US3765021A - Adjustable aperture antenna employing dielectric and ferrimagnetic material - Google Patents

Abstract

In order to obtain wide aperture and high directivity radiation patterns from the same antenna, the radiating structure is made of a non magnetic dielectric and a dielectric ferrimagnetic material. The magnetic part is associated with magnetizing coils energized from an aperture control current source. When the antenna is of the rod type, the rod is made of the non magnetic dielectric which is covered at least partly with a magnetic coating. When the antenna is of the spherical type, a magnetic material core is surrounded with a non magnetic dielectric shell.

Description

Unite States Patent [1 1 Chiron et al.

[ 1 Oct. 9, 1973 1 ADJUSTABLE APERTURE ANTENNA EMPLOYING DIELECTRIC AND FERRIMAGNETIC MATERIAL [75] Inventors: Bernard Chiron; Louis Duffau, both of Paris, France [73] Assignee: Societe Lignes Telegraphiques Et Telephoniques, Paris, France [22] Filed: July 14,1971

[2]] App]. No.: 162,444

[30] Foreign Application Priority Data 7 July 30. 1970 France 7028150 Jan. 14.1971 France 7101105 [52] US. Cl 343/785, 343/787, 343/854, 343/911 L [51] Int. Cl. H0lq 13/00 [58] Field of Search 343/753, 754, 755, 343/785, 787, 854, 911 L [56] References Cited UNITED STATES PATENTS 3,277,489 10/1966 Blaisdell .1 343/785 3,653,054 3/1972 Wen 343/787 2,869,124 1/1959 Marie 343/785 2,973,516 2/1961 Medved 343/787 2,981,945 4/1961 Fyler 343/787 2,921,308 1/1960 Hansen et a1 343/787 Primary Examiner-Eli Lieberman Attorney-Kemon, Palmer & Estabrook [57] ABSTRACT In order to obtain wide aperture and high directivity radiation patterns from the same antenna, the radiating structure is made of a non magnetic dielectric and a dielectric ferrimagnetic material. The magnetic part is associated with magnetizing coils energized from an aperture control current source. When the antenna is of the rod type, the rod is made of the non magnetic dielectric which is covered at least partly with a magnetic coating. When the antenna is of the spherical type, a magnetic material core is surrounded with a non magnetic dielectric shell.

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saw MW 11 ADJUSTABLE APERTURE ANTENNA EMPLOYING DIELECTRIC AND FERRIMAGNETIC MATERIAL BACKGROUND OF THE INVENTION AND PRIOR ART The present invention concerns antenna structures with adjustable aperture. The requirement that the same radar equipment may be used for different types of operations which become necessary in navigation and even in surveillance has led users to seek antennas whose radiation pattern can be modified, while maintaining constant yield. An antenna structure is required which can simultaneously carry out a watch (very wide antenna diagram) and, when the target has been found, carry out the tracking (very selectiye radiation diagram This result can readily be obtained by using two different antennas which are successively switched to the radar apparatus. This solution cannot be envisaged in the case of airborne equipment or simply of mobile equipment owing to the large overall dimensions resulting from the use of two antennas. From the economic viewpoint, this solution is also unsatisfactory. It has therefore been proposed to solve this problem with the aid of a single antenna.

One of the proposed solutions consists in modifying the radiation pattern of the antenna by mechanically deforming the reflector element. This solution has given rise to designs which do not always provide the desired reliability.

The present invention relates essentially to electronically controlled adjustable aperture antennas. As compared with the aforesaid mechanical 'solution, they show not only increased reliability but also advantages in regard both to weight and space requirement and to power consumption and time constant.

BRIEF DISCLOSURE OF THE INVENTION In accordance with the essential features of the invention, the antenna is a partly dielectric, partly ferri magnetic structure associated with magnetizing coils surrounding said ferrimagnetic part and a control current generator feeding said coils. The adjustment of the radiation diagram of the antenna is achieved by changing the magnetizing current in the coils.

In a first embodiment of the invention concerning a directive structure, the dielectric part is a plain rod surrounded at least partly by a ferrimagnetic coating associated with magnetizing coils. In a second embodiment relating to an omnidirectional structure the ferrimagnetic part is an inner core surrounded by a dielectric shell arranged according to Lunebergs law, said inner core being associated with magnetizing coils. According to an additional feature of the invention, the coils associated with the ferrimagnetic part are arranged so as to provide for latching operation, that is the operating conditions are maintained when the magnetizing current is cut off. The latter feature is of particular interest when low power consumption is important.

DETAILED DISCLOSURE OF THE INVENTION The invention will be readily understood with reference to the following description and to the accompanying drawings, in which FIGS. 1 and 2 are diagrammatic illustrations of antennas according to the present invention.

FIG. 3 illustrates the variation of the aperture angle of a dielectric rod type antenna as a function of the length of the ferrimagnetic coating (without magnetizing current).

FIG. 4 illustrates, in the absence of magnetizing current, the variation of the gain as a function of the length of the ferrimagnetic coating.

FIG. 5 illustrates the variation of the aperture angle of the partially coated antenna as a function of the magnetization current.

FIG. 6 illustrates the variation of the gain of the same aerial as a function of the magnetization current.

FIG. 7 illustrates, as a function of the current, the variation of the aperture angle of a second modified embodiment.

FIGS. 8 and 9 are two embodiments of omnidirectionnal Luneberg type antennas according to the invention.

FIGS. 10 & 10A are a variant of the Luneberg type antenna according to the invention which may be switched from a small aperture operation to a wide aperture operation.

FIG. 11 shows the variation of the effective permeability of the core with respect to the magnetizing field.

FIG. 12 shows the variation of the 3 dB beam width with respect to frequency for both a Luneberg dielectric antenna and a Luneberg type antenna according to the invention.

FIG. 13 shows the aperture angle variation with respect to the control current for the antenna of FIG. 8.

FIG. 14 shows the same variation for the antenna of FIG. 10.

FIGS. 15 and 16 show an embodiment of a latching antenna according to the FIG. 10.

In FIG. 1, there is diagrammatically illustrated a frustoconical dielectric structure I constituting what is usually called a dielectric rod antenna. A detailed explanation of the operation of this type of antenna will be found in the work Les Antennes," by J. Thourel, page I88 of the edition dated 1956, published by Dunod.

As is well known, the radiating structure is fed through a coupling device diagrammatically represented by loop 2, from an electromagnetic energy source. The base of the antenna is mounted in a metallic holder 3. In'accordance with the invention, this dielectric radiating structure is at least partially coated with ferrimagnetic material and associated with a magnetizing device.

In FIG. 1, there is denoted by d the length of the antenna provided with a coating of ferrimagnetic material, the total length of the antenna being L. The coating comprises two rings 4 and 5 each associated with a magnetizing winding,4' and 5 respectively. It is to be understood that the length d of the coating may as a variant be provided in the form of a single piece of ferrimagnetic material surrounding the base of the dielectric rod 1. There may also be envisaged the coating of the desired surface of the dielectric structure 1 with ferrimagnetic material by any method known per se (cathode sputtering, deposition by sedimentation, etc.), the manner in which the magnetization conductors are provided being adapted to each case.

The diagram of FIG. 2 illustrates also a dielectric rod structure I. It is provided with a coating of ferrimagnetic material over the whole of its length L. As in FIG. 1, the coating consists of a set of rings 4, 5, 6, 10 each associated with one magnetizing coil. The experimental diagrams of FIGS. 3, 4, 5 and 6 show the influence of the ferrimagnetic coating on the performance of the radiating structure. They have all been obtained with the same structure consisting of a dielectric rod antenna operating in the X band surrounded by a 5 mm thick ferrimagnetic tore made of type 63 O7 ferrite, manufactured by L.T.T. The composition of this ferrite corresponds to the formula 40 Fe O 9 MnO, 46 MgO, 5(TiO NiO), which has been disclosed in the first addition No. 86,409 to French Patent No. 1,354,232, applied for on the July, 24, 1964, by L.T.T.

The diagram of FIG. 3 shows the influence on the antenna aperture angle of the length a' coated with ferrimagnetic material. In order to simplify the diagram, there has been taken as the measure of the length d the rationalised value d/L, which will thus vary between 0 (no ferrimagnetic material coating) and I (completely coated antenna). In the absence of any current magnetizing the ferrimagnetic material, a considerable widening of the radiation pattern is noted when the portion d of the antenna which is coated increases. The bare dielectric rod antenna has an aperture angle at 3 decibels of about 30, and when one-fourth of the antenna is coated with electromagnetic material the aperture angle is 160. The increase in the aperture angle is extremely rapid, and then slows down. When one-half of the length of the antenna is coated, the aperture angle is 185, and it reaches 200 when the antenna is completely coated with magnetic material. 7

The curve of FIG. 4 represents under the same conditions, the variation of the antenna gain as a function of the fraction of the antenna which is coated with ferrimagnetic material. The gain of the dielectric rod antenna is about dB, and it decreases very rapidly to 4 dB when one-fourth of the length of the antenna is coated. The decrease slows down considerably and the gain changes from the value 3 dB when one-half of the length is coated to 2 dB when the antenna is completely coated. It is current practice to characterize the yield of an antenna by the product G,,,,,,,. 0 .0 when the aperture angle 0, and 0 measured in two perpendicular planes, remain small and G is the value of the gain (numerical value, while the ordinate scales of the curves are ratios of the gain, in dB, to the gain of a dipole). In the case of an antenna with axial symmetry such as the dielectric rod antena, 0 0 By calculating the values of the product G 0 from the data of the curves of FIGS. 3 and 4, the following results are obtained It will be seen that the empirical law by which the yield of the antenna is related to the product G,.,,,,,0 shows that this yield is substantially constant up to 6 50. It is well known that this formula is applicable only to small aperture angles. More precise calculation utilising the formula which includes the solid angle affected by the radiation, leads to higher precision, and when applied to the above example of embodiment it shows that the yield of the aerial is substantially constant.

The curve of FIG. 5 shows the variations of the aperture angle ofa dielectric rod antenna whose base is surrounded by a 1 cm thick tore of the same ferrite as stated above over percent of its height as a function of the permeability of the said ferrite, measured by the current flowing through a winding of 12 coils which is mounted directly on the tore. As is shown, a variation of the aperture angle between and 50 can be obtained. An increase of the current provides a reduction of the aperture angle. The curve of FIG. 6 illustrates, for the same antenna, the variation of the gain as a function of the current supplied to the winding. It is also possible to verify that the yield of the antenna is independent of the value of this current.

The above example concerns an antenna operating at a frequency in the neighborhood of 10 GHz. The curve of FIG. 7 has been measured on a dielectric rod antenna operating in the neighbourhood of 6 GHz coated with a single piece of ferrite over 30 percent of its total height. The ferrite used in this embodiment is a Fe Y Gd Al garnet. The thickness of the coating is 1.6 cm.

The embodiment shown in the following figures is of the Luneberg lens type. It is well known that this type of lens is a refracting structure with a spherical symmetry and an index which varies according to the distance to the center.

One of the main properties of these lenses is that the output from a point source located at any point on the sphere surface is a beam of parallel rays. As has been established, the relationship between the refractive index and the permittivity and the permeability of lens medium is as follows Since it is quite impractical to manufacture this sphere with a continuously variable index, the practical embodiments of Luneberg lenses are made of a central core surrounded by several concentric shells the constant permittivity of which decreases as increases their distance from the center of the spherical core.

The study of electromagnetic wave propagation in a ferrimagnetic medium is based on the property of the permeability of the medium of being a tensor which means that the value of the permeability varies with the direction considered. Theoretical considerations are very well developed in the book entitled Microwave ferrites and ferrimagnetics" by B. LAX and K. BUT- TON published by McGraw-Hill Book Co. Inc. in 1962. As explained page 351 the phase constant of a TEM electromagnetic wave propagating within a non lossy medium of permeability u where has been established a magnetic field H perpendicular to the propagation direction is given by z B/B l M0 quency of the wave and H the transverse magnetic field. As seen, any modification of the value of H will produce a variation of the phases ofthe wave propagating within the ferrimagnetic medium. Any modification of the phase will result in a modification of the radiated lobe if the medium is used as a radiating element. The above is a short theoretical explanation of the reason why it is possible to modify the aperture angle in omnidirectional spherical antennas according to the invention.

FIG. 8 shows such an antenna made of a central core 12 of ferrite surrounded by a shell 13 made of polythene loaded with titanium dioxide The permittivity of core 12 is c 14.9 and the permittivity of the shell 3 is e 4.0. The waveguide 14 is terminated by a flange 15 the front face of which has been shaped so as'to be adapted to the spherical shell 13.

The cut view in FIG. 8 shows the small dimension of the waveguide. The electric field of the microwave is within the plane of the figure. A winding 16 surrounds core 12. It is made of four turns and is fed with the aperture control current 1. The radius of core 12 and shell 13 are respectively 32 mm and 41 mm. The ferrite constituting core 12 is an yttrium iron garnet (5 Fe O 3 Y O sold by L.T.T. as ferrite type 6901 and by the Compagnie THOMSON C.S.F. as Type YlO.

The main characteristics of this material are as follows 411M, 1750 gauss (M saturation magnetization moment) AI-l 45 to 60 oersteds tg 8 4.4 10* (8 loss angle) at 9 GHz.

The variation of the effective permeability of this ferrite with respect to the value of the applied magnetic field is shown in FIG. 11. Actually p... corresponds to the first member of equation 4.

FIG. 12 shows the variation of the aperture at 3 dB in degree with respect to the operating frequency. Curve 51 shows the variation for a Luneberg dielectric lens of conventional manufacture and curve 52 shows the same variation for the antenna of FIG. 1 at I 0.

FIG. 13 shows the measured curve giving the aperture of the antenna of FIG. 8 operating at 9,375 MHZ with respect to the value of the aperture control current I. At I 0, the aperture is 52, as shown on curve 52 of FIG. 12, It is about 75 for I 2 Amperes and 100 for I 4 Amperes.

FIG. 9 illustrates another embodiment of the invention in which the antenna comprises a core 12 surrounded by two concentric shells 17 and 17 made also of ferrimagnetic material surrounded by an external shell 18 made of dielectric. The central core 12 and the inner shells 17 and 17 have respective diameters of 32, 41 and 50 mm and are associated respectively with windings 19, 21 and 21 They are fed respectively with currents I, I and 1 which control the permeability of parts 12, 17 and 17 so that the product a 6,. is equal to 12 for core 12, to 9 for shell 17 and to 7.5 for shell 17 The permittivity of outer shell 18 is 2.5 and its diameter 59 mm. The values of currents I, I and I are set by experience. The calculation of these values leads to very intricate equations because each winding establishes a field which is not limited to the core or shell which it surrounds but extends also in the neighbouring volume of ferrimagnetic material. The variation of the value 6;; [LR is obtained in this embodiment by using pure YIG for the core,.a mixture of YIG and polythene for the inner shells l7, and 17 It is also possible to make core 12 and the two inner shells from the same magnetic material (YIG) and to control the value of the /.L product by means of the currents I, 1,, 1 The embodiment shown in FIGS. 10 and 10A is roughly the same as the embodiment of FIG. 8 except for the winding 23. As shown, a central hole 22 has been drilled in the core and the winding 23 is made of a set of semicircular turns which are each located in a diametral plane and which are closed through hole 22.

The core 12 is made ofa ferrite with rectangular hysteresis loop sold as ferrite type 6901 by L.T.T. It has the following characteristics 41rM 1650 Gauss (M, saturation magnetizing moment) AH 90 Oersted tg5 5.10 at 9 GI-Iz.

The dimensions of the antenna are as follows outside diameter 41 mm, core diameter 32mm, shell thickness 4 mm, 6,; of the shell 3.75, diameter of drilled hole 5 mm.

FIG. 14 shows the variation of the aperture angle of such an antenna with respect to the control current I in winding 23 operating at 9,375 MHz. As shown, the variation curve has the shape of any hysteresis loop. When a pulse of 2 Amperes is applied to winding 23 the aperture is about 45 after said pulse has disappeared. On the contrary, a pulse of the same amplitude but with the current circulating in the other direction will set the aperture to about 100. This type of operation which is usually referred to as latching allows to switch the aperture angle from one preset value to another value with a control pulse of a given amplitude. The antenna does not require a continuous current to maintain magnetization of the ferrite material as is the case in the previously described embodiments.

FIGS. 15 and 16 show the radiation diagrams corresponding to the antenna shown in FIG. 10. They are measured radiation patterns.

FIGS. 15 and 16 correspond to measurements made in a plane perpendicular to the propagating direction of the microwave and containing the magnetic field of the wave as established in the wave guide 14. Measurements made in a plane perpendicular to the above plane and to the propagation direction show that within the measurement precision, the same values are obtained.

What we claim 1. An electrically controlled adjustable aperture r.f. antenna structure comprising:

a dielectric non-magnetic radiating unit having an axis of directivity;

a feed for said radiating unit;

magnetic dielectric means integral with said dielectric radiating unit and symmetrically arranged around said axis of directivity;

magnetizing means for said magnetic means; and

means affording connection of a DC current source to said magnetizing means in order to vary the aperture value of the antenna without changing the directivity in accordance with the magnitude of current supplied to said magnetizing means.

2. An electrically controlled adjustable aperture r.f. antenna structure according to claim 1 in which said dielectric non-magnetic radiating unit is a dielectric rod and said magnetic means is a sleeve coating said rod.

5. An antenna according to claim 4 in which said central core consists of an inner sphere surrounded by at least one spherical shell said core and said shell being associated with independent coils.

6. An electrically controlled adjustable aperture r.f. antenna structure according to claim 1 in which said magnetic dielectric means is made ofa rectangular hysteresis loop material.

Claims (6)

1. An electrically controlled adjustable aperture r.f. antenna structure comprising: a dielectric non-magnetic radiating unit having an axis of directivity; a feed for said radiating unit; magnetic dielectric means integral with said dielectric radiating unit and symmetrically arranged around said axis of directivity; magnetizing means for said magnetic means; and means affording connection of a D.C current source to said magnetizing means in order to vary the aperture value of the antenna without changing the directivity in accordance with the magnitude of current supplied to said magnetizing means.

2. An electrically controlled adjustable aperture r.f. antenna structure according to claim 1 in which said dielectric non-magnetic radiating unit is a dielectric rod and said magnetic means is a sleeve coating said rod.

3. An electrically controlled adjustable aperture r.f. antenna structure according to claim 1 in which said dielectric non-magnetic radiating unit is a dielectric rod and said magnetic means consists of a plurality of ferrimagnetic rings stacked on said rod near its feed end.

4. An electrically controlled adjustable aperture r.f. antenna structure according to claim 1 in which said dielectric radiating unit is a luneberg lens and said magnetic dielectric means is the central core of said lens.

5. An antenna according to claim 4 in which said central core consists of an inner sphere surrounded by at least one spherical shell said core and said shell being associated with independent coils.

6. An electrically controlled adjustable aperture r.f. antenna structure according to claim 1 in which said magnetic dielectric means is made of a rectangular hysteresis loop material.

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