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(11) | EP 0 159 301 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Electrically controlled aerial array with reduced side lobes |
| (57) An electrically controlled aerial array for microwave signals comprises a plurality
of aerial elements in the form of wave conductors (V1, V2,...) provided with broadside
slits or slots (S11, S12, ... S21, S22, ...). Each of the conductors (V1. V2, ...) is provided on its inside with a ridge
(R1, R2, ...) of given configuration, and for each wave conductor the ridge has a
given dimension such that different wave conductor wavelengths are obtained. Similarly,
the mutual spacing (d1, d2, ...) of the broadside slits is different for different wave conductors. The positions
of the grid lobes in the radiation diagram of the aerial array are thus upset and
a reduction of the side lobe level may be attained. |
TECHNICAL FIELD
[0001] The present invention relates to an electrically controlled aerial array, i.e. an aerial with a main lobe which may be controlled by varying the phases in the included aerial elements. Such an aerial is used in radar reconnaissance equipment for example.
BACKGROUND ART
[0002] An aerial array of the kind intended here comprises a plurality of aerial elements configured as rectangular wave conductors lying parallel. In particular, the radiation openings in the elements are formed as so-called broad side slits, i.e. longitudinal slits along the wider surface of each wave conductor in the aerial array. It is already known to make the aerial lobes controllable in a plane at right angles to the longitudinal direction of the wave conductors by placing phase shifters in the feed path to each conductor, e.g. according to GB-B1.577.939. alternatingly above and below the centre line of the wave conductors, the illumination function will be phase modulated along the aerial aperture, i.e. along the wave conductors. This gives rise to large side lobe peaks in the aerial array radiation diagram.
[0003] It is known to solve this problem by using radiation openings and elements that lessen or eliminate the occurrence of periodical disturbances in the aperture. For example, edge slits may be used instead of broadside slits, see "Low-Sidelobe Radar Antennas" by H. E. Schrank from "Microwave Journal", July 1983 p 109 ff. Edge slits are difficult to master from the electrical design aspect, particularly due to the strong electromagnetic connection between them, and it is therefore desirable to retain broadside slits to obtain good side lobe suppression.
DISCLOSURE OF INVENTION
[0004] The object of the present invention is to achieve an electrically controlled aerial array of the kind mentioned in the introduction, using broadside slits as radiation elements, the aerial diagram for which shows substantially suppressed side lobes. The invention is characterized as will be seen from the characterizing portion of claim 1.
BRIEF DESCRIPTION OF DRAWINGS
[0005] The invention will now be described in detail, with reference to the accompanying drawings, where Figure 1 illustrates an aerial array with a construction known per se, but with further distinguishing features in accordance with the invention;
Figure 2 illustrates parts of two aerial elements included in the aerial of Figure 1;
Figure 3 is a cross section of an aerial element according to Figure 2; and
Figures 4 and 5 are radiation diagrams.
BEST MODES FOR CARRYING OUT THE INVENTION
[0006] The aerial array in Figure 1 comprises a plurality of aerial elements (4 elements in the Figure) in the form of rectangular wave conductors V1, V2, V3 and V4 lying parallel along their respective long narrow sides. Feed wave conductors M1 and M2-M4 (the latter three being concealed in the Figure) are each connected to one of the wave conductors V1-V4. Each wave conductor is provided with radiation openings in the form of longitudinal slits, S11, S12, ... on the wave conductor VI, S21, S22 on the wave conductor V2, S31, S32, ... on the wave conductor V3 and S41, S42, ... on the wave conductor V4. All the slits or slots shown are so-called broadside slits, i.e. uniformly wide slits or slots made in the wider face of the respective wave conductor. The end portions of the feed wave conductors MI-M4 which are attached to the wave conductors VI-V4 have a feed opening (not illustrated in Figure 1) through which electromagnetic field energy, e.g. within the X band, is fed to each wave conductor VI-V4. The other ends of the feed wave conductors are connected via suitable input feed elements to the phase shifters Fl-F4 (F3 and F4 being concealed in Figure 1) for controlling the phase of the field fed in, relative to a reference phase, e.g. the phase of the field to the wave conductor VI.
[0007] The use of broadside slits or slots with uniform element spacing d (d=d1=d2=, ..) according to Figure 2 gives rise to side lobe peaks in the aerial array radiation diagram, the height of the peaks depending on the directing angle. A radiation diagram is illustrated in Figure 4, in a plane parallel to the wave conductors and through the lobe maximum when the direction is 20° from the direction of the normal. The side lobe peaks are so-called grid lobes corresponding to the element spacing 2d. If the slits S11, S12, ··· S21, S22 ... etc in the wave conductors Vl-V4 had mutually differing element spacing d1=d2= ... instead, the grid lobes from the individual wave conductors V1-V4 would occur at different places in the radiation diagram and would not be added to each other to form the prominent peaks (S1 S2) in Figure 4. According to the invention, different element spacing is achieved by changing the wavelengths of the individual wave conductors.
[0008] Figure 2 illustrates a portion of the aerial array in Figure 1, portions of two wave conductors being depicted. The slits S11, S12, and S13, S14 in the wave conductor V1 have the mutual spacing d2 and the slits S21, S22, S23, S24 etc in the wave conductor V2 have the mutual spacing d2=d1. To attain the intended reduction of the side lobe peaks in Figure 4, the wave conductor wavelength λ varied such that λg is different for each of the conductors VI-V4. This is described below in connection with Figure 3. Different spacings d1, d2 between the slits of the different wave conductors Vl-V4 are obtained as a consequence.
[0009] Figure 3 is a cross section of a wave conductor VI with the slits S11, S12, there also being shown a part of an adjacent wave conductor V2. On its inner surface facing the slits S11, S12 the wave conductor VI is provided with a raised portion or ridge R, situated symmetrically about the symmetrical axis C of the wave conductor. The ridge has two side walls RV1 and RV2 extending at right angles to the inner surface Y of the wave conductor in the longitudinal direction and entire length thereof. The side walls RV1 and RV2 are bridged by a wall RV3 at right angles to them. Both walls RV1 and RV2 have a height h from the surface Y. The wave conductor Vl is a so-called ridge wave conductor wavelength λg for a given wave conductor width a and height b may be varied within given limits by varying ridge height h. The height h is thus constant for a given wave conductor in the group aerial, i.e. for the wave conductor VI the height of the ridge R is equal to h1, for the wave conductor V2 the height of the R2 is h2 (h1≠ h2) and so on. Since the slit spacing d≈λg/2, the grid lobes may be spread out over the lobe angle interval of the aerial, thereby reducing their effect on the side lobe level.
[0010] In Figure 5 is shown a radiation diagram for an aerial array with a ridge wave conductor where this principle is utilised. The diagram in Figure 5 may be compared directly with the one in Figure 4, since apart from the ridges Rl, R2 the aerials are otherwise entirely the same.
[0011] Broadside slits in aerial arrays of the type intended here have large advantages:
a) They have very low losses
b) They are simple and cheap to manufacture
c) Established and well functioning calculation methods are used.
[0012] The inventive aerial array retains the above-mentioned advantages due to the broadside slits, but with reduced side lobes.
[0013] The invention is not restricted to embrace wave conductors VI-V4, where the wave conductor wavelengths λg1, λg2, ... for the different wave conductors have been varied by the measures described in connection with Figure 3. What is essential in the inventive concept is that the wavelengths λg1, λg2 ... have been made different, which results in that the mutual spacing dI, d2, ... must be dimensioned so that d1 ≠ d2 etc. There is thus obtained variation in the positions of the grid lobes for the entire aerial array, which causes a reduction of the side lobe level.
An electrically controlled aerial array comprising at least two juxtaposed aerial
elements, each constituting a preferably rectangular wave conductor (Vl, V2 ...),
one side surface of which is provided with a plurality of radiation openings in the
form of slits or slots (S11, S12 ... S21, S22 ...) in the longitudinal direction thereof, characterized in that for spreading the
grid lobes, coming from the individual aerial elements when electrically directing
the aerial, to different positions in the radiation diagram of the aerial array, the
wave conductors in the aerial are implemented such that the wave conductor wavelength
(λ g) for at least some of the wave conductors (VI, V2) assumes mutually different
values, the mutal spacing (d1, d2 ...) of the broadside slits or slots being different for selected wave conductors.