Log-periodic antenna and associated vehicle

Abstract

The present invention relates to a log-periodic antenna (10) comprising a boom (12) extending in a plane along an elongation axis (E), the boom (12) carrying successive radiating elements (14) arranged along the elongation axis (E), the radiating elements (14) extending along an elevation axis (Z), in which, on at least one portion of the boom (12), the quotient of the distance separating a radiating element (14) from an immediately succeeding radiating element (14) by the distance separating the radiating element (14) from the immediately preceding radiating element (14), is constant, characterised in that at least a plurality of successive radiating elements (14) are arranged alternately on either side of a plane defined by the elongation axis (E) and the elevation axis (Z) and at a non-zero distance from this plane.

Description

[0001] TITLE: Log-periodic antenna and associated vehicle

[0002] The present invention relates to a log-periodic type antenna comprising a mast extending in a plane along an elongation axis, the mast carrying successive radiating elements arranged along the elongation axis, the radiating elements extending along an elevation axis, in which, over at least a part of the mast, the quotient of the distance separating a radiating element from an immediately successive radiating element by the distance separating the radiating element from the immediately preceding radiating element is constant.

[0003] Among directional antennas, log-periodic antennas offer a wide bandwidth thanks to the large number of radiating elements they contain. A log-periodic antenna is a directional antenna comprising an array of radiating elements, in which the distance between successive radiating elements and / or the length of successive radiating elements follows a geometric ratio. Due to the mathematical relationship between their distances, widening the bandwidth of such an antenna requires, for matching purposes, increasing the number of radiating elements it contains. When a wide bandwidth is desired, the number of radiating elements in such an antenna and their spacing can become so large that the antenna's wingspan and length make it impractical to use.

[0004] In particular, when such an antenna is intended to be mounted on a vehicle, the dimensions of the broadband log-periodic antenna can be prohibitive.

[0005] It is known from the prior art to equip a log-periodic antenna with a folding system. However, this requires one or more users to operate the antenna directly to fold it, which can be cumbersome and dangerous, or the antenna incorporates an automatic deployment system. In the latter case, the antenna is more expensive and heavier.

[0006] The aim of the invention is therefore to propose a log-periodic antenna whose wingspan and length are less than those of prior art antennas, for similar performance in terms of matching and radiated power over the bandwidth in particular.

[0007] To this end, the invention relates to an antenna of the aforementioned type characterized in that at least a plurality of successive radiating elements are arranged alternately on either side of a plane defined by the elongation axis and the elevation axis and at a non-zero distance from this plane, and in that, on at least a portion of the mast carrying at least three successive radiating elements, the mast comprises a plurality of straight edges connecting the successive radiating elements, the sequence of angles formed by the successive straight edges on said portion of the mast forming a strictly decreasing sequence.

[0008] According to other advantageous aspects of the invention, the log-periodic antenna comprises one or more of the following features, taken individually or in any technically possible combination:

[0009] - the distances between two successive radiating elements and the elongation axis form a strictly increasing sequence for at least three successive radiating elements;

[0010] - each radiating element consists of a pair of monopoles;

[0011] - the value of the quotient of the distance separating a radiating element from an immediately successive radiating element by the distance separating the radiating element from the immediately preceding radiating element on at least a part of the mast is between 0.8 and 0.98;

[0012] - the successive radiating elements extend alternately in opposite directions of the elevation axis;

[0013] - the monopolies of each pair of monopolies extend in opposite directions of the elevation axis;

[0014] - the monopolies of each pair of monopolies are substantially identical;

[0015] - at least one radiating element has several folds, and is contained in a plane which contains the elevation axis, so as to have a fractal shape;

[0016] - the ratio of the lengths of at least two successive radiating elements is constant;

[0017] - The mast comprises a folded portion on which the radiating elements extend along the elongation axis between a first radiating element numbered 0 and a last radiating element numbered n+1. The distance between the radiating element (14) numbered i and the next radiating element numbered i+1, taken along the direction orthogonal to the plane generated by the elongation axis and the elevation axis, is denoted Xi and satisfies the following relation: X = ai + an, with ai = (i-1) * Ci + ao for i between 2 and n-1, ai = Co + ao for i=1, and a n = C2 + a n -i for i = n, Co, Ci, and C2 being positive and non-zero constants.

[0018] The invention also relates to a vehicle comprising a log-periodic antenna as defined above.

[0019] The invention will become clearer upon reading the following description, given solely by way of non-limiting example, and made with reference to the drawings in which: - [Fig. 1] Figure 1 is a perspective representation of a portion of a log-periodic antenna according to the invention,

[0020] - [Fig. 2] Figure 2 is a top view of the log-periodic antenna of Figure 1,

[0021] - [Fig. 3] Figure 3 is a representation similar to that of Figure 1, of an antenna according to another embodiment, and

[0022] - [Fig. 4] Figure 4 is a schematic cross-sectional view of radiating elements according to the invention.

[0023] The log-periodic antenna 10 includes a mast 12 on which a plurality of radiating elements 14 are mounted. The mast 12 is in particular intended to be supported by a support in order to raise the log-periodic antenna 10. The support is for example mounted on the roof of a vehicle such as a truck.

[0024] The log-periodic antenna 10 is particularly designed to operate in transmission and / or reception in the frequency band [20; 600] MHz, which corresponds to a wavelength between 0.5 and 15 meters.

[0025] The mast 12 extends along an elongation axis E, and the radiating elements 14 extend along a direction parallel to an elevation axis Z perpendicular to the elongation axis E. The plane of the antenna P is defined as the plane generated by the elevation axis Z and the elongation axis E.

[0026] Referring to Figure 1, the mast 12 comprises two electrically conductive lines 16 arranged side by side. A spacing is defined between the lines 16 to ensure a specific impedance to the electromagnetic wave propagating along them. The two lines 16 form the antenna's framework and support the radiating elements 14. An excitation source, not shown, is used to circulate an electromagnetic wave through the mast 12, so as to distribute the electromagnetic wave to the radiating elements 14 forming the antenna 10.

[0027] In the embodiment shown, the radiating elements 14 consist of a pair of monopoles 17A, 17B which are linear monopoles. A linear monopole is formed of a straight rod of constant cross-section.

[0028] Monopoles 17A, 17B of each pair are connected respectively to lines 16A, 16B.

[0029] For a particular radiating element 14, the monopole 17A extends along the elevation axis Z in a direction opposite to that of the monopole 17B substantially in the continuation of each other, being offset by the gap between the two lines 16A, 16B.

[0030] The successive monopoles 17A, 17B along the elongation direction of the same line 16 extend alternately in opposite directions of the elevation axis Z. Advantageously, the monopoles 17A, 17B constituting a radiating element 14 are identical.

[0031] A monopole 17A, 17B extends continuously between a connecting end 20 and a free end 22. The monopoles 17A, 17B are connected to the mast 12 at their connecting end 20.

[0032] The portion of the mast 12 not parallel to the elongation axis E is called the bent portion 24 and the part of the mast 12 parallel to the elongation axis E is called the straight portion 26.

[0033] As shown in Figure 1, each line 16 has a polygonal line shape made up of coplanar segments. Line 16 consists of a segment parallel to the elongation axis E on the straight part 26, and a plurality of angularly offset segments connecting the connecting ends 20 of the different monopoles 17A, 17B on the folded portion 24.

[0034] The segments constituting the polygonal line on the folded portion 24 intersect at a single point with the elongation axis E, giving line 16 a zigzag shape. In the same direction of angle measurement, for example in the trigonometric direction, the angles formed by successive segments of the plurality of segments that constitute line 16 are alternately salient and re-entrant angles.

[0035] The two lines 16A, 16B of mast 12 are offset from each other along the direction orthogonal to plane P of the antenna.

[0036] Lines 16A, 16B, for example, have a tubular shape and are assembled by welding together a plurality of metal tubes, which makes it possible to create the characteristic polygonal line shape of lines 16A, 16B from tubes.

[0037] With reference to Figure 2, the radiating elements 14 located on the folded portion 24 extend between a first radiating element 28 numbered 0 and a last radiating element 30 numbered n+1, the set of radiating elements 14 of the folded portion 24 being located between the first radiating element 28 and the last radiating element 30 along the elongation axis E.

[0038] The radiating elements 14 strictly included between the first radiating element 28 numbered 0 and the last radiating element 30 numbered n+1 have their connection point located on either side and at a non-zero distance from the plane of the antenna P.

[0039] For a radiating element i+1, with i between 0 and n-1, we denote d, i+1 the distance between the radiating element i+1 and the immediately preceding radiating element i. Similarly, we denote d i+1 i+2 the distance between the radiating element 14 numbered i+1 and the immediately following radiating element 14 numbered i+2. The quotient of d i i+1 by i+1 i+2 is equal to a coefficient T which is a strictly positive real number, for at least one value of i between 0 and n-1.

[0040] Alternatively, the quotient of d i i+1 by i+1 i+2 T is equal to any value of i between 0 and n-1.

[0041] In another variant, for any radiating element 14 belonging to the folded portion 24 or to the straight portion 26, the quotient of the distance between the radiating element 14 and the immediately successive radiating element by the distance between the radiating element 14 and the immediately preceding radiating element is equal to T.

[0042] Advantageously, the value of the coefficient T is between 0.8 and 0.98.

[0043] The curve length of a monopole 17A, 17B is defined as the distance obtained by traversing the monopole 17A, 17B from its connecting end 20 to its free end 22. The curve length of a radiating element 14 consisting of a pair of monopoles 17A, 17B is defined as equal to the sum of the curve lengths of the two monopoles 17A, 17B. For a radiating element 14 numbered i, its curve length is denoted Lj.

[0044] The quotient of the curve length L, of the radiating element 14 numbered i, by the curve length L i+1 of the next radiating element is equal to T.

[0045] Alternatively, for a radiating element 14 belonging to the folded portion 24 or to the straight portion 26 and having a successive radiating element 14, the quotient of the curve length of the radiating element 14 by the curve length of the immediately following radiating element 14 is equal to T.

[0046] In the embodiment shown, the connection end 20 of a radiating element 14 on the mast 12 is considered to be located in the middle of the segment defined between the connection ends 20A, 20B of the monopoles 17A, 17B of the radiating element 14.

[0047] In particular, the orthogonal projection of the connecting end 20A of a monopole 17A of a radiating element 14 onto the elongation axis E is equal to the orthogonal projection of the connecting end 20B of the other monopole 17B of the radiating element 14 onto the elongation axis E.

[0048] The distance to the axis, denoted a, is called h the distance between the connecting end 20 of the radiating element 14 numbered i+1 and its orthogonal projection on the elongation axis E, for ie [2; n-1],

[0049] The sequence of distances to the axis a, is a strictly increasing sequence, the mast 12 of the log-periodic antenna 10 presents, on its folded portion 24, a span increasing along the elongation axis E, from the first radiating element 28 to the last radiating element 30. The span of the antenna is defined as the height of the antenna taken along the elevation axis Z.

[0050] In particular, in the embodiment shown, the distance a, satisfies, for i in the interval [2; n-1], the following relation: a, = (i-1)*C1+a0

[0051] And, for i = 1 and i = n: ai = ao+Co a n = has n -i +C2 with C o , O-, , and C2 are positive and non-zero dimensioning constants, whose physical quantity is homogeneous to a length.

[0052] Preferably the size C o is less than the quantity C-, and the quantity C-, is less than the quantity C2, for example C o , C-\ , and C2 have respective values ​​of 20 cm, 30 cm, and 40 cm.

[0053] The value of n, which represents the number of segments of each of the lines 16A, 16B not parallel to the elongation axis E, is notably between 5 and 40, for example equal to 9.

[0054] The distance of i i+1The distance between a radiating element 14, numbered i, and the next element, numbered i+1, along the elongation axis E is denoted Y, and the same distance taken along the axis orthogonal to the plane of the antenna P is denoted X. h for i ranging from 1 to n-1.

[0055] The distances X and Y, for i ranging from 1 to n-1, are equal to:

[0056] Xi = a.+a

[0057] Because the sequence of distances to the axis a, is a strictly increasing sequence, the sequence of distances X, is also a strictly increasing sequence.

[0058] On at least a portion of the mast 12 bearing at least three successive radiating elements, the mast comprises a plurality of straight edges connecting the successive radiating elements 14.

[0059] For example, the portion of the mast corresponds to a part of the folded portion 24, in particular to the folded portion 24, with the exception of the last radiating element 30 and the edge connecting the immediately preceding radiating element 14 to the last radiating element 30.

[0060] In the example shown in Figure 2, each edge is formed from the segments of lines 16A, 16B connecting the successive radiating elements 14.

[0061] The sequence of angles formed by successive straight edges is strictly decreasing. By "angles formed by successive straight edges" we mean the salient angles formed between successive straight edges.

[0062] The decreasing sequence of angles formed by successive edges allows for the preservation of electrical lengths and spatial spacing between successive radiating elements 14, which reduces the propagation delay between these elements and limits the effects of layering. The antenna 10 thus exhibits more homogeneous radiation over its operating band, particularly in the [20; 600] MHz band.

[0063] The first radiating element 28 numbered 0 of the log-periodic antenna 10 is arranged on the elongation axis E, and is located at the junction between the straight portion 26 and the folded portion 24.

[0064] The distance of O between the first radiating element 28 and the immediately following radiating element 14 is chosen, when dimensioning the log-periodic antenna 10, taking into account a wavelength λ of use of the log-periodic antenna 10, preferably such that:

[0065] 0.03*At < d O < o t *With o t a sizing parameter being defined by the following formula: o t = 0.243*T-0.051

[0066] The distance of 0 1 along the axis orthogonal to the plane in the antenna P is denoted a0, and this distance is denoted Y o Therefore, along the elongation axis E, the following relationship is verified:

[0067] The last radiating element 30 numbered n+1 of the log-periodic antenna 10 is arranged on the elongation axis E, and is located at a junction between the straight portion 26 and the folded portion 24.

[0068] We note d n n+1 the distance between the last radiating element 30 and the immediately preceding radiating element 14. We denote a n the projection of d n n+1 on the axis orthogonal to the plane of the antenna P and Y n the projection of d n n+1 on the elongation axis E, which satisfy the relation:

[0069] The length Y, taken along the elongation axis between two consecutive radiating elements 14 belonging to the folded portion 24, is less than the distance d, i+1 between two consecutive radiating elements 14. The length of the folded portion 24 of the log-periodic antenna 10 is equal to the sum of the lengths Y, for i ranging from 0 to n. A log-periodic antenna 10 according to the invention therefore has a mast length 12, taken along the elongation direction E, less than the mast length 12 of a log-periodic antenna of the prior art.

[0070] The log-periodic antenna 10 illustrated in Figure 3 has a first zone 32, similar to the portion of the antenna shown in Figure 1, in which the radiating elements 14 consist of a pair of linear monopoles 17A, 17B, and a second zone 34 in which the radiating elements 14 consist of a pair of folded monopoles 36A, 36B.

[0071] Each folded monopole 36A, 36B is contained in a plane, in which the Z elevation axis is included, and has several folds, which give it a fractal shape.

[0072] For the same curve length, the length taken along the elevation axis Z of a folded monopole 36A, 36B is less than that of a linear monopole 17A, 17B. The folding implemented on the folded monopoles 36A, 36B reduces the span of the log-periodic antenna 10 along the elevation axis Z.

[0073] In the embodiment shown in Figure 3, a folded monopole 36A, 36B of the first radiating element 38 of the second zone 34 has fewer folds than a folded monopole 36A, 36B of the immediately following radiating element 40 of the second zone 34.

[0074] In this embodiment, the sequence of curve lengths L of the radiating elements 14 is a strictly increasing sequence. In particular, when the sequence of curve lengths of the radiating elements 14 is strictly increasing, there exists an index i from which it is particularly advantageous to reduce the length, taken along the elevation direction Z, of the radiating elements 14.

[0075] Figure 4 illustrates radiating elements 14 according to different embodiments, shown in cross-section along a plane including the elevation axis E.

[0076] The radiating elements 14 have two folded monopoles 36A, 36B which extend on either side of the mast 12, shown schematically in cross-section, in which the electric current from the excitation source flows.

[0077] A first pair of radiating elements 42 is shown, in which the folded monopoles 36A, 36B include portions parallel to or perpendicular to the elevation axis E.

[0078] A second pair of radiating elements 44 represents folded monopoles 36A, 36B comprising a plurality of portions not parallel to the elevation axis E and not perpendicular to the elevation axis E.

[0079] The folded monopoles 36A, 36B of the same radiating element 14 are advantageously arranged symmetrically with respect to the mast 12. The log-periodic antenna 10 according to the invention is particularly suitable for integration on a vehicle, in particular the roof of a land vehicle, due to its reduced dimensions in wingspan and length compared to a log-periodic antenna according to the prior art.

Claims

DEMANDS 1. A log-periodic antenna (10) comprising a mast (12) extending in a plane along an elongation axis (E), the mast (12) carrying successive radiating elements (14) arranged along the elongation axis (E), the radiating elements (14) extending along an elevation axis (Z), wherein, over at least a portion of the mast (12), the ratio of the distance separating a radiating element (14) from an immediately successive radiating element (14) to the distance separating the radiating element (14) from the immediately preceding radiating element (14) is constant, characterized in that at least a plurality of successive radiating elements (14) are arranged alternately on either side of a plane defined by the elongation axis (E) and the elevation axis (Z) and at a non-zero distance of this plan, and in that, on at least a portion of the mast (12) bearing at least three successive radiating elements (14),The mast comprises a plurality of straight edges connecting successive radiating elements (14), the sequence of angles formed by the successive straight edges on said portion of the mast (12) forming a strictly decreasing sequence.

2. Log-periodic antenna (10) according to claim 1, wherein the distances between two successive radiating elements (14) and the elongation axis (E) form a strictly increasing sequence for at least three successive radiating elements (14).

3. Log-periodic antenna (10) according to any one of the preceding claims, wherein each radiating element (14) consists of a pair of monopoles (17A, 17B; 36A, 36B).

4. Log-periodic antenna (10) according to any one of the preceding claims, wherein the value of the quotient of the distance separating a radiating element (14) from an immediately successive radiating element (14) by the distance separating the radiating element (14) from the immediately preceding radiating element (14) on at least a part of the mast (12) is between 0.8 and 0.

98.

5. Log-periodic antenna (10) according to any one of the preceding claims, wherein the successive radiating elements (14) extend alternately in opposite directions of the elevation axis (Z).

6. Log-periodic antenna (10) according to claim 3, wherein the monopoles (17A, 17B; 36A, 36B) of each pair of monopoles (17A, 17B; 36A, 36B) extend in opposite directions of the elevation axis (Z).

7. Log-periodic antenna (10) according to claim 3 to 6, wherein the monopoles (17A, 17B; 36A, 36B) of each pair of monopoles (17A, 17B; 36A, 36B) are substantially identical.

8. Log-periodic antenna (10) according to any one of the preceding claims, wherein at least one radiating element (14) has several folds, and is contained in a plane which contains the elevation axis (Z), so as to have a fractal shape.

9. Log-periodic antenna (10) according to any one of the preceding claims, wherein the ratio of the lengths of at least two successive radiating elements (14) is constant.

10. Log-periodic antenna (10) according to any one of the preceding claims, wherein the mast comprises a folded portion on which the radiating elements (14) extend along the elongation axis (E) between a first radiating element numbered 0 and a last radiating element numbered n+1, the distance between the radiating element (14) numbered i and the next radiating element (14) numbered i+1, taken along the direction orthogonal to the plane generated by the elongation axis (E) and the elevation axis (Z), is denoted Xi and satisfies the following relation: X = ai + an, with ai = (i-1) * Ci + ao for i between 2 and n-1, ai = Co + ao for i=1, and a n = C2 + a n -i for i = n, Co, Ci, and C2 being positive and non-zero constants.

11. Vehicle comprising a log-periodic antenna (10) according to any one of the preceding claims.

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