GB2064876A - Slot array antenna - Google Patents

Abstract

A small slot array antenna with a circular aperture 11 and linear polarization is made up of a plurality of slotted waveguides 10u, 12u, 14u, 10l, 12l, 14l having different lengths and widths W14, W12, W10 which decrease radially outwardly. The slots in the waveguides are positioned to optimize amplitude taper in any plane passing through the centre of the array normal to the surface thereof and to maximize the number of slots. Thus the slot pitch increases from guide 14 to guide 12 to guide 10 and the slots of each guide are offset widthwise from the guide axes by amounts which increase from the centre of each guide to its ends. <IMAGE>

Description

SPECIFICATION Slot array antenna This invention pertains to antennae for radio frequency energy wherein a planar array of slotted waveguides is used.

It has been common practice in the art of designing radar antennae for seekers in guided missiles to use a so-called resonant slot array. According to known practice, such an array is formed by mounting a plurality of similarly dimensioned slotted rectangular waveguides in proximity with one another to cover a predetermined aperture. An electrical short circuit is formed across one end of each waveguide to make a resonant structure wherein standing waves can exist to optimize the energisation of the slots. A corporate feed of conventional design is connected to the second ends of the waveguide to allow operation of the resonant slot array either as a transmitting antenna or a receiving antenna, such as a monopulse antenna.

Ordinarily, when a resonant slot array is to be used in a guided missile, it is necessary that a broadside pencil beam be formed so that antenna gain is maximized, with sidelobe levels as low as possible; and that the energy in the beam be linearly polarized, with cross-polarization effects minimized. In order to achieve the foregoing in the limited space available in the cylindrical body of a guided missile, the aperture of the usual slot array is circular in shape, the array itself is mounted so as to be steerable in pitch and yaw and the orientation of all of the slots with respect to the longitudinal axes of the waveguides is maintained constant. Further, if the slot array is to be operated as a monopulse antenna, the number of waveguides and disposition of slots is such that an equal number of slots is located in each quadrant of the aperture.In addition, the constraints on any slot array which must be met to avoid grating lobes or reduction in efficiency must be observed. This is to say, for a given frequency of operation, proper attention must be given frequency of operation, proper attention must be given to the dimensions of the waveguides, the spacing between slots and the position of the electrical short circuit in each one of the waveguides. Thus, in a typical application wherein the aperture of an antenna in a guided missile may have a diameter of 12.5 cm, a slot array for x-band operation may have a maximum of 20 slots when known techniques are used to design such an array. Because the antenna gain of any slot array is directly related to the number of slots, the antenna gain of the array is limited.

Another problem is encountered with the conventional slot array wherein similarly dimensioned waveguides are used. Because the positions of the slots in each waveguide (along the length of such guide) are fixed, it is difficult to produce a symmetrical pencil beam. As a result, the quality of performance of the conventional slot array varies, depending upon the direction of a target from the boresight.

In view of the foregoing background of this invention, it is a primary object hereof to provide a slot array array wherein, with a circular aperture, the number of slots may be increased beyond the largest number possible when known techniques are applied in the design of such an array.

The antenna according to the invention is defined in claim 1, to which reference should now be made.

The antenna according to the invention can be used as a monopulse antenna. The spacing between slots can be obtimized to produce a symmetrical pencil beam.

For a more complete understanding of this invention, reference is now made to the following description of the accompanying drawing wherein the single Figure is a plan view of an antenna embodying this invention.

Referring now to the figure, the disposition of the slots (numbered below) in six waveguides 10u, 12u, 14u,14t,12t and 10t of an exemplary resonant slot array (intended for use at X-band frequencies in a monopulse radar in a guided missile where the largest allowable aperture 11 is circular in shape with a diameter of 12.5 cm) is shown. Because the slots in the waveguides 10t,12e and 13t are disposed in the same way as the slots in the waveguides 10u, 12u and 14u, and because the dimensions of similarly numbered waveguides are the same, only the latter waveguides will be described. Also, the thickness of each wall in each waveguide and the conventional corporate feed have not been shown.

Starting with waveguide 14u, the width W14 (meaning the inside dimension of the broad wall of a rectang ular waveguide) is chosen so that the cut-off frequency of the dominant TE01 mode in the guide is near the low end of the X-band. The depth (meaning the inside dimension of the narrow wall of a rectangular waveguide) is chosen so that the cut-off frequency of the next higher mode (the TElo or TE20 mode) is above the highest frequency in the X-band.

Here a conventional rectangularwaveguide (having inside dimensions of 22.9 x 10.2 mm) for use in the X-band is employed.

It will be recognized that the waveguide wavelength of X-band energy within the waveguide 1 4u is longer than the wave-length of such energy in free space and that the slots must be spaced at distances determined by the waveguide wavelength.

Even so, with an aperture 12.5 cm in diameter, with shunt slots it is still possible to have six slots spaced at half-wavelengths (measured in the waveguide along the axis 14u) and an electrical short circuit spaced one-quarter wavelength from the last slot, say slot 14u(3').

Slots 14u(3), 14u(1) and 14u(2') are pitched at intervals of one wavelength along the axis 14u.

Similarly, slots 14u(2), 14u(1') and 14u(3') are pitched at intervals of one wavelength and are staggered at one-half wavelength intervals relative to the slots 14u(3), 14(1) and 14u(2'). In addition, slots 14u(1) and 14u(1 ') are equally spaced on opposite sides along the yaw axis, from the axis 14u; slots 14u(2) and 14u(2') are also similarly spaced; and finally slots 14u(3) and 14u(3') are also similarly spaced. It will now be recognised that the slots 1 4u(1) to 14u(3') constitute a linear array with amplitude taper along the pitch axis, with the centreline of the beam produced by the array broadside to the waveguide 14u (meaning orthogonal to the plane defined by the pitch axis and the yaw axis).Further, the first sidelobe (measured along the pitch axis) is determined in accordance with the selected amplitude taper.

Waveguide 12u is dimensioned so that its width W12 is less than the width W14 of the waveguide 14u.

Therefore, the wavelength of energy in the waveguide 1 2u is greater than that in the waveguide 14u.

In consequence, then, the spacing (measured along axis 12u) between the various slots 1 2u(1), 1 2u(2), 12u(l'), 12u(2') is greater than the spacing of the corresponding slots in the waveguide 14u. It should be noted that, if energy is fed to the same end of waveguide 12u as waveguide 14u, the electrical short circuit (not shown) in the former would be disposed one-quarter wavelength from slot 12u(2p) so the sense of the electric field at each pair of corresponding slots in the two waveguides would be appropriate along the yaw axis.

The spacing (measured along the yaw axis) of each one of the slots 12u(l ), 12u(2), 12u(l'), 12u(2') is determined to allow an amplitude taper along the yaw axis (without affecting polarization). Therefore, each slot in the wave-guide 12u is at a greater distance from the axis 12u' than the distance of the corresponding slot in the waveguide 14u from the axis 14u'.

Waveguide 10u is dimensioned so that at least two slots 10u(l), 10u(l') may be fitted in the zone defined by the free side of the waveguide 1 2u and the circle defining the largest allowable aperture 11. The width of this waveguide 1 0u is 18.8 mm.As would be expected, because the wavelength of energy within the waveguide 10u is greater than the wavelength within the waveguide 14u or 12u, slots 10u(1) and 10u(1 ') are further apart than slots 12u(1) and 12u(l') or 14u(1) and 14u(l'). Also, the position ofthe electrical short circuit (not shown) near slot 10u(l') is determined by the wavelength of the energy in the waveguide 10u and the senses of the electric fields at slots 1or(1) and 1or(1') are, respectively, the same as the senses of the electric fields at slots 14u(1) and 14u(1').

In order to provide the desired amplitude taper along the yaw axis, the amount of energy fed into each one of the waveguides 14u, 12u, 10u, 14e, 12P, 10t is adjusted in any convenient manner in the corporate feed. In addition, the positions (measured along the yaw axis) of slots 10u(l), 10u(l') are changed to contribute to the desired amplitude taper so that the shape of the beam along the yaw axis is optimized and sidelobes are reduced to a minimum.

It will now be evident to one of skill in the art that, for a slot antenna with a small diameter, say 12.5 cm, circular aperture when designed for X-band operation, the flexibility in design offered by using waveguides of different widths is advantageous. That is to say, because the relative positions of corresponding slots in the different waveguides may be adjusted without changing the orientations of the slots, antenna gain may be maximized for a pencil beam with relatively small sidelobes and without affecting the polarization of the beam. In this connection, it should be noted that the length of each slot also may be adjusted to modify the phase distribution across the aperture. With knowledge of the effect of changing the length of a resonant slot, conventional empirical techniques may be followed to adjust the phase distribution across the aperture for any particular case. Thus, as here where the width of the beam measured along the yaw axis is to be the same as the width of the beam along the pitch axis, the lengths of the slots may be changed to optimize the phase distributions along such axes.

Claims (4)

1. A linearly polarized slot array antenna having a substantially circular aperture, comprising a first plurality of rectangularwaveguide resonators juxtaposed with their narrow walls abutting and dimensioned substantially to cover a first half of the circular aperture, the widths of the broad walls of the waveguide resonators decreasing radially outwardly, a second like plurality of rectangular waveguide resonators covering the second half of the circular aperture, and a plurality of radiating slots formed through the broad walls of the rectangular waveguide resonators, the slots being parallel one to another with the centre of each slot lying in a plane of maximum electric field within its corresponding rectangular waveguide resonator from the centreline of the resonator increasing from the centre to each end of the resonator to provide amplitude taper along the lengths of the resonators.

2. A linearly polarized slot array antenna according to claim 1, wherein the distances from the longitudinal axis of each rectangular waveguide resonator to the centres of its slots increases from the centre to each end of the resonator to provide amplitude taper along a line orthogonal to the lengths of the resonators.

3. A linearly polarized slot array antenna according to claim 1 or 2, wherein the ratio between the diameter of the aperture and the wavelength of radio frequency energy at the design frequency of the antenna is about 5:1.

4. A linearly polarized slot array antenna substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.