GB2040587A

GB2040587A - Improvements in or relating to circuit arrangements for detecting sighting errors in aerials for a telecommunication systems

Info

Publication number: GB2040587A
Application number: GB7940950A
Authority: GB
Original assignee: Societa Italiana Telecomunicazioni Siemens SpA
Current assignee: Italtel SpA
Priority date: 1978-11-27
Filing date: 1979-11-27
Publication date: 1980-08-28
Also published as: BR7907537A; IT1160267B; JPS5577206A; FR2442518A1; GB2040587B; DE2947762A1; IT7830211A0

Abstract

A circuit arrangement is provided for detecting sighting errors in an aerial for a telecommunication system such as an earth-satellite system. First and second differential phase shifters DP1, DP2 are in series in a wave guide G, G1 between an aerial horn H and a receiving apparatus and each produces a differential delay of 90 DEG between orthogonally polarised fields. OMT extracts modes polarized in two directions at right angles to each other. Coupling means AD withdraws from the wave guide G two higher energized modes from aerial signal, and OMT1 injects the higher modes into a further wave guide G2 according to two orthogonal planes. G2 is connected via a third differential phase shifter DP, which causes a differential delay of 180 DEG , to OMT2 which extracts modes polarized according to two directions at right angles to each other and rotated through 45 DEG with respect to those of OMT1. Each output of OMT2 is connected to a coherent detector of the receiving apparatus to which one of the output signals of OMT is sent as a reference signal. The sighting error signals are used for control of the aerial orientation. <IMAGE>

Description

SPECIFICATION Improvements in or relating to circuit arrangements for detecting sighting errors in aerials for a telecommunication system The present invention reiates to a circuit arrangement which makes it possible to obtain from a control signal a sighting error in an aerial for a very high frequency telecommunication system, for instance in a receiving and transmitting aerial for an earth station of a satellite telecommunication system.

For such telecommunication systems which are increasingly used, signals of very high frequency of the order of GHz are usually employed (at present the band ranging from 4 to 6 GHz is used but experiments are being made to assess the possibility of using 1 2 to 14 GHz and even higher frequencies). Such signals have the advantage of allowing highly directive aerials to be used which are required to send, to the aerial of a target (satellite or a station on earth) located tens of thousands kilometers away, a fraction of the emitted energy large enough to be picked up and detected by the receiving circuits of the target.

High directivity of the aerials and the large distances involved require as perfect as possible an alignment between a satellite and an aerial in a station on the earth.

To this end the satellite generates a control signal that is received by the aerial and processed so as to obtain error signals which are suitably amplified and designed to control motors arranged to rotate the aerial about a pair of axes, i.e. usually a vertical and a horizontal axis.

According to the invention, there is provided a circuit arrangement for detecting sighting errors in an aerial for a telecommunication system, comprising first and second differential phase shifters arranged in series on a wave guide path connecting a horn of the aerial to a receiving apparatus assembly, the first and second phase shifters being arranged to cause a differential delay of 900, first means for extracting modes polarized in two directions at right angles to each other, coupling means disposed between the horn and the first differential phase shifter and arranged to withdraw from the same section of the wave guide path two higher energized modes of the signal picked up by the aerial and to send them to insertion means for injecting the higher modes into a section of a further wave guide according to two orthogonal planes, the said section being connected by way of a third differential phase shifter arranged to cause a differential delay of 1800 to second means for extracting modes polarized according to two directions at right angles to each other and rotated through 450 with respect to those of the insertion means (OMT1), each output of the second means being connected to a coherent detector belonging to the receiving apparatus to which one of the output signals of the first means is sent as a reference signal.

It is thus possible to provide a structurally simple and reliable circuit arrangement for telecommunication systems effecting both linear polarization independently of the polarization angle, and right-hand or left-hand circular polarization, and in which no further line-up is required when the transmitting system is switched from linear to circular polarization and vice versa.

The invention will be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a partial block diagram of a receiving station including a circuit arrangement constituting a preferred embodiment of the invention; Figure 2 illustrates a receiving aerial and the components of the electric field associated with the received electromagnetic wave; Figures 3 and 4 show the modes energized by the electric field of Figure 2; Figure 5 shows a linearly polarized electromagnetic field; Figures 6, 7 and 8 show the positions of three differential phase shifters DP1 DP2, DP respectively for a linear, right-hand circular and left-hand circular polarization; Figures 9 and 10 respectively illustrate the directions of the outputs of first and second extraction means (OMT, OMT2);; Figures 11 and 12 illustrate the calculation of the fields at the outputs of the first and second extraction means in the case of linear polarization; and Figures 13 and 14 illustrate the calculation of the fields available at the outputs of the first and second extraction means in the case of circular polarization.

Figure 1 shows a block diagram of an illuminator for a telecommunication aerial having an automatic sighting device.

With reference to Figure 2, the case when the aerial is correctly sighted is considered. If z is the aerial axis and x and y two axes perpendicular to each other and to the z axis, the axes x and y extend parallei to the opening plane of the aerial. Should the aerial have a sighting error, the position z' taken by its axis can be described by two angular errors Ex and EV respectively lying in planes xz and VZ Ex and Ey are the components extending parallel to the axes x and y of the electric field associated with an electromagnetic wave received by the aerial. The components of the illuminator of Figure 1 are described below.

The energy received by the aerial is collected by a horn H and applied to a circular cross-section cylindrical wave guide G. The fields Ex and Ey of Figure 2 energize the guide G according to infinite modes, the diameter of the guide G being such as to permit propagation of the modes TE", TM01, TE21.

Figure 3 shows the distributions of the electric field occurring in the section 1-1 of Figure 1 with the modes TE11 and TE21. The two distributionsTE'117 and TE"11 are respectively energized by the fields Ey and Ex of Figure 2 in any sighting condition, the two distributions TE'21 and TE"2, being also energized by Ev and Ex when the aerial has a sighting error.

A mode coupler C' extracts the energy associated with the mode TE'2, from the guide G and sends it by way of an attenuator A' and a phase shifter P' to a wave guide R,. A mode coupler C't extracts the energy associated with the mode TE"21 from the guide G and sends it by way of an attenuator A" and a phase shifter P" to a wave guide R"1.

A circular cross-section cylindrical wave guide G1 permits only the propagation of the modes TE'11 and TE"11z which respectively extend in directions y# and x# of Figure 3 at right angles with one another. The mode TM01 is thus totally reflected and reradiated again by the horn H together with all the modes which cannot be propagated in the guide G.

An orthogonal mode transducer OMT, transfers the energy passing through the guides R'1 and R", to a circular cross-section cylindrical wave guide G2 which permits the propagation of the mode TE" only. The energy associated with the modes TE'2, and TE"2, is thus transferred to the guide G2 so as to be propagated in it according to respective modes TE'11 A and TE"11 A perpendicular to one another and directed along the directions xi, and Yea of Figure 4.

The directions x# Y# x#, y# defined above are taken as a reference to define the angular position of the components to be described of Figure 1.

With a given field distribution, it is possible to define a biunivocal correspondence between this distribution and a vector. According to this correspondence the following table indicates on the left side the field distributions in question, in the middle the amplitudes of their respective vectors, and on the right side their respective directions in the space: TE'11# Y1::=HEy V (1) TEX, X, = HE, (2) TE'11# Y#=K(Ey#y - Ex#x) y# (3) TE"11# X#=K(Ey#x+Ex#y) XA (4) In Figure 1 the reference sections are 2-2 for the first two vectors and 5-5 for the second two vectors. The above relationships are known and easily demonstratable and apply for small sighting errors of the aerial, H and K being complex constants.

In particular, the vectors XA and have the same constant K only when the modes TE21 are taken from the same section of the guide G by means of coupling means AD. By using, as in the embodiment shown in Figure 1, two adjacent mode couplers (C' and C") equality of the constants K in the relationships (3) and (4) can be obtained by means of the attenuator A' and A" and the phase shifters P' and P". These four components, however, are not indispensable and could be totally or partly eliminated.

A differential phase shifter DP, causes a time delay of 90 in a field polarized parallely to a direction a1-a1 with respect to a field polarized parallely to the orthogonal direction b 1-b1; a,--a, and b1-b1 are termed below the said differential phase shifting directions.

A differential phase shifter DP2 causes a phase shifting of 900 in time delay with respect to its phase-shifting directions a2-a2 and b2-b2. A differential phase shifter DP causes a phase shifting of 1 800 in time delay with respect to its differential phase-shifting directions a a and bb.

The three above described differential phase shifters can rotate about their own axis by means of rotatable couplings, not shown in Fig. 1. Their angular position depends on the type of polarization of the field received by the aerial.

Three possible cases will be considered, while phase shiftings (which are equal to each other for the two polarizations) caused by the wave guide sections are not taken into consideration as they have no effect on the final results.

Linear polarization - If the field received by the aerial has an amplitude E and is inclined by an angle cr with respect to the axis y of Fig. 2 (see Fig. 5), the angles formed with the reference direction and the differential phase-shifting directions of the phase shifters are indicated in Figs. 6a, 6b, 6c.

This means that the differential phase shifters DP, and DP2 have been preliminarily orientated in the directions x and y, whereas the phase shifter DP has been rotated through 22.50 with respect to the transducer OMT,. The three differential phase shifters are then simultaneously rotated through an angle equal to half the polarization angle by means of circuits designed to detect the said polarization angle and belonging to receiving apparatus RIC; such circuits are not shown in the drawings as they are of a conventional type.

Circular polarization - The angles formed between the reference directions and the differential phase displacement or shifting directions of the phase shifters are shown in Fig. 7 for right-hand polarization, in Fig. 8 for left-hand polarization. The positions of the phase shifters DP, and DP2 are normally used in the telecommunication systems in which circular polarization is adopted, the phase shifter DP being rotated through 450 with respect to the transducer OMT,.

Still with reference to Fig. 1, OMT is an orthogonal mode transducer which sends to wave guides R' and R" the energy propogated in the wave guide G,, the two modes TE" being polarized in respective directions r, and r" as indicated in Fig. 9 which shows that the direction r' is parallel to Grand the direction r" is parallel to x.

Finally, OMT2 is a transducer of orthogonal modes which sends to wave guides R'2 and R"2 the energy propagated in the wave guide G2 according to two modes TE1, respectively polarized in the directions r'2 and r"2 indicated in Figure 10.

As shown in the drawings, the direction r'2 is rotated through 450 with respect to VA and the direction r"2 is rotated through -45 with respect to y#.

It will be demonstrated that the signals available in the wave guides R', R'2, R"2 have characteristics such as to permit the aerial to be automatically sighted when the polarization of the received signal is of the following type: linear, right-hand circular, left-hand circular. When the polarization is changed from one of these types to another, it is sufficient to modify the angular position of the three differential phase shifters, no further re-alignment being required either for the illuminator or for a self-collimating receiver connected to the wave guides R', R'2, R"2.

Linear polarization -- Should this polarization have the characteristics shown in the drawings, one obtains: Ey = E cos a (5) Ex = E sin a (6) With reference to Figure 11 and by writing the relationships (5) and (6) in (1) and (2) in the section 2-2 of Figure 1, one obtains: Y= HE cos a (7) X= HEsin a (8) By projecting along the directions y, and x, in the section 2-2 of Figure 1, one obtains::

After passing through the differential phase shifters DP, and DP2, in the section 4-4 of Figure 1 one obtains

By projecting along the directions y, and x, still in the section 5-5 of Figure 1, one obtains:

With reference to Figure 12, by substituting the relationships (5) and (6) in the relationships (3) and (4) in the section 5-5 of Figure 1, one obtains: Y#=KE(#y cos α-#x sin α) (11) X#=KE (#x cos α + #y sinα) (12) By projecting along the directions y,X, and x,, still in the section 5-5 of Figure 1, one obtains:

After passing through the differential phase shifter DP, in the section 6-6 of Figure 1 one obtains:

In the same section S6, by projecting along the directions r'2 and r"2 and bearing in mind the relationships (11) and (12), one obtains

i.e.

r'2 = KEEy (13) r''2 = KE#x (14) Right-handcircular polarization -- Thefield received by the aerial can be indicated as follows:

Bearing in mind the angular position of the differential phase shifters as indicated in Figure 7, reference should now be made to Figure 13.By replacing the relationships (15) and (16) in (1) and (2), in the section 2-2 of Figure 1, one obtains:

By projecting along the directions y,z and x,, still in the section 2-2 of Figure 1, one obtains:

After passing through the phase shifter DP1, in the section 3-3 of Figure 1, one obtains:

In the same section 3-3, by projecting along the directions y# and x# one obtains

The same relationships apply even after the passage through the differential phase shifter DP2; thus one obtains in the section 4 --4 : r'=HE (17) r"=O (18) With reference now to Figure 14, by replacing the relationships (15) and (16) in (3) and (4) of Figure 1 one obtains:

By projecting along thee directions y1 and in the same section 5-5 of Figure 1 one obtains:

Left-hand circular polarization -The field received by the aerial can be indicated as follows:

Bearing in mind the angular position of the differential phase shifters as shown in Figure 8, reference should now be made to Figure 1.By substituting the reiationships (21) and (22) in the (1) and (2), in the section 2-2 of Figure 1 one obtains:

By projecting along the directions Y1 and x,, in the same section 2-2 one obtains:

After passing through the differential phase shifter DP1, in the section 3-3 of Figure 1 one obtains:

in the same section 3-3, by projecting along the directions y# and x# one obtains:

After passing through the differential phase shifter DP2,in the section 4--4 of Figure 1 one obtains: : r' = HE (23) r" = O (24) With reference now to Figure 14, by replacing the relationships (21) and (22) in the (3) and (4), in the section 5-5 of Figure 1 one obtains:

By projecting along the directions y1# and x1# in the same section 5-5 of Figure 1, one obtains:

After passing through the differential phase shifter DP in the section 6-6 of Figure 1, one obtains:

As known, in a self-collimination receiver, after amplification and conversion at a suitable frequency, the signals r', r'2, r"2 from the wave guides R', R'2, R"2 of the illuminator are sent to two coherent detectors in which r' is used as a reference and r'2, r"2 are signals to be detected.By taking advantage of the known characteristic of the coherent detectors which detect only the component in phase relationship with the reference signal, whereas the component in quadrature is eliminated, in view of the relationships (9), (13), (14), (17), (19), (20), (23), (25), (26) the output signals by and Ax of the two coherent detectors are of the type: Linear polarization Ay = AEy Ax = Aex (27) Circular polarization

where A is a constant. When passing from linear polarization to circular polarization in any direction, only a variation of 3 dB in the amplitude of the signals by and Ax is obtained, which does not affect the good operation of the entire self-collimation system.

The relationships (27) and (28) apply only when the reference signal is in phase relationship with the real part of the signal to be detected. In practice, this does not occur for a multiplicity of possible causes, e.g. the fact that the chain of amplification and frequency translation of the signal r' differs from, and is independent of, that of the remaining two signals.

It is thus common to arrange a phase shifting circuit upstream of a coherent detector, such circuit requiring a preliminary calibration to align the signal to be detected with the reference signal. It should be noted that in the case af linear polarization, imperfect alignment results only in a decrease in the amplitude at the output signal, which makes the preliminary calibration particularly easy.

Phase shifting circuits and coherent detectors are not shown as they are generally part of the receiving circuits indicated by the block RIC.

Claims

1. A circuit arrangement for detecting sighting errors in an aerial for a telecommunication system, comprising first and second differential phase shifters arranged in series on a wave guide path connecting a horn of the aerial to a receiving apparatus assembly, the first and second phase shifters being arranged to cause a differential delay of 900, first means for extracting modes polarized in two directions at right angles to each other, coupling means disposed between the horn and the first differential phase shifter and arranged to withdraw from the same section of the wave guide path two higher energized modes of the signal picked up by the aerial and to send them to insertion means for injecting the higher modes into a section of a further wave guide according to two orthogonal planes, the said section being connected by way of a third differential phase shifter arranged to cause a differential delay of 1800 to second means for extracting modes polarized according to two directions at right angles to each other and rotated through 450 with respect to those of the insertion means (OMT1), each output of the second means being connected to a coherent detector belonging to the receiving apparatus to which one of the output signals of the first means is sent as a reference signal.

2. A circuit arrangement as claimed in Claim 1, in which the coupling means comprises two adjacent mode couplers, each of which is arranged to withdrawn one of the higher modes, there being provided wave guide paths connecting the outputs of the mode couplers to the inputs of the insertion means and comprising amplitude and phase correction means arranged to re-establish the amplitude and phase relationships existing between the said modes in the same section of the wave guide path.

3. A circuit arrangement as claimed in Claim 1 or 2, for use in a linear polarization telecommunication system, in which the third differential phase shifter is rotated through 22.50 with respect to the insertion means, the three differential phase shifters being rotated through an angle equal to half the polarization angle in response to circuits belonging to the receiving apparatus and capable of detecting the polarization angle.

4. A circuit arrangement as claimed in Claim 1 or 2, for use in a circular polarization telecommunication system, in which the third differential phase shifter is rotated through 450 with respect to the insertion means.

5. A circuit arrangement substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.