Note: Descriptions are shown in the official language in which they were submitted.
SPEC I F I CATI ON
. . _
Field of the Invention
-
The present invention relates to a branching
network for the separation of signals with a very high
frequency in double polarization and, more particularly,
to a signal separator for microwaves which comprises two
n3dB~-type couplers with reject-band type filters inserted
between them, the filters reflecting the signais coming
from the first 3dB-type coupler in one of the frequency
bands and allowing the transit of the other band. The
filters are such as not to alter the amplitude and phase
relationships in the signals pertaining to each of the two
frequency bands.
Backaround of the Invention
Branching or separation networks of the type
with which the present invention is concerned have utility
in the telecommunication field, more particularly to the
field of satellite telecommunications. The device can
form an element of the illumination system (~FEED~) for
terrestrial antennas, since frequency recovery is
generally required for this type of communications, given
the high cost of duplica-ting antennas of large dimensions.
While this description will refer repeatedly to
linearly polarized signals, it should be understood that
the principles involved can also be applied to
telecommunication systems operating with circular
polarization.
--1--
A linearly polarized signal can be transformed
into a circularly polarized signal or vice versa, by means
of a 90 differential phase shifter (polarizer) with its
differential phase-shift axis located at 4~ with respect
to the linearly polarized signal.
It is also known that in the waveguide
connected to the antenna for signal transmission and
extraction, four channel groups are present, na~nely, two
transmission channel groups which are isofrequential and
differ from one another by their polarization, and two
reception channel groups which are isofrequential but in a
frequency band dif~erent from that of the transmission
channels, and also differing ~rom each other by their
polarization.
In order to separate the transmission channels
from the reception channels it is necessary to use
frequency separation. This separation can be based on
polarization and can be efected by using orthogonal-mode
transducers (OMT) or others known to those skilled in the
art.
In satellite telecommunications systems~
devices such as "Od~ type couplers, besides separating
the two bands by their frequencies, extract and supply to
different outputs the channels of one of the two bands,
separated according to their polarization.
A double OdB coupler consists oE a central
waveguide (having a circular or square cross-section)
wherein both the Erequencies with both polarizations can
propagate and of four lateral waveguides symmetrically
arranged with respect to the central waveguid~, connected
--2--
~ ~ ~(3~
thereto by a plurality of coupling waveguides, arrange-3 so
as to allow the transit of the signals present on one o~
the polarization planes.
The signals present in the central waveguide
are thus separated according to their polarization. One
pair of lateral guides symmetrically arranged with respect
to the central guide, receives the vertically polarized
signals, and the other pair of lateral guides receives the
horizontally polarized signals. In a more general sense,
one pair of lateral guides receives the signals present in
one plane of polarization, while the other pair receives
the signals present in a polarization plane perpendicular
to the first.
Each coupling guide allows the transfer into
the lateral guides of a fraction of the energy of the
signals present in the central guide, which can propagate
into the coupling guide. By suitably dimensioning the
couplers, the complete transfer of the mentioned energy
from the central guide to the selected lateral guides can
be achieved. Based on the reciprocity principle, the
above-mentioned coupler can also accommodate dual mode
operation: the delivery to one of the pairs of opposite
lateral guides of two signals with e~ual amplitudes and
phases, these being totally transferred to the central
guide, where they are summed.
The coupling waveguides can be dimensioned as
high-pass filters of poor ef~iciency. If the reception
band is sufficiently remote from the transmission band, it
is possible to separate the two bands by dimensioning the
coupling waveguides so that only the band with the highest
frequency can propagate (for instance the transmission
band) while the other band propaga-tes undisturbed in the
central waveguide.
There are OdB couplers capable of separating
the reception band from the transmission band, even when
the two bands are close to each other.
The frequency selectivity which allows the llse
of the OdB coupler in only one of the two bands, is
achieved due to the rejection cavities located in one wall
of each of the lateral guides and facing the coupling
guides, but the rejection cavities are capable o
operating only when acting upon very narrow frequency
bands, which limits the use of this device, because when
very wide frequency bands must be rejected, the cavities
will produce a different phase relationship in the lateral
guides, departing from the synchronism principle to the
point where these couplers cannot be useful. Another
circuit arrangement uses bandpass filters inserted between
3dB type couplers to separate the reception band from the
transmission band. Here the ~ilters with lamellar
structure make the construction of this device complicated.
In addition, when the reception band is very
close to the transmission band, the desired selectivity
cannot be achieved. Thus it may be said that prior art
separators have the following disadvantages:
major insertion losses, due to the ma~imum
rejectivity required in the filters;
large and cumbersome construction and weight;
a very complex construction; and
di~iculties is use o~ the device for wide and
close frequency bands.
~ 3~ ~
Objects of the Invention
It is the principal object of this invention to
provide a microwave branching network for the separation
of polarized high-frequency signals whereby the
disadvantages of earlier systems are obviated.
Another object of this invention is to provide
a separator connectible to a terrestrial antenna of a
satellite communications system for separating
transmitting and receiving bands with respective
polarizations so that the insertion losses are negligible
and the device is of simple construction, small size and
little weight.
Summary of the Invention
These objects and others which will become
apparent hereinafter for the separation of very high
frequency microwave signals in double polariza-tion are
achieved with a device which comprises a central waveguide
permitting propagation of the signals of both bands with
respective polarizations and four lateral waveguides
connected symmetrically to the central waveguides by
respective coupling waveguides allowing transit of the
signals of the two bands having the same polarization.
Respective reject-band or high-pass filtering
means is connected to the control waveguide and to the
four lateral waveguides to permit transit of only one
frequency band each, the filtering means being symmetrical
with respect to both polarization planes.
Connected to these filtering means is a second
double 3dB coupler with central waveguide, drom lateral
waveguides and respective coupling waveguides as described.
g~
The device of the present invention has been
found to be capable of overcoming the disadvantages of the
earlier devices with the following results:
negligible insertion losses (~0.20 dB in the
band 10.7 + 14.5 GHz);
less cumbersome construction and a lower
weight, due to its particular configuration, which ~ill be
urther descri'oed below;
simple construction;
agreement between the controlling elements;
optimum reproducibility;
utility of the device even with wide frequency
bands close to each other.
More specifically the device of the invention
consists of a double coupler of the OdB-type, comprising
in turn two identical sections in the form of double
couplers of the 3dB-type with filtering elements of the
reject-band or the high-pass type inserted between them
and capable of separating two ~re~uency bands even if they
are very wide or close to each other.
The circuit according to the invention ~or the
separation of two frequency bands for very high frequency
signals in double polarization comprises:
a first double coupler of the 3dB type
including a central waveguide, capable of permitting the
propagation of signals from both bands, according to each
of the two polarizations tV and H as described below to
represent vertical and horizontal polarizations,
respectively) and four lateral waveguides, disposed
symmetrically with respect to the central waveguide, being
~ 3~ ~
connected the~eto by a multiplicity of coupling waveguides
capable of allowing the transit of signals from both bands
having the same polarization;
filtering structures of the reject-band type or
the high~pass type received in the square-cross section
central guide with square cross-section/ allowing the
transit of signals of only one of the frequency bands;
a second 3dB coupler analogous to the first.
Brief Description of the Drawing
The invention is now going to be described with
respect to one of its preferred embodiments which is given
as an illustrative, but nonlimiting, example, with
reference to the accompanyiny drawing in which:
FIG. la is an axial (longitudinal) section
through a device according to the invention;
FIG. lb is a section taken along the line Ib-Ib
of FIG. la;
FIG. lc is an end view of the device;
FIG. 2 is a block diagram illustrating the
principles ~or single plane polarization;
FIG. 3 is a similar diagram for a transit mode
of the filters; and
FIG. 4 is another diagram showing the
principles of operation in a mode in which the filters are
rejecting.
Specific Descri~io
FIG. 1 shows the configuration of the device in
one of its possible embodiments and in which the device
--7--
comprises a flange 1 having connection bores 2 for
connecting the separator with the remaining parts of the
antenna illuminator. Within the device is a set of
lateral rectangular guides 3, a central guide 4, and a
supporting structure 5 of polygonal shape (so as to reduce
the weight of the device). The filtering structures with
cut-off guides in the reception band (high-pass filters)
are represented at 6.
In the block diagram of FIG. 2, the circuit
arrangement for a single polarization (V or indeed H)
comprises a reception end or port 8, a common guide 9 for
transmission/reception, the ~irst 3dB coupler 10, the
high-pass filters 11, a second coupler 1~, the
transmission end or port 13 and a terminal 14 for high
frequency.
It is to be noted that a double 3dB coupler can
also have a dual mode of operation, i.e. the energy of a
field applied to the inputs of a pair of lateral guides
can be distributed over the lateral guides and the central
guide.
The present device is a circuit for the
separation of signals by their frequencies and
polarization. In the embodiment shown in FIG. 2, the ends
or ports 8 and 13, respectively the reception and
transmission ports, are uncoupled. While the signals
coming from the port 13 in the band TX are sent to the
antenna 15~ the signals received by the antenna 15 in the
band RX are inserted into the reception port 8.
This diagram refers to only one of the
polarizations (for instance polarization in the V-plane
--8--
polarization of FIG. lb) and is completely separate
(isolation ~ 35d~) from the other polarization plane (~ o~
FIG. lb). These features are a result of the symmetry of
its structure. The mode of operation of the structure
with the frequency accepted by the filters is shown in
FIG. 3.
FIG. 4 refers, as already mentioned, to the
signals present in the band rejected by the filters.
In order to achieve greater graphic and
descriptive clarity, the attenuation and the phase shi~t
introduced by the double 3d~ couplers and the filtering
sections 17 have been considered zero, as well as the
angle o~ reflection (the reflected signal has the same
amplitude and phase as the incident angle); in practice,
the couplers, as well as the filtering sections, may
modify the amplitude and/or the phase of the transitting
signals and/or of the reflected signals.
With reference to FIG. 3, a unitary amplitude
signal applied in B will generate at the outputs C and C'
of the first coupler 1~ two signals of an amplitude 1/~ ,
phase-shifted with respect to each other by ~/2 (the
signal in C anticipates the one in C') these signals pass
through the filtering sections (which have been omitted
being transparent to the passed signal) and reach the
inputs D and D' of the second coupler 12.
By application o the overlapping effect it can
easily be seen that the entire signal applied in B is
available at the exit of the lateral guide E', while at
the exit E of the central guide 4 there is no signal.
~ lith reference to Fig. la, the two
isofrequential signals comprised in the band passing
through the fil~ering sections can still be extracted, in
accordance ~ith their polarization, at the output of the
two pairs of lateral guides of the second coupler 12.
In the diagram of FIG. 4 the filtering sections
have been substituted by short circuits, since the filters
work in the rejection band. A signal of unitary amplitude
applied in B will generate at the outputs C and C' of the
first coupler 10 two signals with an amplitude of 1/
phase-shifted with respect to each other by ~/2.
The signals will be reflected by the short
circuit towards the first coupler (as mentioned before,
the signals have not been ph~se-shifted).
By applying the overlapping effect, it can be
easily seen that the entire signal applied in B is
available at the output B'.
The device according to the invention separates
the transmission band from the reception band,
~0 respectively in the two polarizations perpendicular to
four rectangular guides.
The high-pass filter section are constructed
with stretches of cut-off waveguides 16 for the lower
frequency band of sufficient length to achieve the desired
degree of selectivity or rejection, these stretches being
preceded by adapter sections 17 for the high frequency
~and.
--10--
