GB2243491A - Frequency-scanned antenna arrays - Google Patents

Abstract

A means for scanning a beam over a two-dimensional sector in space comprising a two-dimensional array (8) of radiating elements excited by a signal of variable frequency via a single travelling wave feed line (7). The lengths of the lines from each element to its junction with the feedline are made equal and the lengths of feed line between each of the junctions are equal. Alternatively, fig. 5 phase shifting sections (10a, 10b, 10c, 11a, 11b) positioned between adjacent radiating elements are set so that the array can be scanned in raster fashion. <IMAGE>

Description

FREQUENCY-SCANNED ANTENNA ARRAYS This invention relates to frequency-scanned antenna arrays and has particular applications in the field of radar.

Most known frequency-scanned antennas are formed by using a travelling wave feed to excite a linear or two-dimensional array. In a simple linear array where a terminated feed line is connected to a plurality of radiating elements, the angle of the main beam 6 is given by sin e = n + d/lf a/io where n is an integer, d is the element spacing and lf and lo are the wavelengths in the feed line and in free space respectively. Usually n = -1 so that 6 is dependent on wavelength, which in a given medium is inversely proportional to frequency. It is thus clear that changing the frequency will change 0. This is the basis cf frequency-scanned antenna action.

In its more general form the length of feed line between each element may exceed d. -This can be implemented by coiling the line or by various forms of line loading. The effect is to increase the rate of frequency scan so that scanning over a given range of e may be accomplished by a smaller change of frequency.

Two-dimensional arrays, which can be formed by placing many such lines together may, as shown in Fig 1, comprise several terminated feed lines la-id incorporating radiating elements 2 and a corporate feed 3.

This configuration will produce a pencil beam in a fixed plane only, ie the 0=0 plane.

It is an object of the present invention to provide a frequency-scanned antenna array which enables the beam to cover a two-dimensional sector in space.

According to this invention a two-dimensional frequency-scanned antenna array comprises: radiating elements arranged in a plurality of rows and a plurality of columns, the elements comprising each column being connected together by lengths of conducting line providing equal phase differences between adjacent elements; and a single travelling wave feed-line for supplying one element in each of the columns with an excitation signal of variable frequency, the lengths of feed line between adjacent elements providing equal phase differences between elements; the array being capable of covering an infinite lattice by simple translation in two directions by steps of X1, Y1 units and X2, Y2 units respectively; and the ratio between phase differences along the lengths of conducting line and feed line nX/ny substantially satisfying the following equations: nxXl + nyYX = N and nxX2 + nyY2 = N where N is the total number of elements in the array.

The said travelling wave feed line may be adapted to supply all the elements comprising one of the rows with said excitation signal.

According to another aspect of the invention, a two-dimensional frequency-scanned antenna array comprises radiating elements arranged in a plurality of rows and a plurality of columns, each element being connected by substantially equal lengths of conducting line to a single travelling wave feed-line for supplying the elements with an excitation signal of variable frequency, said conducting lines being connected to the feed-line in a defined order at substantially equispaced intervals.

The single travelling wave feed could take the form of a waveguide, coaxial line or stripline such as microstrip or triplate.

The radiating elements may comprise means such as horns, dipoles or microstrip patches.

In order to understand how the invention performs two-dimensional scanning, it is convenient to contrast its method of operation with that of a known two-dimensional scanning arrangement. It is known that arrays can form a set of beams covering a two-dimensional sector by feeding with an orthogonal beamformer. Each input to the beamformer corresponds to a beam in space which is created by application of the beam former output in certain well-defined sequences to the radiating elements. In Fig 2, a beam former 4 having input ports a to f and array output ports 0 to 5 is connected as shown to a six-element array 5. The rules for the interconnections between the beamformer and array are well-established and are as follows: a. The array size and shape must be such that by simple translation of the array in two directions an infinite lattice would be covered.This is illustrated in Fig 3 for a seven-element array and is known as the covering condition. b. Similar elements in each array must have the same feeding number. In addition the other elements must be in the correct sequence, corresponding to the phase progression obtained at the output of an orthogonal beamformer. Fig 3 shows translational vectors XX, Y1 and X2, Y2 corresponding to an element 6. The correct sequencing is achieved by specifying integer increments in the lattice formed by the array in the two lattice directions as nx and ny, the values of which are given by: nxXl + nyY1 =N and nxX2 + nyY2 N where N is the total number of elements.For the case shown in Fig 3, 3nx + 2ny = 7 and nx + 3ny = 7 so that nx = 1 and ny = 2. For the array of Fig 2, nx = 2 and ny = 3.

The relationship between this sequencing and the beam former output is important in understanding the invention. A beamformer output usually consists of equal amplitude signals whose phase progressively increases across the output. In other words, for input to beam port 1, the outputs have phase 0, a, 2a, 3a, 4a, etc where a is a progressive phase constant.

For input to beam port 2, the outputs have phase 0, 2a, 4a, 6a, 8a, etc and so on for all beam inputs. The phase progressions when applied to a planar array will give rise to beams covering a two-dimensional sector.

It will be noticed that the phase increase at each port of a beamformer is monotonic with beam port excitation. The phase increase at each element of a travelling wave array is also monotonic. Hence the invention employs a travelling wave feed line which is frequency-scanned, in lieu of a beamformer, to create the 2-D beamset by operation of the antenna at monotonically increasing frequencies. Thus in principle at f1 a phase excitation of 0, a, 2a, 3a, etc could be obtained, and at f2 this could be 0, 2a, 4a, 6a, etc and so on. Where f1 > f2 an orthogonal beamset may be obtained by operation on a number of discrete frequencies.

By way of example, embodiments of the invention will now be described with reference to Figs 4, 5 and 6 of the drawings, which show, schematically, three alternative frequency-scanned antenna arrays in accordance with the invention and to Fig 7, which illustrates the performance of a simulated array.

In the embodiment of Fig 4, a travelling wave feed line 7 is used to feed a two-dimensional array 8 similar to the one shown in Fig 2. In Fig 4, the six radiating elements comprising the array 8 are equi-spaced and the lengths of the lines from each element to its junction with the feed line 7 are made to be equal. Similarly the lengths of feed line between each of the said junctions are equal. By making connections from the feed line 7 to the radiating elements as shown and by varying the frequency of excitation on the feed line 7, two-dimensional scanning can be achieved.

An alternative implementation, shown in Fig 5, makes use of the regular nature of the phase sequencing noted in the examples of Fig 2 and Fig 3.

One travelling wave feed line 9 feeds a six-element aray, similar in layout to that of Fig 2. In Fig 2, the horizontal phase advance is a factor of 2, that is nx = 2, and the vertical advance is a factor of 3, that is ny = 3. Thus, feed lines can be used to provide this action as shown in Fig 5. Phase shifting sections 10a, lOb, lOc, lia and lib ensure that the ratio of vertical to horizontal phase advance for a given frequency change is 3:2. This implementation can be realised using planar technology such as microstrip patches connected by microstrip feed lines, triplate stripline slot radiators with triplate lines or waveguide slots and lines.

In a similar manner a 19-element hexagonal-shaped array, such as that illustrated in Fig 6, can be fed; the relative phase lengths of the interconnections, shown as dashed lines, are calculated in the same manner.

Fig 7 shows the results of computations for an array of 100 elements arranged in a 10 x 10 square. In this example, the phase sequence factors, obtained for a 94 element hexagonal array of the shape shown in Fig 7, are nx = 6.409 and ny = 4.273. These are then applied to the 10 x 10 array rather than the 94 element array to simplify the computation.

This approximation serves to show the robustness of the concept in that the change in the array shape and size does not significantly affect the scanning properties. The array elements are spaced by 0.5 free space wavelengths and the ith beam is selected by applying inx and iny extra phase change per element in the x and y directions respectively.

Fig 7 shows the computed main beam peak positions for 0 < i < 108.

It is clear that the beam has scanned over most of the hemisphere above the array, and has eventually scanned back to the broadside position.

That it only reaches broadside for i = 109, not i = 101, is due to the above approximations. Also, as in normal arrays, the beam is severely degraded when scanning far out from broadside. Thus coverage out beyond e = 60 is not regular. In addition to speed computation the beam position was found to an accuracy of only about 4", and this accounts for some variations in the scan paths when compared with the computed ideal.

Nevertheless the computations are seen to successfully validate the concept of two-dimensional scanning.

Claims (4)

1. A two-dimensional frequency-scanned antenna array comprising a plurality of rows and a plurality of columns of radiating elements, the elements comprising each column being connected together by lengths of conducting line providing equal phase differences between adjacent elements and a single travelling wave feed-line for supplying one element in each of the columns with an excitation signal of variable frequency, the lengths of feed line between adjacent elements providing equal phase differences between the elements of adjacent rows, the array being capable of covering an infinite lattice by simple translation by steps of X1, Y, units and X2, Y2 units respectively and the ratio between phase differences along the lengths of conducting line and feed line nx/ny substantially satisfying the following equations: : nxX1 + nyY1 = N and nxX2 + nay2 N where N is the total number of elements in the array.

2. An antenna array according to Claim 1 in which the said travelling wave feed line is adapted to supply the elements comprising one of the rows with said excitation signal.

3. A two-dimensional frequency-scanned antenna array comprising radiating elements arranged in a plurality of rows and a plurality of columns, each element being connected by substantially equal lengths of conducting line to a single travelling wave feed-line for supplying the elements with an excitation signal of variable frequency, said conducting lines being connected to the feed-line in a defined order at substantially equispaced intervals.

4. An antenna array substantially as hereinbefore described with reference to Figures 4, 5, 6 or 7 of the drawings.