EP0762529B1 - Iris polarizer for an antenna primary source - Google Patents

Description

The present invention relates to an iris polarizer for primary antenna source.

In many cases, we would like to have a antenna that can operate either in linear polarization or in polarization circular. This is particularly the case of the antennas used in monitoring the air traffic. As we know, the use of circular polarization allows in particular to overcome, to a large extent, disturbances due to rain. An essential element of the antenna is then the polarizer used in the primary source, which allows switching from a polarization to the other ensuring, in the case of circular polarization, a differential phase shift of substantially 90 ° between the components of the electric field.

Generally and in particular for large powers, the polarizer is of the iris type, that is to say that it has inside a waveguide section a succession of regularly reactive elements spaced. These reactive elements are often conductive strips symmetrically entering the waveguide section and located in transverse planes, that is to say perpendicular to the longitudinal axis.

Depending on the orientation of the electric field with respect to the irises, these can present a capacitive or inductive susceptance. When the electric field is at 45 ° relative to the irises, we can decompose it into two orthogonal components equal in amplitude. By planning the arrangement of the irises, their number and the dimensions of the guide, we can get a differential phase shift between these two components equal to 90 ° whence a wave with circular polarization at the output of the polarizer. Optimized differential phase shift on a frequency band can be obtained by defining very precisely the polarizer settings. Now some of these are very difficult to define by calculation, which results in successive realizations of polarizer to develop and optimize it so Experimental. This is of course long and costly. In addition, the processes being complex, it is difficult to keep the manufacturing tolerances during mass production resulting in degradation performs.

A polarizer of this type where screws are used in place of iris but where the same problems are found is described, for example, in US patent US 4,672,334. This patent describes a polarizer comprising for each frequency band, in a guide section of waves, a pair of rows of diametrically conductive screws opposed. The insertion of these screws can be adjusted to optimize only certain sizes of the polarizer.

An iris polarizer is described in the Soviet patent XP002003899 where no adjustment means after manufacture is not included.

An object of the invention is therefore a polarizer free of these disadvantages thanks to a simple possibility of adjustment and adaptation to a given frequency band.

According to the invention, there is provided an iris polarizer for source antenna primary of the type including a waveguide section longitudinal with a succession of regularly reactive elements spaced apart forming irises and acting as capacitive susceptances or inductive according to the direction of the linear polarization of the field electric inside said waveguide section, as defined in the revendications.

• la figure 1 est le schéma d'une source primaire d'antenne connue avec un polariseur à iris ;
• les figures 2a et 2b représentent des diagrammes explicatifs ;
• les figures 3 à 5 sont des courbes relatives à un polariseur à iris optimisé ;
• la figure 6 est un schéma en coupe longitudinale et transversale d'un polariseur selon l'invention ;
• les figures 7 à 9 sont des courbes relatives au polariseur de la figure 6 lorsque le guide d'ondes a une section trop faible ; et
• les figures 10 à 12 sont des courbes relatives au polariseur de la figure 6 lorsque le guide d'ondes a une section trop grande.

The invention will be better understood and other characteristics and advantages will appear from the following description and the accompanying drawings, in which:

• Figure 1 is the diagram of a known primary antenna source with an iris polarizer;
• Figures 2a and 2b show explanatory diagrams;
• Figures 3 to 5 are curves relating to an optimized iris polarizer;
• Figure 6 is a diagram in longitudinal and cross section of a polarizer according to the invention;
• Figures 7 to 9 are curves relating to the polarizer of Figure 6 when the waveguide has a too small section; and
• Figures 10 to 12 are curves relating to the polarizer of Figure 6 when the waveguide has too large a section.

Figure 1 schematically represents a primary source known antenna such as those mentioned above.

This source 1 includes a transition 2 to a section of waveguide 3 of a polarizer. The waveguide can be of section square, rectangular or circular. The simplest solution that has been shown here is the circular section. This waveguide section 3 comprises a succession of reactive elements 5 which are irises. We prefer often use irises although screw solutions or quartz blades by example can be used. However the iris solution is more efficient, better suited to large powers and above all requires fewer elements than the other reactive element solutions because an iris can have both capacitive and inductive susceptibility, hence a greater simplicity.

An iris may consist of two conductive flaps penetrating at inside the guide symmetrically to the longitudinal axis of the guide and arranged in a transverse plane perpendicular to this axis.

The waveguide section 3 can be rotated by a geared motor assembly 4 (hence the advantage of a circular guide) of so that it rotates 45 °. Section 3 is followed by a transformer 6 then a phase shifter 7 connected to a horn 8.

Figures 2a and 2b show the equivalent diagram of an iris 5 for a square or circular guide depending on the polarization direction of the field electric. The iris comprises two conducting flaps 51 and 52 in a plane transverse to guide 3 and entering the guide of a given height h.

In the case of FIG. 2a, the direction of the electric field (represented by an arrow) is perpendicular to the edges of the iris. In this case, susceptance B _VS is capacitive.

On the other hand, in the case of FIG. 2b where the electric field is parallel to the edges of the iris, the susceptance B _L is inductive.

If now guide section 3 is rotated 45 ° by compared to the electric field having the direction of figure 2a or 2b, one can consider the incident wave as decomposable into two components orthogonal of the same amplitude, one of which is parallel to the edges of the iris and the other perpendicular to these same edges. If the polarizer requires these two components a 90 ° differential phase shift, we obtain at output a wave with circular polarization.

To optimize the differential phase shift on a band of given operating frequencies, the dimensions of the guide (side d of a square guide for example) and sinking h of the irises. In general there is provision for a sinking law for the different irises which can be a law in cosine or according to a distribution of Tchebychev and which allows to minimize the TOS in the operating band.

Figure 3 shows the variations in capacitive susceptances (BC curve) and inductive (BL curve) of all the irises as a function of the frequency.

For the iris of rank n whose penetration is h _n , the capacitive susceptance is given by: B cn Y o = K 1 (h not , d). 1 λg (d, f) where B _cn is the capacitive susceptance, Yo the characteristic susceptance of the guide, λg the wavelength in the guide as a function of the dimensions of the guide and the frequency and K ₁ (h _n, d) a constant depending on the dimensions of the guide and iris depression. The capacitive susceptance is inversely proportional to the wavelength in the guide.

Similarly, the inductive susceptibility B _Ln of the iris is given by: B Ln Y o = K 2 (h not , d). λg (d, f)

This value is proportional to the wavelength in the guide.

The constants K ₁ and K ₂ are difficult to determine analytically.

However by approximation in the case of the square guide and by supposing that h is very small compared to d and λg, one can determine the differential phase shift of a simple iris by the approximate relation:

The first term depends on the size of the guide and the depth of the iris; the second term depends on the size of the guide and the frequency. We can generalize this relation and the differential phase shift introduced by the set of N iris is then written:

The variation of this phase shift with the frequency is represented in FIG. 4 and that of the ellipticity rate in decibels is represented in FIG. 5. To optimize the polarizer, it is necessary to minimize the ellipticity rate in the operating band (here between 2.6 and 3.6 GHz) on the one hand by equalizing the performance at the two extreme frequencies and on the other hand by adjusting the penetration of the irises so as to center the variation in the differential phase shift  _T over 90 °.

The first condition, applied to the second term of relation (1), leads to the determination of the dimension d of the guide: 2d 2 = λgmax.λgmin where λgmax and λgmin are the wavelengths in the guide for the extreme frequency values of the band.

When we therefore want to make a polarizer in a given frequency band, we must first choose the dimension d according to relation (2). But, as we have seen, this determination arising from that of the constants K ₁ and K ₂ is only approximate. Only a series of experimental achievements makes it possible to specify these factors, which, as already mentioned, increases the costs.

To remedy this, FIG. 6 represents the diagram of a polarizer according to the invention. This polarizer comprises a waveguide section (3), circular in the example described, of diameter d determined approximately. In this guide, irises 5 are placed whose sinking h _n is determined from d : the sinking h _n of the irises obeys a law which can be a Chebyshev distribution to obtain the best possible TOS in the band. The whole is adjusted to also obtain a differential phase shift substantially equal to 90 °.

In addition to the irises, there are adjustable screws arranged in pairs 30, 31 symmetrical about the axis of symmetry longitudinal 32, this in order to avoid the generation of higher modes. The longitudinal spacing between two adjacent pairs is substantially equal to (2n + 1) λg / 4 for the center frequency of the band functioning in order to obtain a good adaptation, this supposing that there are at least two pairs of screws.

Finally the screws for example inside the guide are arranged towards one end of the guide 3, where the irises are the least depressed, so to guarantee the best power handling. In the case of Figure 6, the screws have been arranged in the median plane P of the irises containing the axis 32, that is to say the plane of the figure for the longitudinal section. The traces of plane P and the orthogonal plane P 'passing through the longitudinal axis 32 have been shown on the cross section passing through the screws 30 of FIG. 6.

We can assume that, in a first case, the value d of the guide 3 is too low. Figures 7 to 9 illustrate this case.

In the absence of the screws 30, 31, the curves are obtained in solid lines. We immediately see that we end up with a shift of the operating band towards the high frequencies (cf. figure 9 curve e ).

By observing the curves BC and BL of figure 7 which represent variations in capacitive and inductive susceptances, we note that the differential phase shift in low frequencies is mainly due to the inductive effect of the irises which has a great dispersion. The addition of screw according to figure 6, i.e. in plane P, as represented on the right of figure 7, brings a capacitive effect which is added to the capacitive susceptibilities of the irises. We can then reduce the sinking of the iris which reduces the inductive effect and therefore the dispersion (see curves BC 'and BL 'in dotted line in Figure 7). By playing on the driving of the screws and iris, we obtain an optimized differential phase shift curve (curve Φ ' in Figure 8) and a re-centered operating frequency band (curve e 'in Figure 9).

In the same way, FIGS. 10 to 12 illustrate the case where the section of the waveguide 3 and therefore the dimension d are too large. The solid lines correspond to the results in the absence of screws. We end up here with a shift towards the low frequencies. Here, we observe (curves BC and BL of FIG. 10) a dispersion towards the high frequencies of the capacitive and inductive suceptances due to the irises and a lower slope for the inductive susceptance. To remedy this, provision is made for adjustable screws 30, 31 arranged in the plane P '(FIG. 6) as shown on the right in FIG. 10.

The dotted curves in Figures 10 to 12 reflect the results obtained. The action of the screws reduces the phase shift provided by the inductive susceptances, which increases the depression of the iris to increase the slope of the curve BL '(Figure 10). We can thus optimize the differential phase shift and re-center the operating band, like the shows the curve e 'in figure 12.

It is clear that the invention thus makes it possible to produce a polarizer for a given operating band without having to search experimentally by a succession of fabrications the dimensions of the guide.

Of course, the invention is in no way limited to the examples of realization described. In particular, it would be possible to replace the screws adjustable by other reactive elements, such as bars, having a essentially capacitive or essentially inductive susceptance. Of even, depending on the embodiment envisaged, it is possible to use different types of screws and in particular conventional screws if there are no related problems to strong powers.

Claims (7)

1. Iris polariser for an antenna feed of the type that includes a longitudinal waveguide section (3) comprising a series of regularly spaced reactive elements (5) which form irises and act as capacitive or inductive susceptances according to the direction of the linear polarisation of the electric field inside said waveguide section, characterised in that there is provision for disposing in said waveguide section, as a complement to said irises, adjustable elements (30, 31) exhibiting a susceptance that is essentially of a given type, capacitive or inductive, so as to recentre the operating frequency band of said polariser while keeping the differential phase shift of said polariser substantially equal to 90° within said frequency band, so as to minimise the axial ratio of the wave at the polariser's exit.
2. Polariser according to claim 1, characterised in that said adjustable elements are constituted by screws or bars having an adjustable depth of penetration into the waveguide section (3).
3. Polariser according to claim 2, wherein said reactive elements (5) are each constituted by two conducting flaps which symmetrically penetrate the interior of the waveguide section (3) by a given height (h_n) in a plane transverse to the longitudinal waveguide section, characterised in that said adjustable elements are constituted by at least two pairs of height-adjustable screws (30, 31), the screws of each pair being disposed symmetrically with respect to the longitudinal axis of symmetry (32) of the waveguide section (3) and the distance between the two pairs along said axis substantially equalling an uneven multiple of one-quarter the wavelength in the waveguide section at the central frequency of said operating frequency band.
4. Polariser according to claim 3, characterised in that the head of the screws (30, 31) inside the waveguide section is spherical in shape.
5. Polariser according to either of claims 3 and 4, characterised in that since the transverse dimension (d) of the waveguide section (3) in the median plane (P) of the irises containing said axis of symmetry (32) is smaller than the optimal dimension for the desired operating frequency band, said optimal dimension equalising the axial ratio obtained at the two ends of said frequency band, said screws (30, 31) are disposed in said median plane.
6. Polariser according to either of claims 3 and 4, characterised in that since the transverse dimension (d) of the waveguide section (3) in the median plane of the irises containing said axis of symmetry is bigger than the optimal dimension for the desired operating band, said optimal dimension equalising the axial ratio obtained at the two ends of said frequency band, said screws (30, 31) are disposed in the plane (P') containing said axis of symmetry and orthogonal to said median plane (P).
7. Polariser according to any one of claims 1 to 6, characterised in that said adjustable elements (30, 31) are disposed towards one of the ends of said waveguide section (3).

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