FR2742003A1 - ANTENNA AND ANTENNA ASSEMBLY FOR MEASURING MICROWAVE ELECTROMAGNETIC FIELD DISTRIBUTION - Google Patents

Abstract

An antenna includes a coaxial cable (12), a half-wave dipole antenna structure (13) connected to the end of the cable, and a reflector plate (11). The end of the outer conductor of the cable is divided by slits (14) into two shells (12a, 12b) over a length of <T364> / 4. One element (13a) of the dipole is connected to the center conductor (12c) and to the end of a shell (12a), while the other element (13b) is connected to the other shell (12b). The reflector plate (11) is substantially orthogonal to the cable and is located at a distance of <T364> / 4 from the end of the cable.

Description

ANTENNA AND ANTENNA ASSEMBLY FOR MEASURING A DISTRIBUTION OF

The present invention relates to an antenna which is preferably applied to the observation of an electromagnetic field distribution, and it relates more particularly to a polar dipole antenna.

Linearly designed for use in the microwave band. Measuring a spatial distribution of an electromagnetic field plays an important role in developing protective measures against electromagnetic interference and reducing unnecessary electromagnetic radiation. In addition, a spatial distribution of electromagnetic field is also measured when viewing images of radio wave sources using the principle of holograms.

As shown in Fig. 1, when measuring a spatial distribution of an electromagnetic field, the electromagnetic field of a desired observation frequency band is generally observed using an XY scanner, 81, and by causing an antenna 83 connected to a receiving cable 82 to perform a scanning movement in the X and Y directions, in the plane of observation of the electric field, 84. The antenna 83 can be used with a loop antenna or a dipole antenna when measuring frequencies below the microwave band, such as frequencies of a few hundred megahertz or less. However, when measuring the spatial distribution of the electromagnetic field in the microwave band, the cross polarization due to the common mode current cannot be attenuated or canceled with a loop antenna or a dipole antenna. and the measurement tends to be influenced by reflected waves from the antenna scanning system. It follows that one uses a horn antenna or a waveguide with or- end.

green, for the observation antenna 83, to perform a measurement in the microwave band. The antenna which is used for example to measure a spatial distribution of an electromagnetic field is more particularly called a "probe antenna". An explanation of the common mode current of a dipole antenna will first be given with reference to Figure 2. Although parallel lines can be used to feed an antenna.

With a dipole antenna in a low frequency band, a coaxial cable must be used at frequencies above a few hundred megahertz, since the high level of losses which occurs at these frequencies prevents the use of parallel lines. Therefore, a dipole antenna 92 is connected to one end of a coaxial cable 91, as shown in Fig. 2. In other words, an element 92a of the dipole antenna 92 is connected to the center conductor of the cable. coaxial 91, and the other element 92b is connected to the outer conductor. As a result, the field component which is due to the dipole antenna 92 is oriented in the longitudinal direction of the dipole antenna 92, i.e. in the vertical direction of the figure. In addition, the coaxial cable 91

which is connected to the dipole antenna 92 is an unbalanced line, and therefore a common mode current flows through the coaxial cable 91. The field direction which is due to this common mode current is oriented in the longitudinal direction of the coaxial cable 91, that is to say in the horizontal direction of the figure.

Figures 3A-3F are graphical representations showing the directional characteristics of the dipole antenna 92 described above, at 10 GHz. Figures 3A and 3B show the directivity, i.e. the field diagram, (Co-Pol), in a plane perpendicular to the longitudinal direction of the dipole antenna 92, Figures 3C and 3D show the directional characteristic in cross polarization (X-Pol)

, 30 and Figures 3E and 3F show the directivity divided by the directional characteristic in cross polarization, i.e. the selectivity of

polarization (Co / X). In this type of prior art antenna, the cross-polarization component is high, and the selective observation of a component of only specific polarization is problematic. As shown in Figure 4, when the observation frequency

drops below a few hundred megahertz, the common mode current can be attenuated relatively easily by loading the coaxial cable 91 with a ferrite ring 94, but in the microwave band it is still difficult to attenuate the common mode current 5 by charging the cable with a ferrite ring.

In the present description, in a half-wave dipole antenna which is fed from its center, elements are called the parts of conductors located on either side of the feed point. In addition, the base sides of the elements are the feed points. As described above, the measurement of a spatial distribution of electromagnetic field in the microwave band necessitated the use of a horn antenna or a waveguide with orifice.

green, and neither a dipole antenna nor a boom antenna could be used.

key. However, a horn antenna has a strong directivity in a forward direction, and therefore it cannot observe electromagnetic waves over a wide region or from a wide range of angles. On the other hand, although an open-ended waveguide has a wide directivity as compared with a horn antenna, its field diagram is complex and therefore the correction of the observation data according to the directional diagram is com. - plex. Two-dimensional observation of electromagnetic fields is used not only to find the distribution of electromagnetic field in an observation plane, but also to find the current distribution or directivity of radiation on the surface of an object. observed, by inverse Fourier transformation or transformation of

Inverse fresnel of observational data, as when picking up a hertzian wave hologram. An antenna used when making this type of observation must have a wide and regular directivity. Both a horn antenna or an open ended waveguide have the problem of not being sufficiently applicable for air wave holography. In addition, a horn antenna and an open-ended waveguide are both large and heavy, expensive in themselves, and furthermore, they do not have the necessary compatibility for installation on a scanner. or an XY scan plate. The object of the present invention is to provide an antenna

linearly polarized, having a light, compact and relatively simple structure, which has excellent compatibility with a scanning structure and which can accurately measure a spatial distribution of electromagnetic field, for example in the microwave band. In addition, the present invention also aims to pro-

curing an antenna intended to receive electromagnetic waves over a wide region and from a wide range of angles, which is not affected by a reflection error from the scanning system, and which, in addition, has almost zero directivity in the rear direction.

The objects of the present invention can be achieved by an antenna having a coaxial line and a dipole antenna structure connected to the end of the coaxial line, wherein: a pair of slots extending in a longitudinal direction of the coaxial line 15 are formed on an outer conductor of the coaxial line, in a region of a length determined from the end of the coaxial line, thereby to split the outer conductor into two shells; an element constituting the dipole antenna structure is connected at its base side to a center conductor of the coaxial line and to one end of a shell of the outer conductor; and the other element constituting the dipole antenna structure is connected from the side of its base to one end of the other shell of the outer conductor.

When considering installation on a scan tray, a dipole antenna and a loop antenna are obviously superior to an open ended horn antenna or waveguide, both because of their low weight and ease of use. Further, in order to achieve the objects of the present invention, one must also consider the attenuation of the common mode current which occurs in a dipole antenna or a loop antenna, and the reduction of the directional gain in the backward direction. . As described above, a common mode current results from the connection of an unbalanced line to a balanced antenna, and one must therefore consider the insertion of a balanced / unbalanced transformer circuit, or "balun". However, increasing the radius of the loop of a loop antenna increases the cross-polarization component, while reducing the radius of a loop antenna.

the loop reduces the resistive component in the impedance and complicates the adaptation by the balun. Therefore, in the present invention, the dipole antenna and the end of the coaxial line are inherently connected by means of a distributed constant balun which is formed by quarter wave lines. In addition, a structure having a reflector plate is adopted to reduce the directional gain towards

the back. In concrete terms, slits of a fixed length are formed in the outer conductor at the end of the coaxial line, thereby forming a split-end configuration in the outer conductor of that region, and the outer conductor of these split portions is used as a distributed constant balun. In this way, besides the fact that one can reduce the sensitivity to cross polarization, one can also obtain a linearly polarized antenna which is lightweight and of small bulk. With such an antenna, one can accurately measure a spatial distribution of electromagnetic field in the microwave band.

In the present invention, the dipole antenna is typically a half-wave dipole antenna, and the elements of the dipole antenna are typically disposed so as to be substantially orthogonal to the longitudinal direction of the line. coaxial. You can use a coaxial cable for the coaxial line. Furthermore,

if we denote by. the wavelength which corresponds to the central frequency which is used in the antenna, the determined length is preferably fixed at? / 4. To attenuate the directional gain in the rear direction, a reflector plate is incorporated which is formed by a conductor and which is substantially orthogonal to the longitudinal direction of the line.

coaxial gne, so as to come into electrical contact with the outer conductor at a position located at a determined distance from the end of the coaxial line, and each element may be arranged parallel to the surface of the reflector plate. In the case where a reflector plate is incorporated, a configuration can be adopted in which two dipole antennas corresponding to two orthogonal polarization components are respectively disposed at a distance from each other on the same reflector plate. On the other hand, in a sys-

teme in which multiple reflections between the reflector plate and an observed object present a problem, instead of incorporating a reflector plate, a radio wave absorber can be attached to the outer surface of the coaxial line, at a distance of the determined region of ion described above.

To widen the observation frequency band, a method can also be employed in which an ordinary dipole antenna intended for wideband operation is used, such as a method using a butterfly-type dipole antenna. Other characteristics and advantages of the present invention will be better understood on reading the following description of embodiments, given by way of non-limiting examples. The remainder of the description refers to the accompanying drawings, in which:

Fig. 1 is a principle perspective view illustrating a method of measuring a spatial distribution of an electromagnetic field; Figure 2 illustrates a common mode current that appears in a dipole antenna connected to a coaxial line; Figures 3A and 3B are diagrams showing the direction of the antenna shown in Figure 2, respectively in polar coordinate representation and rectangular coordinate representation; Figures 3C and 3D are diagrams showing the cross-polarized directional characteristic of the antenna shown in Figure 2, respectively in a polar coordinate representation and a rectangular coordinate representation;

Figures 3E and 3F are diagrams showing the polarization selectivity of the antenna shown in Figure 2, respectively.

ment in a representation in polar coordinates and in a representation in rectangular coordinates; Fig. 4 is a perspective view showing a prior art dipole antenna in which a common mode current is attenuated; Fig. 5A is a perspective view showing the structure 35 of an antenna according to the first embodiment of the present invention.

invention; Figure 5B is a plan view of the essential part of the antenna shown in Figure 5A; Fig. 6 is an equivalent circuit diagram of the antenna 5 according to the first embodiment; Figs. 7A and 7B are diagrams showing the directionality of the antenna of the first embodiment, respectively in polar coordinate representation and in rectangular coordinate representation;

Figs. 7C and 7D are diagrams showing the cross-polarized directional characteristic of the antenna of the first embodiment, respectively in polar coordinate representation and in rectangular coordinate representation; Figs. 7E and 7F are diagrams showing the polarization selectivity of the antenna of the first embodiment, respectively in polar coordinate representation and in rectangular coordinate representation;

Fig. 8 is a perspective view showing the structure of an antenna for normal observation, equipped with a balun and a reflector plate; FIGS. 9A and 9B are diagrams showing the directivity of the antenna shown in FIG. 8, respectively in polar coordinate representation and in rectangular coordinate representation; Figs. 9C and 9D are diagrams showing the cross-polarized directional characteristic of the antenna shown in Fig. 8, respectively in polar coordinate representation and in rectangular coordinate representation; Figures 9E and 9F are diagrams showing the polarization selection of the antenna shown in Figure 8, respectively in polar coordinate representation and in rectangular coordinate representation;

Fig. 10 is a perspective view showing the structure of an antenna according to the second embodiment of the present invention; 2742003

Fig. 11 is a perspective view showing the structure of an antenna according to the third embodiment of the present invention; and Fig. 12 is a perspective view showing the structure of an antenna assembly according to the fourth embodiment of the present invention. First embodiment

The antenna 10 according to the first embodiment of the present invention, which is shown in Figures 5A and 5B, is a probe antenna which is suitable for measuring a spatial distribution of an electromagnetic field. This antenna 10 is constructed by inserting one end of a coaxial cable 12 into a reflector plate 11, through the rear side of the plate and substantially into the central part thereof, so that the coaxial cable 12 is erected perpendicularly. -

15 dicularly to the reflector plate 11. The reflector plate 11 is formed by a conductor and has dimensions sufficiently greater than the wavelength X, denoting by X the central wavelength of the object's signals. observation of the antenna, that is to say the wavelength which corresponds to the nominal central frequency. In

In the example shown here, the reflector plate 11 is a square having sides 2X in length. The protrusion length of the coaxial cable 12 from the reflector plate 11 is approximately X14, and the outer conductor of the coaxial cable 12 is electrically connected to the reflector plate 11 at the point of penetration through the plate. reflector 11. Further, the end portion of the outer conductor of the coaxial cable 12, from the point of penetration, is divided into two outer conductor shells, 12a and 12b, by a pair of slots 14 extending into the projection direction of the coaxial cable 12.

A half-wave dipole antenna, 13, which is parallel to the reflector plate 11, is connected to the end of the coaxial cable 12 so as to be fed from the center. In concrete terms, the base of one element 13a of the dipole antenna 13 is connected to the end of an outer conductor shell 12a, and the base of the other element 13b is connected to the end of the. other outer conductor shell 12b. Further, the center conductor 12c of the coaxial cable 12 is connected to the base of an element.

ment 13a. The elements 13a and 13b are practically a length of .14 and they are basically equal lengths. Fig. 5B is a top view of the antenna of the reflector plate 11, and it shows the connection relationship between each of the elements 13a and 13b of the dipole antenna 13 and each conductor of the coaxial cable 12.

In antenna 10, an outer conductor shell 12a also performs the function of a quarter-wave constant-region balun which performs an asymmetric / symmetrical transformation and attenuates a common mode current. The spacing between the dipole antenna 13 and the reflector plate 11 is here set at about ./4 in order to maximize the gain of the antenna and, furthermore, to allow adaptation by the balun. The function of the reflector plate 11 is to attenuate the backward directional gain, and although there are no particular limitations on its size, a size of A x A or less tends to increase the size. size of a side lobe of the backward directivity pattern, and is not conducive to measurements where reflection from the scanning system is a problem.

FIG. 6 is an equivalent circuit diagram of this antenna 10. If we denote by Zi the impedance of the antenna 10, by ZO 20 (= 120 x [n]), the wave impedance of l free space, and through E sin and the signal source 21 of the observed object, there is a single-distance reflector.

tance of λ / 4 of antenna 10, at a point of reflection 22. An image signal source 23, represented by - E sin, t is therefore apparently visible from this antenna 10, on the side opposite to the point of reflection 22. As a result, the receiving voltage of the dipole antenna 13 is doubled, and furthermore, the charge of the balun is opened. Figures 7A and 7B show the results obtained when measuring the directivity (Co-Pol) at 10 GHz of the antenna 10 constructed in this way. In addition, the characteristic in cross polarization (X-Pol)

30 to 10 GHz is that shown in Figures 7C and 7D. When determining the polarization selectivity (Co / X) on the basis of these results, the diagrams which are shown in Figures 7E and 7F are obtained. From these results, it can be seen that the antenna 10 produces almost no cross-polarization, i.e. it has almost zero sensitivity to the cross-polarization component, and it exhibits.

polarization selectivity over a wide range of angles. In addition, as shown in Figures 7A and 7B, there is virtually no backward directional gain. Further, the frequency bandwidth of this antenna is about 40% of the nominal central wavelength X. The reason why the frequency bandwidth of this antenna is larger than that of A conventional dipole antenna depends on the resonance of the balun and the dipole antenna, in addition to the resonance of the dipole antenna.

The results of comparing the antenna 10 of the present embodiment with an antenna constructed by connecting a half-wave dipole antenna to a coaxial cable by means of an ordinary distributed constant balun will now be described. FIG. 8 is a perspective view showing an antenna 30 equipped with an ordinary distributed constant balun and a reflector plate 31. The reflector plate 31 of the antenna 30 which is shown in FIG. 8 is formed by a conductor, and the latter has a size sufficiently greater than the wavelength., denoting by. the wavelength which corresponds to the nominal center frequency. The antenna 30 is constructed by feeding a coaxial cable 32 into the reflector plate 31, from the rear thereof and substantially into its central part, so that the coaxial cable 32 protrudes about ./ 4 from the reflector plate 31 and perpendicular to the latter. At the end of the coaxial cable 32, the center conductor is connected to the base part of one element 33a of a dipole antenna 33, and the outer conductor is connected to the base part of the other element 33b. of the dipole antenna

33. The outer conductor of the coaxial cable 32 is electrically connected to the reflector plate 31 at the point of entry into the reflector plate 31. In this case, instead of forming slits in the outer conductor, a conductive linear element 34, with a length of about ./4, is arranged parallel to the coaxial cable 32, so that the base part of an element 33a is short-circuited to the reflector plate 31. This linear element 31 performs the function of a balun. Figures 9A and 9B show the 10 GHz directivity of antenna 30 shown in Figure 8, and Figures 9C and 9D show the cross-polarized directional characteristic of antenna 30.

The

Figures 9E and 9F show the polarization selectivity of antenna 30. As these figures clearly show, an antenna using an ordinary balun has greater sensitivity to cross-polarized components and poorer polarization selectivity than the antenna. 10 of the present embodiment. In particular, the sensitivity to the components of cross-polarization towards both sides is greater in the antenna 30, and as a result, the polarization selectivity can only be guaranteed within a narrow range. This is because the incorporation of the linear element 34 constituting a balun forms a folded dipole antenna, and such a folded dipole antenna is sensitive to cross polarized components. On the contrary, the outer conductor of the coaxial cable is divided and used as a balun in the antenna 10 of the present embodiment, which has the effect of eliminating a parasitic folded dipole antenna, and therefore of attenuating. sensitivity to

15 cross polarization components. Second Embodiment The antenna of the present invention combines a dipole antenna with a distributed constant balun, and therefore desired characteristics can only be readily obtained in the nominal frequency band and within the range of frequencies. adjacent frequency bands. Therefore, a dipole antenna for use in radio communication or-

As a dipole antenna having a butterfly configuration, it can be viewed as a means of widening the frequency band of an antenna of the present invention. Fig. 10 shows the second embodiment of an antenna according to the present invention which uses a butterfly dipole antenna 15. Employing this structure, the butterfly dipole antenna 15 has a capacitive impedance. - tive on the lower side and resonate with the balun section, which has the effect of widening the frequency band. On the other hand, the polarization selectivity becomes a little less good. It is experimentally found that the antenna of the second embodiment allows about 25% broadening of the bandwidth of the lower side, compared to the antenna of the first embodiment, but conversely, it decreases by 5 dB per day. polarization selectivity, which goes from 25 dB to 20 dB. Further, the bandwidth shows no change on the top side.

laughing. Third Embodiment In the antennas of the first and second embodiments, a reflector plate is used to attenuate the directional gain towards the rear, but depending on the applications, structures in which a reflector plate is used. is not incorporated are also possible. For example, it is not necessary to incorporate a reflector plate in cases where an antenna is elongated and extended, such as in observing near fields in microwave holographic measurements. Omitting a reflector plate can prevent multiple reflections between an antenna and the object of observation.

vation. In such a case, the selective directivity in the forward direction is obtained by the distance between the antenna and the observation object, and reflected waves are received only weakly, due to the attenuation by the distance. Fig. 11 shows the structure of an antenna according to the third embodiment of the present invention, in which a reflector plate is not incorporated. In this antenna, an absorber 16 for radio waves can be wrapped around the coaxial cable 12, from the lower end of the slit region 14 to the base side of the coaxial cable 12, to attenuate a current. common mode that circulates through the coaxial cable 12.

Fourth Embodiment In a case where an antenna having a structure which includes two dipole antennas having mutually orthogonal polarization directions, is used to observe spatial conditions of electromagnetic waves, including polarization information, a monolithic structure simplifies the installation of the an-

tenne on the scanning system. Fig. 12 is a perspective view showing an antenna assembly 40 of the fourth embodiment of the present invention, which is constructed in this manner. The antenna assembly 40 uses two antennas which are equivalent to the antenna shown in the first embodiment, in association with a reflector plate 41. The centers of the dipole antennas 42 and 43, which respectively correspond to the component of vertical polarization and the component of horizontal polarization, are

separated by a distance of 2Ly, and the polarization directions, i.e.

say the longitudinal directions, of dipole antennas 42 and 43, are mutually orthogonal. When using this type of antenna assembly 40, each polarization component can be observed by selectively receiving the output signal from each of the dipole antennas 42 and 43. Further, when performing manipulated observation, for example, for a microwave hologram, inter-antenna coordinate correction can be performed for the measured data, in accordance with the 2Ly separation of the dipole antennas 42 and 43.

Finally, in the present embodiment, each dipole antenna 42 and 43 may take the form of a butterfly dipole antenna, which is shown in the second embodiment. It goes without saying that numerous modifications can be made to the device described and shown, without departing from the scope of the invention.

Claims (11)

1. Antenna having a coaxial line (12) and a dipole antenna structure (13) connected to one end of this coaxial line (12), characterized in that: a pair of slits (14) extending in a longitudinal direction of the coaxial line (12) are formed on an outer conductor of the coaxial line (12), in a region of determined length, from the aforementioned end of the coaxial line (12) and they divide the outer conductor in two shells (12a, 12b); an element (13a) constituting the dipole antenna structure (13) is connected on the side of its base to a central conductor (12c) of the coaxial line (12) and to one end of a shell (12a) the outside conductor; and the other element (13b) constituting the dipole antenna structure (13) is connected from the side of its base to one end of the other shell (12b) of the outer conductor. 2. Antenna according to claim 1, characterized in that the central wavelength which is used in the antenna is A, and the lon- determined heat is ./4. 3. Antenna according to claim 2, characterized in that the length of each of the elements (13a, 13b) is practically ./4. 4. Antenna according to claim 1, characterized in that a reflector plate (11) which is formed by a conductor and which is substantially orthogonal to the longitudinal direction of the coaxial line (12), is incorporated so as to be connected. electrically to the outer conductor at a position whose distance from the end of the coaxial line (12) is equal to the determined length, and each of the elements (13a, 13b) is arranged parallel to the surface of this reflector plate (11). 5. Antenna according to claim 4, characterized in that the central wavelength which is used in the antenna is B, and the Ion- determined value is X14. 6. Antenna according to claim 1, characterized in that an absorber (16) for radio waves is attached to the outer surface of the coaxial line (12) elsewhere than in a region extending over the length determined from the end of the coaxial line (12). 7. Antenna according to claim 6, characterized in that the central wavelength which is used in the antenna is A, and the lon- determined heat is ./4. 8. An antenna assembly comprising first and second coaxial lines (12), first and second dipole antenna structures (42, 43) which are arranged so that their polarization directions are mutually orthogonal, and which are respectively. connected to the first and second coaxial lines (12), and a reflector plate (41); characterized in that: the first and second coaxial lines (12) pass respectively through the reflector plate (41) at two different locations on the reflector plate (41), a protrusion length of each of the first and second coaxial lines (12) is a determined length, and further, an outer conductor of each of the first and second coaxial lines (12) and the reflector plate (41) are electrically connected; a pair of slots (14) extending in a longitudinal direction of each of the first and second coaxial lines (12) are formed in the outer conductor of each of the first and second coaxial lines (12), in a region which s 'extends over the length determined from the end of the first and second coaxial lines (12), and they each divide the outer conductors into two shells (12a, 12b); an element constituting the first dipole antenna structure (42) is connected at its base side to a center conductor (12c) of the first coaxial line (12) and to one end of a shell (12a) of the outer conductor of the first coaxial line (12); and the other element constituting the first dipole antenna structure (42) is connected from the side of its base to one end of the other shell (12b) of the outer conductor of the first coaxial line (12); an element constituting the second dipole antenna structure (43) is connected at its base side to a central conductor of the second coaxial line (12) and to one end of a shell (12a) of the conductor outside of the second coaxial line (12); and the other element constituting the second dipole antenna structure (43) is connected from the side of its base to one end of the other shell (12b) of the outer conductor of the second coaxial line (12); and each of these elements is arranged so as to be parallel to the surface of the plate. reflector (41). 9. Antenna assembly according to claim 6, characterized in that the reflector plate (41) is constituted by a conductor, and the outer conductors are electrically connected to the reflector plate (41) at their points of penetration through. the reflector plate (41). 10. Antenna assembly according to claim 6, characterized in that the center wavelength which is used in the antennas (42, 43) is X, and the determined length is ./4. 11. Antenna assembly according to claim 7, characterized in that the length of each of the elements is substantially 14.