therefore, there can be achieved the antenna
 device in which the mounting operation is easy, and which has the good
 stability.
 The installation of the antenna of the above construction on the mounting
 object (particularly on a vehicle (truck)) will be described with
 reference to the drawings.
 FIG. 8 is a side-elevational view showing the installation of the antenna
 of the third embodiment on the vehicle, and FIG. 9 is an enlarged view
 showing an antenna-mounting portion in the third embodiment of the
 invention.
 Reference number 13 denotes a vehicle body (mounting object), and the
 above-mentioned antenna 14 is installed on a box-like metal body 13a of
 the upper part of the vehicle body 13. More specifically, through holes
 13b are formed respectively through the box-like metal body 13a aligned
 respectively with the mounting holes 12 in the antenna 14, and a bolt 15a
 of fixing means 15 is passed through the aligned mounting hole 12 and
 through hole 13, and the bolt is tightened using a nut 15b threaded on the
 distal end of the bolt, thus mounting the antenna 14 on the vehicle body
 13.
 In this case, preferably, the ground plate 6 of the antenna 14 is held in
 direct contact with the box-like metal body 13a of the car body 13. This
 will be described below. In many cases, the antenna 14 of this embodiment
 is mounted on the mounting surface of a box-like metal body of a car or a
 container (mounting object) in parallel relation thereto. Generally, when
 an antenna is mounted in close proximity to a box-like metal body
 (mounting object) of a car or a container, the characteristics of the
 antenna are often adversely affected even if the antenna is beforehand so
 adjusted that the antenna characteristics become optimum at a
 predetermined frequency. The reason is that the antenna impedance is
 influenced by the box-like metal body, on which the antenna is mounted,
 and is decreased, so that the loss increases because of the mismatching
 with an impedance of a feeder.
 In order to prevent such antenna characteristics change, the ground plate 6
 of the antenna 14 is exposed, and is adapted to be in direct contact with
 the box-like metal body 13a of the vehicle body 13. With this
 construction, the ground plate 6 of the antenna 14 and the box-like metal
 body 13a can be kept at the same potential. and therefore a change of the
 antenna characteristics due to the influence of the box-like metal body
 13a can be almost eliminated. The ground plate 6 of the antenna 14 and the
 box-like metal body 13a need only to be held in electrical contact with
 each other, and therefore the ground plate 6 and the box-like metal body
 13a may be bonded together by an electrically conductive bonding material,
 instead of using the fixing means 15. In this case, the installation of
 the antenna 14 on the vehicle body 13 can be effected easily, besides here
 is no need to provide the mounting holes 12 in the round plate 6.
 Therefore, the construction of the round plate 6 can be simplified, and
 the antenna can be achieved which has high productivity and a low-cost
 design, and can be used easily.
 Although not shown in the drawing, a cushioning member may be provided
 between the antenna 14 and the vehicle body 13. In this case, an impact
 due to vibrations, developing during the movement of the car or the
 container (on which the antenna 14 is installed), is not reached the
 antenna 14 directly, and therefore the antenna 14 is not damaged by such
 impact, then the reliability of the antenna 14 can be enhanced.
 Preferably, the fixing means 15 is made of metal in order that the ground
 plate 6 and the box-like metal body 13a can be positively held in
 electrical contact with each other. This is effective particularly where
 the cushioning member is interposed between the antenna 14 and the vehicle
 body 13 and where the antenna 14 and the vehicle body 13 are not held in
 direct contact with each other.
 Reference number 16 denotes fixing means-assisting member. Preferably, the
 fixing means-assisting member 16 has a ring-shape, and is made of a
 material (e.g. rubber) which has a waterproof property so as to prevent
 the intrusion of water into the inside of the mounting object through the
 fixing means 15, and has a certain degree of elasticity. Such fixing
 means-assisting member 16 effectively suppresses the intrusion of water.
 Specifically, a washer, made of rubber, can be used as the fixing
 means-assisting member.
 Next, the thickness of the antenna 14 to be mounted on the box-like metal
 body 13a will be studied. FIG. 10 is a graph showing the correlation
 between the distance between the radiation elements and the ground plate
 and a loss of the antenna in the third embodiment of the invention.
 In an antenna in which the distance between the antenna board 1 and the
 ground plate 6 is close as in the antenna of this embodiment, a copper
 loss (loss due to a heat loss of the copper element, which is indicated by
 B in FIG. 10) is in inverse proportion to the distance H (hereinafter, the
 distance H) between the ground plate 6 and the radiation elements 2
 provided the width of the radiation element 2 is constant, and the loss
 decreases with the increase of the distance H. A radiation loss (loss due
 to radiation, which is indicated by A in FIG. 10), is in proportion to the
 distance H between the radiation elements 2 and the ground plate 6
 provided the width of the radiation element 2 is constant, and the loss
 increases with the increase of the distance H. Usually, the receiving
 sensitivity of the antenna greatly varies, depending external factors,
 though it somewhat varies, depending on the intensity of the target radio
 waves and the corresponding frequency. Therefore, in order that the
 antenna can be kept in a usable condition under any circumstances, it is
 necessary that the internal loss (the sum of the copper loss and the
 radiation loss; A+B in FIG. 10) should be kept to the minimum. Generally,
 the allowable internal loss is not more than 1 dB, and particularly when
 transmitting and receiving weak radio waves in a satellite communication
 or the like, the allowable internal loss is not more than 0.5 dB. In view
 of this, the distance H is as follows;
 1.div.250.ltoreq.H.div..lambda..ltoreq.1.div.80, and preferably
 1.div.200.ltoreq.H.div..lambda..ltoreq.1.div.100 (where .lambda.
 represents a wavelength), the good antenna efficiency can be obtained, and
 therefore the antenna of high reliability can be provided which can
 positively operate under a wide range of conditions of use.
 For example, in a mobile satellite communication system which effects a
 ground-satellite-ground data communication using a satellite orbiting in a
 low orbit, assume that this antenna 14 is used in Obcomb communication to
 which 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 are
 assigned in WARC' 92 (Meeting of World Radio communication Association,
 1992). The range of variation of the wavelength (.lambda.) is
 2000.ltoreq..lambda..ltoreq.2190 (mm), and therefore good antenna
 characteristics can be achieved at all wavelength bands In the range in
 which the range of obtained with the shortest wavelength (.lambda.=2000),
 overlaps the range H obtained with the longest wavelength (.lambda.=2190).
 This range is 8.76.ltoreq.H.ltoreq.25 (mm), and preferably
 10.95.ltoreq.H.ltoreq.20 (mm).
 If the distance H is in the above range, the sum of the radiation loss and
 the copper loss is suppressed to the minimum, and therefore the antenna
 can be achieved which has a small loss and good antenna characteristics as
 a whole. Besides, the thickness of the antenna 14 is very small, and
 therefore the antenna can be easily installed on a container or the like
 designed to be used in a stacked condition. More specifically, when
 freight containers are stacked together, a gap, formed between the
 containers, is 1 to 2 inches at the largest, and when the antenna 14 is
 used for Obcomb communication, the antenna 14 can be easily formed into a
 thickness within this range. Therefore, the antenna can be achieved which
 being mounted on the container, enables the stacking of the containers.
 Besides, by short-circuiting the ground plate 6 and the box-like metal
 body of the container together, the change of antenna impedance can be
 suppressed to the minimum even when the container is stacked on the
 antenna 14, and therefore the antenna can be achieved which has good
 antenna characteristics even in a stacked condition.
 Fourth Embodiment
 Next, a fourth 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 fourth embodiment, and FIGS. 28 and 29 are perspective
 views showing the construction of radiation elements in the fourth
 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 .lambda.g1, .lambda.g2, . . . .lambda.gn 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 21 and 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 i-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 shared between two wavelengths.
 If the lengths of the transmission lines 27 and 28 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 be of course 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 dimension of the each portion of the radiation
 elements 21 and 22, the radiation elements 21 and 22 radiate radio waves
 into the air at the desired resonance 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:
EQU Le1.apprxeq.C/4f1 (Formula 1)
EQU Le2.apprxeq.C/4f2 (Formula 2)
 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 the fourth 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 .lambda.g1 and .lambda.g2
 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
 portion 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 (.fwdarw.) 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:
EQU Z=V.div.I (Formula 3)
 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 Z0 of the feeder, such as a coaxial cable, can be achieved.
 Fifth Embodiment
 Next, a fifth 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 fourth
 embodiment. FIGS. 33A and 33B are plan view showing the construction of
 radiation elements of the fifth 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 .OMEGA., from Z.apprxeq.5 to 75.OMEGA.). 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 broken 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.