EP1325537B1 - Improvements to transmission/reception sources of electromagnetic waves for multireflector antenna - Google Patents

Description

The present invention relates to a source antenna (E) / reception (R), hereinafter called E / R source which can be placed at the point focal point of an antenna system and more particularly at the focal point of a Cassegrain type double reflector antenna. A possible application for this I / O source is in communication systems by satellite using the C, Ku or Ka bands.

The published European patent application EP1 162 686 A1 forming part of the state of the art within the meaning of section 54 (3) claims priority of French national patent application FR2810163 registered under the number 00 07424.

In French patent application n ° 00 07424 filed on June 9, 2000 on behalf of THOMSON multimedia with the title "Improvement to antennas - source of emission / reception of waves electromagnetic ", we proposed a hybrid E / R source consisting of a propeller network excited by a printed supply circuit, surrounding a longitudinal radiation antenna such as a propeller or a "polyrod".

In order to minimize interactions between emission sources and reception, it is advantageous to use the propeller network for reception and the longitudinal radiation source for emission. However, in reception the losses of the printed supply network affect doubly the link budget. Indeed, the G / T merit factor of the antenna is reduced on the one hand due to the reduction in the gain G of the antenna, and on the other hand, the increase in the noise temperature T of the system linked to the losses dissipative from the power network. From this point of view, the solution proposed in patent application 00 07424 allows, using a network of propellers rather than a network of patches, to improve the factor G / T of the antenna.

Furthermore, in French patent application 00 07424, the substrate on which the printed propeller supply network is engraved and which includes the antenna reception circuits is arranged perpendicular to the axis of radiation of the propellers. So in a Cassegrain structure, in order to avoid blockages by the LNB (Low Noise Block in English), it is necessary to place the focus of the double system reflector at the top of the main reflector. This constraint on the Cassegrain system geometry requires the use of a source too directive which has the effect of increasing the level of the side lobes of the antenna system.

i) de la diffraction par le réflecteur secondaire 3. L'énergie diffractée a un niveau absolu en dB égal à (G-Edge). G est le gain de la source primaire défini essentiellement par sa directivité. Pour un fonctionnement optimal du système d'antenne à double réflecteur, Edge est de l'ordre de 20 dB. Le niveau des lobes secondaires issus de cette diffraction est de l'ordre de la valeur de (G-Edge),

ii) des lobes secondaires I rayonnés par la source même 2 et n'interceptant pas le réflecteur secondaire 3. Si la source primaire 1 présente un niveau de lobes secondaires égal en dB à SLL, alors le niveau absolu des lobes secondaires du système d'antenne issus des lobes secondaires de la source primaire est égal à (G-SLL).

Une solution pour réduire les lobes d'un système Cassegrain est de réduire G. Cependant, comme illustré en figure 2, pour réduire G et maintenir une valeur de Edge optimale ( de l'ordre de 20 dB ), il faut que la focale 2' du système d'antenne soit située entre le réflecteur principal 1 et le réflecteur secondaire 3.Indeed, as illustrated in FIG. 1 which schematically represents a Cassegrain structure comprising a main reflector 1, a source 2 and a secondary reflector 3 facing the source 2, the secondary lobes mainly come from:

i) diffraction by the secondary reflector 3. The diffracted energy has an absolute level in dB equal to (G-Edge). G is the gain of the primary source defined essentially by its directivity. For optimal operation of the dual reflector antenna system, Edge is around 20 dB. The level of the secondary lobes resulting from this diffraction is of the order of the value of (G-Edge),

ii) secondary lobes I radiated by the same source 2 and not intercepting the secondary reflector 3. If the primary source 1 has a level of secondary lobes equal in dB to SLL, then the absolute level of the secondary lobes of the system antenna from the secondary lobes of the primary source is equal to (G-SLL).

One solution to reduce the lobes of a Cassegrain system is to reduce G. However, as illustrated in Figure 2, to reduce G and maintain an optimal Edge value (of the order of 20 dB), the focal length 2 'of the antenna system is located between the main reflector 1 and the secondary reflector 3.

The object of the present invention is to remedy this problem by proposing an E / R source structure having its phase center between the main reflector and the secondary reflector without inducing blockage for the functioning of the antenna system with two reflectors. It thus makes possible reduction of the side lobes of the antenna system.

On the other hand, reducing the SLL level of the side lobes of the primary source also makes it possible to reduce the secondary lobes of the system antenna.

The present invention also aims to propose a new E / R source structure which reduces the side lobes of sources of emission and reception.

In addition, unlike a focusing system based on homogeneous lens, a dual reflector antenna system has a perfectly defined focal point and requires for E / R sources a perfect coincidence of their phase centers.

Thus, the present invention also aims to provide a E / R source structure which allows to perfectly match the phase centers of emission and reception sources.

The subject of the present invention is therefore a source of emission / reception (E / R) of electromagnetic waves for antenna with multi-reflector of the Cassegrain type comprising means with longitudinal radiation operating in a first frequency band and a network of n Traveling wave type radiating elements operating in a second frequency band with the n radiating elements arranged symmetrically around the longitudinal radiation means, the network and the longitudinal radiation means having a phase center substantially common, characterized in that the network of n elements radiating is excited by a waveguide forming a slice-shaped cavity pineapple "of rectangular cross section.

According to one embodiment, the network of n elements radiating is a circular network and the guide forms a cavity in the form of "Pineapple slice". In this case, the waveguide is sized to such that, D being the mean diameter of the circular network:

D = nλg / 2 where n represents the number of radiating elements and λg the guided wavelength at the operating frequency.

λg = λ0 [εr - (λ0 / λc) ² ] ^{- ½} with λc the cut-off wavelength of the rectangular guide for the fundamental mode TE01, λ0 the wavelength in vacuum and εr the permittivity of the filling dielectric material guide.

λc = 2a (εr) ^½ where a is the width of the rectangular guide. To obtain good directivity of the source, D is chosen such that: 1.3 λ0 <D <1.9 λ0.

The rectangular guide above is excited by a connected probe reception circuits (LNA (Low Noise Amplifier in English), mixer, etc ...) by a coaxial line.

1) de réduire les lobes secondaires et arrières de l'antenne à rayonnement longitudinal,

2) de faire coïncider les centres de phase des sources d'émission et de réception, et

3) d'améliorer les performances d'isolation entre les sources d'émission et de réception.

Furthermore, for transmission, the longitudinal radiation antenna which can be constituted either by a “polyrod” excited by a circular or square guide or by a long helix excited by a coaxial line, located in the center of the array has a sort rear cavity which allows:

1) to reduce the side and rear lobes of the longitudinal radiation antenna,

2) to match the phase centers of the emission and reception sources, and

3) improve the insulation performance between the emission and reception sources.

Finally to reduce the secondary lobes of the propeller network, a second conical cavity surrounds it.

figure 1 déjà décrite est une représentation schématique d'un système Cassegrain selon l'art antérieur,

figure 2 déjà décrite est une représentation schématique correspondant à celle de la figure 1 et expliquant un des problèmes que cherche à résoudre l'invention,

figure 3 est une représentation schématique d'un système Cassegrain comportant une source conforme à la présente invention,

figures 4a et 4b représentent respectivement une vue en coupe et une vue de dessus d'un système-source selon un mode de réalisation de la présente invention,

figure 5 est une vue en coupe de détail d'une hélice utilisée dans le système des figures 4,

figure 6 est une courbe donnant les résultats du couplage du guide rectangulaire aux hélices en fonction de la fréquence,

figure 7 est une vue identique à celle de la figure 4a représentant le système réalisé pour simulation,

figures 8,9 et10 sont des courbes donnant des résultats de simulations réalisées avec le système-source de la figure 7, et

figure 11 représente une autre mode de réalisation d'un système-source selon la présente invention.

Other characteristics and advantages of the present invention will appear on reading the description given below of various embodiments, this description being made with reference to the attached drawings in which:

FIG. 1 already described is a schematic representation of a Cassegrain system according to the prior art,

FIG. 2 already described is a schematic representation corresponding to that of FIG. 1 and explaining one of the problems that the invention seeks to solve,

FIG. 3 is a schematic representation of a Cassegrain system comprising a source in accordance with the present invention,

FIGS. 4a and 4b respectively represent a sectional view and a top view of a source system according to an embodiment of the present invention,

FIG. 5 is a detailed sectional view of a propeller used in the system of FIGS. 4,

FIG. 6 is a curve giving the results of the coupling of the rectangular guide to the propellers as a function of the frequency,

FIG. 7 is a view identical to that of FIG. 4a representing the system produced for simulation,

FIGS. 8, 9 and 10 are curves giving results of simulations carried out with the source system of FIG. 7, and

FIG. 11 represents another embodiment of a source system according to the present invention.

For simplification, in the figures the same elements bear the same references.

We will now describe with reference to Figures 3 to 11 different embodiments of the present invention.

Figure 3 schematically shows a sectional view of the E / R source 10 object of the invention, placed at the focal point FP of a system antenna with double reflector located between the two reflectors 1 and 3.

un encombrement, un poids et un coût réduits en même temps qu'une bonne isolation électrique entre les voies d'émission et de réception grâce à une isolation physique entre les deux voies.

The source / transmit antenna object of the invention has, compared to more conventional solutions using waveguide technology, the following advantages, namely:

reduced dimensions, weight and cost, as well as good electrical insulation between the transmission and reception channels thanks to physical insulation between the two channels.

i) Elle permet de réduire encore les pertes de la source constituée du réseau d'hélices grâce aux très faibles pertes de son réseau d'alimentation utilisant un guide rectangulaire monomode, connu pour ces pertes minimales, et dont la longueur est réduite en moyenne au demi-périmètre du réseau circulaire.

ii) Elle apporte une solution à faible coût au problème des lobes secondaires trop élevés des antennes à double réflecteur de type Cassegrain :

• en permettant de placer le centre de phase du système source hybride entre le réflecteur principal et le réflecteur secondaire
• en réduisant les lobes secondaires des sources primaires d'émission et de réception.

iii) Elle permet de faire parfaitement coïncider les centres de phase des sources d'émission et de réception et permet ainsi de positionner la source primaire de manière optimale en émission et en réception.

In addition, compared to the system described in French patent application 00 07424:

i) It makes it possible to further reduce the losses of the source made up of the propeller network thanks to the very low losses of its supply network using a single-mode rectangular guide, known for these minimum losses, and the length of which is reduced on average to half-perimeter of the circular network.

ii) It provides a low-cost solution to the problem of excessively high side lobes of Cassegrain type double reflector antennas:

• by allowing the phase center of the hybrid source system to be placed between the main reflector and the secondary reflector
• by reducing the secondary lobes of the primary sources of emission and reception.

iii) It makes it possible to perfectly match the phase centers of the emission and reception sources and thus makes it possible to position the primary source optimally in transmission and in reception.

We will now describe in more detail, with reference in Figures 4 to 10, a preferred embodiment of the present invention.

• le réseau de n éléments rayonnants du type à ondes progressives est constitué de huit hélices 11. Elles sont placées sur la circonférence d'un cercle de diamètre D fonctionnant dans une seconde bande de fréquences. Elles sont montées sur la face supérieure 15a d'un guide d'onde 15 en forme de « tranche d'ananas ».
• l'antenne à rayonnement longitudinal située au milieu du réseau est un « polyrod » 12,

Figures 4a and 4b respectively show a sectional view and a top view of the source system object of the invention. In this particular case:

• the array of n radiating elements of the traveling wave type consists of eight helices 11. They are placed on the circumference of a circle of diameter D operating in a second frequency band. They are mounted on the upper face 15a of a waveguide 15 in the form of a “pineapple slice”.
• the longitudinal radiation antenna located in the middle of the array is a “polyrod” 12,

As shown in Figures 4a and 7, the rear cavities 13 and 14 making it possible to reduce the radiation of the lateral lobes for the "Polyrod" and for the propeller network are conical.

The rectangular waveguide 15 in the form of a "slice pineapple ”is excited by a coaxial line 16. The radiating helices 11 are in turn coupled by probe 17 to the cavity in a rectangular guide.

For optimal excitation of the propellers, these are placed in the middle of the straight section of the guide in maximum field planes, namely open circuit plans.

FIG. 5 shows the detail and the dimensioning of a propeller 11 excited at 12 GHz mounted on a waveguide 15 of straight section polygonal, more particularly rectangular with dimensions a and b.

Figure 6a presents simulations showing the result of the coupling of the rectangular guide to the propellers according to the invention as well as the adaptation of the guide cavity to the central frequency of 12 GHz., in the case of 4 propellers such as 11-2, 11-3, 11-4, 11-5 compared to port A1 (Figure 6b).

• 
  ( dans le cas d'un réseau de 8 hélices 11) ; λ_g est la longueur d'onde guidée à la fréquence de fonctionnement ;
  
  λ_c est la longueur d'onde de coupure du guide rectangulaire pour le mode TE10 et λ0 est la longueur d'onde dans le vide ;
     λ_c = 2a(ε_r)^1/2; a est la largeur du guide rectangulaire
     ε_r = permittivité du matériau diélectrique remplissant le guide
• Par ailleurs pour un éclairement optimal du réflecteur secondaire, la directivité de la source primaire varie entre +/- 20° et +/- 30° à -20 dB. Ces valeurs de directivité sont obtenues pour des diamètres moyens D tels que :
  
  λ₀ étant la longueur d'onde dans le vide

Thus, the dimensioning of the rectangular guide 15 is done as follows:

• 
  (in the case of a network of 8 propellers 11); λ _g is the guided wavelength at the operating frequency;
  
  λ _c is the cut-off wavelength of the rectangular guide for TE10 mode and λ0 is the wavelength in vacuum;
  λ _c = 2a (ε _r ) ^1/2 ; a is the width of the rectangular guide
  ε _r = permittivity of the dielectric material filling the guide
• In addition, for optimal illumination of the secondary reflector, the directivity of the primary source varies between +/- 20 ° and +/- 30 ° at -20 dB. These directivity values are obtained for average diameters D such that:
  
  λ ₀ being the wavelength in a vacuum

For D fixed by the directivity of the source, equations (I) and (III) allow to deduce a relation between λ _g and λ ₀ . Taking this relation into account in (II), we deduce a. To minimize losses in the rectangular guide, the height b of the rectangular guide is chosen to be approximately half its width. So let b ∼a / 2.

In general, to minimize losses and cost, the guide is chosen empty (ε _r = 1). However, if the guide is too wide, or if there is a need to clear more space in the middle for the placement of the polyrod 12 with its rear cavity 13, it suffices to fill the guide with a dielectric material of permittivity ε _r > 1. The width of the guide is reduced by a factor (ε _r ) ^-1/2 .

For the dimensioning of the external cavity, the quantities Δ, α and h are adjusted so as to reduce the level of the secondary lobes of the propeller network.

For the interior cavity 13, the diameter d _c is given by the dimensioning of the rectangular guide 15, and more particularly by its width a. As shown in FIG. 7, the depth d is such that the phase center FP of the "polyrod »12 (which is about 1/3 the length of the polyrod) coincides with the phase center FH of the propeller network 11 (ie in the middle of the propeller network and about 1/3 of the length of l 'propeller). Thus, with reference to FIG. 7, and starting from an origin situated on the base and in the center of the conical cavity of depth d, the point Fp is at a height of approximately Lp / 3 where Lp is the total length of the polyrod 12 counted from the origin. To make the phase centers coincide, the points Fh must be at the same height as Fp, which results in the relation: d + Lh / 3 = Lp / 3 therefore d = (Lp - Lh) / 3; or
Lh is the length of each of the propellers 11.

The dimensions of each of the propellers 11 operating in mode longitudinal at the central frequency, as well as those of the central polyrod in depending on the directionality sought, are given by formulas classics known to those skilled in the art.

Finally the shape of the rear cavity of the central polyrod can be changed. Thus, instead of a conical shape 13, the rear cavity can have a cylindrical or similar shape.

FIG. 7 represents a particular embodiment of the source transmission / reception object of the invention. The emission part is made up of polyrod 12 and operates in the 14-14.5 GHz band. The reception part operates in the 11.7-12.5 GHz band and consists of a network of 8 propellers 11 located on a circle with diameter D = 42 mm, or approximately 1.7λ0 where λ0 represents the wavelength in vacuum at the center frequency of the receiving strip is therefore λ0 = 24.7 mm.

For this realization, the shape of the polyrod 12 was first of all optimized. Then the three types of interior cavities (namely a cavity cylindrical, a cylindrical cavity with traps and a conical cavity), all of depth d = 30 mm (i.e. approximately (Lp-Lh) / 3 = (110-30) / 3 = 26.6 mm) so as to make the phase centers of the two coincide sources have been simulated. The conical cavity allowed for this configuration to get the best result. The adaptation of the polyrod in the target band (14-14.5 GHz) as well as the radiation patterns obtained in the presence of the conical cavity are given in FIG. 8.

We then carried out the optimization of the angle ∝ and the height h of the outer conical cavity 14, with respect to the secondary lobes of the polyrod. The best result is then obtained for ∝ = 45 ° and h = 25 mm. The Figure 9 shows the simulation results of the adaptation curve and the radiation patterns obtained for these values of ∝ and h. We can note a significant reduction in the side lobes in the presence of the outer cavity.

Finally, Figure 10 shows the radiation patterns of the network of eight propellers, all 30 mm long, evenly spaced on a circle with diameter D = 42 mm or about 1.7λ0 where λ0 represents the wavelength in vacuum at the center frequency of the band reception.

Optimization of the secondary lobes of the receiving source through the outer cavity leads to optimal values of h = 25mm and ∝ = 40 °. These values are slightly different from those obtained for optimization of the secondary lobes of the emission source (h = 25 mm and ∝ = 45 °). These are the values obtained for the emission source that have been privileged, given the stronger constraints on the diagram resignation.

In FIG. 11, an alternative embodiment of the longitudinal radiation source. In this case, the source is constituted by a propeller 12 mounted in a conical cavity 13 and coupled by a probe 17 to Tx supply.

In the embodiments shown, the polarizations of the sources at transmission and reception are circular and can be from same or opposite direction.

Obviously for those skilled in the art, the 12 'propeller can be positioned in a cylindrical cavity like the polyrod.

The present invention can be modified in numerous ways. manners without going beyond the scope of the claims below.

Claims (9)

1. Electromagnetic wave transmission/reception (T/R) source for a multireflector antenna of the Cassegrain type comprising longitudinal-radiation means (12, 12') operating in a first frequency band and an array of n radiating elements (11) of the travelling-wave type operating in a second frequency band with the n radiating elements arranged symmetrically around the longitudinal-radiation means, the array and the longitudinal-radiation means having an approximately common phase centre, characterized in that the array of n radiating elements is excited by a waveguide (15) forming a cavity in the shape of a "slice of pineapple" of polygonal cross section.
2. Source according to Claim 1, characterized in that the array of n radiating elements is a circular array.
3. Source according to Claims 1 and 2, characterized in that the waveguide (15) has dimensions such that, D being the mean diameter of the circular array :

   D = nλ_g/2 where n represents the number of radiating elements and λ_g represents the wavelength of the guided wave at the operating frequency ;

   λ_g = λ₀[ε_r - (λ₀/λ_c)²]^-½ , where λ_c is the cut-off wavelength of the reactangular waveguide for the TE₀₁ fundamental mode, λ₀ is the wavelength in vacuo and ε_r is the permittivity of the dielectric filling the waveguide ; and

   λ_c = 2a(ε_r)^½, where a is the width of the rectangular waveguide.
4. Source according to Claim 3, characterized in that D is chosen such that: 1.3λ0 < D < 1.9λ0.
5. Source according to any one of Claims 1 to 4, characterized in that the waveguide is filled with a dielectric of permittivity ≥ 1.
6. Source according to any one of Claims 1 to 5, characterized in that the radiating elements of the travelling-wave type are helices (11).
7. Source according to any one of Claims 1 to 3, characterized in that the longitudinal-radiation means consist of a longitudinal-radiation dielectric rod or "polyrod" (12) whose axis is coincident with the radiation axis, the said rod being excited by means comprising a waveguide.
8. Source according to any one of Claims 1 to 3, characterized in that the longitudinal-radiation means consist of a device (12') in the form of a helix whose axis is coincident with the radiation axis, the said device being excited by means comprising a coaxial line.
9. Source according to either of Claims 7 and 8, characterized in that the longitudinal-radiation means are surrounded by a cavity (14) that reduces the side lobes.

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