EP0014692B1

EP0014692B1 - Mode coupler in an automatic angle tracking system

Info

Publication number: EP0014692B1
Application number: EP19800850014
Authority: EP
Inventors: Karl Erik Anders Molker; Barry Kenneth Watson; Per Bertil Ingvarsson
Original assignee: Telefonaktiebolaget LM Ericsson AB
Current assignee: Telefonaktiebolaget LM Ericsson AB
Priority date: 1979-02-07
Filing date: 1980-02-06
Publication date: 1983-11-30
Also published as: SE419906B; DE3065740D1; SE7901055L; EP0014692A2; EP0014692A3

Description

Field of the Invention

The present invention relates to a mode coupler according to the first part of claims 1, included in a microwave antenna automatic angle tracking system. The tracking system can be included in a satellite for telecommunication between one or several transmitting ground stations and a number of receiving ground stations, the tracking system keeping the antenna of the satellite directed towards a certain area of the ground by means of a so- called beacon signal from one of the transmitting ground stations.

Description of Prior Art

It is previously known that higher order modes arise in microwave antennas, the modes having odd character when the receiving radiation is obliquely incident on the antenna. The number of modes obtained is determined by the cut-off wavelength of the respective mode in the feeding wave guide and if this is greater than the actual wavelength. The strength of the odd modes in relation to the even basic mode for a certain antenna only depends on the angle deviation and is for a small amount approximately proportional to this deviation. By "mode coupler" is in the present application meant an arrangement which can separate the basic mode from the various higher order modes and deliver output signals proportional to the strength of the various higher modes.
A known tracking system with the property to detect deviation (the pointing error) between the reflector axis of a ground station and the direction of a target by separating the higher order modes from the basic mode is described in the U.S. patent 3.936.838. The mode coupler according to this patent shows two pairs of waveguide arms which are connected to a tapered portion of a main waveguide so that the arms in each pair are placed oppositely to each other and so that the two pairs are disposed in a plane perpendicular to each other. Hybrid couplers are connected to the output of the waveguide arms in order to obtain the asymmetrical mode signals from the main waveguide in phase and 90° out of phase. By means of the hybrid network the error signals can be created to determine the pointing error previously described. When the pointing error is zero only the basic mode is created in the main waveguide. Otherwise higher modes arise in the waveguide among which one, namely the TM_o1 is employed to form error signals together with the basic mode and which are allowed to control the regulating circuits in the antenna system of the ground station so that the pointing error will become zero. In the known mode coupler asymmetrical or difference mode waves are coupled through the side wall coupling apertures into a funnelshaped member. The asymmetrical modes are, for example the TE₂₁ + TM_o1 modes and the TE₁₂ mode in a circular wave guide. In this known mode coupler, however, the basic mode TE₁₂ in the main wave guide is not coupled out and no reference signals are extracted. To obtain good accuracy in this system a very good phase equality between the signal paths is required.
Another known system, operating according to the same principle and acknowledged in the first part of claim 1, is described in U.S. Patent 3.821.741. A first and a second higher mode, TM_o1 and TE_21' respectively and the basic mode TE₁₁ are employed to create the error signals. This known system contains a mode coupler, consisting of three wave guide sections, which together form the circular wave guide connected to the feeding horn of the system. Two of these wave guide sections are each provided with a pair of rectangular wave guides, which are connected to the main wave guide with a certain aperture to couple out the required higher modes to form difference signals. Together with the sum signal, extracted in another place in the system, the error signals are formed. The employment of two higher modes TM_o1 and TE₂₁ make it possible for the system to work with not fully circularly polarized signals and to deliver difference signals, insulated between themselves, for two perpendicular planes, which implies lower demands regarding the phase equality between the signal paths compared with previous system. However, the different coupling-out points from the circular wave guide of the two higher modes will imply difficulties in maintaining the phase equality between the difference and the sum signals at varying temperature or frequency. Phase differences between these signals will lead to varying sensitivity upon reception and particularly at off-boresight tracking to a direct pointing error.

Summary of the Invention

In a system of the above mentioned kind, error signals in two planes, for example the elevation and the azimuth planes, are to be created which are dependent only on the difference angles of the target in these planes but not on variations in the strength of the beacon signal (the fading) and the polarization (the depolarization) within certain limits. Such variations arise, for example, through the influence of the atmosphere on the wave propagation. The system should also be as insensitive as possible to changes in the characteristics of the components which inevitably arise, for example in time or at varying temperature. It is of importance partly to employ appropriate wave guide modes, partly to couple out these in appropriate manner to create the sum- and the difference signals and partly to let the signals be conducted the same or possibly equal paths from the antenna to the receiver. Otherwise, varying sensitivity at reception or eventual pointing error is obtained. Particularly at off-boresight tracking, also pointing errors are obtained as a consequence of the sensitivity variation.
According to the present anvention, two higher modes TM_o1 and TE₂₁ and the basic modes TE_11V and TE_11H in a smooth wave guide, or E₀₂ and HE₂₁ and HE_11Vand HE_11H in a corrugated wave guide are employed and all the employed modes are coupled out in the same section of the circular main wave guide, whereupon from these modes, two sum and two difference signals are formed and guided through a common receiver channel, whereby the difficulties in keeping the signals equal in phase are avoided. In the receiver, the error signals are produced in the two planes by employing the sum and difference signals belonging to the respective planes. The object of the present invention is thus to provide a mode coupler, which conducts two higher modes and two basic modes in order to create two difference signals (for example denominated Δx and Δy) and two sum signals (for example Ex and Σy), the obtained modes and the sum-difference-signals travelling an equal path through the tracking system and, in addition, in pairs (Σx, Δx and Σy, Δy) showing the same influence from the depolarization of the beacon signal. The invention is then characterized as appears from the characterizing part of claim 1.

Brief Description of the Drawings

The invention will be more fully described below with reference to the accompanying drawings, where

Fig. 1 shows a frequency diagram which indicates the position of the transmitter channel, the receiver channel and the beacon frequency of the tracking system in which the mode coupler according to the invention is included.
Fig. 2 shows a simplified field pattern of the two higher modes and the basic mode in the main wave guide included in the tracking system.
Figs. 3a-3f illustrate schematically how the error signals in the tracking system are formed from the two higher modes, shown in Fig. 2.
Fig. 4 shows a perspective view of the mode coupler according to the invention.
Fig. 5 shows a side view of the part of the mode coupler according to Fig. 4 which filters away the two higher modes.
Fig. 6 shows more fully a rectangular wave guide arm which is included in the mode coupler according to Fig. 4.
Fig. 7 shows schematically a wave guide network which illustrates the creation of a reference signal and error signals. Preferred Embodiments

Before the description of the mode coupler according to the present invention, the known principle for forming the error signals when tracking by means of two higher wave guide modes will be more fully illustrated. The tracking system, in which the proposed mode coupler is included, represents a part of the telecommunication equipment of a satellite, which receives, treats and transmits the signals received from the ground station to a number of other ground stations. The transmission is carried out by RF-signals and for that purpose two frequency bands are selected, which are schematically shown in Fig. 1. The band f₁―f₂ (for example 400 MHz) with the center frequency f_T forms the transmitter channel for a certain ground station and the band f₃―f₄ with the center frequency f_R forms the receiver channel for the station. The transmission of the transmitter and the receiver channels is carried out within the GHz-band and by means of a reflector antenna, with a horn antenna as feeder. For transmission of the direction information, there is a beacon frequency f_b, for example 12.498 GHz. The two bands f₁―f₂ and f₃―f₄ can either be situated below the beacon frequency f_b, as shown in Fig. 1, or above this frequency. In certain cases only one band is used (an antenna for transmitting from or receiption to a satellite only).
In Fig. 2 the two higher modes are shown in more detail. These arise in the main wave guide of the tracking system, the guide being connected to the antenna. The modes are employed to indicate any deviation between the directions of the reference signal of the antenna and the target direction.
Moreover the two basic modes TE_11V, and TE_11H (smooth-walled wave guide; HE_11V and HE_11H for corrugated wave guide) are shown. These are employed for transmitting the telecommunication signals and for normalization of the difference signals. The principle of the tracking is that the two modes TE₂₁ and TM₀₁ in a smooth-walled wave guide (or corresponding modes HE₂₁ and E₀₂ in a corrugated wave guide), according to Fig. 2, which have the same amplitude and phase in the y-direction, and opposite phase but the same amplitude in the x-direction are added to form the error signals Δx and Δy. The principle is generally shown in the figures 3a-3f. The figures 3a-3c show the addition of the two modes, a signal representing the error in the y-direction (the elevational error) being obtained, while the figures 3d-3f show subtraction of the two modes, a signal representing the error in the x-direction (the azimuthal error) being obtained.
The structure of the mode coupler in the tracking system appears from Fig. 4. It consists of a circular wave guide 1, which is connected to the feeding horn, not shown, in the tracking system, and to a polarization unit, not shown. The main wave guide 1 can either constitute an integral part of the feeding horn or consist of a separate part connected to the feeding horn. The wave guide walls can either be formed by smoothed or corrugated surfaces. Four rectangular wave guide arms 2a-2d are provided to the circular main wave guide 1, which each are coupled to the wave guide 1 by the apertures 3a-3d. The apertures 3a-3d are arranged relatively displaced 90° around the outer circumference of the wave guide 1. In Fig. 4, the front part 11 of the wave guide 1 is shown in section in order to make the positions of the apertures clear.
Characteristic for the proposed mode coupler is that the rectangular wave guide arms 2a-2d are connected to the main wave guide 1 in the same section. The front part 11 constitutes the feeding horn or is connected to the horn in known manner, this part of the circularly polarized wave guide field in the TE₁₁-mode passing the two communication channels f₁―f₂, f₃―f₄ and the beacon frequency signal f_b according to Fig. 1. According to known principles, the two higher modes TE₂₁ and TM₀₁, arising in the main wave guide at a certain angle deviation of the incoming signals to the tracking system are employed to form the error signals Δx and Δy. The coupling of these higher modes is carried out by exciting the TE₁₀-mode in the wave guide arms 2a―2d, from the field in the main wave guide 1 by coupling openings in the form of the apertures 3a-3d. The apertures 3a-3d are preferably of rectangular form and their dimension is chosen in such a way that a sufficient coupling of the TE₁₀-mode is obtained from the wave guide 1 to the rectangular wave guide arms 2a-2d. The inner sectional area of these will, however, be dimensioned so that the TE₁₀-mode propagates in the wave guides 2a-2d at the frequency f_b in known manner. The amplitude and the phase of the TE₁₀-mode occurring in the rectangular wave guides 2a-2d, is then determined by the amplitude and the phase position of the higher modes TE₂₁, TM₀₁ and the basic mode TE₁₁, occurring in the main wave guide 1, all the modes giving a certain contribution to the field in the rectangular wave guides 2a-2d.
Fig. 5 shows more in detail the circular wave guide 1 of the mode coupler in which, for the sake of clarity, the rectangular wave guide arms 2a-2d are omitted. The rear part 12 of the wave guide 1 shows a tapered part 13, containing two sections I and II. The section I is situated a distance d₁ from the section containing the wave guides 2a-2d, the distance d₁ being chosen for a certain value of the radius R, so that a standing wave for the TE₂₁-mode is created in the wave guide part, limited by the distance d₁ and so that the maximum value of the mode coincides at the apertures 3a-3d or close to these. The section II is situated a distance d₂ from the mentioned sections, the distance d₂ being chosen so that a standing wave for the TM₀₁-mode is created in the wave guide over this distance d₂ thus causing a maximum at the apertures 3a-3d or close to these. The distances d₁ and d₂ then form an even (but not necessarily the same) number of quarter-wavelengths for the modes TE₂₁ and TM₀₁, respectively. The taped part 13 of the circular wave guide forms a mode filter for filtering away the non-desired modes TE₂₁ and TM₀₁, which are reflected back to the apertures 3a-3d at the short circuit planes formed by the sections I and II and, according to the above have a maximum value at the apertures. The mode filter 13, however, does not influence the TE₁₁-mode, which passes the telecommunication signals within the frequency bands f₁―f₂, f₃―f₄, which thus appear across the outlet of the mode coupler. The tapered part 13 has a non-linear contour, which can be determined by experimental measurements.
As previously mentioned, the coupling out from the wave guide field in the circular main wave guide 1 is carried out by means of the four apertures 3a-3d and the TE₁₀-mode is excited in the four wave guide arms 2a-2d. In Fig. 6 such a wave guide arm is shown more in detail. It consists of a broader part, connected to the main wave guide, and has the sectional dimensions indicated by a and b in Fig. 6. The dimension b₁ is chosen in such a way that all frequencies of the field in the wave guide 2a from f₁ up to f_b can be propaged in the wave guide as a TE₁₀-mode.
The wave guide 2a has a tapered part which is principally limited by the sections III and IV. At the section III, on the distance d₃ from the aperture 3a, the dimension b₂ of the wave guide is such that the center frequency f_T of the band f₁―f₂ is reflected, i.e. b₂ is such that the section III forms a short-circuit plane for the TE₁₀-mode at the frequency f_T. According to the same principle, the distance d₄ for the section IV should be chosen in such a way that the dimension b₃ is such that the center frequency f_R of the band f₃―f₄ is reflected. The wave guide sections III and IV thus define a frequency filter for filtering away the telecommunication signals, the distances d₃ and d₄ forming a number (not necessarily the same) of half wave lengths for the TE₁₀-mode at the center frequencies f_T and f_R, i.e. the sections III and IV form the short circuit planes, transformed to the apertures 3a-3d, for the frequencies f_R and f_T respectively. Thus, from the wave guide 2a and the remaining wave guides 2b-2d, only the signals corresponding to the TE₁₁-mode which has the frequency f_b are obtained and, furthermore, the signals corresponding to the two modes TE₂₁ and TM_o1 at the frequency f_b in the main wave guide 1.
In Fig. 7 a wave guide network is schematically shown, consisting of so called magic-T's, which are connected to the four rectangular wave guide arms 2a-2d for recovering the error signals Δx, Δy and the reference signals Σx and Σy. Each connection point m₁―m₄ represents a magic T with two inputs 1 and 2 and a sum- and a difference output s and Δ, respectively. From Fig. 7 the signals, appearing on the outlets of the four wave guide arms 2a-2d, are shown, which are derived from the four modes TE₂₁, TM₀₁ and TE₁₁H, TE_11V, appearing in the main wave guide 1, cf. Fig. 2. From these the error signals Δx and Δy are recovered from the modes TE₂₁ and TM₀₁, while the reference signals Σx and Σy are recovered from the TE_11H- and TE_11V-modes respectively. Fig. 7 shows that the signals Δx and Δy are separated in three different stages. In the first stage the signals, derived from the TE₂₁- and TM₀₁-modes in the connection points m1 and m2, are separated. In the second stage, a separation of the two modes TE₂₁ and TM₀₁ is carried out in the connection point m3 and in the third stage the desired error signals Δx and Δy are formed by vectorial addition in the connection points m4 as shown in Fig. 3. Between m3 and m4, phase- and amplitude adjustments can be introduced (trimming and temperature compensation). In some cases this is not necessary, the connection points m3 and m4 being omitted. Instead, correct phase- and amplitude properties can be obtained by accurate choice of the distances d₁ and d₂ in Fig. 5, of the difference in phase propagation between the TE₂, and TM_oi-modes when propagating from the orifice of the horn, and of the dimensions of the apertures 3a-3d in Fig. 5.

Claims

1. A mode coupler in an automatic angle tracking system in a telecommunication system for transmitting one or more communication frequency bands (f₁―f₂, f₃―f₄) and a certain beacon frequency (f_b), including a microwave antenna in which a basic mode and two modes of higher order are produced when the radiation received by the antenna is incident in a plane which does not coincide with the plane perpendicular to the antenna reference axis, said modes of higher order being used to create a first error signal (Δx) by subtracting the two higher order modes and a second error signal (Δy) by adding said modes, the error signals representing a measure of the deviation between said planes, a smooth or corrugated circular main wave guide (1) which forms part of or is connected to the feeding horn of said antenna and in which said basic mode (TE_11V, TE_11H or HE_11V, HE_11H) and said higher order modes TE₂₁ TM₀₁ or HE₂₁, E₀₂) propagate and which shows four apertures (3a-3d) arranged in a plane perpendicular to the wave guide axis, the apertures (3a-3d), preferably being of rectangular cross-section, four wave guide arms (2a-2d) of rectangular cross-section arranged circumferentially and symmetrically around the main wave guide in the plane perpendicular to the wave guide axis, and a wave guide network including a first and a second hybrid network (m₁ and m₂ respectively) each having two inputs and a sum and a difference output, the two inputs being connected to the exit ports of two opposite wave guide arms (2a, 2c and 2b, 2d respectively), characterized in that said apertures (3a-3d) are arranged in a plane perpendicular to the wave guide axis and are dimensioned so that a certain wave guide mode (TE₀₁) is created whose amplitude and phase is determined by the amplitude and phase of the basic mode (TE₁₁ or HE₁₁) in the main guide (1) and the two higher order modes (TE₂₁, TM₀₁ or HE₂₁, E₀₂), and that reference signals (Σx, Σy) associated with said error signals are created from the basic mode, said pair of reference signals (Σx, Σy) being formed across the difference outputs (Δ), and said pair or error signals (Δx, Δy) being formed from the sum outputs which originate from said higher order modes, both the two reference and the error signals being formed from the basic modes (TE_11V, TE_11H or HE_11V, HE_11H) and from the higher order modes (TM₀₁, TE₂₁ or HE₂₁, E₀₂) extracted from said wave guide arms.

2. A mode coupler according to claim 1, characterized in that a part (12, 13, 14, 15) of the circular main wave guide, which is situated between the wave guide arms (2a-2d) and its exit port, is designed as a mode filter for independent reflection of each of the higher order modes (TE₂₁, TM₀₁ or HE₂₁, E₀₂), while the basic modes (TE₁₁ or HE₁₁) in communication bands pass through.

3. A mode coupler according to claim 2, characterized in that the mode filter is a tapered portion (13) of the circular main portion (12) and the end portion (15) with a smaller inner radius, the tapered portion having two sections (I, II) constituting shortcircuit planes for each of the two higher order modes (TE₂₁ TM₀₁or HE₂₁, E₀₂) whose distances (d₁ and d₂, respectively) to said plane perpendicular to the wave guide axis are chosen to give the desired coupling of the higher modes into the apertures (3a-3d).