Slotted waveguide antennas and arrays.

0001The present invention relates to slotted waveguide antennas employing waveguide radiators having sidewall slots and arrays of such radiators.

0002The merits of slotted-waveguide array antennas, which are capable of providing well controlled, very low side lobe, radiation patterns, are well known. For linearly polarised arrays two methods of slotting the waveguide are commonly used: namely staggered axial shunt slots in the broad wall, or inclined sidewall slots. Unfortunately arrays formed from slots of this type suffer from two basic radiation pattern errors. These are firstly periodic errors associated with the requirement either to stagger the longitudinal shunt slots or to incline the sidewall slots oppositely, and secondly cross-polarisation errors particularly in the case of the sidewall slots because of their inclination.

0003Where sidewall slots are used, the slots should be normal to the longitudinal axis of the waveguide in order to prevent cross-polarisation. The slots should also be symmetrical with regard to longitudinal axis to avoid periodic errors. The latter requires means inside the waveguide to ensure that radiation from the symmetrical slots occurs.

0004The radiation patterns of waveguides with symmetrical sidewall slots contain grating lobes because adjacent slots radiate in phase only when separated by a distance equal to one or more wavelengths of the mode within the waveguide and it is an object of the present invention to provide in phase radiation when the sidewall slots are separated by half of the guide wavelength.

0005According to a first aspect of the present invention there is provided an elongated waveguide radiator which in cross section is divided into first and second waveguide portions by a septum, having a plurality of first elongated slots in an external wall of the radiator each of which extends on both sides of the septum and is orthogonal to the longitudinal axis of the radiator, and a plurality of elongated second slots, each of which extends from a corresponding one of the first slots into the septum, and has a longitudinal axis which is inclined to a line in the septum normal to the longitudinal axis of the radiator, the direction of the angle of inclination alternating along the radiator.

0006The inclination of the second slots can be as required to provide a desired control of the radiation magnitude from adjacent slots but the second slots must be inclined to said line in the septum (that is second slots must not be parallel to the longitudinal axis of the radiator) or radiation between adjacent slots will not be in phase when they are separated by half of the guide wavelength. Arrays may be constructed using a plurality of radiators according to the first aspect of the invention.

0007According to a second aspect of the invention there is provided an antenna array comprising two or more elongated waveguide radiators each of which in cross section is divided into first and second waveguide portions by a septum, has a plurality of first elongated slots in an external wall of the radiator each of which extends on both sides of the septum and is orthogonal to the longitudinal axis of the radiator, and has a plurality of elongated second slots, each of which extends from a corresponding one of the first slots into the septum, wherein the separation between the first slots in each radiator is substantially equal to 10 times the distance from any first slot in the array to the nearest adjacent first slot.

0008In both first and second aspects of the invention the first and second waveguide portions are usually excited in antiphase.

0009Certain embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:- <ul id="ul0001" list-style="none"><li>Figure 1 is used in explaining radiators according to the invention and shows part of a waveguide,</li><li>Figure 2 shows a radiator according to the invention,</li><li>Figure 3 is a schematic diagram showing how phase reversal occurs between the slots of Figure 2,</li><li>Figure 4 shows another radiator according to the invention,</li><li>Figure 5 shows ways of stacking radiators to form arrays according to the invention, and</li><li>Figure 6 shows a way of feeding antenna arrays according to the invention.</li></ul>

0010In Figure 1 part of a slotted waveguide radiator 10 comprises two waveguides 11 and 12 each of the usual dimensions required to support the TE₁₀ mode and thus for the X-band the breadth of the waveguide 10 is about two and a quarter centimetres while its overall height is in the region of two and a half centimetres. Thus the radiator 10 can be regarded as being bifurcated along the H plane. There are two slots 13 and 14 in the sidewall of the part of the waveguide radiator shown and a septum 15 separating the waveguides 11 and 12 has slots 16 and 17 contiguous with the slots 13 and 14, respectively. The slots 13, 14, 16 and 17 are part of a series of such slots along the radiator 10. For an X-band array the slots are about 0.2 cm wide and spaced at about 4.5 cm along the radiator.

0011Since slots 13 and 14 are orthogonal to the direction of propagation they would not radiate in a waveguide having no septum and further if the waveguides 11 and 12 were excited in phase the septum 15 would have no effect. Thus the waveguides 11 and 12 are normally excited in antiphase, so that the septum slots 16 and 17 are strongly excited by the "odd" TE₁₀ mode since they significantly interrupt the septum wall currents of this mode. The fields produced in the slots 16 and 17 if they are each about a quarter of the free space wavelength long induce field patterns in the slots 13 and 14 causing them to radiate parasitically. However these slots can vary in length in the range one-eighth to half a free space wavelength. The slots 13 normally have an overall length of half the free space wavelength at the centre of the band to be propagated in the waveguides 11 and 12, although they may be as short as a quarter of the free space wavelength. Hence the slots 13 and 14 are resonant as are each upper half of the slots 13 and 14 together with the corresponding slots 16 and 17, and each lower half of the slots 13 and 14 together with the corresponding slots 16 and 17. Non- resonant slot lengths may be used if some pattern shaping is desired; for example a cosine distribution or other tapering of field strength across the antenna aperture. Other out of phase excitations than antiphase can sometimes prove useful for the waveguides 11 and 12.

0012The waveguide radiator 10 generates horizontal polarisation when the radiator is mounted horizontally, and as the slots 13 and 14 are orthogonal to the axis of the radiator, significant cross-polarisation does not occur. The slots 13 and 14 are regularly spaced and symmetrical with respect to the wall of the radiator 10 so periodic errors are avoided.

0013However a disadvantage of the arrangement of Figure 1 in comparison with conventional inclined-edge slot radiators is that 180⁰ phase reversals between adjacent slots (to allow radiation in phase at half guide wavelength slot spacing) is not possible since such reversal is usually obtained by oppositely inclining alternate slots. Thus for a waveguide having a normal broadwall dimension, substantially more than one free space wavelength spacing between slots is required to procure approximately broadside radiation and as a result more than one principal radiation lobe occurs, that is grating lobes are formed.

0014The grating lobe problem can be largely overcome by the waveguide of Figure 2. This waveguide also has slots 13 and 14 but these slots radiate in phase when separated axially by half of one guide wavelength and so reduces the problem of grating lobes. As before the waveguides 11 and 12 are fed in antiphase but now septum slots 25 and 26 extend axially along the radiator 10 from the slots 13 and 14, respectively, and are configured to provide the required phase reversal. At each slot 13, 14, the direction in which the slots 25, 26 extend in the axial direction of the radiator 10 is reversed. Referring to Figure 3 where currents in the septum 15 are indicated by chain dashed arrows 27, the electric fields parasitically excited in the slots 25 and 26 are as shown by the arrows 28 and 29, respectively. Between two adjacent slots two 180 phase changes occur, one because the slots are separated by half a guide wavelength and one due to the directions of the slots 25 and 26. It can be seen that the electric field directions excited in the slots 13 and 14 are in phase due to these two reversals and at half guide wavelength spacing, therefore, the slots radiate in phase. Again the slots 25 and 26 are each approximately a quarter of a free space wavelength long at the centre of the band of frequencies to be propagated and other dimensions are the same as those of the waveguide of Figure 1.

0015Since the slots 13 and 14 remain at right angles to the direction of propagation the problem of cross-polarisation is largely avoided and since the slots are symmetrical in relation to the centre line of the waveguide radiator no significant periodicity error arises in the beam pattern.

0016As shown in Figure 4, the septum slots 25 and 26 may be inclined to the longitudinal axis of the waveguide at angles up to, but not including, 90⁰ (when they become equivalent to the slots 16 and 17). Traversing the waveguides in the longitudinal direction the slots are inclined first in the 'forward' direction and then in the 'backward' direction for adjacent slots. Such slots provide only partial phase reversal and although such a phase change is usually a disadvantage, it can sometimes be useful to give control of radiation strength if it is required to keep the lengths of the sidewall slots and septum slots invariant; for example in constructing a narrow-band resonant array.

0017For slot arrays for which a phase reversal mechanism is not possible, as with the radiator of Figure 1, the grating lobe problem can also be largely overcome by stacking radiators. Figure 5 shows three possible ways 20, 21 and 22 of stacking, on a background of equilateral triangles each having sides d. Schemes 20 and 21 are particularly suitable for this purpose. In each of the three stacked arrays shown, slots as indicated by lines transverse to the waveguides are separated by a distance d. The object of stacking is to create a planar array in which slots radiate in phase and are spaced by less than 0.7 λ_o, where A₀ is the free space wavelength. This condition avoids grating lobes within + or - 90⁰ of the normal to the axes of the waveguides, allows a finite beam width and avoids large internal reflections within the waveguides. Thus d should be less than 0.7 but for simplicity this condition can be expressed as d approximately equal to λ_o/2. The array 20 is made up of three radiators 10 of the type shown in Figure 1 while the arrays 21 and 22 are each formed by two such radiators but, in practice, arrays of this type usually comprise many more radiators. Corresponding waveguides in the radiators making up the arrays are fed in phase.

0018In the array 20 the separation between adjacent slots in the same waveguide radiator is d 10 and thus the separation d between slots in the stacked array is just over a third of the guide wavelength (λ_g). Since the A is greater than λ_o, d is, as required, approximately equal to λ_o/2. If the waveguide width is chosen to be that of a standard waveguide the direction of the beam is almost broadside. However as is apparent from Figure 5 this increase in slot separation is achieved at the expense of the sidewall dimension (b) of the waveguide radiator. As a consequence the sidewall dimension has to be reduced to less than 0.2 of a free space wavelength. After provision is made for wall thickness and the septum, the inside dimension of each waveguide 11 and 12 is such that manufacture is difficult.

0019The array 21 overcomes this problem but requires the addition of phase compensation between adjacent slots to reduce λ_g to be equal to or a little larger than λ_o in order to place the main beam close to broadside. The separation between the slots in each waveguide radiator making up the array is about A giving the required array slot separation of about λ_o/2. Phase compensation can be obtained for example by dielectric loading (this is partially or completely filling the waveguide with dielectric), the use of periodically spaced metallic fins or irises, oversize waveguides or discrete phase shifters. The latter is usually preferable in view of attenuation loss, weight associated with dielectric or periodic loading and overmoding associated with an oversize waveguide. If the individual waveguide radiators are of standard width then the required additional phase shift between slots which is needed to ensure approximately broadside radiation is of the order of 90°. Since the separation between adjacent slots is approximately one free space wavelength, sufficient space is available for the insertion of an inductive post phase shifter between slots.

0020In the array 22 the sidewall dimension b is enlarged to normal size. For slot arrays containing no phase reversals this arrangement requires that the waveguide radiator be even more heavily loaded so that λ_g approaches λ_o/2. While radiators as used in the arrays 20 and 21 are not suitable for use individually those of the array 22 may be used singly.

0021The array 20 can be fed in the way shown in Figure 6. A feed waveguide 31 has three slots which couple into bifurcated waveguides 32, 33 and 34 by way of 3dB power splitters 35, 36 and 37. Since the power splitters introduce a 90° phase shift between the two portions of each bifurcated waveguide, the upper portions include phase shifters 38, 39 and 40 to give the 180 phase difference to induce the "odd" mode in the waveguides. The phase shifters may comprise shaped dielectric inserts providing 90° of phase shift at the centre-band frequency but which are also matched to the waveguides. Matched loads 42 to 48 are provided for the waveguides 31 to 34. Other radiators according to the invention can be fed in similar ways.

0022Arrays 21 and 22 are more easily implemented with waveguide radiators of the type shown in Figures 2 and 4 since loading is not required. The distance S between slots in each radiator is, in effect, λ_g/2, and for the array 21 λ_g should equal about twice λ_o. This condition can easily be met for example by operating close to the cut-off frequency of the waveguide. For the array 22 if d is allowed to become equal to 0.7 λ_o then λ_g should be approximately 1.4 λ_o which again can easily be arranged. It will be realised that there are many other ways of putting the invention into effect than those specifically described. For example the slots in the septum between the waveguides can be of other shapes provided, as far as the arrangement similar to Figure 3 are concerned, the electric field and adjacent slots are in opposite directions.

0023The waveguides may be filled with dielectric or may be periodically loaded to reduce the guide wavelength, thus minimising the grating lobe problem and also permitting frequency scanning of the main radiated beam. The sidewall slots and the septum slots may be considerably shorter than their 'resonant' values if a particular aperture field shaping is desired. The slots in the waveguide walls may extend from the narrow walls into the broad walls.