EP2264832B1 - Secondary reflector for a double reflector antenna - Google Patents

Description

The present invention relates to radio frequency (RF) antennas with dual reflectors. These antennas comprise a concave primary reflector of large diameter and a secondary reflector ("sub-reflector" in English) convex of smaller diameter located near the focus of the primary reflector. These antennas operate indifferently in transmitter mode or in receiver mode, corresponding to two opposite directions of RF wave propagation. In what follows, the description is given either in transmission mode or in reception mode of the antenna, according to which allows to better illustrate the described phenomena. It should be noted that all the reasonings apply to the antennas as well in reception as in emission.

Double reflector antennas, especially those known as Cassegrain type, are used to produce compact systems. The double reflectors comprise a concave primary reflector, usually parabolic, and a convex secondary reflector of smaller diameter which is placed in the vicinity of the focus, on the same axis of revolution as the primary reflector. The primary reflector is pierced at its top and a waveguide is inserted along its axis. The end of the waveguide faces the secondary reflector. In transmission mode, the RF waves transmitted by the waveguide are reflected by the secondary reflector to the primary reflector.

The secondary reflector must be maintained in the vicinity of the focus of the primary reflector. One of the possible ways is to fix the secondary reflector at the end of the waveguide, then serving as a mechanical support. In this case, the secondary reflector usually comprises a dielectric body (frequently plastic) substantially conical and transparent to RF waves. The outer surface of substantially conical general shape of the secondary reflector faces the primary reflector. The convex inner surface of the secondary reflector is coated with a treatment for reflecting the RF waves in direction of the primary reflector through the dielectric body. This coating is usually made of metal. Multiple reflections of RF waves occur between the end of the waveguide and the primary reflector, involving the secondary reflector.

In order to reduce these reflections, it has been proposed to introduce local perturbations, in the form of reliefs, on the external surface of the secondary reflector facing the primary reflector. However, these reliefs have a lesser effect on overflow losses ("spillover" in English).

In the antenna transmission mode, for example, the overflow losses, expressed in dB, correspond to the energy reflected by the secondary reflector towards the primary reflector, and whose path ends beyond the outer diameter of the reflector. primary. These losses lead to pollution of the environment by RF waves. These overflow losses should be limited to levels defined by standards.

A first proposed solution is to attach to the periphery of the primary reflector a skirt which has the shape of a cylinder, of diameter close to that of the primary reflector and of suitable height, lined internally with a layer absorbing RF radiation. In addition to the resulting congestion, this known solution has the inconvenient today troublesome cost of the material of the skirt, as well as the cost of assembling this skirt on the primary reflector.

Another solution is to provide a secondary reflector whose outer surface has a profile according to a particular curve. However, this solution allows a low overflow loss value only if the secondary reflector has a large size. In addition, the reflection losses are reduced only for a narrow frequency band. Indeed, the waveguide produces a large beam of radiation with a low gain. In order to reduce overflow losses, the secondary reflector must therefore have a large diameter or be placed very close to the waveguide. However, in the latter case, the performance relating to the reflection loss is affected.

The document US 6724349 proposes to provide the inside of the waveguide transverse sections of different diameter and to provide on the convex surface of the secondary reflector faces perpendicular to its axis.

The object of the present invention is to propose a secondary reflector of a double reflector antenna whose overflow losses are significantly reduced, without presenting the drawbacks of the prior art, particularly in terms of size and cost.

The overflow losses for the transmission mode of an RF antenna correspond to values of the illumination angle (measured with respect to the axis of revolution of the secondary reflector) of the primary reflector by the secondary reflector for which RF waves from the waveguide are reflected by the secondary reflector in a direction which is outside the perimeter of the primary reflector.

The object of the present invention is a dual reflector antenna secondary reflector according to claim 1.

dans laquelle

• A est une constante numérique,
• a_w est le premier rayon de la première extrémité du réflecteur secondaire,
• p_o est le deuxième rayon de la deuxième extrémité du réflecteur secondaire.
• L est la hauteur totale du réflecteur secondaire,
• z est la hauteur considérée du réflecteur secondaire, et
• p_(z) est le rayon d'une section placée à la hauteur z du réflecteur secondaire.

Preferably the outer surface of the horn has a profile defined by the following relation: $$\rho \left(z\right)={\mathrm{at}}_{\omega }+\left({\rho }_0-{\mathrm{at}}_{\omega }\right)\left\{\frac{\left(1-\mathrm{AT}\right)\left(z+\mathrm{The}\right)}{\mathrm{The}}+\mathrm{AT}{\mathrm{<figure-callout\; id="2"\; label="sin"\; filenames="imgf0001.png,imgf0003.png"\; state="\{\{state\}\}">sin</figure-callout>}}^2\left(\frac{\pi \left[z+\mathrm{The}\right]}{2\mathrm{The}}\right)\right\}$$

in which

• A is a numerical constant,
• _w is the first radius of the first end of the secondary reflector,
• p _o is the second radius of the second end of the secondary reflector.
• L is the total height of the secondary reflector,
• z is the considered height of the secondary reflector, and
• p _(z) is the radius of a section placed at the height z of the secondary reflector.

According to a first variant of the constant A is equal to zero (A = 0). In this case, the outer surface of the horn has a substantially linear profile and the horn has substantially the shape of a truncated cone.

According to a second variant embodiment, the constant A is different from zero (A ≠ 0). In this case, the outer surface of the horn has a continuous profile of particular shape.

With such a profile, the horn has better efficiency, lower cross polarization, side lobes having a lower level and a frequency band of increased width.

According to another variant, the outer surface of the horn is metallized. It may for example be covered with a reflective metal, such as silver.

The invention also proposes a double reflector antenna comprising a secondary reflector as previously described.

The invention has the advantage of significantly reducing the losses by overflow and to have low losses by reflection.

• la figure 1 montre une coupe schématique d'un mode de réalisation d'un réflecteur secondaire selon l'invention.
• la figure 2 représente en coupe axiale schématique d'un réflecteur secondaire selon le mode de réalisation préféré de l'invention.
• la figure 3 est une vue en perspective coupée du réflecteur de la figure 2,
• la figure 4 montre en perspective le réflecteur de la figure 2.
• la figure 5 est une comparaison de la largeur du faisceau sortant d'un guide d'onde de l'art antérieur et celui sortant du cornet du réflecteur de la figure 2, le ? S en dB est donné en ordonnée, et en abscisse l'angle θ mesuré par rapport à l'axe de W-W' du réflecteur secondaire en degrès,
• la figure 6 montre la perte par réflexion du réflecteur de la figure 2, la perte par réflexion R en dB est donné en ordonnée, et en abscisse la fréquence v en GHz,
• la figure 7 reprèsente le diagramme de radiation du réflecteur de la figure 2, le gain G en dB est donné en ordonnée, et en abscisse l'angle θ en degrés.

Other characteristics and advantages of the present invention will appear on reading the following description of an embodiment, given of course by way of illustration and not limitation, and in the accompanying drawing in which:

• the figure 1 shows a schematic section of an embodiment of a secondary reflector according to the invention.
• the figure 2 is a schematic axial section of a secondary reflector according to the preferred embodiment of the invention.
• the figure 3 is a perspective cut view of the reflector of the figure 2 ,
• the figure 4 shows in perspective the reflector of the figure 2 .
• the figure 5 is a comparison of the width of the beam emerging from a waveguide of the prior art and that coming out of the horn of the reflector of the figure 2 , the ? S in dB is given in ordinate, and in abscissa the angle θ measured with respect to the axis of WW 'of the secondary reflector in degrees,
• the figure 6 shows the reflection loss of the reflector of the figure 2 , the reflection loss R in dB is given on the ordinate, and on the abscissa the frequency v in GHz,
• the figure 7 represents the radiation diagram of the reflector of the figure 2 , the gain G in dB is given in ordinate, and in abscissa the angle θ in degrees.

In the embodiment of the invention illustrated on the figure 1 , in axial section, is shown a secondary reflector 1 of a double reflector antenna connected to a waveguide 2 . The waveguide 2 is here a hollow metal tube, for example aluminum. Its diameter, about λ _g / 2.5 where λ _g is the wavelength of the guided RF signal, is chosen so as to allow propagation of the TE11 mode only, before the appearance of higher modes. The secondary reflector 1 comprises a dielectric body 3 whose end 4 of smaller diameter is connected to the waveguide 2 . The dielectric body 3 of the secondary reflector 1 is for example a dielectric material such as plastic. The opposite end 5 of the dielectric body 3 is dug to form a reflective inner concave surface 6 for the RF signal. The inner surface 6 of the secondary reflector 1 is for example a surface of revolution described by a polynomial equation around an axis of revolution W-W ' The inner surface 6 may be covered with a reflecting metal, such as silver. The outer surface 7 of the secondary reflector 1 is the surface placed facing the primary reflector. The outer surface 7 is a surface of revolution about the axis of revolution WW ".

The end 4 of the dielectric body 3 of the secondary reflector 1 is externally provided with a bulge, here in the shape of a truncated cone, forming a horn 8 ("horn" in English) whose outer surface 9 has a continuous profile. The outer surface 9 of the horn 8 is smooth and metallized to be reflective. The junction 10 of the secondary reflector 1 and the waveguide 2 is located at the smaller diameter end of the horn 8 . Part of the dielectric material of the dielectric body 3 extends to penetrate inside the waveguide 2, to ensure the mechanical maintenance and the radio-electric transition between the waveguide 2 and the secondary reflector 1 .

permettant des transitions douces entre un guide d'onde et la surface interne réfléchissante 23 du réflecteur secondaire 20, We will now consider the figure 2 illustrating an advantageous embodiment of a secondary reflector 20 , Although a horn of substantially conical shape solves the problem, better results can be obtained with a horn 21 whose outer surface 22 has a profile defined by the relationship : $$\rho \left(z\right)={\mathrm{at}}_{\omega }+\left({\rho }_0-{\mathrm{at}}_{\omega }\right)\left\{\frac{\left(1-\mathrm{AT}\right)\left(z+\mathrm{The}\right)}{\mathrm{The}}+\mathrm{<figure-callout\; id="2"\; label="AT\; \; sin"\; filenames="imgf0001.png,imgf0003.png"\; state="\{\{state\}\}">AT\; </figure-callout>}{\sin }^{}{}^2\left(\frac{\pi \left[z+\mathrm{The}\right]}{2\mathrm{The}}\right)\right\}$$

allowing smooth transitions between a waveguide and the reflecting inner surface 23 of the secondary reflector 20,

dans laquelle

• A est une constante numérique,
• a_w est le premier rayon de la première extrémité du réflecteur secondaire,
• ρ_o est le deuxième rayon de la deuxième extrémité du réflecteur secondaire,
• L est la hauteur totale du réflecteur secondaire,
• z est la hauteur considérée du réflecteur secondaire, et
• ρ_(z) est le rayon d'une section placée à la hauteur z du réflecteur secondaire.

The Figures 3a and 3b shows the comparison between a truncated cone-shaped horn 30 ( fig.3a ) and a horn 31 having a profile outer surface defined by the following relation ( Fig. 3b ): $$\rho \left(z\right)={\mathrm{at}}_{\omega }+\left({\rho }_0-{\mathrm{at}}_{\omega }\right)\left\{\frac{\left(1-\mathrm{AT}\right)\left(z+\mathrm{The}\right)}{\mathrm{The}}+\mathrm{<figure-callout\; id="2"\; label="AT\; \; sin"\; filenames="imgf0001.png,imgf0003.png"\; state="\{\{state\}\}">AT\; </figure-callout>}{\sin }^{}{}^2\left(\frac{\pi \left[z+\mathrm{The}\right]}{2\mathrm{The}}\right)\right\}$$

in which

• A is a numerical constant,
• _w is the first radius of the first end of the secondary reflector,
• ρ _o is the second radius of the second end of the secondary reflector,
• L is the total height of the secondary reflector,
• z is the considered height of the secondary reflector, and
• ρ _(z) is the radius of a section placed at the height z of the secondary reflector.

The horn 31 with such a profile has a better efficiency, a lower cross polarization, side lobes having a lower level and a frequency band of increased width.

On the Figures 4a and 4b , a secondary reflector 40 connected to a waveguide 41 is shown in perspective. The figure 4a is a partial section. The secondary reflector 40 comprises a bulge, forming a horn 42 having a continuous profile as described by the aforementioned equation, placed at its smaller diameter end cooperating with the waveguide 41.

The figure 5 is a comparison of the width of the beam emerging from a waveguide of the prior art and that of the horn having a profile as described by the equation previously mentioned placed at the end of smaller diameter of the secondary reflector. The amplitude of the signal S is given for a frequency of 7.8 GHz in the vertical plane (curves 50 and 51 ) and in the horizontal plane (curves 52 and 53 ) as a function of the opening angle θ of the waveguide. wave (curves 51 and 53) or the horn at its end of larger diameter (curves 50 and 52 ) respectively. When an antenna has a horn having a profile as described by the aforementioned equation, its diagram is more directive, with reduced overflow losses.

We see on the figure 6 the low reflection loss R 60 of the secondary reflector as a function of the frequency ν of the transmitted / transmitted PF signal.

FIG. 7 represents the radiation pattern of the secondary reflector respectively in the horizontal plane (curve 70 ) and in the vertical plane (curve 71 ). It can be seen, for an opening angle ρ around -120 ° and + 120 ° by relative to the axis of revolution WW 'of the reflector, areas 72 and 73 respectively in which the overflow losses are significantly improved over the antecuric art.

Claims (6)

1. Secondary reflector (1) for a double reflector antenna comprising:

   - a first end (4) having a first diameter,

   - a second end (5) having a second diameter longer than the first diameter,

   - a convex, reflective inner surface (6) positioned at the second end (3) having an axis of revolution (W-W'),

   - an outer surface (7), having the same axis of revolution as the inner surface (6), connecting the first end (4) to the second end (5),

   - a dielectric body (3) extending between the first and second ends (4, 5) and limited by the inner surface (6) and the outer surface (7),
   characterised in that the dielectric body (3) has, at the level of the first end (4), a bulge forming a horn (8) having an outer surface (9) with a continuous profile, the end of which having the shorter diameter is designed to be coupled to the end of a cylindrical waveguide (2), the outer surface (9) of which is reflective.
2. Secondary reflector (1) according to claim 1, wherein the horn (8) has a substantially truncated cone shape.
3. Secondary reflector (1) according to claim 1, wherein the profile of the outer surface (9) of the horn (8) is defined by the formula: $$p\left(z\right)=a_w+\left(p_o-a_w\right)\left\{\frac{\left(1-A\right)\left(z+L\right)}{L}+A{\sin }^2\left(\frac{\pi \left[2+L\right]}{2L}\right)\right\}$$

   

   
   where

   A is a digital constant,

   a_w is the first radius of the first end of the secondary reflector,

   ρ_o is the second radius of the second end of the secondary reflector,

   L is the total height of the secondary reflector,

   z is the considered height of the secondary reflector, and

   ρ_(z) is the radius of a cross-section positioned at the height z of the secondary reflector.
4. Secondary reflector (1) according to claim 3, wherein the constant A is different from zero.
5. Secondary reflector (1) according to one of the previous claims, wherein the outer surface (9) of the horn (8) is metallic.
6. Double reflector antenna comprising a secondary reflector (1) according to one of the previous claims.

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