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(11) | EP 0 249 303 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | A dipole array |
| (57) An antenna adapted to scan in one or more planes comprising an array of dipoles in
which the arms of adjacent dipoles are not parallel, but are at right angles to each
other, which arrangement results in a reduction in mutual coupling in the E plane.
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[0002] A well known undesirable characteristic of such antennas is that strong coupling exists between adjacent dipoles. It is difficult to predict the nature of the coupling in any particular design and therefore it is also difficult to select the correct phase and amplitude values to be applied to each dipole in order to achieve a required beamshape. This problem is set out in a paper entitled "Mutual Coupling in Two-Dimensional Arrays" by J. Blass and S.J. Rabinowitz published by the Institute of Radio Engineers Western Convention Record Vol. l Part l pages l34-l50.
[0003] This invention provides an antenna comprising an array of dipoles each comprising two arms arranged at an angle to each other so that adjacent arms of adjacent dipoles are not parallel.
[0004] By arranging the arms of the dipoles such that adjacent arms of adjacent dipoles are not parallel, it has been found that mutual coupling between adjacent dipoles can be considerably reduced.
[0005] It is envisaged that each dipole arm will normally make, with a plane passing through all the dipoles in the array, the same angle as does each other dipole arm. However, this is not a necessary feature of the invention and it would be possible, perhaps for some specialist purpose, for one arm of each dipole to be, for example, parallel to that plane.
[0006] The problem of mutual coupling is particularly apparent where the antenna is adapted to scan in one or more planes and the invention is therefore of particular value for use in such antennas.
[0007] The dipoles may be formed in a particularly simple manner by shaped extensions of ground planes of a triplate structure which feeds energy to the dipoles.
[0008] Preferably, adjacent arms of adjacent dipoles are at 90 degrees to each other. This arrangement has been found to maximise the reduction in mutual coupling. Of course, adjacent arms of adjacent dipoles may be at other angles to each other, but the resulting reduction in mutual coupling will probably be less.
[0009] Each dipole may have a second crossed dipole associated with it, each such pair of crossed dipoles being connected to a feed system designed to impose a relative phase shift between them. In this way the antenna can radiate or receive circularly or eliptically polarised radiation.
[0010] Two ways in which the invention may be performed will now be described by way of example, with reference to the accompanying drawings in which:
Figure l shows in very diagrammatic form, a part of a dipole array forming an antenna constructed in accordance with the invention designed for transmitting and receiving vertically polarised radiation;
Figure 2 shows one of an array of pairs of crossed dipoles forming another antenna, constructed in accordance with the invention for transmitting and receiving circularly or eliptically polarised radiation.
Figure 3 shows very schematically part of an alternative configuration of a dipole array forming an antenna constructed in accordance with the invention.
[0011] Referring to figure l, there is shown a transmitter l which feeds a signal of microwave frequency to a beam forming network 2. The latter, which is of conventional design, applies the signal, with different phase and amplitude adjustments, designed to produce a beam having a specified shape in azimuth, to co-axial lines 3. The adjustments made at 3 change periodically to scan the beam in azimuth.
[0012] The co-axial lines 3 lead to corresponding triplates 4 each of which has two outer conductors and an inner conductor having a number of branches leading to respective vertically spaced dipoles. This inner conductor is designed to split the available energy and to impose appropriate phase shifts in such a way as to achieve a desired vertical beam shape in a manner known per se.
[0013] The front edges of the outer conductors of each triplate are machined to form a pair of projections 5 each having arms 6 separated by a slot 7. These projections extend through holes 8 in a reflector 9.
[0014] A conductive rod l0 connects together the projections of each pair on one side of slot 7. A second conductive rod ll connects the projections on the other side of the slot back together and to the end l2 of a branch of the inner conductor.
[0015] The arms 6 of each pair of projections define a dipole, and, as will be seen from the drawing, these arms are arranged at an angle, in this case 45 degrees to a plane in which the dipoles lie, this being parallel to the plane of the reflector 9. The arms of each dipole also define an angle relative to each other, this angle being 90 degrees in the illustrated embodiment.
[0016] The angles of the arms of the dipoles are of significance because adjacent arms of adjacent dipoles are, unlike similar known arrays, not parallel. On the contrary they are at right-angles to each other. This has been found to reduced mutual coupling between them to a significant extent.
[0017] With reference to the alternative construction of Figure 2 there is shown a ground plane l3 and four angular elements l4A, l4B and l5A, l5B, each having an upright attached to the ground plate and a dipole arm which defines an angle of 45 degrees with the ground plane and an angle of 90 degrees with an adjacent arm of an adjacent dipole (not shown).
[0018] Arms of elements l4A and l4B define a dipole which is fed by a co-axial cable l6 passing through a hole l7 in the reflector. The outer conductor of the cable l6 is soldered at l8 to one arm and the inner conductor l6A is soldered at l9 to the other arm. Arms of elements l5A, l5B define a second dipole arranged in a plane at right angles to the plane of the first dipole. It is fed by a similar co-axial cable 20 passing through a hole 2l and having similarly connected inner and outer conductors. Signals applied to the cables l6 and 20 are subjected to relative phase shifts so that the structure illustrated will radiate or receive radiation polarised in a desired manner, for example circularly polarised.
[0019] Although the embodiments specifically described comprise an array of elements arranged in vertical columns and horizontal rows, other configurations may be used. For instance, the elements may be arranged to form a triangular grid in which there is vertical alignment between respective elements in alternate horizontal rows, but no vertical alignment between respective elements in adjacent horizontal rows (Figure 3). When the elements are so arranged, it is found that, as well as there being a reduction in mutual coupling in the E plane, there is also a reduction in mutual coupling in the H-plane. This is of particular significance where the beam is scanned in the H-plane as is the case in Figure l.
1. An antenna comprising an array of dipoles, each comprising two arms arranged at
an angle to each other, so that adjacent arms of adjacent dipoles are not parallel.
2. An antenna according to claim l in which each arm of each dipole is at an angle
to a plane passing through all the dipoles.
3. An antenna according to claim l or 2 connected to a feed system designed to cause
the direction of maximum gain of the antenna to sweep in a plane passing through the
said adjacent dipoles.
4. An antenna according to any preceding claim in which adjacent arms of adjacent
dipoles are at 90 degrees to each other.
5. An antenna according to any preceding claim in which each of the aforementioned
dipoles has a second dipole associated with it, the first mentioned and the second
dipoles being arranged in orthogonal planes and being connected to a feed system designed
to impose a relative phase shift between them.
6. An antenna substantially as described with reference to Fig. l of the accompanying
drawings and substantially as illustrated therein.
7. An antenna substantially as described with reference to figure 2 of the accompanying
drawings and substantially as illustrated therein.