The present invention relates to an electrically controlled antenna array, i.e. an antenna with a main lobe which may be controlled by varying the phases in the included antenna elements. Such an antenna is used in radar reconnaissance equipment for example.
An antenna array of the kind intended here comprises a plurality of antenna elements configured as rectangular wave guides 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 guide in the antenna array. It is already known to make the antenna lobe controllable in a plane at right angles to the longitudinal direction of the wave guides by placing phase shifters in the feed path to each guide, e.g. according to GB-B1.577.939. alternatingly above and below the centre line of the wave guides, the illumination function will be phase modulated along the antenna aperture, i.e. along the wave guides. This gives rise to large side lobe peaks in the antenna array radiation diagram.
It is known to solve this problem by using radiation openings and elements that reduce 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 coupling between them, and it is therefore desirable to retain broadside slits to obtain good side lobe suppression.
A slot array antenna with slotted wave guides is previously known from US-A-4.423.421. This antenna is not provided with adjustable phase shifters to vary the phase of the electromagnetic field to the wave guides in the array. The spacing between the slots in one wave guide is different from that of another wave guide in order to produce a symmetrical pencil beam with maximized antenna gain and reduced sidelobes, but the pencil beam has a constant lobe direction.
The object of the present invention is to achieve an electrically controlled antenna array of the kind mentioned in the introduction, using broadside slits as radiation elements, the antenna diagram of which shows substantially suppressed side lobes. The invention is characterized as will be seen from the characterizing portion of claim 1.
The invention will now be described in detail, with reference to the accompanying drawings, where Figure 1 illustrates an antenna array with a construction known per se, but with further distinguishing features in accordance with the invention;
The antenna array in Figure 1 comprises a plurality of antenna elements (4 elements in the Figure) in the form of rectangularwave guides V1, V2, V3 and V4 lying parallel along their respective long narrow sides. Feed wave guides M1 and M2-M4 (the latter three being concealed in the Figure) are each connected to one of the wave guides Vl-V4. Each wave guide is provided with radiation openings in the form of longitudinal slits, S11, S12,... on the wave guide V1, S21, S22 on the wave guide V2, S31, S32, ... on the wave guide V3 and S41, S42, ... on the wave guide 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 guide. The end portions of the feed wave guides Ml-M4 which are attached to the wave guides V1V4 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 guide V1V4. The other ends of the feed wave guides are connected via suitable input feed elements to the phase shifters F1―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 guide VI.
The use of broadside slits or slots of the type shown in Figure 2 with uniform element spacing d (d=d1=d2=, ...) gives rise to side lobe peaks in the antenna 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 guides and through the lobe maximum when the direction is 20° from the direction of the normal. The side lobe peaks are so-called grating lobes whose periodicity correspond to the double element spacing 2d. If the slits S11, S12, ... S21, S22... etc in the wave guides V1―V4 had mutually differing element spacing d1≠d2≠ ... instead, the grid lobes from the individual wave guides 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 wavelength of the individual wave guides.
Figure 2 illustrates a portion of the antenna array in Figure 1, portions of two wave guides being depicted. The slits S11' S12, and S13, S14 in the wave guide V1 have the mutual spacing d1 and the slits S21, S22, S23, S24 etc in the wave guide V2 have the mutual spacing d2≠d1. To attain the intended reduction of the side lobe peaks in Figure 4, the wave guide wavelength Àg varied such that Àg is different for each of the guides V1―V4. This is described below in connection with Figure 3. Different spacings d1, d2 between the slits of the different wave guides V1―V4 are obtained as a consequence.
Figure 3 is a cross section of a wave guide VI with the slits S11, S12, there also being shown a part of an adjacent wave guide V2. On its inner surface facing the slits S11, S12 the wave guide VI is provided with a raised portion or ridge R1, situated symmetrically about the symmetrical axis C of the wave guide. The ridge has two side walls RV1 and RV2 extending at right angles to the inner surface Y of the wave guide 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 h1 from the surface Y. The wave guide V1 is a so-called ridge wave guide wherein the wavelength λg for a given wave guide width a and height b may be varied within given limits by varying the ridge height h1. The height h1 is thus constant for a given wave guide in the group antenna, i.e. for the wave guide VI the height of the ridge R1 is equal to h,, for the wave guide V2 the height of the ridge R2 is h2 (h1≠h2) and so on. Since the slit spacing d≈λg/2, the grating lobes may be spread out over the lobe angle interval of the antenna, thereby reducing their effect on the side lobe level.
In Figure 5 is shown a radiation diagram for an antenna array with a ridge wave guide 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 R1, R2 the antennas are otherwise entirely the same.
Broadside slits in antenna arrays of the type intended here have large advantages:
The inventive antenna array retains the above- mentioned advantages due to the broadside slits, but with reduced side lobes.
The invention is not restricted to embrace wave guides V1―V4, where the wave guide wavelengths λg1, λg2, ... for the different wave guides 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 d1, d2, ... must be dimensioned so that d1≠d2 etc. There is thus obtained variation in the positions of the grating lobes for the entire antenna array, which causes a reduction of the side lobe level.