Antenna Array With Pattern Compensation During Scanning

Abstract

An array is made of elemental radiators with an adjustable aperture controlled by a magnetizing field established within a magnetic section of the radiator. The broadening of the radiation pattern resulting from large amplitude scanning is compensated through aperture control of the individual radiators patterns in the array by means of the magnetizing field within said radiator.

Description

BACKGROUND OF THE INVENTION AND PRIOR ART

The present invention concerns an antenna array with electronic scanning.

It is known to perform electronic scanning of the lobe or a switching of the lobe of an antenna array by varying the relative phase of the currents feeding identical primary antennas or radiators arranged as an array. It is also known that the usable scanning angle is limited by the widening of the pattern due to scanning.

Copending application Ser. No. 162,444 filed on July 14, 1971 and assigned to the same Assignee discloses antennas in the form of a Luneberg sphere, of which at least the central spherical core consists of ferrimagnetic material and which comprise means for varying the permeability of the said core. This results in a variation of the aperture of the antenna. Another variable-aperture antenna structure having purely electronic control, which consists of a dielectric rod at least partially coated with ferrimagnetic material associated with coils is also described.

The object of the invention is to compensate, in the course of the widening of the aperture due to electronic scanning in antenna arrays of radiators according to said application.

BRIEF SUMMARY OF THE INVENTION

In accordance with the invention, the aperture compensated antenna array consists of at least two radiators as described in said of one of copending application, comprising means for varying the relative phase of the microwave currents feeding said radiators, and means for correlatively varying the magnetic field applied to the ferrimagnetic elements of the radiators in such manner that the aperture of the lobe remains constant in the course of the scanning.

A constant aperture array is the general aim, but it is to be understood that it is possible to adjust the correlation between the phase difference which controls the scanning angle and the magnetic field which controls the aperture angle, so as to follow a preset law between these two angle values.

In a preferred embodiment of the invention, the phase difference in the feeding currents is obtained by means of variable phase shifters, for example of the ferrite type, to which an adjustable current is applied, and the desired aperture is obtained by means of an adjustable current supplied to the coils surrounding the ferrimagnetic material parts of the elemental antennas. The constancy of the aperture or the desired "scanning angle/aperture angle" law is therefore obtained by adjusting currents in relation to relative phase, which often amounts to adjusting currents in relation to other currents.

DETAILED DISCLOSURE OF THE INVENTION

A particular embodiment of the invention will now be described in detail by way of non-limiting illustration with reference to the accompanying drawings, in which:

FIGS. 1a and 1b illustrate an array of primary antennas of the prior art, intended to show the variation of the aperture angle in the course of the scanning,

FIG. 2 shows the scanning lobe obtained with the antenna of FIGS. 1a and 1b,

FIG. 3 illustrates a Luneberg sphere having a ferrimagnetic core in accordance with the above-mentioned patent application Ser. No. 162,444.

FIG. 4 illustrates an antenna array with lobe aperture and gain compensation in accordance with the invention,

FIG. 5 illustrates the variation of the scanning angle .alpha. as a function of the phase difference .phi. for the antenna of FIG. 4,

FIG. 6 illustrates the variation of the lobe aperture .theta. as a function of the control current of the ferrimagnetic Luneberg spheres for two different values of the scanning angle,

FIG. 7 illustrates the variation of the current controlling the aperture of the individual patterns of the elemental radiators as a function of the scanning angle for obtaining a constant aperture of the resultant pattern, and

FIG. 8 illustrates an antenna array according to the invention.

FIGS. 1a and 1b illustrate a three-dimensional array of elemental antennas of the prior art, made of 24 antennas forming a front curtain and a rear curtain. The front curtain is fed as indicated in FIG. 1a. The feed of the rear curtain is similar, but the currents are 90.degree. out of phase. It is known (see for example "Principles of Radar," McGraw-Hill Book Company, Inc.1946, pages 9 - 60) that the intensity of the resultant field at a point of a direction which is at an angle .beta. to the axis Ox and the angle .alpha. to the axis Oz perpendicular to the array (.alpha.+.beta.= 90.degree.) and remote from r of the array is

.epsilon..sub..theta..sub.m = (60 I.sub.m /r) F.sub.o (.beta.) F(2.5/2) (.beta.) F(3.5/2) (90) G.sub.1/4 (2.1/4)(.alpha. ) G.sub.p (2.2/2)(.beta.),

where I.sub.m is the amplitude of the supply current of the primary antennas, and

G.sub.p.sup.N,n (.alpha.) = [sin N (p-n cos.alpha.).pi.]/[sin (p-n cos.alpha.).pi.]

F.sup.N,n (.alpha.) = G.sub.O.sup.N.n (.alpha.),

where N is the total number of radiators in a curtain of the array, n is the distance between adjacent radiators expressed in wavelengths, and p is the phase difference between adjacent antennas expressed in fractions of a cycle. In the formula giving .epsilon..sub..theta..sub.m, p denotes more precisely the phase difference between the left-hand half of the array and the right-hand half of the array.

It is possible to obtain lobe scanning by varying p, and FIG. 2 shows the position of the main lobe with p successively taking the values:

p = 0,1/6, 1/3, 1/2, 2/3, 5/6, 1.

It will be seen that it is possible to obtain a sweep of .+-. 20.degree. by varying p from 0 to .+-. 1/2. However, for the extreme positions .+-. 20.degree., the secondary lobe has become as large as the main lobe and if the value is limited to .+-. 14.degree., the aperture angle at 3dB has changed from 22.degree. at the centre to 30.degree. at the edges. The invention has for its object to keep this angle constant irrespective of the scanning angle.

Referring to FIG. 3, the reference numeral 1 denotes a Luneberg sphere composed of a ferrimagnetic spherical core 2 and of a polyethylene shell 3 in the form of a hollow sphere, loaded with titanium dioxide. The permittivity of the sphere 2 is .epsilon..sub.R = 14.9 and the permittivity of the spherical shell 3 is .epsilon..sub.R =4.0. A waveguide 4 terminated by a flange 5 whose front face is profiled in the form of a spherical surface is applied to the surface of the sphere 1. The waveguide 4 is seen on its smaller side in FIG. 3. The microwave electric field is in the plane of FIG. 3 and the microwave magnetic field is perpendicular to the plane of FIG. 3. It is obvious that any other feed of the lens may be used.

A winding 6 surrounds the sphere 2 and is locked between the latter and the spherical shell 3. This antenna has been more completely described in the aforesaid patent application.

Referring now to FIGS. 4, 10 and 11 are two antennas in the form of a Luneberg sphere, of which the core consists of iron-yttrium garnet (5Fe.sub.2 O.sub.3, 3Y.sub.2 O.sub.3.). The diameter of the core is 14 mm and that of the sphere is 22 mm. The distance between the centres of the spheres is 30 mm. The spheres are fed with a wave at 9 Ghz produced by generator 19 through waveguide 12, which is doubled into two lines 13 and 14 to feed antennas 10 and 11 respectively. It will be seen that the diameter of the Luneberg spheres having a core consisting of ferrimagnetic material is here slightly smaller than the wavelength of the radiated guide. Two ferrite phase shifters 15 and 16 are located on lines 13 and 14 receiving through the leads 17 and 18 phase-control currents I.sub.1 and I.sub.2. The windings of antennas 10 and 11 are series supplied with the same aperture control current I. Currents I.sub.1, I.sub.2, I are adjusted in accordance with experimentally determined curves in such manner that the aperture angle does not vary in the course of the scanning.

FIG. 5 illustrates for the antenna of FIG. 4 the variation of the angle of rotation of the lobe .alpha. as a function of the phase difference .phi. introduced by the phase shifters 15 and 16 (the measure bear on lobes G.phi. .sup.2.2/2 transverse to the direction joining the centres of the antennas). This law of variation depends upon the distance between the antennas. For mechanical reasons, the optimum value has not been used here.

FIG. 6 illustrates the variation of the aperture of the lobe .theta. as a function of the current flowing through the coils of the ferrite Luneberg spheres in the case where the number of turns of each coil is 10. The lower curve corresponds to .alpha. = 0 (.phi. = 0; the two antennas are excited in phase). The upper curve corresponds to .alpha. = 12.degree. (.phi.= 100.degree.).

Let it be assumed that an aperture .theta. of 28.degree. is to be maintained in the course of the electronic scanning. It will be seen that the current I flowing through the windings of the cores of the Luneberg lenses is 2A (point M). In order to maintain this aperture in the course of the scanning, i.e. when .phi. increases in accordance with the curve of FIG. 5, the current in the lenses must be reduced in accordance with a law experimentally defined from the set of curves .theta. as a function of I for the successive values of .alpha.. It will be seen that, for the maximum value of the scanning angle (.alpha. = 12.degree.) in the example under consideration, I = 1.3 A (point P).

The angle .alpha. depends upon the phase difference angle .phi. which in turn, in the case of phase shifters having a ferrimagnetic core, depends upon the value of a phase shift control current (I.sub.1 + I.sub.2) (FIG. 7). It will therefore be seen that each value of .alpha. corresponds to values of (I.sub.1 + I.sub.2) and I for a given .theta.. Hence, it is possible to supply the phase shifters and the radiators with two control currents which are functions of the same angle, for example by means of two potentiometers having a common shaft of the "function generator" type.

FIG. 8 illustrates the front curtain of an antenna according to the invention derived from the antenna having an array of primary radiators according to FIG. 1. The rear curtain is absolutely identical to the front curtain, but is supplied with a wave 90.degree. out of phase with that with which the front curtain is fed.

In the array of FIG. 8, all the primary radiators are ferrite Luneberg antennas of the type of FIG. 3. There are 12 such antennas for the front curtain and 12 for the rear curtain. The vertical and horizontal spacings between antennas are not the same as in the ideal case of FIG. 1, because the antennas employed have a diameter which is greater than half the wavelength and therefore cannot be less than one wavelength apart.

This disadvantage does not arise when the elemental antennas are of the rod type as described in the aforesaid patent application. All the antennas 21, 22, 25, 26, 29, 30 of the left-hand half of the front curtain are fed substantially in phase. All the antennas 23, 24, 27, 28, 31, 32 of the right-hand half of the front curtain are fed substantially in phase, but with a phase difference of p (in fractions of a cycle) with respect to the antennas of the left-hand half. This phase difference is produced by the phase shifters 15 and 16 situated on two waveguides 13 and 14 respectively, which are connected to the signal source 19 by a common waveguide 12. The waveguide 13 is divided into two waveguides 7 and 7' and the waveguide 14 is bifurcated into two waveguides 8 and 8'. The attenuators 33-36 are introduced into the waveguides 7-8, 7'-8'.

Scanning is obtained by varying the phase difference between the left-hand and right-hand halves of the front and rear curtains by means of phase shifters 15 and 16 and homologous phase shifters of the rear curtain (not shown). The aperture control of the lobes of the individual radiators of the array of radiators is obtained by the supply of the coils of the ferrimagnetic-core Luneberg spheres. The ferrimagnetic-core spheres have been interconnected in a series of three.

The variation of the current for controlling the permeability of the ferrite cores of the antennas and the concomitant variation of the current controlling the phase difference are effected by the control circuit 37.

It will be apparent to the person skilled in the art, from the description of FIGS. 4 and 8, how it is possible to construct arrays of scanning antennas of any desired structure with spheres having ferrimagnetic cores or dielectric rods having a ferrimagnetic coating, as primary radiators. Of course, the microwave supply to the antennas, shown through of waveguides in the examples, may be effected by means of other types of transmission lines, such as the coaxial lines, micro-strip.

Claims

What we claim is:

1. An electronically scannable antenna array comprising:

a plurality of elemental radiating units each having a dielectric non-magnetic radiating unit having an axis of directivity,

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

magnetizing means for said magnetic means;

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;

a common feed source for said radiating units;

a controllable phase shifting means between said source and said radiating units;

a control unit for said phase shifting means; and

means for controlling the D. C. current supplied to said magnetizing means of each elemental radiating unit in order to vary the aperture value of the elemental beam radiated by said unit so as to compensate for the aperture variation of the beam radiated by said array due to scanning.

2. An electronically scannable antenna array as defined by claim 1 in which said phase shifting means are ferrimagnetic phase shifters and said control unit is a current generator.

3. An electronically scannable antenna array as defined by claim 1 in which said elemental radiating units are Luneberg-type antennas having a ferrimagnetic core and at least one dielectric shell surrounding said core.