EP2828931B1 - Compact helical antenna with a sinusoidal profile modulating a fractal pattern - Google Patents

Description

GENERAL TECHNICAL FIELD

The invention relates to helical type antennas. In particular, it relates to printed quadrifilar helix type antennas. Such antennas find application in particular in the band telemetry L (operating frequency between 1 and 2 GHz, typically around 1.5 GHz) for payloads of stratospheric balloon.

STATE OF THE ART

The printed helix antennas have the advantage of being simple and inexpensive to manufacture.

They are particularly suitable for L-band circular polarization telemetry signals used in stratospheric balloon payloads.

They also offer a good ellipticity rate and therefore a good circular polarization over a wide range of elevation angles.

The patent EP 0320404 describes a printed antenna of the helix type and its method of manufacture.

Such an antenna includes four radiating strands in the form of metal strips obtained by removing material from the metallization of both sides of the strips of a metallized zone of a printed circuit. The printed circuit is intended to be wound helically around a cylinder.

These antennas although offering good performance are however cumbersome.

Compact helical antennas comprising meandering radiating strands have been proposed to reduce the size of antennas of this type.

Article: Y. Letestu, A. Sharaiha, Ph. Besnier "A size reduced configuration of printed quadrifilar helix antenna," IEEE workshop on Antenna Technology: Small Antennas and Novel Metamaterials, 2005, pp. 326-328, March 2005 describes such compact antennas.

However, although a gain in the order of 35% (height reduction) on the footprint has been obtained, the performances, in particular in cross polarization and backward radiation, are degraded showing the limits of the use of such reasons as to reduce the size of antennas of this type.

The document FR 2 916 581 discloses a helix-type antenna comprising radiating strands consisting of a repetition of a fractal pattern.

However, the use of these patterns does not significantly reduce the size of the antenna.

Furthermore fractal patterns composed of straight segments have a much smaller number of degrees of freedom on which the designer can play to adjust and optimize the performance of the compact antenna. In addition, at given antenna height, far fewer solutions with these patterns exist.

PRESENTATION OF THE INVENTION

The invention makes it possible to reduce the size of the known type of helix antennas and in particular to reduce the height of such antennas. The invention is defined by independent claim 1.

To this end, according to a first aspect, the invention relates to a helix-type antenna comprising a form of revolution and a plurality of radiating strands helically wound around the form of revolution, characterized in that each radiating strand is defined by a repetition of a fractal pattern comprising segments constituted by a sinusoidal curve.

• chaque segment correspond à une demi-période d'une courbe sinusoïdale définie par $y\left(x\right)=S.k.L\prime .\sin \left(\frac{\pi }{L\prime }.x\right)$
  
  où : S est un entier à valeur dans {-1; +1}, k est le rapport entre l'amplitude de la sinusoïde et sa demi-longueur d'onde ;
• chaque segment du motif fractal à une longueur identique ;
• le fractal est du type Von Koch dont chaque ligne droite est remplacée par un segment sinusoïdal ;
• les brins rayonnants sont chacun constitués par une zone métallisée déterminée, enroulée en hélice sur la surface latérale d'un manchon, tel que l'axe directeur de chaque brin est distant de l'axe du brin suivant d'une distance déterminée, définie selon toute perpendiculaire à toute ligne directrice du manchon comme la distance entre deux points, chacun défini par une intersection entre l'axe d'un brin et une perpendiculaire à toute ligne directrice du manchon ;
• la forme de révolution est cylindrique ou conique ;
• l'antenne comprend quatre brins rayonnants identiques ;
• la longueur d'un brin déroulé est de l'ordre de $k.\frac{\lambda }{4}$
  
  où λ est la longueur d'onde de fonctionnement de l'antenne.

The invention is advantageously completed by the following features, taken alone or in any of their technically possible combination:

• each segment corresponds to half a period of a sinusoidal curve defined by $\mathrm{there}\left(x\right)=S.k.\mathrm{The}\text{'}.\sin \left(\frac{\pi }{\mathrm{The}\text{'}}.x\right)$
  
  where: S is an integer with a value in {-1; +1}, k is the ratio between the amplitude of the sinusoid and its half-wavelength;
• each segment of the fractal pattern to an identical length;
• the fractal is of the Von Koch type, each straight line of which is replaced by a sinusoidal segment;
• the radiating strands are each constituted by a determined metallized zone, helically wound on the lateral surface of a sleeve, such that the direction axis of each strand is distant from the axis of the next strand by a determined distance, defined according to any perpendicular to any guide line of the sleeve as the distance between two points, each defined by an intersection between the axis of a strand and a perpendicular to any guide line of the sleeve;
• the form of revolution is cylindrical or conical;
• the antenna comprises four identical radiating strands;
• the length of a strand unwound is of the order of $k.\frac{\lambda }{4}$
  
  where λ is the operating wavelength of the antenna.

PRESENTATION OF FIGURES

• la figure 1 illustre de manière schématique en développé une antenne hélice de type connu comprenant des brins rayonnants rectilignes ;
• la figure 2 illustre de manière schématique une vue de face d'une antenne hélice de type connu comprenant des brins rayonnants rectilignes ;
• les figures 3a, 3b et 3c illustrent un motif de référence de type Von Koch avec des segments rectilignes et avec des segments constitués par une courbe sinusoïdale ;
• les figures 4a, 4b et 4c illustrent respectivement un premier motif de référence, fractal d'ordre 1, un fractal d'ordre 2 et un fractal d'ordre 3 ;
• les figures 5a, 5b et 5c illustrent respectivement, un second motif de référence, fractal d'ordre 1, un fractal d'ordre 2 et un fractal d'ordre 3 ;
• les figures 6a, 6b et 6c illustrent respectivement, un troisième motif de référence, fractal d'ordre 1, un fractal d'ordre 2 et un fractal d'ordre 3 ;
• les figures 7a et 7b illustrent respectivement, un quatrième motif de référence, fractal d'ordre 1 et un fractal d'ordre 2 ;
• les figures 8a et 8b illustrent respectivement, un motif de référence, fractal d'ordre 1 et un fractal d'ordre 2 pour des motifs des brins rayonnants, selon un cinquième mode de réalisation ;
• les figures 9a, 9b, 9c illustrent un motif de référence de type Von Koch avec des segments constitués par une courbe sinusoïdale selon plusieurs réalisations ;
• la figure 10 illustre une réalisation d'une antenne de type hélice selon l'invention.

Other features and advantages of the invention will become apparent from the description which follows, which is purely illustrative and nonlimiting, and should be read with reference to the accompanying drawings in which:

• the figure 1 schematically illustrates in developed a helical antenna of known type comprising straight radiating strands;
• the figure 2 schematically illustrates a front view of a known type of helix antenna comprising straight radiating strands;
• the Figures 3a, 3b and 3c illustrate a Von Koch type reference pattern with rectilinear segments and segments consisting of a sinusoidal curve;
• the Figures 4a, 4b and 4c respectively illustrate a first reference pattern, first order fractal, a second order fractal and a third order fractal;
• the Figures 5a, 5b and 5c respectively illustrate, a second reference pattern, first order fractal, a second order fractal and a third order fractal;
• the Figures 6a, 6b and 6c respectively illustrate, a third reference pattern, first order fractal, a second order fractal and a third order fractal;
• the Figures 7a and 7b respectively illustrate a fourth reference pattern, first order fractal and a second order fractal;
• the Figures 8a and 8b respectively illustrate, a reference pattern, first order fractal and a second order fractal for patterns of the radiating strands, according to a fifth embodiment;
• the Figures 9a, 9b, 9c illustrate a Von Koch type reference pattern with segments constituted by a sinusoidal curve according to several embodiments;
• the figure 10 illustrates an embodiment of a helical type antenna according to the invention.

DETAILED DESCRIPTION OF THE INVENTION General structure of the antenna

The Figures 1 and 2 respectively illustrate a developed view and a front view of a helical antenna comprising four radiating strands helically wound.

Such an antenna comprises two parts 1, 2.

Part 1 comprises a conductive zone 10 and four radiating strands 11, 12, 13 and 14.

In part 1, the helical type antenna comprises four radiating strands 11, 12, 13, 14 helically wound in a shape of revolution around a sleeve 15, for example.

On this part, the strands 11-14 are connected on the one hand in short circuit at a first end 111, 121, 131, 141 strands to the conductive zone 10 and secondly in a second end 112, 122, 132, 142 of the strands to the feed circuit 20.

The radiating strands 11-14 of the antenna may be identical and are for example four in number. The antenna is in this case quadrifilar.

The sleeve 15 on which the antenna is wound is shown in dotted line on the figure 1 to constitute the antenna as represented on the figure 2 .

The radiating strands 11-14 are oriented so that a support axis AA ', BB', CC 'and DD' of each strand forms an angle α with respect to any plane orthogonal to any directing line L of the sleeve 15.

This angle α corresponds to the helical winding angle of the radiating strands.

The radiating strands 11-14 are each constituted by a metallized zone.

On the Figures 1 and 2 the metallized zones of part 1 are symmetrical bands with respect to a guide axis AA ', BB', CC ', DD' strands.

The distance d between two successive strands is defined along any perpendicular to any line L of the sleeve 15 as the distance between two points, each defined as the intersection of the said perpendicular with an axis of the strands.

For example, to obtain a symmetrical quadrifilar antenna, this distance d will be fixed at a quarter of the perimeter of the sleeve 15.

The substrate supporting the metal strips is helically wound on the lateral surface of the sleeve 15.

According to one embodiment of such an antenna, the two parts 1, 2 are formed on a printed circuit 100.

The radiating strands 11-14 are then metal strips obtained by removal of material on each side of the strips of a metallized zone, on the surface of the printed circuit 100.

The printed circuit 100 is intended to be wound around a sleeve 15 having a general shape of revolution, such as a cylinder or a cone, for example.

Part 2 of the antenna comprises a supply circuit 20 of the antenna.

The supply circuit 20 of the antenna is constituted by a transmission line of the meander-shaped ribbon line type, ensuring both the function of distribution of the supply and adaptation of the radiating strands 11-14 of the antenna.

The supply of the radiating elements is at equal amplitudes with a progression of phases in quadrature.

The reduction of the size of the helix type antennas as represented on the Figures 1 and 2 is obtained by using for the radiating strands of part 1 of the antenna particular patterns that are going to be described below. Part 2 of the antenna is of known type and will not be more detailed.

Patterns of radiating strands

The radiating strands are constituted by a fractal, comprising segments constituted by a sinusoidal curve.

A segment is an elemental element of the fractal pattern.

The figure 3a illustrates a reference pattern of a Von Koch fractal having three elementary elements 30, 31, 33. Such a pattern is a first order fractal. figure 3a the elementary element is a rectilinear segment.

Fractals have the property of self-similarity, they are formed of copies of themselves at different scales. They are self-similar and very irregular curves.

A fractal is composed in particular of reduced replicas, of the reference pattern.

A fractal is generated by iteration of steps of reduction of the reference pattern then application of the pattern obtained to the reference pattern.

The higher orders are obtained by applying to the middle of each segment of the reference pattern this same reduced reference pattern, and so on.

The reference pattern may be simple or alternating with respect to a direction axis of the pattern.

The choice of the pattern itself is guided by the radiation performance of the antenna.

For the generation of the Von Koch fractal one can refer to http://www.mathcurve.com/fractals/koch/koch.shtml.

To reduce the height of the antenna while keeping the same operating frequency (resonance) each rectilinear segment of the fractal pattern is replaced by a sinusoidal segment.

Such a replacement makes it possible to increase the deployed length of the radiating strand for a given height or to reduce the height of the antenna for a given deployed length.

The resonance frequency of the antenna is fixed by the extended length of the radiating strands. This extended length is a function of the propeller parameters (height, radius and number of revolutions) and the geometry of the pattern used.

The figure 3b illustrates a reference pattern used for the strands of the helix antenna, each segment 30 ', 31', 32 ', 33' of the fractal pattern is constituted by a sinusoidal segment.

In the case of figure 3a there is a first-order von Koch fractal pattern composed of four straight segments of identical length L '/ 3, L being the' horizontal 'length of the pattern. In the case of figure 3b each segment of length L '/ 3 of the Von Koch pattern (that of the figure 3a ) is replaced by a sinusoidal segment (half a sinusoidal period).

• la taille de chaque répétition du motif de référence (ordre 1 du motif fractal) ;
• le nombre de répétition que l'on appelle nombre de cellules ;
• l'itération du fractal que l'on appelle ordre du fractal.
• En outre, un brin de l'antenne est défini par les paramètres suivants :
• longueur déployée ;
• l'angle α correspondant à l'angle d'enroulement en hélice du brin rayonnant ;
• longueur de la cellule L ;

All segments of the pattern have the same length.
A fractal pattern is defined by three parameters:

• the size of each repetition of the reference pattern (order 1 of the fractal pattern);
• the number of repetitions known as the number of cells;
• the iteration of the fractal called fractal order.
• In addition, one strand of the antenna is defined by the following parameters:
• deployed length;
• the angle α corresponding to the helical winding angle of the radiating strand;
• length of the cell L;

où : S est un entier à valeur dans {-1; +1}, constant sur un segment, k est le rapport entre l'amplitude de la sinusoïde et sa demi-longueur d'onde (demi-période) Ainsi, comme on le comprend, la sinusoïde modulant le motif fractal est définie sur une période.The sinusoid that modulates the fractal profile can be defined in particular by the following functional $\mathrm{there}=S.k.\mathrm{The}\text{'}.\sin \left(\frac{\pi }{\mathrm{The}\text{'}}.x\right)$

where: S is a value integer in {-1; +1}, constant on a segment, k is the ratio between the amplitude of the sinusoid and its half-wavelength (half-period) Thus, as we understand it, the sinusoid modulating the fractal pattern is defined on a period.

On the figure 3b the pattern is such that S = +1 while on the figure 3c the pattern is such that S = -1.

Thus this reference pattern is constituted by a succession of alternating sinusoid arcs constituting a fractal pattern.

The function can be defined segment by segment or by adopting a curvilinear coordinate along the pattern.

In the box of the figure 3b , the above defined functional was applied in segments of two segments (segments 30, 31 on the one hand and segments 32, 33 on the other hand).

In the case of figure 3a the central segments make an angle of 60 °. To get the reason for the figure 3b the functional is first applied to two rectilinear segments and is oriented by 60 °. Figures 9a, 9b and 9c a pattern for different values of k for S = + 1.

The parameter k makes it possible to increase the length deployed for each corresponding segment of the fractal Von Koch: instead of having a short rectilinear segment, there is a sinusoidal segment of greater length. The larger the amplitude of the sinusoid, the larger the length deployed. However, care must be taken to avoid overlapping radiating strands when k takes too high values.

Other types of fractal patterns in which each segment is replaced by a sinusoidal curve can also be envisaged.

The FIGS. 4a, 5a, 6a, 7a and 8a illustrate a reference pattern (fractal order 1) whose segments are straight.

On the figure 4a the reference pattern is a triangle in which the base is deleted.

On the figure 5a the reference pattern is a square in which the base is deleted.

On the figure 6a the reference pattern comprises two isosceles trapezes in opposition and spaced from the width of the small base, in which the large base has been removed. The angle θ between a side extending from the small base to the large base.

On the figure 7a the reference pattern comprises two equilateral triangles in opposition and spaced from the width of one side, in which the base has been removed

The Figures 4b, 5b and 6b, 7b and 8b respectively illustrate the order 2 of a fractal pattern following an iteration of the reference patterns of the FIGS. 4a, 5a, 6a, 7a, 8a , respectively.

The Figures 4c, 5c, 6c respectively illustrate the order 3 of a fractal pattern following two iterations of the reference patterns of the Figures 4a, 5a, 6a .

In the case of certain reasons, in particular those of the type represented in FIGS. 4a, 6a and 7a crosses between lines of the same cell are possible.

To avoid such crossings we can play on the angle β (see FIGS. 4a, 6a and 7a ).

The angle β is the angle between the first inclined segment and the deleted base.

The adjustment of this angle β reduces the length of the strands.

In the case of a Von Koch type pattern, there is a ratio between the deployed length and the length of the pattern at the order 1 of 4/3. At order 3 this ratio is (4/3) 3 which is low.

n étant l'ordre de la courbe fractale. De cette façon, on peut déployer une longueur de brin dans une même longueur. On appelle ce motif de référence un motif « Von Koch modifié ».
Comme précédemment chaque segment constituant les motifs fractals ci-dessus décrits est constitué par une courbe sinusoïdale. Pour des raisons de lisibilité, ces motifs ne sont pas représentés mais au vue de la description ci-dessus, l'homme du métier comprend comment aboutir à l'antenne hélice dont les brins rayonnants sont constitués par un motif fractal dont les segments sont constitués par un segment sinusoïdal.To obtain a larger reduction one can play on the angle β. The equilateral triangle of the Von Koch pattern then becomes isosceles instead of being equilateral and the two triangle segments become longer than those of the initial equilateral triangle (to length The constant). Their length is L '/ (6.cos β) and the ratio of the length deployed on the length L' is given by $${\left(\frac{\frac{\mathrm{The}\text{'}}{3}+2\frac{\mathrm{The}\text{'}}{6\cos \beta }+\frac{\mathrm{The}\text{'}}{3}}{\mathrm{The}\text{'}}\right)}^{\mathrm{not}}={\left(\frac{2\cos \beta +1}{3\cos \beta }\right)}^{\mathrm{not}}$$

n being the order of the fractal curve. In this way, one can extend a strand length in the same length. This reference pattern is called a "Von Koch Modified" pattern.
As before, each segment constituting the fractal patterns described above is constituted by a sinusoidal curve. For reasons of readability, these patterns are not shown but in view of the description above, the skilled person understands how to achieve the helix antenna whose radiating strands are constituted by a fractal pattern whose segments are constituted by a sinusoidal segment.

Example of realization and performances

A helix type antenna comprising a Von Koch type fractal whose segments have been replaced by sinusoidal segments has been realized and tested. The figure 10 illustrates an embodiment of such an antenna.

In particular, the performance of such an antenna was measured and compared to a quadrifilar helix (reference) antenna comprising rectilinear strands, the antenna having a height of 514 mm.

The table below lists the different parameters used for the radiating strands. The basic fractal is a motif of Von Koch. Order 1 1 1 1 1 2 2 2 Number of cells 3 3 3 3 4 2 2 3 α (degrees) 52 52 49 52 52 43 43 50 Cell length (mm) 155 150 140 135 108 250 243 190 k 0.5 0.5 0.7 0.7 0.7 0.7 0.7 0.7 s -1 1 -1 1 1 -1 1 -1 Height (mm) 285 276 252 249 265 205 198 254 LHC (dB) 0.88 0,952 0.8 0.97 0.95 0.15 0.202 0.93 RHC (dB) -10.3 -10.2 -10.3 -11.8 -10.8 -10.2 -11.1 -10.0 S11 (dB) -6.1 -6 -6.9 -6.9 -6.3 -6.2 -6.3 -6.2 Efficiency 66 64 60 62 62 50 50 55 Relative size (%) 55.4 53.7 49 48.4 51.6 39.9 38.5 49.4 Max Gain (dB) 2.44 2.41 1.91 2.23 2.26 0.37 0.36 1.6 There is a reduction in the height of the antenna. In the table above the relative size (%) is calculated as the ratio between the height of the compact antenna and the height of the reference antenna (514 mm).

In addition, it is found that the best performances are obtained with the antenna based on the Von Koch pattern with sinusoidal segments of order 2 and with two cells. This antenna has the same 137MHz pattern and resonance frequency (144MHz). In addition, its height is 198 mm (relative size 38.5%), a reduction of 61.5% of the height of the reference antenna.

Claims (8)

1. Helical type antenna comprising a form of revolution and a plurality of radiating strands helically wound around the form of revolution, characterised in that each radiating strand is defined by a repetition of a fractal pattern comprising segments, a sinusoidal arc having been applied to each segment of the fractal pattern, so that each radiating strand has a pattern constituted by a succession of alternate sinusoidal arcs, each arc having as its guiding axis the segment of the fractal pattern.
2. Helical type antenna according to claim 1, wherein each segment corresponds to a half-period of a sinusoidal curve defined by $y\left(x\right)=S.k.L\prime .\sin \left(\frac{\pi }{L\prime }.x\right)$

   

   where: S is an integer having a value within {-1; +1}, k is the ratio between the amplitude of the sinusoid and its half-wavelength, L' is the horizontal length of the pattern.
3. Helical type antenna according to either claim 1 or claim 2, wherein each segment of the fractal pattern has an identical length.
4. Helical type antenna according to any one of the preceding claims, wherein the fractal is of the Von Koch type, each straight line of which is replaced by a sinusoidal segment.
5. Antenna according to any one of the preceding claims, wherein the radiating strands are each constituted by a specific metallised zone, helically wound on the lateral surface of a sleeve (15), such that the guiding axis (AA', BB', CC', DD') of each strand is distant from the axis of the following strand by a specific distance (d) defined according to any perpendicular to any guiding line (L) of the sleeve (15) as the distance between two points, each defined by an intersection between the axis of a strand and a perpendicular to any guiding line (L) of the sleeve (15).
6. Antenna according to any one of the preceding claims, characterised in that the form of revolution (15) is cylindrical or conical.
7. Antenna according to any one of the preceding claims, characterised in that the antenna comprises four identical radiating strands.
8. Antenna according to any one of the preceding claims, wherein the length of an unwound strand is approximately $k.\frac{\lambda }{4},$

   

   where λ is the operating wavelength of the antenna.

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