EP0041077B1 - Antenna-feeding system for a tracking antenna - Google Patents

Description

The invention relates to an antenna feed system for circularly polarized signals with an exciter whose aperture cross section is symmetrical to at least one main axis, and a device for coupling higher wave types as storage signals for tracking the antenna, the excitation of which is proportional to the deviation of the antenna main axes from a received circularly polarized beacon signal .

Future communications satellites will be required to illuminate a very specific area of the earth and, as little as possible, shine onto neighboring areas, especially when it comes to supplying neighboring countries with TV programs.

In order to prevent the radiation field emitted by a satellite antenna from migrating to neighboring areas, the alignment of the transmitting antenna must be stabilized. For example, from G. Mörz: Analysis and synthesis of electromagnetic wave fields in reflector antennas with the help of multi-type waveguides, Diss. D82, TH- Aachen (1978), p. 46 ff., A transmitting antenna known that works as a monopulse sensor. Here, in fact, the transmitting antenna also serves as a receiving antenna for a beacon signal which is emitted by a beacon station arranged in the center of the prescribed illumination area. Depending on the main axis deviation of the exciter of the satellite transmitter antenna from the received beacon signal, higher wave types are excited, which are coupled via a mode coupler located directly behind the exciter and used as storage signals. A linearly polarized signal is used as the beacon signal. In the following, however, an antenna feed system with a device for coupling higher wave types as storage signals for circularly polarized signals is to be specified, whereby the exciter can also have a shape symmetrical only to a main axis of the aperture surface in order to generate an elliptical radiation field on earth. As a further condition, it must be taken into account in the present system that the frequency of the received signal, which is composed of the beacon signal and a possibly additionally transmitted message signal, is very much higher than that of the transmitted signal (fg = 17.3 ./. 18, 1 GHz, f _s = 1 11.7. /. 12.5 GHz). Because of the condition fE »f _s , it is difficult to couple the higher wave types already in the exciter, since the path of the exciter can not be dimensioned so small that the higher wave types are forced to total reflection, which is a prerequisite for selective coupling of the higher wave types . Otherwise, a very complex and space-consuming coupling device is necessary. Such a coupling device is described for example in DE-B-26 08 092.

The invention is based on the object of creating an antenna feed system for circularly polarized signals with an exciter, the aperture cross section of which is symmetrical to at least one main axis, in the manner mentioned at the outset, the two filing signals suitable for antenna tracking and obtained with simple means after the multimode -Monopul principle generated, it has a high polarization purity of the transmitted message signals and affects the required minimum attenuation of the message signals as little as possible.

According to the invention, the object is achieved in that a polarization converter, which contains amplitude and phase compensation devices, is arranged between the exciter and the device for coupling higher wave types, that the coupling of the higher wave types in a polarization switch connected to the polarization converter for separating two orthogonally polarized signals happens, which has two message signal inputs and outputs assigned to the two polarization directions for coupling the basic wave types and two outputs for coupling two different fields, which are created by additive and subtractive superimposition of two higher wave types and which contain the storage information of the antenna from the desired direction in Cartesian orientation .

Appropriate embodiments of the above arrangement can be found in the subclaims.

The fact that, according to the invention, the coupling structure for coupling the higher wave types is not arranged in the exciter, but behind it, does not interfere with the excitation of the hybrid modes advantageously used by grooved exciters (see DE-C-26 16 125). They are preferably used because they best meet the high requirements with regard to area efficiency and freedom from cross-polarization, as well as the adjustment of the lobe shapes in the E and H sections of the radiation diagrams. A further advantage of this antenna feed system is the arrangement of the polarization converter between the exciter and the coupling structure. On the one hand, it does not interfere with the excitation of the hybrid modes and, on the other hand, it is possible to provide it with means for the interference effects of the exciter on the two storage signals and compensate for the polarization purity of the transmitted message signals.

Based on the embodiment shown in the drawing, the invention will now be explained in more detail. Show it

• Figur 2 das Blockschaltbild des Antennenspeisesystems,
• Figur 3a, b den Polarisationswandler,
• Figur 4 die Polarisationsweiche mit Modenkoppler und
• Figur 5a, b, c das realisierte Antennenspeisesystem in verschiedenen Ansichten.

Figure 1a, b, c the formation of independent Storage signals for rectangular and elliptical excitation aperture,

• FIG. 2 shows the block diagram of the antenna feed system,
• 3a, b the polarization converter,
• Figure 4 shows the polarization switch with mode coupler and
• Figure 5a, b, c, the realized antenna feed system in different views.

First, the creation of the independent filing signals in the antenna feed system will be explained; for one exciter with a rectangular and one with an elliptical aperture. Figure la shows the field types that are excited in the excitation horn with a rectangular or elliptical cross section and smooth wall. With the rectangular cross section, there are the two shaft types H ₁₁ and E ₁₁ and with the elliptical cross section, the shaft types H ₂₁ and E ₀₁ (based on the designation of the shaft types in the circular waveguide). Depending on the storage of the circularly polarized beacon signal B with respect to the main axes of the antenna, which is illuminated by the feed system, the wave types H ₁₁ and E ₁₁ or H ₂₁ and E ₀₁ are superposed in a certain way.

In the case of a non-rectangular cross section (eg elliptical) of the exciter horn throat, the necessary transition from the throat cross section to the cross section of the polarization switch converts the higher wave types containing the storage information into the corresponding wave types of the input waveguide of the polarization switch (e.g. into the H ₁₁ and E ₁₁ waves) . Ideally, as shown in FIG. 1b, when the beacon signal B is deposited Ax, the two wave types in the polarization switch are superimposed in phase opposition with mode couplers, resulting in a field in the x direction. When the beacon signal is deposited Ay, the two wave types are superposed in phase, which, as can be seen from FIG. 1, results in a resulting field in the y direction. Only when the two higher wave types are superposed in the phase direction described above are the coupled signals independent of one another in their storage information.

If, for example, the rectangular exciter horn has a groove structure, two wave types are no longer excited, which are superimposed on one another to obtain independent placement signals, but instead the hybrid shaft type HE ₂₁ is obtained with x placement and the HE ₁₂ hybrid shaft type with y placement, each with clear placement information. This case should not be dealt with in any more detail, however, since no major changes need to be made to the feed system. When a transition from a grooved exciter to a waveguide with a smooth edge is required, each type of hybrid shaft in turn breaks down into the types of shaft H ₁₁ and E ₁₁ described above.

FIG. 2 shows the block diagram of an antenna feed system for circularly polarized signals, the exciter 1 of which is symmetrical only with respect to a main axis of the aperture area, in this embodiment it is rectangular. A polarization converter 2, to which a polarization switch 3 with mode coupling is connected, is arranged behind the exciter with the interposition of a transition to cross-sectional adaptation. The transmission signal S is fed into the input a of the polarization switch 3. The storage signals Δ1 and Δ2 are present at the outputs b and c, which generally contain mixed storage information rather than unambiguous information. The mixing of the storage information is due to different transmission properties of the higher wave types in the waveguides, which has the consequence that the phase-correct superposition of the wave types and thus also the independence of the storage signals is lost. There is an interference that contributes to the coupling of the storage signals. given by the different propagation constants of the excitation horn for the two higher wave types.

The different phase rotations of the excitation horn in both its main planes have a disruptive effect on the circularly polarized message signals to be transmitted. The incoming circularly polarized field is elliptically distorted by the different phase rotations. Another interference may result from a different antenna gain in the two main planes of the horn. Here, too, the circular polarization deteriorates to an elliptical one. Gain and phase differences can also be caused by the reflector material of the antenna.

The polarization converter 2 located behind the exciter 1 in which these disturbances occur contains means for compensating for the described amplitude and phase errors. A specific embodiment of such a special polarization converter is described below. This polarization converter and the subsequent polarization switch 3 likewise effect coupling of the storage signals by differently influencing the H ₁₁ and E ₁₁ waves. But irrespective of the individual causes of coupling, the signals Δ1 and Δ2 at the outputs b and c of the polarization switch are decoupled again with a mode coupler by means of a downstream correction coupler 4, for example in the form of a directional coupler normally used. The unmixed storage signals Ax and Δy are then at the outputs of the correction coupler. The correction coupler can be dispensed with if the exciter fulfills certain phase conditions for the higher wave types. For example, the desired superposition of the higher wave types H ₂₁ and E ₀₁ , the then the decoupled storage signals Δx and Ay appear directly at the outputs of the polarization switch 3, are predetermined by the length of the excitation horn. It is therefore possible to generate a field configuration by means of a specific length specification of the excitation horn, which compensates for the interference from the exciter, polarization converter and polarization switch. The length of the horn must be selected so that the individual fields H ₂₁ and E ₀₁ to be overlaid cause a mutual phase angle of 0 ° or a multiple of 180 ° for the corresponding waves at the mode couplings. This phase relationship can also be set by specifying the length of the exciter horn throat, which does not necessarily have to have the same cross-sectional shape as the exciter aperture. For example, in the case of an exciter with a horn having an elliptical aperture, the horn throat has a circular cross section (see DE-A-29 39 562.8). The cross section of the horn throat must then be adapted to the cross section of the polarization transducer with a waveguide transition.

At the output d of the polarization switch 3 is the received signal E, which is broken down in a downstream crossover 5 into the reference signal 1: from the beacon signal and a possibly additionally transmitted message signal N. A control variable for tracking the antenna can be derived from the comparison between the reference signal Z and the storage signals Ax and Ay derived from the beacon signal.

In addition to the reference signal 1: and the message signal N at the gate d of the polarization switch 3, an interference signal S _{1 appears} , which is composed of undesired portions of the transmission signal S reflected in the exciter or on the antenna reflector b. This reflected interference signal S ,, which would deteriorate the polarization purity of the radiation field without special compensation measures, is separated by the crossover 5 from the received signal and received by an absorber 7.

Figures 3a, b show the implementation of a polarization converter with means for polarization conversion for amplitude and phase compensation. FIG. 3a shows the front view and FIG. 3b shows the longitudinal section A-A of the polarization converter. In the case of pathogens with identical propagation and radiation properties for the main orthogonal types, as is the case with pathogens that are symmetrical to two main axes of the aperture surface (e.g. round or square pathogens), the coupling means are adjusted in their combination in the polarization converter that a fed-in, linearly polarized wave at the output of the polarization converter is split into broadband into two orthogonal waves (Ex, Ey) with the same amplitude and 90 ° phase difference (3.01 dB coupling). These waves then form the components of a circularly polarized wave. Pathogens with unequal propagation and radiation properties for the main orthogonal types, namely those that are only symmetrical to a main axis of the aperture area (e.g. rectangular or elliptical), have identical propagation and radiation properties in the two main planes only if the radiation diagram of the first main type in the E level is identical to that of the second main type in the H level and vice versa (EH adjustment). In practice, this condition is usually not sufficiently fulfilled, so that the profit distinctions lead to an amplitude difference (Ex ≠ Ey), particularly in the main beam direction, which is equivalent to a degradation of the circular to an elliptical field. In the present exemplary embodiment, the means for polarization conversion and amplitude compensation consist of two bevels 8 and 9 with grooves 8 'and 9', which are arranged in two diagonally opposite corners of the square polarization converter, and one which engages in grooves 8 'and 9'. diagonal dielectric plate 10. The bevels have an inductive effect and the diagonal dielectric plate has a capacitive character. These two capacitive and inductive coupling means together have an almost frequency-independent coupling behavior. In practice, it can happen that the antenna-related gain difference is frequency-dependent, so that the amplitude compensation must also be made frequency-dependent. This can take place with an increase in the coupling with the frequency with the aid of a predominantly capacitive coupling and with a decrease with a predominantly inductive coupling. For a lower inductive coupling, a thicker or longer dielectric plate is used in connection with smaller bevels in the corners, whereas for a stronger inductive coupling a shorter or thinner plate is used in connection with enlarged bevels. If the frequency dependence is very strong, one of the two coupling means (bevels or dielectric plate) can also be omitted or the dielectric plate can be arranged on a diagonal other than the bevels. In order to reduce the self-reflection of the inductive and capacitive coupling means, the bevels 8, 9 and the plate 10 can be constructed in steps over the length (λ / 4 transformers).

The amplitude compensation is achieved by dimensioning the coupling means described above, which lie in diagonal planes, in such a way that an uneven splitting of a wave fed into the two main planes of the quadratic polarization converter is achieved. As a result, the output shaft is not circular, but elliptically polarized, the main axes of the polarization ellipse lying parallel to the central axes of the square output cross section of the polarization converter. The wave components Ex and Ey of the elliptical polarized wave are 90 ° out of phase with each other, but are no longer the same in amount. The amounts of the wave components Ex and Ey can thus be influenced in such a way that a difference in amount between Ex and Ey, caused, for example, by different antenna gains in the x and y planes, can be compensated for; ie the elliptically polarized output wave of the polarization converter in turn generates a circularly polarized field in the radiation field of the exciter in the main beam direction.

In addition to the amplitude compensation, a phase compensation is provided in the polarization converter, which compensates for phase shifts between Ex and Ey caused by a rectangular or elliptical exciter.

The phase compensation is provided by a further dielectric plate 11, which is arranged either horizontally or vertically, in front of the diagonally extending plate 10, depending on whether Ex is to be influenced in relation to Ey or Ey in relation to Ex in phase. Deviating from this, e.g. the phase correction can also be carried out with a rectangular waveguide section placed at the start of the excitation side in front of the square polarization converter, in which a side length is reduced compared to that of the polarization converter (not shown in the drawing). Both means - dielectric plate and rectangular waveguide section - can be used together to compensate for the frequency response of the phase error. Depending on the amount and direction of the frequency response, one or the other compensation means must predominate.

As a polarization switch with mode coupling, the switch described in DE-A-26 51 935 is used for the present application.

This polarization switch with mode coupling, which is shown in FIG. 3, begins with a square waveguide 12 in which the two orthogonally polarized waves of the H ₁₀ and H ₀₁ type exist. The polarization converter must be connected to this. In the square waveguide 12, two coupling windows 13 and 14 are arranged, which are embedded in the E position transversely to the square waveguide. The width of the coupling window is about half as large as the side length of the square waveguide cross section. The energy of the H ₁₀ shaft that is coupled out at the coupling windows is passed on via a rectangular hollow body 15, 16. Both rectangular waveguides 15 and 16 open into a waveguide branch (double-T branching) which, according to the designation in the block diagram in FIG. 2, forms the input a for the transmission signal S and a waveguide gate b for energy components of the higher wave types H ₁₁ and E ₁₁ . The signal coupled to the waveguide b has been designated 41 in FIG.

The coupling window 13 and 14 are each provided with an electrically conductive rod 17 and 18, which is inserted into the side walls of the square waveguide 12. They are a countermeasure to suppress the resonances of higher waveforms that usually occur due to the enlargement of the waveguide space at the level of the coupling window.

The signal of the H _oj type is passed through a separating structure 19 in the square _waveguide 12 to the output d, at which the received signal appears. The door structure 19 consists of a plate arranged between the upper and lower walls of the square waveguide, which, viewed in the direction of expansion, begins near the rear edges of the coupling window. Towards the front, the separating plate 19 is approximately circularly tapered on both sides and extends into a tip 20. This enables the H ₁₀ -type wave arriving from the square waveguide 12 to be deflected into the rectangular waveguides 15 and 16 with low resistance and reflection. The directional damping of the coupling arrangement for the H ₁₁ and E ₁₁ shaft can be influenced over the length of the tip 20. Their length is set to the highest directional damping.

At the end of the separating plate there is a further waveguide coupling c, also for energy components of the higher shaft types H ₁₁ and E ₁₁ . The signal coupled here has been designated 42 in FIG. 2, the block diagram of the entire antenna feed system. The waveguide outputs c and d, together with the partial waveguides formed by the separating structure 19, represent a folded double-T branch.

Finally, a possible structural design of the antenna feed system will be described with reference to FIGS. 5a, b and c. The names of the individual elements of the antenna feed system correspond to those of the block diagram in FIG. 2.

The polarization converter 2 with amplitude and phase compensation is connected to the exciter 1. This is followed by the polarization switch 3 with mode coupling with the input a for the transmission signal S, the outputs b and c for the storage signals Δ1 and Δ2, which are generally still coupled and which are broken down into the uncoupled storage signals Ax and Ay with the aid of the correction coupler 4, and the output d for the received signal E. The reference signal Σ is split off from the received signal with the crossover 5. At the gate d 'of the crossover 5 is the interference signal S ₁ and a possibly additional transmitted message signal N, which, which is not shown here, would still have to be separated from the interference signal via a further crossover. The interference signal S ₁ is finally fed to an absorber not included in the drawing.

The correction coupler 4 can only fulfill its function if its coupling damping is adapted to the coupling of the storage signals Δ1 and Δ2 and a defined phase relationship of 90 ° is set at its input. This phase relationship is e.g. by selecting the length of the waveguide leading from the waveguide output b to the correction coupler 4.

It should be noted that the components of the antenna feed system, such as polarization converters, polarization switches with mode couplings in their central waveguide section, can also be formed from a round waveguide.

The arrangement of the antenna feed system according to the invention naturally also works with a round exciter as the limit case of the elliptical exciter; in this case there is no need for amplitude and phase compensation in the polarization converter.

Further options for modification result from the wiring of the inputs and outputs for the message signals. For example, with the aid of additional switch circuits, a receive signal can also be obtained from the transmit input a or a transmit signal can be fed into the output N.

Claims (8)

1. Aerial feed system for circularly polarised signals with an exciter (1), the aperture cross-section of which is symmetrical to at least one main axis, and an equipment (3) for the coupling of higher wave types as deviation signals for the tracking of the aerial, the excitation of which wave types takes place proportionally to the deviation of the main aerial axes from a received circularly polarised beacon signal, characterised thereby, that a polarisation converter (2), which contains amplitude and phase compensating devices (8, 8', 9, 9', 10, 11), is arranged between the exciter (1) and the equipment (3) for the coupling of higher wave types and that the coupling-in of the higher wave types occurs in an in itself known polarisation filter (3), which is connected to the polarisation converter (2) and for the separation of two orthogonally polarised signals, wherein this filter (3) possesses two communication signal inputs or outputs (a, d) for the coupling of the fundamental wave types and two outputs for the coupling of two different fields which have arisen through additive and subtractive superimposition of two higher wave types and contain the deviation data of the aerial from the target direction in Cartesian orientation.

2. Aerial feed system according to claim 1, characterised thereby, that the polarisation converter (2) consists of a square wave guide, that it possesses a coupling attenuation differing from 3.01 dB for the amplitude compensation, wherein bevels (8, 9) in two diagonally opposite edges of the wave guide and a dielectric plate (10), engaging in grooves (8', 9') and being between two diagonally opposite corners, are arranged as coupling means and that a further dielectric plate (11) is arranged in the wave guide horizontally or vertically between the wave guide walls for the phase compensation.

3. Aerial feed system according to claim 1 or 2, characterised thereby, that a portion of rectangular cross-section is provided at the end at the exciter side in the polarisation converter (2) of otherwise square cross-section for the phase compensation.

4. Aerial feed system according to claim 1 or 2, characterised thereby, that the coupling means in the polarisation converter (2) are so dimensioned that they counteract the frequency dependence, caused by the exciter, of the gain difference and of the phase difference of the waves in both the main planes.

5. Aerial feed system according to claim 1, characterised thereby, that a frequency filter (5), which separates the reference signal (Σ) originating from the received beacon, an additional communications signal (N) and an interference signal (S), which comes into being through reflection of transmitted signal components at the aerial, one from the other is connected to the output (d) of the polarisation filter (3) and that the output of the frequency filter, at which the interference signal (S₁) appears, is terminated by an absorber (7).

6. Aerial feed system according to claim 1, characterised thereby, that a correction network for the coupling-out of the deviation signals is connected to the outputs (b, c) for the deviation signals (Δ 1, A2) of the polarisation filter (3).

7. Aerial feed system according to claim 6, characterised thereby, that a directional coupler is used as correction network.

8. Aerial feed system according to claim 1, characterised thereby, that the length of the exciter horn throat is so dimensioned that the wave guide wave types employed for obtaining the deviation data are so set in their phase position one relative to the other that the mutually independent deviation signals are present directly, without use of a correction network, at the outputs (b, c) of the polarisation filter (3).

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