EP0575211B1 - Radiating element of an antenna with wide bandwidth and antenna array comprising such elements - Google Patents

Description

The invention relates to a micro-ribbon type antenna with a small thickness but a large passband.

• milieu à propagation guidée (câbles, lignes, guides d'onde...),
• milieu à propagation libre (espace homogène ou non, isotrope ou non ...).

It will be recalled here that, in the radio frequency domain, an electromagnetic wave, in particular characterized by its wavelength λ (ratio of the speed of light to the frequency of the transmitted signal), energy carrier and generally information carrier , can spread in different environments, the main ones being:

• medium with guided propagation (cables, lines, waveguides ...),
• medium with free propagation (homogeneous space or not, isotropic or not ...).

An antenna can be considered as an interface between these two types of medium, allowing the transfer, total or partial, of electromagnetic energy from one to the other. The transmitting antenna passes this energy from a guided propagation medium to a free propagation medium and the receiving antenna reverses the direction of the energy transfer between the media. In the following, we will generally refer implicitly to an antenna working on the emission. However, the principle of equivalence guarantees the reciprocity of all the properties stated with an antenna at reception.

Called circuit (s) or antenna feed device, the set of components of all or part of the medium with guided propagation, directing or collecting the electromagnetic energy to be transferred and comprising passive or active elements, reciprocal or not.

We often associate with an elementary antenna, one or more geometrical points, called phase center, from which the electromagnetic wave seems to come, for a given direction, in the case of an antenna considered as working on the emission.

The resonance of the antenna is manifested at the frequency (s) for which the transfer of the energy transmitted from the supply line to space via the antenna is optimal, which results mathematically by the fact that, at the resonance frequency fr the complex impedance Z at the input of the antenna has a zero imaginary part and a maximum real part.

In microwave we are used to representing the place of the impedances Z (as a function of frequency) on an abacus called SMITH abacus where each resonance manifests itself in the form of a loop.

With current measurement means this resonance is "seen" through the adaptation which characterizes the transfer of energy from the supply line to the antenna. This vision of the behavior of the antenna can be called the response of the antenna and is quantified using the losses by mismatch or the Stationary Wave Rate (TOS) in English Voltage Standing Wave Ratio (VSWR) - defined above. below.

If Z is the impedance at the point where the adaptation measurement is made, and Zc the characteristic impedance of the supply line (the generally accepted standard is Zc = 50 Ohms), then setting z = Z / Zc we call coefficient of reflection or losses by mismatch the complex relationship: $$\text{ρ = (z-1) / (z + 1)}$$

The standing wave rate is then defined by: $$\text{TOS = | (1 + | ρ |) / (1- | ρ |) |}$$

• le Taux d'Onde Stationnaire (TOS) qui rend compte de la qualité de l'adaptation, c'est-à-dire la quantité d'énergie transmise de la ligne d'alimentation à l'antenne (il est d'autant plus proche de l'unité que cette qualité est bonne),
• le diagramme de rayonnement qui figure la distribution dans l'espace du champ électromagnétique E porteur de l'onde,
• auquel sont associés des paramètres classiques (le gain, la directivité, le rendement, l'ouverture à -3 dB, la probabilité de couverture ....).

In short, the antenna is characterized by a certain number of performances including:

• the Stationary Wave Rate (TOS) which accounts for the quality of the adaptation, i.e. the amount of energy transmitted from the feed line to the antenna (it is all the more close to unity that this quality is good),
• the radiation diagram which shows the distribution in space of the electromagnetic field E carrying the wave,
• which are associated with conventional parameters (gain, directivity, efficiency, aperture at -3 dB, probability of coverage, etc.).

By convention, the radiation diagram is represented in a frame of reference centered at a point on the antenna (if possible its phase center), and supplied in the form of "sections" in a standard spherical coordinate system (θ, φ). A so-called "constant coupe" cut is the variation curve of the E field, projected onto a given polarization (either (Eθ, or Eφ), θ varying from 0 to 180 ° (or from -180 to + 180 °). even, a so-called "constant coupe" cut is the variation curve of the E field, projected on a given polarization (either Eθ, or Eφ), φ) varying from 0 to 360 °.

An association of elementary antennas is called antenna array when these have common parts in their supply circuits or when a coupling exists between these elementary antennas rendering the overall radiation pattern of the array, in a frequency range data, dependent on that of each of the antennas or radiating elements.

The network obtained by the distribution of antennas similar to one or more given elementary antennas, on a given surface, is often called array antenna, generally implying a notion of geometric repetition of the elementary antennas.

They are generally used to obtain a radiation pattern with high directivity in a given direction relative to the network.

The spacing Δ between the phase centers of the elementary antennae of the array, related to the wavelength λ _o in air or vacuum, is often a critical parameter.

For example, for values of Δ / λ _o > 0.5, the appearance of large lobes of networks, outside the useful radiation area, penalizes the energy transmission budgets in the medium with free propagation.

As for micro-ribbon technology, it consists of stacking several layers of conductive or dielectric materials such as for example a layer of dielectric substrate (glass - PTFE for example) coated on its underside (or side I) with a conductive sheet (copper, gold, etc.) called a ground plane and carrying on its upper face (or side S) a conductive sheet partially cut according to a given geometric design (we commonly speak of patterns, aerials or "patches").

• soit de guider une onde électromagnétique (ligne micro-ruban),
• soit de rayonner un champ électromagnétique (antenne micro-ruban).

This set allows:

• either to guide an electromagnetic wave (micro-ribbon line),
• or to radiate an electromagnetic field (micro-ribbon antenna).

• soit l'interface air-substrat,
• soit l'interface air-conducteur-substrat.

The propagation medium for surface currents is:

• either the air-substrate interface,
• or the air-conductor-substrate interface.

où ε_r est la constante diélectrique du substrat.In the first case, we can define (see the book MICROSTRIP ANTENNAS by IJ BAHL and P. BHARTIA, published by ARTECH HOUSE in 1980), the "effective" dielectric constant of the medium by: $${\text{ε}}_{\text{e}}\text{=}\frac{{\text{ε}}_{\text{r}}\text{+1}}{\text{2}}$$

where ε _r is the dielectric constant of the substrate.

où h est l'épaisseur de ce substrat
et W est la largeur du ruban ("strip" en anglais) conducteur.In the second case, we write $${\text{ε}}_{\text{e}}\text{=}\frac{{\text{ε}}_{\text{r}}\text{+1}}{\text{2}}\text{+}\frac{{\text{ε}}_{\text{r}}\text{- 1}}{\text{2}}\frac{\text{1}}{\sqrt{\text{(1 + 12}\frac{\text{h}}{\text{W}}\text{)}}}$$

where h is the thickness of this substrate
and W is the width of the conductive strip.

One can generally provide various types of components and other elements (possibly active) on the S face of the structure.

• on peut prévoir dans une bonne mesure le diagramme de rayonnement,
• le dimensionnement de ces motifs ou aériens pour résonner à une fréquence donnée est bien connu.

By definition, a micro-ribbon antenna is an element of geometric shape made of conductive material attached to the face S of a dielectric layer. We often choose a rectangular or circular shape for the following reasons:

• the radiation diagram can be predicted to a good extent,
• the dimensioning of these patterns or aerials to resonate at a given frequency is well known.

A rectangular micro-ribbon pattern can be compared to a certain extent to two parallel slits coinciding with two edges of the so-called radiating rectangle. The selection of those of the edges of a rectangular pattern which must radiate (and a contrario of those which must not radiate) is done by an appropriate choice of the area of the rectangle which is connected to the supply circuit.

In general we feed the rectangular pattern near or on the median connecting the sides that we want to radiate. In this way, the mode excited in the resonator produces a good quality linear polarization. The direction of this polarization is perpendicular to the radiating edge of the pattern.

This connection can be made through the dielectric substrate, or on the periphery of the pattern, by a microstrip line carried by the side S (we sometimes speak of coplanar supply) as described in particular in document FR-2.226 .760.

It is essentially the distance L between these edges (called "length" of the pattern) which will determine the resonant frequency of the antenna.

Equations, even abacuses, have been developed and constructed on this subject.

où : $${\text{ε}}_{\text{e}}{\text{= 0,5.(ε}}_{\text{r}}{\text{+1) + 0,5.(ε}}_{\text{r}}\text{-1)/}\sqrt{\text{(1+12.h/W)}}$$

• ε_e est la constante diélectrique du substrat diélectrique
• h est la hauteur (ou épaisseur) de ce substrat substrat
• λ_o est la longueur d'onde dans l'air associée à fr (c'est-à-dire le rapport entre la vitesse de la lumière et cette fréquence)
• et
• W est la largeur du motif, par exemple définie selon cet ouvrage précité par la formule : $$\text{W =}\frac{\text{1}}{{\text{λ}}_{\text{o}}\sqrt{{\text{2 (ε}}_{\text{r}}\text{+1)}}}$$
  

Thus, by way of example, we find in the work MICROSTRIP ANTENNAS by IJ BAHL and P. BHARTIA, published by ARTECH HOUSE in 1980, that to resonate at the frequency fr a rectangular pattern must have a length L such that: $$\text{L =}\frac{\text{1}}{{\text{2.λ}}_{\text{o}}\text{.}\sqrt{{\text{(ε}}_{\text{e}}\text{)}}}\text{+ 0.412}\frac{{\text{(ε}}_{\text{e}}\text{+0.3) (W + 0.264.h)}}{{\text{(ε}}_{\text{e}}\text{-0.258) (W + 0.8h)}}$$

or : $${\text{ε}}_{\text{e}}{\text{= 0.5. (Ε}}_{\text{r}}{\text{+1) + 0.5. (Ε}}_{\text{r}}\text{-1) /}\sqrt{\text{(1 + 12.h / W)}}$$

• ε _e is the dielectric constant of the dielectric substrate
• h is the height (or thickness) of this substrate substrate
• λ _o is the wavelength in air associated with fr (i.e. the ratio between the speed of light and this frequency)
• and
• W is the width of the pattern, for example defined according to this aforementioned work by the formula: $$\text{W =}\frac{\text{1}}{{\text{λ}}_{\text{o}}\sqrt{{\text{2 (ε}}_{\text{r}}\text{+1)}}}$$
  

The choice of the width W will to a good extent condition the quality of the radiation, namely its efficiency and its shape (radiation diagram).

où $${\text{K = 8,794/(f}}_{\text{r}}\sqrt{{\text{ε}}_{\text{r}}}$$

Still according to the aforementioned document, the radius of a circular pattern is obtained using the formula below: $$\text{R = K}\sqrt{\text{{1 +}\frac{\text{2h}}{{\text{π.ε}}_{\text{r}}\text{.K}}\text{. [ln (}\frac{\text{π.K}}{\text{2h}}\text{) + 1.7726]}}}$$

or $${\text{K = 8.794 / (f}}_{\text{r}}\sqrt{{\text{ε}}_{\text{r}}}$$

• série,
• parallèle,
• combinaison des deux précédents.

Any micro-ribbon motif can be used as part of a network such as:

• series,
• parallel,
• combination of the previous two.

• peu épaisses,
• légères,
• peu coûteuses (fabrication rapide et facile)
• que l'on peut "conformer" pour les appliquer à des structures développables, cylindriques ou coniques par exemple.

This technology makes it possible to obtain antennas (or antenna array):

• not very thick,
• light,
• inexpensive (quick and easy to make)
• that we can "conform" to apply them to developable structures, cylindrical or conical for example.

The micro-ribbon antenna is in fact an electronic resonator which, by construction, has a high quality factor Q. Therefore, the antennas developed in this technology always have a low bandwidth, that is to say that the resonance occurs only punctually, that is to say at the frequency for which the antenna was sized and at frequencies very close to the latter.

For example, a conventional rectangular micro-ribbon antenna sized to resonate at 1600 MHz on a substrate 1 mm thick and with dielectric constant εr = 2.2 can only be used in a frequency band of the order of 1% the value of the resonance frequency, which is insufficient for most applications (remote information, etc.).

Several methods have already been proposed to overcome this difficulty, a review of which is presented in the article BANDWIDTH EXTENSION TECHNIQUES IN PRINTED CONFORMAL ANTENNAS by A. HENDERSON, J.R. JAMES and C.M. HALL (Military Microwaves 1986).

• 1 - la surface conductrice du motif,
• 2 - la partie du plan de masse correspondant à la projection perpendiculaire de cette surface sur le plan de masse,
• 3 - les murs magnétiques coïncidant avec les bords du motif à travers toute l'apaisseur de substrat et dont la hauteur est cette épaisseur,

   épaissir la couche de diélectrique revient à allonger les murs magnétiques, ce qui tend à accroître la bande passante de la cavité.The simplest method to widen the bandwidth of the antenna is to thicken the dielectric layer. Indeed if we consider the resonant structure as a cavity whose walls (magnetic walls) would be:

• 1 - the conductive surface of the pattern,
• 2 - the part of the ground plane corresponding to the projection perpendicular to this surface on the ground plane,
• 3 - the magnetic walls coinciding with the edges of the pattern across the entire thickness of the substrate and the height of which is this thickness,

thickening the dielectric layer amounts to lengthening the magnetic walls, which tends to increase the bandwidth of the cavity.

• faible augmentation de la bande passante,
• augmentation des pertes ohmiques dans le substrat,
• génération d'ondes de surfaces,
• accroissement de l'encombrement de l'antenne.

This method has the following disadvantages:

• small increase in bandwidth,
• increased ohmic losses in the substrate,
• generation of surface waves,
• increase in the size of the antenna.

The most common concept is to stack non-powered radiating elements (with their associated dielectric layer) on the powered element. These elements are called "parasitic elements". Each of these elements i is dimensioned to resonate at a frequency Fi close to the frequency Fa of the element supplied. The electromagnetic coupling between these elements and the powered element ensures the transfer of energy to the "parasites". The frequency response of the set is the envelope of the responses of each of the elements.

• épaisseur accrue, ce qui peut être rédhibitoire si on demande à l'antenne d'être mince, notamment si elle doit être conformée (application aéronautique, lanceur),
• discontinuités et inhomogénéités mécaniques affectant les performances de l'antenne si elle doit subir des agressions mécaniques ou thermiques (antenne embarquée sur aéronef, missile ou satellite),
• difficultés lors de la fabrication de l'antenne pour respecter les dimensions et positions relatives des différentes couches (impact sur les performances radio-électriques).

The disadvantages of this so-called multilayer structure and the structures derived therefrom are the following:

• increased thickness, which can be prohibitive if the antenna is asked to be thin, in particular if it must be conformed (aeronautical application, launcher),
• mechanical discontinuities and inhomogeneities affecting the performance of the antenna if it has to undergo mechanical or thermal attack (antenna on board an aircraft, missile or satellite),
• difficulties in manufacturing the antenna to respect the dimensions and relative positions of the different layers (impact on radio performance).

There is therefore a need for certain applications to develop a broadband antenna in a single-layer structure (a single layer of dielectric) making it possible to avoid the above drawbacks.

It has already been proposed to place two rectangular parasitic patterns along the non-radiating sides of a fed rectangular pattern, or even four rectangular parasitic patterns along the sides of this pattern so as to allow a strong coupling between the opposite sides of these reasons. We can refer in this regard to document WO-89/07838 or to the article "Non-radiating Edges and Four Edges Gap-Coupled Multiple Resonator Broad Band Microstrip Antennas" by G. KUMAR and K.C. GUPTA published in I.E.E.E. Transactions on Antennas and Propagation Vol. AP 33 No. 2, February 1985. Preferably, the parasitic patterns, of dimensions at least close to those of the central pattern, are four in number.

The networking of such antennas consists in reproducing periodically, in one or even two directions of a plane, groups of three (preferably five) patterns of which only one is supplied, which poses problems of congestion : it is difficult to satisfy for example a spacing constraint of the type Δ <0.5 λ _o , since between two fed patterns there are two parasitic patterns separated by a substantial space; in addition, the supply can only be done by a line in an underlayer located under the ground plane (see in particular document WO-89/07838 which is the only one of the two aforementioned documents to expressly provide for such placing network). We therefore find the geometric or mechanical drawbacks inherent in the multi-layer technique.

The same type of drawbacks, in particular, are found with the concepts proposed by documents US-4,933,680 and GB-2,067,842.

• le diamètre extérieur de l'anneau est beaucoup plus grand que celui du disque correspondant (c'est-à-dire ayant la même fréquence centrale de résonance). Ce concept est donc incompatible avec le souhait d'une distance faible entre centre de phase (par exemple Δ/λ_o < 0.5),
• la large bande passante n'est obtenue que poour un mode d'excitation bien particulier (TM12), nécessitant de connecter la source d'énergie en des points précis de l'anneau, à des distances précises de ses bords interne et externe : ce type d'alimentation n'est pas compatible avec le souhait d'une alimentation coplanaire.

It has also been proposed (see summary review cited above) a micro-ribbon ring pattern which allows obtain a bandwidth worth three times the bandwidth obtained for a full microstrip disk. However, this concept has the following drawbacks:

• the outer diameter of the ring is much larger than that of the corresponding disc (that is to say having the same central resonant frequency). This concept is therefore incompatible with the wish for a small distance between phase centers (for example Δ / λ _o <0.5),
• the wide bandwidth is only obtained for a very particular excitation mode (TM12), requiring the connection of the energy source at precise points of the ring, at precise distances from its internal and external edges: this type of feeding is not compatible with the wish for coplanar feeding.

Document US-A-4,987,421 likewise proposes a micro-ribbon antenna comprising a radiative ring situated on a dielectric opposite to a ground conducting plane, with a supply opening through the dielectric, inside of the ring, and a microwave circuit also located inside the ring and connecting the power supply to the internal edge of the ring; there may be an upper dielectric separated by an intermediate blade of air or dielectric, with a parasitic pattern placed opposite the ring. The microwave circuit preferably includes an adaptation circuit and an amplifier. When an unpowered conductive element is placed inside the ring, it is in principle connected to the ground plane through the dielectric. The antennas thus obtained are compact and have low losses but are complex, without the possibility of feeding the patterns; nothing is planned for obtaining a large bandwidth.

As for document US-A-4,157,548, it relates to twin antenna patterns, that is to say pairs of identical patterns arranged on either side of a dielectric support, so that each motif serve as a ground plan to the other. A large number of shapes are proposed, and when a rectangular pattern is proposed inside another rectangular pattern, it is presented as equivalent to connect them to each other or to keep them without contact. Nothing is planned to increase the bandwidth.

• bande passante élargie par rapport au cas de motifs connus d'encombrement équivalent,
• faible épaisseur globale (c'est-à-dire notamment faible épaisseur de diélectrique),
• faisabilité en structure mono-couche (une seule couche de diélectrique et de préférence une seule couche de chaque côté du diélectrique) aussi bien qu'en structure multi-couches,
• possibilité de conformer l'antenne combinée à une tenue mécanique acceptable,
• possibilité d'emploi d'un réseau d'alimentation coplanaire, c'est-à-dire rapporté sur la même face du circuit que les motifs rayonnants,
• lors d'une mise en réseau, possibilité de respecter des contraintes d'espacement sévères (par exemple : Δ/λ_o < 0,5) entre les centres de phase des éléments voulues pour des raisons d'encombrement ou pour un meilleur contrôle du diagramme de rayonnement,
• facilité de fabrication.

The object of the invention is to overcome the aforementioned drawbacks by proposing an elementary pattern of micro-ribbon antenna combining the following advantages:

• wider bandwidth compared to the case of known patterns of equivalent bulk,
• small overall thickness (that is to say in particular small dielectric thickness),
• feasibility in a single-layer structure (a single layer of dielectric and preferably a single layer on each side of the dielectric) as well as in a multi-layer structure,
• possibility of conforming the antenna combined with acceptable mechanical strength,
• possibility of using a coplanar supply network, that is to say attached to the same face of the circuit as the radiating patterns,
• during a networking, possibility of respecting severe spacing constraints (for example: Δ / λ _o <0.5) between the phase centers of the desired elements for reasons of space or for better control of the radiation pattern,
• ease of manufacture.

The invention proposes for this purpose an elementary micro-ribbon antenna comprising a dielectric substrate of constant thickness along, on one side by a conductive metal layer forming ground plane and on its other side by a radiating pattern electrically connected to a line d feed, this pattern being formed of a conductive loop of constant width l , this antenna being characterized in that this loop surrounds an internal parasitic pattern which is not supplied by being separated from this internal parasitic pattern by a slot continuous closed on itself of constant width e suitable for ensuring a coupling between the loop and the internal parasitic pattern.

It should be noted that such an elementary pattern presents essential differences compared to the teachings of document US Pat. No. 4,771,291 which relates to a dual frequency micro-ribbon antenna pattern.

First of all, this document, taking note of the narrowness of the passbands of the known micro-ribbon antenna patterns, notes that the operation which is sought in practice is not done on a continuous band, but at two or more frequencies. discreet. This document is therefore not concerned with obtaining a large bandwidth, which is already sufficient to distinguish it from the invention.

On the other hand, this document implements a completely particular mode of supply since it is to the ground plane that the signals having the radio frequencies envisaged are applied, which is completely incompatible with the principle of coplanar feeding.

In addition, this document teaches to make slots in the patterns, generally in combination with pins crossing the dielectric in very precise locations for the short-circuiting of these patterns with the ground plane (which again prevents a coplanar feeding). The special case of a C-shaped slot is provided with the formation of a rectangular pattern (no other shape is envisaged) connected to a conductive line which surrounds it. The fact that it is repeatedly specified that this pattern and this line are connected in parallel goes completely against the invention which distinguishes a powered band and a non-powered pattern surrounded by this band by simply being electromagnetically coupled. It may be noted in this connection that this document aims to be able to neglect the coupling effect.

It will be appreciated that the invention lends itself very well to construction by printed circuit, since it allows all of the supply lines, supplied strips and solid non-supplied (or parasitic) patterns to be manufactured on one side. , without any crossing of the dielectric. This is very advantageous when several patterns of the aforementioned type are mounted in a network.

The invention also provides a network antenna formed of a plurality of elementary patterns formed by a powered strip surrounding a solid pattern, being separated by a closed loop slot, whether these patterns are mounted in series, in parallel or according to a mixed serial / parallel configuration. Such an antenna lends itself very well to a severe space constraint such as Δ / λ _{o of} less than 1 or even 0.5.

• le rapport l/e vaut entre 1/5 et 5, l'une au moins des grandeurs l ou e étant au moins approximativement comprise entre 0,001 et 0,1 fois le rapport Δ_o/ $\sqrt{{\text{ε}}_{\text{e}}}$
  
  si λ_o est la longueur d'onde associée à la fréquence de fonctionnement de l'antenne et ε_e la constante diélectrique effective du milieu de propagation constitué par le substrat et le motif,
• l'une au moins des grandeurs l ou e est au moins approximativement comprise entre 0,003 et 0,05 fois le rapport λ_o/ $\sqrt{{\text{ε}}_{\text{e}}}$
  
  ,
• le motif parasite interne est circulaire, la boucle conductrice et la fente lui étant concentriques,
• le diamètre du motif parasite interne vaut au moins approximativement 0,5 fois le rapport λ_o/ $\sqrt{{\text{ε}}_{\text{e}}}$
  
  ,
• le motif parasite interne est polygonal,
• le motif parasite interne est carré,
• le côté du motif parasite interne vaut au moins approximativement 0,5 fois le rapport λ_o/ $\sqrt{{\text{ε}}_{\text{e}}}$
  
  ,
• la ligne d'alimentation (5, 15, 25) est coplanaire audit motif.

According to preferred teachings of the invention, possibly combined:

• the ratio l / e is between 1/5 and 5, at least one of the magnitudes l or e being at least approximately between 0.001 and 0.1 times the ratio Δ _o / $\sqrt{{\text{ε}}_{\text{e}}}$
  
  if λ _o is the wavelength associated with the operating frequency of the antenna and ε _e the effective dielectric constant of the propagation medium constituted by the substrate and the pattern,
• at least one of the magnitudes l or e is at least approximately between 0.003 and 0.05 times the ratio λ _o / $\sqrt{{\text{ε}}_{\text{e}}}$
  
  ,
• the internal parasitic pattern is circular, the conductive loop and the slot being concentric with it,
• the diameter of the internal parasitic pattern is worth at least approximately 0.5 times the ratio λ _o / $\sqrt{{\text{ε}}_{\text{e}}}$
  
  ,
• the internal parasitic pattern is polygonal,
• the internal parasitic pattern is square,
• the side of the internal parasitic pattern is worth at least approximately 0.5 times the ratio λ _o / $\sqrt{{\text{ε}}_{\text{e}}}$
  
  ,
• the supply line (5, 15, 25) is coplanar with said pattern.

• la figure 1 est une vue de principe, en perspective, d'un motif élémentaire d'antenne conforme à l'invention,
• la figure 2 est une courbe d'impédance en fonction de la fréquence rapportée dans une abaque de SMITH pour une antenne conforme à la figure 1, mais non optimisée,
• la figure 3 est la courbe d'impédance de cette antenne, également rapportée dans une abaque de SMITH, après optimisation,
• la figure 4 est une vue d'un motif rayonnant conforme à la figure 1 mais de forme circulaire,
• la figure 5 est une représentation dans une abaque de SMITH de la courbe d'impédance d'un élément d'antenne conforme à la figure 4, dans la gamme de fréquence 2,3 GHz 2,4 GHz,
• la figure 6 est un référentiel associé à un élément d'antenne de motif conforme à la figure 4, permettant de définir des coupes du diagramme de rayonnement,
• les figures 7A à 7F représentent les coupes φ=0 et φ=90° pour 2,3 GHz, pour 2,35 GHz, et pour 2,4 GHz, respectivement,
• la figure 8 est une vue d'un motif rayonnant conforme à la figure 1, mais de forme carrée,
• la figure 9 est une antenne-réseau formée d'un alignement de 24 éléments d'antenne identiques conformes à celui de la figure 4,
• les figures 10A et 10B donnent la réponse en fréquence de l'antenne-réseau de la figure 9, dans un diagramme TOS/fréquence, et dans une abaque de SMITH, respectivement, dans la gamme de fréquence 2,29 GHz - 2,42 GHz,
• la figure 11 est une vue éclatée d'une antenne-réseau ceinturant un corps cylindrique et formée de quatre antennes-réseau conformes à la figure 10, et
• les figures 12A et 12B sont des coupes φ=0 et φ=90° dans le référentiel de la figure 11, du diagramme de rayonnement de l'antenne de cette figure 11.

Objects, characteristics and advantages of the invention appear from the following description, given by way of nonlimiting example, with reference to the appended drawings in which:

• FIG. 1 is a principle view, in perspective, of an elementary antenna pattern according to the invention,
• FIG. 2 is an impedance curve as a function of the frequency reported in a SMITH chart for an antenna conforming to FIG. 1, but not optimized,
• FIG. 3 is the impedance curve of this antenna, also reported in a SMITH chart, after optimization,
• FIG. 4 is a view of a radiating pattern in accordance with FIG. 1 but of circular shape,
• FIG. 5 is a representation in a SMITH abacus of the impedance curve of an antenna element conforming to FIG. 4, in the frequency range 2.3 GHz 2.4 GHz,
• FIG. 6 is a frame of reference associated with a pattern antenna element in accordance with FIG. 4, making it possible to define sections of the radiation diagram,
• FIGS. 7A to 7F represent the sections φ = 0 and φ = 90 ° for 2.3 GHz, for 2.35 GHz, and for 2.4 GHz, respectively,
• FIG. 8 is a view of a radiating pattern in accordance with FIG. 1, but of square shape,
• FIG. 9 is a network antenna formed by an alignment of 24 identical antenna elements in accordance with that of FIG. 4,
• Figures 10A and 10B give the answer in frequency of the array antenna of FIG. 9, in a TOS / frequency diagram, and in a SMITH chart, respectively, in the frequency range 2.29 GHz - 2.42 GHz,
• FIG. 11 is an exploded view of a network antenna encircling a cylindrical body and formed by four network antennas in accordance with FIG. 10, and
• FIGS. 12A and 12B are sections φ = 0 and φ = 90 ° in the reference frame of FIG. 11, of the radiation diagram of the antenna of this FIG. 11.

The block diagram of an antenna element according to the invention is given in FIG. 1.

This antenna element, referenced 1 as a whole, comprises a dielectric substrate 2 bordered, on its lower face (or I) by a conductive metal layer 3 forming a ground plane and, on its upper face (or S) of a pattern 4 produced in micro-ribbon technology of conductive material and connected to a supply line 5 preferably coplanar with pattern 4.

The substrate 2 is in practice homogeneous of constant thickness.

According to a variant not shown, this pattern can be supplied by direct contact with a cable passing through the substrate while being isolated with respect to the ground plane 3.

According to the invention, the pattern 4 is formed of a conductive loop 6 of constant width l surrounding a solid internal pattern 7 isolated (that is to say not connected) from the loop, the outer edge of which follows the inner edge of the loop at a non-zero constant distance e so as to form a continuous slit 9 closed on itself of constant thickness e .

It will be appreciated that the internal pattern 7 is not directly supplied, and is only coupled to the internal loop: it therefore behaves like an internal parasite.

The outline of this internal parasite is completely arbitrary in Figure 1. In practice, this outline has a shape simple geometric (circle, square, rectangle, polygon with possibly rounded corners, ellipse, oval ...).

This pattern 4 for its dimensioning, can be analyzed as being a conventional pattern adapted to resonate at a desired frequency (when powered) surrounded by a conductive loop which degrades the quality factor, that is to say which widens the peak, that is to say which widens the bandwidth.

In other words, the central frequency of the antenna element (or elementary antenna) 1 is defined by the shape and the dimensions of the internal parasite 7 using the conventional rules (equations or abacuses) of dimensioning, for example those mentioned above, given by the aforementioned work "Microstrip Antennas" by BAHL and BARTHIA.

The width e of the slot is chosen so as to ensure a strong coupling between the loop 6 supplied and the parasitic pattern 7. The width l of the conductive loop is chosen in particular so as to allow good coupling through the slot while along it.

The frequency response of the pattern 4 depends of course on the exact choice of the dimensions of the internal parasite, of the slot and of the loop. Depending on the specifications imposed on the antenna element (or of a network antenna independently of the individual performance of the antenna elements) the final dimensioning is done for example by iteration from an arbitrary starting dimensioning .

• la rapport l/e est compris entre 1/5 et 5 environ,
• l'une au moins de ces grandeurs l ou e est au moins approximativement comprise entre 0,001 et 0,1 (de préférence entre 0,003 et 0,05) fois le rapport λ_o/ $\sqrt{{\text{ε}}_{\text{e}}}$
  
  .

For example, after having dimensioned (see above) the internal parasite as a function of the target central frequency, the quantities l and e can be arbitrarily chosen while respecting the inequalities defined above, if λ _o is the length wave associated with said center frequency and ε _e is the effective dielectric constant of the propagation medium formed by the antenna element (see above):

• the l / e ratio is between approximately 1/5 and 5,
• at least one of these quantities l or e is at least approximately between 0.001 and 0.1 (preferably between 0.003 and 0.05) times the ratio λ _o / $\sqrt{{\text{ε}}_{\text{e}}}$
  
  .

The power supply mode of the loop influences the behavior of the elementary antenna, mainly on its main polarization (which is in practice parallel to a fictitious line connecting the feed point to a central point of the parasite internal 7).

From such a principle design, it is within the reach of the skilled person to conduct an optimization as a function of the particular constraints of the targeted specifications. Thus, for example, as is known, the optimization process with a view to satisfying a given TOS coefficient (for example 2 or even 1.5) leads those skilled in the art to adjust the dimensions so as to bring about, in a abacus known as SMITH abacus, the largest possible part of the impedance curve, for a given frequency range (f ₁ , f ₂ ), of the antenna element (or of the antenna -network if necessary) in a circle of radius the smaller the required TOS is low. The larger the part of the curve contained in the circle, the wider the bandwidth.

By way of example, the optimization process will lead to passing from curve A in FIG. 2, which barely intercepts the circle C representative of the targeted TOS, to curve B in FIG. 3, a whole loop of which is contained in the circle C (it is recalled in this connection that in the abacus of SMITH each loop corresponds to a resonance).

FIG. 4 shows a pattern 14, in accordance with pattern 4 of FIG. 1, but for a circular shape: this pattern 14 includes an internal parasite 17 of diameter D separated from a surrounding circular loop 16 by a circular slot 18.

The loop 16 is supplied by a coplanar line 15.

$${\text{e = λ}}_{\text{o}}\text{/( 87.}\sqrt{{\text{ε}}_{\text{e}}}\text{)}$$

$${\text{D = λ}}_{\text{o}}\text{/( 2.}\sqrt{{\text{ε}}_{\text{e}}}\text{)}$$

By way of example, the design of the pattern can be chosen, at the start of the iteration process, by the following approximate formulas (to within 20% for example): $${\text{l = λ}}_{\text{o}}\text{/ (175.}\sqrt{{\text{ε}}_{\text{e}}}\text{)}$$

$${\text{e = λ}}_{\text{o}}\text{/ (87.}\sqrt{{\text{ε}}_{\text{e}}}\text{)}$$

$${\text{D = λ}}_{\text{o}}\text{/ (2.}\sqrt{{\text{ε}}_{\text{e}}}\text{)}$$

These orders of magnitude make it possible to obtain a first order dimensioning of the element in a secure manner, that is to say a starting point with a view to development by iterations.

The optimization of the dimensioning depends, as has been said, on the targeted performances, for example of a constraint concerning the TOS.

• l = 0,5 mm
• e = 1 mm
• D = 47 mm

As an example in the case of a dielectric substrate with a thickness of 2.28 mm (for example made of TLX material from TACONIC in the United States) and of coefficient DK = 2.55 (where DK denotes the dielectric constant) with ground planes and patterns of thicknesses equal to 35 µm and made of copper, the optimization of the dimensioning, for radiation in the range 2.3 GHz - 2.4 GHz has resulted in:

• l = 0.5 mm
• e = 1 mm
• D = 47 mm

In this example 1 is worth 0.5 e. Other tests have shown that satisfactory results for other values such as 1 = 3e.

) de la ligne d'alimentation auprès de sa connection à la boucle conductrice.FIG. 5 shows the impedance curve thus obtained between the points F1 and F2 corresponding respectively to 2.3 GHz and 2.4 GHz, after adaptation using a quarter wave device of any suitable known type not shown, such as for example a widening over a distance of λ _o / (4. $\sqrt{{\text{ε}}_{\text{e}}}$

) from the supply line near its connection to the conductive loop.

It is observed that the frequency response is very regular and homogeneous over the entire band envisaged (TOS <2). This shows that the phenomenon involved does not result from a succession of resonances but from a single resonance whose quality has been "degraded".

FIG. 6 represents a frame of reference associated with a pattern antenna element conforming to that of FIG. 4 in which the sections of the radiation diagram given by FIGS. 7A to 7F are defined, for φ = 0 and φ = 90 ° and for 2.3 - 2.35 - 2.4 GHz, that is to say three frequencies in the frequency band envisaged above.

These main sections of the radiation diagram measured at the center frequency of 2.35 GHz show that it is at least equivalent in quality (that is to say hemispherical in character and stable in shape as a function of frequency) at diagram of a classic micro-ribbon pattern with narrow bandwidth.

The pattern of Figure 4 therefore meets the objectives of the invention.

FIG. 8 represents a pattern comparable to that of FIG. 1, but of square shape: this pattern, marked 24 as a whole, includes an internal parasite 27 on side L separated from a square conducting loop 26 which surrounds it, width l , through a slot 28 of thickness e .

As an example of the initial dimensioning, the same rules are used as for the pattern in FIG. 4, replacing D by L.

• l = 1 mm
• e = 0,5 mm
• L = 47 mm

pour les mêmes matériaux que dans l'exemple de motif circulaire précité, et en explorant sensiblement la même gamme de fréquences.Acceptable performances were contained by taking:

• l = 1 mm
• e = 0.5 mm
• L = 47 mm

for the same materials as in the aforementioned circular pattern example, and by exploring substantially the same frequency range.

In fact a circular shape may seem preferable to a rectangular or square shape (see polygonal) insofar as, during an emission at high power, the corners present a predisposition to the formation of an arc electric capable of locally destroying the antenna element.

As stated above, the invention is generalized to other forms of internal parasite such as polygons with possibly rounded corners, ellipses, ovals in particular.

It has already been indicated above that the dimensioning of the element must take into account its future application.

For example, if the element is to be used repeatedly in a network, the bandwidth of the entire network will be a function of the bandwidth of the element, but will not necessarily be the same.

For example, if the distance between the elements (all identical) of a parallel network is such that the couplings between patterns are not negligible, then the response of the network will be different from the response of each element taken individually. It is generally observed that the network resonance loop is smaller than that of the isolated element. In this case, it makes sense to use an element with a slightly oversized resonance loop (like loop A in Figure 2).

• lorsque l croît, e étant constante,
• lorsque e croît, l étant constante,

les effets observés sont similaires à savoir que la boucle de la résonance du motif s'agrandit.The directions of variation are as follows:

• when l increases, e being constant,
• when e increases, l being constant,

the effects observed are similar, namely that the loop of the resonance of the pattern becomes larger.

FIGS. 9 to 12B present the final application of the concept of elementary antenna presented above to a networking of the optimized element.

The network of FIG. 9 is of the parallel type with only one dimension. However, this application being shown by way of nonlimiting example, it is very possible to use the element which is the subject of the invention on a network of series type or a two-dimensional network, flat or shaped.

FIG. 9 represents an array antenna 50 formed of 24 optimized elements 14 in accordance with FIG. 4.

These 24 elements are supplied from a point O by an at least partly coplanar network comprising a divider by 2, marked 51, supplying two other dividers by 2, marked 51A and each supplying two dividers by 2, marked 52 - same supplying each two dividers by 3 marked 53.

The frequency responses of this array antenna are presented in Figures 10A and 10B, where the frequencies designated by 1, 2 and 3 correspond respectively to 2.29 GHz, 2.42 GHz, and 2.3576 GHz.

It is observed that the bandwidth for a TOS less than 2 is 115 MHz, which represents 4.9% of the central frequency which is higher than what would be obtained with a conventional element of full circular shape and of the same size.

• les éléments soient équi-répartis uniformément sur la structure à une même hauteur,
• les éléments sont alimentés de manière équi-phase et équi-amplitude à une tolérance donnée près.

Several networks 50A, 50B, 50C and 50D conforming to that of FIG. 9, are then applied to a cylindrical structure so that:

• the elements are evenly distributed evenly on the structure at the same height,
• the elements are supplied in an equi-phase and equi-amplitude manner to a given tolerance.

This arrangement makes it possible to obtain a very omnidirectional radiation pattern, which is the objective in most telemetric applications. In order to optimize this radiation diagram, a calculation of the optimal number of elements can be performed by software. In general, this calculation leads to a result close to that mentioned above, namely a distance between successive elements at most close to half the wavelength in air (Δ / λ _o , <0.5). The number of elements must also take into account the supply network and the constraints associated with it (power dividers, etc.).

In the case presented in FIG. 11, on a cylinder 100 of radius equal to one meter, at 2350 MHz, we distribute 96 elements on the structure, that is to say four networks 50 (denoted 50A to 50D). As indicated above, it is necessary to provide for each network 50 three stages dividing by two and a stage dividing by three (24 = 2 ³ x 3) to supply the elementary patterns.

The dividing stages making it possible to distribute the signal to the four sub-networks are of the coaxial type. The other stages internal to the sub-networks are of the micro-ribbon type, included in the coplanar supply as shown in FIG. 9.

Note in Figure 9 that the divider by three has the following feature: each of the branches of the divider has the same length to within λ _o . In fact, the middle branch has any length l while the lateral branches have a length L = l + λ _o where λ _o is the wavelength in air at the central frequency of the useful band (here: 2350 MHz). The "equi-phase" nature of food is no longer strictly respected. We admit an error of +/- 12 ° over the entire useful band.

This kind of consideration is obviously to be taken into account on a case by case basis depending on the type of application. For example, for a similar network on a cylinder of radius equal to 650 mm, at the same frequency, the number of elements to be distributed would be equal to 64 and a supply network with 6 dividers by two would fulfill the requested mission.

It will therefore be noted that the element described lends itself very well to any networking with conventional spacing constraints between the radiating elements of the network.

On figure 12 are provided the sections Θ + 90 ° and Φ = 0 ° of the radiation diagram of the cylindrical antenna measured in the reference of figure 11.

We observe that the diagram of this antenna belt presents a very omnidirectional character. The energy distribution of the radiation is very homogeneous, which completely corresponds to the needs associated with telemetric links.

• appliquées sur un plan,
• appliquées sur une forme cylindrique,

pour tout système de télécommunication. L'application ci-dessus a été développée pour une application télémétrique sur un mobile.The antennas presented can be used:

• applied on a plan,
• applied on a cylindrical shape,

for any telecommunication system. The above application was developed for a telemetry application on a mobile.

• possibilité de travailler avec une seule couche de diélectrique Technologie de l'antenne très simple : élimination des risques de délamination de couches et des problèmes de collages et de tenue mécanique),
• faible épaisseur (conservation de caractéristiques aérodynamiques).

This concept lends itself particularly well to application on a mobile because of its qualities:

• possibility of working with a single layer of dielectric Very simple antenna technology: elimination of the risks of delamination of layers and problems of bonding and mechanical strength),
• thin (conservation of aerodynamic characteristics).

Claims (13)

1. Elementary microstrip antenna (1) comprising a constant thickness dielectric substrate (2) having on one side a conductive metal layer (3) forming a ground plane and on its other side a radiating patch (4, 14, 24) electrically connected to a feed line (5), the patch (4, 14, 24) being formed by a conductive loop (6, 16, 26) of constant width l, the antenna being characterised in that this loop surrounds an inner parasitic patch (7, 17, 27) which is not energised and is separated from said inner parasitic patch by a closed continuous slot (8, 18, 28) of constant width e adapted to bring about coupling between the loop and the inner parasitic patch.
2. Elementary antenna according to claim 1 characterised in that the ratio l/e is between 1/5 and 5/1, l or e being at least approximately between 0.001 and 0.1 times the ratio λ_o/ $\sqrt{{\text{ε}}_{\text{e}}}$

   

   where λ_o is the wavelength at the operating frequency of the antenna and ε_e is the effective dielectric constant of the propagation medium comprising the substrate and the patch.
3. Elementary antenna according to claim 2 characterised in that l and/or e is at least approximately between 0.003 and 0.05 times the ratio λ_o/ $\sqrt{{\text{ε}}_{\text{e}}}$

   

   .
4. Elementary antenna according to any one of claims 1 to 3 characterised in that inner parasitic patch (14) is circular and the conductive loop (16) and the slot (18) are concentric with it.
5. Elementary antenna according to any one of claims 1 to 4 characterised in that the diameter of the inner parasitic patch (14) is at least approximately 0.5 times the ratio λ_o/ $\sqrt{{\text{ε}}_{\text{e}}}$

   

   .
6. Elementary antenna according to any one of claims 1 to 3 characterised in that the inner parasitic patch (24) is polygonal.
7. Elementary antenna according to claim 6 characterised in that the inner parasitic patch is square.
8. Elementary antenna according to claim 7 characterised in that the side length of the inner parasitic patch (24) is at least approximately 0.5 times the ratio λ_o/ $\sqrt{{\text{ε}}_{\text{e}}}$

   

   .
9. Elementary antenna according to any one of claims 1 to 8 characterised in that the feed line (5, 15, 25) is coplanar with said patch.
10. Antenna (50) formed by an array in at least one direction of elementary antennas (14) according to claim 1 fed in series and/or in parallel.
11. Antenna according to claim 10 characterised in that the elementary antennas are fed by a feed array which is at least in part coplanar with the patches.
12. Antenna according to claim 10 or claim 11 characterised in that the elementary antennas are fed at the same phase and the same amplitude.
13. Antenna according to any one of claims 10 to 12 characterised in that it is formed on a cylinder by a annular series of equi-distant elementary antennas disposed in a plane transverse to the cylinder.

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