[0001] The present invention relates to a system for the alignment of polarisation of an
electromagnetic wave with waveguide differential phase shifters, and in particular
a system of the type set out in the preamble of claim 1.
[0002] In satellite communication systems using linearly polarised signals the antenna unit
of a ground station must allow for alignment of the polarisation vector of the signal
with the direction of polarisation of the characteristic guided mode of the waveguide
connection of the transceiver apparatus coupled to the antenna.
[0003] In order to perform such function the antenna unit must be equipped with a system
able to rotate the polarisation vector of a linearly polarised signal.
[0004] In the prior art this is achieved by arranging a common waveguide 180° differential
phase shifter in the antenna feed unit and by conveniently rotating this phase shifter
until achieving the alignment condition.
[0005] A differential phase shifter is typically formed by a section of square waveguide
provided with a series of irises disposed at regular intervals along two parallel
opposite sides of the guide so that the orthogonal linear components of the electric
field vector E in the reference system of the waveguide are subject to a relative
phase displacement of 180°. At the output of the phase shifter a rotation of the electric
field polarisation vector with respect to the input polarisation direction is obtained
as a function of the angle established between the electric field vector at the input
and the polarisation reference direction of the phase shifter, defined according to
the axes of the Cartesian reference system of the waveguide.
[0006] The phenomenon will be understandable with reference to Figures 1a and 1b in which
the polarisation vector of the input signal is decomposed into its orthogonal components
in the reference system of the phase shifter.
[0007] The square waveguide forming part of the 180° differential phase shifter according
to the prior art, provided with irises I along two opposite sides, is indicated WG,
and the polarisation vector of an incident wave at the input port of the phase shifter
is indicated E.
[0008] OXY is the reference system of the communication system, for example of a device
for processing the signal downstream of the phase shifter, with respect to which it
is intended to align the polarisation vector of the incident wave.
[0009] At the input of the phase shifter the polarisation vector of the incident wave has
an angle θ with respect to the reference system OXY.
[0010] To obtain alignment of the vector E with respect to the vector system OXY the differential
phase shifter is oriented, with respect to the system OXY, at an angle equal to θ/2.
[0011] By decomposing the polarisation vector E into the orthogonal components E
x, E
y of the reference system of the phase shifter, aligned with the sides of the wave
guide, it can be seen that at the output of the phase shifter the component E
x will be phase displaced by 180° with respect to the component E
y. The result of the vector addition will therefore be a vector E' aligned with one
of the axes of the system OXY.
[0012] A polarisation alignment system such as that described is based on rotating the phase
shifter coupled to the antenna unit through an angle equal to half the misalignment
angle. This represents a complication and a limitation in various applications.
[0013] For example, some antenna units separate the transmission and reception signals and
bands, as shown in the prior art system in Figure 2.
[0014] A receiving section is indicated 10 and comprises a signal extractor 12 coupled to
a radiation horn 14 of the antenna feed unit and a low-noise amplifier 16 connected
on one side to the signal extractor 12 and on the other to a radio frequency cable
(not shown) for transmission of the signal to known processing circuits. Alignment
with the incident signal is obtained by rotating the unit formed by the radiation
horn 14 and the receiving section 10 with respect to the fixed transmission section
20 connected to the ground station.
[0015] To align the transmission signal coming from the waveguide of a high power amplifier
(not illustrated) with the radiation horn of the feed unit rotated to achieve alignment
with the incident signal, an alignment system with a 180° phase shifter is used, the
differential phase shifter being indicated PS in the drawing.
[0016] This constructional arrangement requires the use of two rotary joints RJ1 and RJ2
respectively interposed between the phase shifter PS and the receiving section 10
and between the phase shifter PS and the transmission section 20 coupled together
through a complex 2:1 reduction gear mechanism which permits the rotation of the phase
shifter through an angle equal to half the angle of rotation of the receiving section
10 with respect to the fixed transmission section 20.
[0017] A man skilled in the art will recognise without difficulty that the arrangement of
Figure 2 is mechanically complicated.
[0018] The object of the invention is to provide a polarisation alignment system which is
mechanically simpler, and which does not require the use of two rotary joints with
a complex reduction gear mechanism between them.
[0019] To this end the subject of the invention is a polarisation alignment system having
the characteristics set out in Claim 1. Particular embodiments of the invention are
defined in the dependent claims.
[0020] The invention also comprehends an antenna unit for a receiver-transmitter station
of a communication system including a polarisation alignment system, as defined in
Claim 8.
[0021] In summary, the present invention is based on the principle of forming the 180° phase
shifter by means of a pair of 90° phase shifter components separated by a rotary joint,
each of these being arranged to be fixed, or mounted without any degree of rotational
freedom, to the respective signal guiding structure, that is to say, in the exemplary
embodiment, to the receiving section and to the transmitting section of the antenna
unit respectively.
[0022] In an alternative embodiment it is envisaged that only the phase shifter associated
with the receiving section of the antenna unit would be coupled to the associate guiding
structure without any degree of rotational freedom, whilst the phase shifter associated
with the transmitting section is coupled to it with the possibility of switching between
at least two angular positions, corresponding to different operating modes of the
system.
[0023] Further characteristics and advantages of the invention will be explained in more
detail in the following detailed description of an embodiment, given by way of non-limitative
example, with reference to the attached drawings, in which:
Figures 1a and 1b are vector diagrams which show the operation of a prior art polarisation
alignment system with a 180° differential phase shifter;
Figure 2 is a block diagram of an antenna unit with a prior art polarisation alignment
system;
Figure 3 is a block diagram of an antenna unit with a polarisation alignment system
according to the invention; and
Figure 4 is a block diagram of a variant of the antenna unit of Figure 3 for operation
with double polarisation.
[0024] A polarisation alignment system according to the invention, for example for a receiver-transmitter
antenna unit of a satellite communication system, is described hereinafter with reference
to Figure 3.
[0025] Elements or components identical or functionally equivalent to those illustrated
in Figure 2 have been indicated with the same reference already utilised in these
Figures in the description of the prior art.
[0026] A receiving section of an antenna feed unit of a ground station is again indicated
10 and comprises a signal extractor 12 coupled to the radiation horn 14 of the antenna
and a low-noise amplifier 16 connected on one side to the signal extractor 12 and
on the other to a radio frequency cable (not shown) for transmission of the signal
to known processing circuits.
[0027] It further includes a first fixed 90° differential phase shifter PS1, the output
port of the signal extractor 12 being aligned with the diagonal of the square waveguide
forming the phase shifter.
[0028] A fixed transmission section is again indicated 20 and comprises a second fixed differential
phase shifter PS2 mounted to form an angle of 45° with respect to the orientation
of the transmission waveguide of the system 20.
[0029] A single rotary joint RJ is interposed between the two fixed phase shifters PS1 and
PS2.
[0030] Advantageously, alignment with the incident signal is obtained by rotating the integral
unit formed by the radiation horn 14 and the receiving section 10 through an angle
equal to the angle formed between the direction of polarisation of the incident wave
and the direction of polarisation of the system.
[0031] It is well known that a 90° phase shifter permits conversion of a linearly polarised
signal into a circularly polarised signal (and vice versa) if it is disposed at an
angle of 45° with respect to the direction of polarisation of the incident wave.
[0032] Therefore, in the system configuration described above, a linearly polarised signal
received by a radiation horn of the suitably aligned antenna feed unit is converted
into a circularly polarised signal at the output of the phase shifter PS1.
[0033] The circularly polarised signal is transmitted to the second phase shifter PS2 through
the rotary joint RJ where it is again converted into a linearly polarised signal aligned
with the waveguide of the transmission section 20.
[0034] Consequently, the system according to the invention makes it possible to obtain alignment
of polarisation by rotating only the receiving section, and therefore the first phase
shifter fixed to it, through an angle equal to the angle formed between the incident
linearly polarised signal and the reference system of the ground station.
[0035] The performance of the system in terms of purity of the polarisation is the same
as that of the prior art system of Figure 2. In particular the ratio between the phase
error of the phase shifting components and the cross polarisation due to this error
is unchanged.
[0036] Advantageously, in a variant embodiment, the system forming the subject of the invention
can be used also to form a dual polarisation, both linear and circular, antenna feed
unit, thereby permitting a rapid switching between the two modes.
[0037] With reference to Figure 4, this can be achieved by providing a further rotary joint
RJ' to allow rotation of the second phase shifter component PS2 with respect to the
fixed transmission section 20. In fact, a circularly polarised signal can be obtained
from the system described by aligning the second phase shifter to the waveguide of
the transmission section, then effecting a phase displacement through only 90° between
the orthogonal vector components of the linearly polarised input signal by means of
the first phase shifter PS1.
[0038] Naturally, the principle of the invention remaining the same, the embodiments and
details of construction can be widely varied with respect to what has been described
and illustrated purely by way of non-limitative example without by this departing
from the scope of protection defined in the appended claims.
1. A system for alignment of the polarisation of an electromagnetic wave between a first
and a second guiding structure of a communication system operating with linearly polarised
signals, the first guiding structure being orientable to be aligned with the direction
of polarisation of an external signal and the second structure being fixed with respect
to a general reference system, the system comprising waveguide differential phase
shifter means operable to cause a total phase displacement through 180° between the
orthogonal components of the electric field vector of the signal input to said means,
the system being
characterised in that it comprises:
first 90° phase shifter means (PS1) arranged to be fixedly coupled to the said first
guiding structure, orientated in such a way as to form an angle between its reference
polarisation direction and the direction of polarisation of the said first structure;
and
second 90° phase shifter means (PS2) arranged to be coupled to the said guiding structure,
and orientated in such a way as to form an angle between its reference polarisation
direction and the direction of polarisation of the said second structure;
the said first and second phase shifter means (PS1, PS2) being coupled together for
rotation in such a way as to allow rotation of the first orientable guiding structure
with respect to the second fixed guiding structure through a predetermined angle corresponding
to the original angle of misalignment between the direction of polarisation of the
external signal and the polarisation direction of the said second fixed guiding structure,
assumed as a reference polarisation direction of the communication system.
2. A system according to Claim 1, characterised in that said first and second phase shifter means (PS1, PS2) are coupled together by means
of a waveguide connection rotary joint (RJ).
3. A system according to Claim 1 or Claim 2, characterised in that the said first and second phase shifter means (PS1, PS2) comprise waveguide differential
phase shifter devices in which the said waveguide is a square waveguide.
4. A system according to Claim 3, characterised in that the said first phase shifter means (PS1) are arranged to be coupled to the said first
guiding structure orientated in such a way as to form an angle of 45° between its
reference polarisation direction and the polarisation direction of the said structure,
and in that the said second phase shifter means (PS2) are arranged to be coupled to the said
second guiding structure orientated in such a way as to form an angle of 45° between
its reference polarisation direction and the polarisation direction of the said structure.
5. A system according to Claim 4, characterised in that the said differential phase shifter devices (PS1, PS2) are formed by means of a waveguide
comprising a plurality of capacitive and/or inductive irises which cause a partial
obstruction of the transverse section of the guide.
6. A system according to Claim 4, characterised in that it includes a further waveguide connection rotary joint (RJ') interposed between
the second phase shifter means (PS2) and the second guiding structure.
7. A system according to Claim 6, characterised in that the said second phase shifter means (PS2) are arranged to be orientated with respect
to the second guiding structure in such a way as to present its reference polarisation
direction aligned to the polarisation direction of the said structure, the system
thus operating as a circular polarisation polarising system.
8. An antenna unit for a receiver-transmitter station of a communication system, comprising
a receiving section (10) fixed to an antenna feed device (14) and orientable with
it for alignment with the polarisation direction of an incident electromagnetic wave,
and a transmitting section (20) fixed with respect to a general reference system,
characterised in that it includes a system according to Claims 1 to 7 for alignment of the polarisation
between the receiving section (10) and the transmitting section (20).