EP0241921B1 - High-efficiency antenna - Google Patents

Description

The present invention relates to a high efficiency antenna.

In terms of high-efficiency radiating elements, the source commonly known as "Short Backfire" is known. It consists of a dipole or two dipoles (crossed if there are two) and a cylindrical reflector. The performances of this type of antenna were studied by HW EHRENSPECK and are described in the article "A New Class of Medium - Size, High-efficiency Reflector Antennas" (IEEE Transactions on Antennas and Propagation "of March 1974). document describes a type of reflector antenna allowing a greater directional gain than those obtained with conventional reflector antennas of the same area. An antenna of this type comprises a cylindrical reflector and a feed system located in the center of this Such an antenna is analyzed as the combination of two radiating sources whose maximum radiation and mutual phase relationships can be adjusted simply to obtain greater directional gain in a direction perpendicular to the bottom surface of the reflector. of directional gain is explained on the basis of a virtual extension of the radiation opening beyond the physical dimensions of the r Reflector.

avec A : aire de la surface considérée
et λ : longueur d'onde.The author compares the effectiveness of this type of source, to the maximum theoretical efficiency of a radiating opening of the same dimension. This efficiency is defined with respect to Dmax (maximum directivity of a radiating opening)

Dmax = $\frac{\text{4A}}{\text{λ2}}$

with A: area of the surface considered
and λ: wavelength.

For certain characteristics of the reflector (Diameter d = 2.45 λ, height h = 0.57 λ), the directivity measured is greater than the maximum directivity of the equivalent radiating aperture. This phenomenon, according to the aforementioned works, is reproduced for values of diameter: d <0.75 λ.

If we consider the minimum value of the reflector diameter, dmin, for a "Short backfire" type antenna dmin must be compatible with the size of the dipole: dmin> 0.75λ.

There is therefore an incompatibility between the two relationships relating to the diameter of the reflector.

The subject of the invention is a structure which makes it possible to take advantage of the advantageous phenomenon which appears for d <0.75λ, while having a reflector diameter d ≲ 0.7λ.

To this end, the invention provides a high-efficiency antenna comprising a reflector consisting of a bottom and walls of direction perpendicular to this bottom and of cylindrical shape, inside which is disposed a device radiating in the direction perpendicular to it. bottom, the radiating device comprising at least one wire wound helically on either side of the axis of symmetry of the reflector which is the axis of radiation of said device, each wire being short-circuited at the reflector by its end located on the side of the bottom of this reflector and being fed by its other end, characterized in that at least one coaxial cable situated inside the helix formed by the radiating wire (s) makes it possible to supply this (or ces) radiating wire (s), and in that the diameter (d) of the reflector (11) is less than 0.75 λ; λ being the wavelength for which the antenna is built.

• Le diamètre du réflecteur n'est pas limité par l'encombrement du dipole,
• La directivité de l'élément rayonnant est modulable, contrairement à une antenne "Short Backfire" dont la directivité du dipole est constante.

The advantages of the present invention compared to “Short Backfire” type antennas are as follows:

• The diameter of the reflector is not limited by the size of the dipole,
• The directivity of the radiating element is modular, unlike a "Short Backfire" antenna whose directivity of the dipole is constant.

More precisely, the subject of the invention is an antenna in which four coaxial cables are arranged inside a qudrifilar helix, the central cores of the first two coaxial cables, which are power cables, being connected by first circuits metallic to the central cores of the other two coaxial cables, a metal plate arranged at a certain distance from, this end making it possible to connect the central cores of these last two coaxial cables and the outer conductors of the four coaxial cables of such that the impedance present at the output of the two supply cables is 50 Ω, the external parts relative to the reflector of these four coaxial cables being connected to the four wires by the intermediary of four second metal circuits.

Advantageously, the invention relates to an antenna in which the first metal circuits are produced by metal layers arranged on either side of a first printed circuit, the second metal circuits being produced by metal layers arranged on a first face. a second print superimposed by its second face with one of the faces of the first printed circuit.

• la figure 1 illustre une vue en perspective schématique d'une antenne selon l'invention, le réflecteur étant représenté en coupe ;
• la figure 2 illustre une vue partielle en perspective de la source rayonnante de l'invention ;
• la figure 3 illustre une vue en coupe selon le plan III-III de la source rayonnante représentée à la figure 2.

The characteristics and advantages of the invention will moreover emerge from the description which follows, by way of nonlimiting example, with reference to the appended drawings in which:

• Figure 1 illustrates a schematic perspective view of an antenna according to the invention, the reflector being shown in section;
• FIG. 2 illustrates a partial perspective view of the radiating source of the invention;
• FIG. 3 illustrates a sectional view along plane III-III of the radiating source shown in FIG. 2.

The present invention relates to a radiating element, consisting of a quadrifilar helix 10, placed in a reflector 11 having a bottom 8 and side walls 9 of direction perpendicular to this bottom and of cylindrical shape. The dimensions of the reflector 11 are optimized to obtain the superposition in phase, in the direction of the radiation axis Δ, of the components radiated by the propeller 10 on the one hand, and by the upper edge 12 of the reflector 11 on the other go.

The antenna according to the invention comprises four coaxial cables 13, 14, 15, 16 which are in directions parallel to the axis of the radiation Δ.

The central cores 20 and 21 of the first two coaxial cables 13, 14, which are power cables, are connected by circuits 24 and 25 to the central cores 22 and 23 of the coaxial cables 15 and 16. These central cores 22 and 23 are short-circuited to the outer conductors of the coaxial cables 13, 14, 15, 16 by a metal plate 26 at a distance e such that the impedance presented at the output of the coaxial cables 13 and 14 is 50 Ω.

The quadrifilar propeller 10 consists of four radiating strands 27, 28, 29, 30 which are fed from the top; their base being short-circuited to the reflector 11, which allows it to be assimilated, at first analysis, to a curved dipole.

These radiating strands 27, 28, 29 and 30 are connected to the external conductors of the coaxials 13, 14, 15, 16 by the circuits 31, 32, 33, 34. These radiating strands are short-circuited at the reflector 11 via 'a plate 17. Depending on the value of the diameter d of the reflector, the height h of the reflector is optimized.

In FIGS. 1, 2 and 3 the circuits 24, 25 which make it possible to connect the central cores 20 and 21 of the cables 13 and 14 to the cores 22 and 23 of the cables 15 and 16, are represented by metallic deposits made on either side another of a first printed circuit 35; the circuits 31, 32, 33 and 34 which allow the external conductors of the coaxial cables 13, 14, 15 and 16 to be connected to the radiating strands 27, 28, 29, and 30 are metallic deposits made on a first face of a second printed circuit 36, the second face of which is superimposed on one of the faces of the first printed circuit 35.

Such an antenna is mainly used at frequencies below 4 GHz. Unlike the antennas of the known art, comprising a dipole located inside a cylindrical reflector whose directivity is given, in the antenna according to the invention it is possible to act, at a given frequency, on the diameter / height of the reflector.

In the case where the above-mentioned antenna is used as the radiating element of a network antenna, a value range of the diameter d is particularly advantageous. It is defined by the relation giving the inter-element distance L = 0.67 λ <L <0.54 λ.

For these values, the lobes of networks do not appear. This results in optimal antenna efficiency. However, the reduction in this distance L generally contributes to significantly increasing the coupling between elements. In the case of the present invention, the criterion of coherence of the radiation coming from the propeller 10 on the one hand, and from the upper edge 12 of the reflector 11 on the other hand, correlates the reduction in diameter d, with an increase in the height h of the reflector 11, and therefore improves the direct insulation between elements. With such a network antenna, the optimization criterion is to obtain maximum directivity in the axis of the radiation. But this criterion could be to obtain a maximum ellipticity rate on a blanket given, for example with a multidirectional antenna.

It is understood that the present invention has only been described and shown as a preferred example and that its constituent elements can be replaced by equivalent elements without, however, departing from the scope of the invention.

Thus the radiating element 10 can also be a helix composed of one or more wires: monofilar, bifilar, etc. This helix can also be of conical shape.

Claims (6)

1. A high-efficiency antenna comprising a reflector (11) constituted by a bottom (8) and walls (9) extending perpendicularly to the bottom and cylindrical in shape, a radiating device (10) for radiating perpendicular to said bottom being placed inside said reflector, the radiating device (10) including at least one wire (27) wound helically about the axis of symmetry of the reflector which is also the radiating axis (△) of said device, each wire being short circuited to the reflector at its end situated adjacent to the bottom of the reflector (11) and being fed from its opposite end, the antenna being characterized in that at least one coaxial cable (13) situated inside the helix formed by the radiating wire(s) serves to feed the radiating wire(s), and in that the diameter d of the reflector (11) is less than 0.75 λ, where λ is the wavelength for which the antenna is constructed.
2. An antenna according to claim 1, characterized in that each coaxial cable is parallel to the radiating axis (△).
3. An antenna according to any preceding claim, characterized in that the radiating device (10) comprises four wires (27, 28, 29, 30) wound to form a four-wire helix.
4. An antenna according to any preceding claim, characterized in that four coaxial cables (13, 14, 15, 16) are disposed inside a four-wire helix (10), with the central cores (20, 21) of two first coaxial cables (13, 14) constituting feed cables and being connected by first metallic circuits (24, 25) to the central cores (22, 23) of the other two coaxial cables (15, 16), a metal plate (26) disposed at a certain distance (e) from said end serving to interconnect the central cores (22, 23) of the latter two coaxial cables (15, 16) and the outer conductors of all four coaxial cables (13, 14, 15, 16) in such a manner that the output impedance presented by the two feed cables (13, 14) is 50 Ω, and in that the outer portions of said four coaxial cables (13, 14, 15, 16) relative to the reflector are connected to the four wires (27, 28, 29, 30) via four second metallic circuits (31, 32, 33, 34).
5. An antenna according to claim 4, characterized in that the first metallic circuits (24, 25) are constituted by metal layers disposed on opposite faces of a first printed circuit (35), and in that the second metallic circuits (31, 32, 33, 34) are constituted by metal layers deposited on a first face of a second printed circuit (36) which is superposed via its second face on one of the faces of the first printed circuit (35).
6. An antenna according to any preceding claim, characterized by the fact that the helical wires (27, 28, 29, 30) constitute a conically-shaped helix.

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