Patent Description:
Direction finding (DF) antenna arrays are known. One such a known DF array is a so-called differentially fed loop array. This array comprises two differentially fed loop antennas collocated, but rotated <NUM> degrees with respect to each other. Each of these loops are fed at their respective left and right corners, on the same horizontal plane and with opposite polarity or out of phase, i.e. with signals that are reverse polarity to each other.

Another known DF array is a dipole antenna array which is shown in <FIG> hereof and which comprises five dipole elements, each comprising a respective centre feed-point, and which are arranged parallel to one another at the five corners of a pentagon. It has been found that the low-frequency DF sensitivity and cross-polarisation isolation of these arrays are not satisfactory for at least some applications. It is known that in low frequency applications, DF antenna arrays often suffer from significant coupling to a mounting structure or other nearby conductive structures. The coupling to these structures will typically have a significant impact on the ability of the DF array to accurately determine the direction of arrival of incoming signals. Low frequency, in this context, refers specifically to those frequencies where the individual elements making up the array can be considered to be electrically small compared to the wavelength of an incoming signal that is to be received. <CIT> discloses a direction finding antenna system comprising a plurality of dipoles.

Accordingly, it is an object of the present invention to provide a direction finding antenna array with which the applicant believes the aforementioned disadvantages may at least be alleviated or which may provide a useful alternative for the known DF antenna arrays.

According to the invention there is provided a direction finding antenna array according to independent claim <NUM>.

The first end of the first antenna element is connected by the first and second electrical connections to the first end of adjacent second and third dipole elements respectively in the array and the second end of the first dipole element is connected by the third and fourth electrical connections to the second end of the adjacent second and third dipole elements respective.

The antenna array may comprise n dipole elements, wherein n ≥ <NUM>. In some embodiments n = <NUM> and in other embodiments n > <NUM>, including, but not limited to n = <NUM>.

In one embodiment, the antenna array may further comprise at least fourth dipole antenna element and a fifth dipole antenna element, each comprising respective first ends, respective second ends and a respective feed-point, wherein the first, second, third, fourth and fifth linear dipole elements are arranged in spaced relationship relative to one another, wherein the first end of the fourth dipole element is connected by fifth and sixth electrical connections to respectively the first ends of the third and fifth dipole elements and wherein the second end of the fourth dipole element is connected by seventh and eighth electrical connections to respectively the second ends of the third and fifth dipole elements, wherein the first end of the fifth dipole element is connected to the first end of the second dipole element by a ninth electrical connection and wherein the second end of the fifth dipole element is connected to the second end of the second dipole element by a tenth electrical connection.

The first to fifth dipole elements may be located at respective corners of a pentagon.

Each of the dipole elements may be linear in configuration.

The linear dipole elements may be arranged vertically and parallel to one another.

Each electrical connection may comprise an elongate conductor.

In some embodiments, at least some electrical connections may comprise a filter circuit.

The respective feed-points may located in a common plane.

Also included within the scope of the invention is a direction finding system comprising a direction finding antenna array as defined above and wherein the respective feed points are connected to respective inputs of coherent receivers of a receiver arrangement.

The receiver arrangement may comprise a processor for executing a computer program comprising correlative direction-finding algorithms.

The invention will now further be described, by way of example only, with reference to the accompanying diagrams wherein:.

An example embodiment of a direction finding (DF) system <NUM> and an antenna array <NUM> therefor are shown in <FIG>.

The DF system <NUM> comprises the DF antenna array <NUM> connected to a receiver arrangement <NUM>. The DF antenna array comprises at least a first dipole antenna element <NUM>, a second dipole antenna element <NUM> and a third dipole antenna element <NUM>. The dipole elements comprise respective first ends <NUM>, <NUM>, <NUM>, respective second ends <NUM>, <NUM>, <NUM> and a respective feed-point <NUM>, <NUM>, <NUM>. The first, second and third dipole elements are arranged in spaced relationship relative to one another in a non-linear pattern. In respect of each dipole element in the array (and taking dipole element <NUM> as an example), the first end <NUM> is connected by first and second electrical connections <NUM>, <NUM> to the first end of each of two adjacent dipole elements <NUM>, <NUM> in the array and the second end <NUM> is connected by third and fourth electrical connections <NUM>, <NUM> to the second end of each of the two adjacent dipole elements <NUM>, <NUM>.

The receiver arrangement <NUM> comprises at least three receivers having respective inputs <NUM>, <NUM>, <NUM> which are respectively connectable to the feed-points <NUM>, <NUM>, <NUM>, respectively.

In a preferred example embodiment as shown in <FIG>, the antenna array <NUM> further comprises a fourth dipole antenna element <NUM> and a fifth dipole antenna element <NUM>. The dipole elements comprise respective first ends <NUM>, <NUM>, respective second ends <NUM>, <NUM> and a respective feed-point <NUM>, <NUM>. The first, second, third, fourth and fifth dipole elements are arranged in spaced relationship relative to one another. The first end <NUM> of the fourth dipole element <NUM> is connected by fifth and sixth electrical connections <NUM>, <NUM> to respectively the first ends <NUM>, <NUM> of the third and fifth dipole elements. The second end <NUM> of the fourth dipole element <NUM> is connected by seventh and eighth electrical connections <NUM>, <NUM> to respectively the second ends <NUM>, <NUM> of the third and fifth dipole elements. The first end <NUM> of the fifth dipole element <NUM> is connected to the first end <NUM> of the second dipole element <NUM> by a ninth electrical connection <NUM> and the second end <NUM> of the fifth dipole element <NUM> is connected to the second end <NUM> of the second dipole element <NUM> by a tenth electrical connection <NUM>.

In this preferred embodiment, the receiver arrangement <NUM> further comprises fourth and fifth receivers with respective inputs <NUM>, <NUM> which are respectively connected to the respective feed-points <NUM>, <NUM>.

The dipole element need not be, but preferably are linear in configuration.

As shown in <FIG>, the first to fifth linear dipole elements <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM> are located at respective corners of a pentagon. The first to fifth dipole elements are arranged vertically and parallel to one another. First parts of each linear dipole element extend from the respective feed-point upwardly to terminate at the first ends and second parts extend from the respective feed-point downwardly to terminate at the second end.

Each of the first to tenth electrical connections <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM> comprises a linear elongate conductor. In other embodiments, at least some of the elongate electrical connections may comprise a filter circuit.

The respective feed-points <NUM>, <NUM>, <NUM>, <NUM>, <NUM> are located in a common plane.

There is hence provided a closed structure comprising the vertically extending linear dipole elements <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM> and the horizontally extending elongate electrical connections <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM> between the ends of adjacent linear dipole elements. The resulting loop-like elements (such as the loop-like element formed by linear dipole element <NUM>, elongate electrical connection <NUM>, linear dipole element <NUM>, and elongate electrical connection <NUM>) in the structure is not a pure loop construction, in the sense that it will not work as a single loop element. The arrangement has, as a necessary feature, connections of a loop-like element to its neighbouring loop-like elements to form a closed structure <NUM> consisting of many (in this example embodiment five (<NUM>)) of these loop-like elements. This structure is clearly distinguishable from other loop structures such as a differentially fed loop, and other traditional loop designs which are always used as stand-alone elements.

The receiver arrangement <NUM> may comprise a processor (not shown) and the processor may execute a computer program comprising correlative direction-finding algorithms. All the feed points <NUM> to <NUM> of the array <NUM> are directly connected to respective coherent receivers of the arrangement <NUM> and allow for multiple combinations to be generated in software (after digitization). In contrast, differentially fed loops impose a fixed <NUM> degree phase shift which is then combined into a single feed point into a receiver.

The fact that each of the feed points <NUM> to <NUM> are connected to a respective coherent receiver is of particular importance, since the physical/galvanic connection between the linear dipole elements allows for multiple feed points along the array to act as pairs that create unique and information rich sources. For example, adjacent feed points <NUM> and <NUM> create a small loop that will have a significantly different radiation pattern when combined, compared to feed points <NUM> and <NUM>, which create a much larger loop-like structure compared to the combined feed points <NUM> and <NUM>. In both cases, the resulting radiation pattern, and consequently the nature of the information available to the DF algorithm, is fundamentally different from those of either a traditional loop or dipole array.

Although the electrical inter-connection of neighbouring dipole elements in DF dipole arrays is generally considered by those skilled in the art to be detrimental to the overall performance of the array, it has been found by the applicant, surprisingly so, that the low-frequency performance, more particularly the low-frequency DF sensitivity, the cross-polarisation isolation and susceptibility to coupling to conductive array mounting structures of the antenna array <NUM> is substantially better than that of the prior art dipole array in <FIG>. This unexpected improved performance is graphically illustrated by the simulated results in <FIG>, where the sensitivity of the prior art dipole array of <FIG> is shown in broken lines and the sensitivity of the array <NUM> is shown in solid lines. The improvement at frequencies smaller than <NUM> is self-evident. In the simulation an array <NUM> with dipole elements of <NUM> in length (which is λ/<NUM> at <NUM>) and an array diameter of <NUM> were used.

The simulated results in <FIG> exhibit an unexpected improvement in cross-polarisation isolation of roughly <NUM> dB of the antenna array <NUM> over that of the conventional dipole array for frequencies below <NUM>, where the isolation of the prior art dipole array of <FIG> is shown in broken lines and the isolation of the array <NUM> is shown in solid lines.

Claim 1:
A direction finding antenna array (<NUM>) comprising at least a first dipole antenna element (<NUM>), a second dipole antenna element (<NUM>) and a third dipole antenna element (<NUM>), each comprising respective first ends (<NUM>, <NUM>, <NUM>), respective second ends (<NUM>, <NUM>, <NUM>) and a respective feed-point (<NUM>, <NUM>, <NUM>), wherein, the at least first, second and third dipole elements are arranged in spaced relationship relative to one another in a non-linear pattern, characterized in that, in respect of each dipole element (<NUM>, <NUM>, <NUM>) in the array: a) the first end (<NUM>) is connected by first and second electrical connections (<NUM>, <NUM>) to the first end of each of only two adjacent dipole elements (<NUM>, <NUM>) respectively in the array and b) the second end (<NUM>) is connected by third and fourth electrical connections (<NUM>, <NUM>) to the second end of each of only the two adjacent dipole elements (<NUM>,<NUM>) respectively.