The high costs of satellites and associated components have forced designers to make the most of the possibility of resorting to the so-called "recovery" of the frequencies, allowing the use of a single antenna for several pairs of signal channels of one and the same frequency, referred to hereinafter as isofrequential; the frequency of the transmission band differs from that of the reception band while the two channels of each pair differ from one another in the adopted polarization.
The present description consistently refers to linearly polarized signals, yet the following observations may also be applied to telecommunication systems operating with a circular polarization; as is well known to those skilled in the art, a linearly polarized signal may be transformed into a circularly polarized signal (and vice versa) for instance by placing the phase-shift axis of a 90.degree. differential phase shifter (polarizer) at 45.degree. with respect to the plane of the linearly polarized signal.
Let us consider a waveguide connected to an antenna and accommodating four groups of channels: two groups of isofrequential but differently polarized transmission channels and two groups of differently polarized isofrequential reception channels operating in a frequency band different from that of the transmission channels.
In order to separate the transmission channels from the reception channels it is necessary to use frequency discrimination; the polarization discrimination may be effected through orthogonal-mode transducers (OMT) or other devices well known in the art. In satellite systems there are utilized devices termed 0 dB double couplers that, besides separating the two bands by their frequencies, extract and supply to different outputs the channels of one of the two bands differing in their polarization.
A 0 dB double coupler consists of a central waveguide (having a circular or square section) where both frequency bands with both polarizations may propagate, and of four collateral waveguides that are symmetrically placed with respect to the central guide connected thereto by respective coupling waveguides so positioned as to give passage to signals present in one or the other polarization plane. The signals present in the central waveguide are thus separated according to their polarization: one pair of collateral guides, symmetrically disposed with respect to the central guide, receive the signals polarized in one (e.g. vertical) plane while the other guide pair receives those that are polarized in a second (e.g. horizontal) plane orthogonal to the first one.
Each coupling guide allows the transfer into the collateral guides of an energy portion of the signals present in the central guide that may propagate into the coupling guide; a complete transfer of the wave energy from the central guide to the collateral guides is obtained by suitably dimensioning the coupler.
Conversely, the coupler can be operated in a mode which is the dual of the one just described: the delivery of two signals equal in amplitude and phase into a pair of opposite collateral guides causes these signals to be fully transferred to the central guide where they are summed.
The coupling waveguides can be dimensioned as high-pass filters of poor efficiency: if the reception band is sufficiently remote from the transmission band, the two bands may be separated by so dimensioning the coupling waveguides that only the band having the higher frequency may propagate therein (e.g. the transmission band) whereas the other band propagates undisturbed in the central waveguide.
Commonly owned Italian Application No. 22821 A/81, filed 9 July 1981, relates to 0 dB couplers adapted to separate the reception band from the transmission band also when such bands are close to each other; the requisite frequency selectivity, allowing utilization of a 0 dB coupling in only one of the two bands, is obtained by way of rejection cavities formed in a wall of each lateral guide and placed in front of the coupling guides.
The rejection cavities are designed to be able to operate properly in only very narrow frequency bands; that reduces the field of utility of the device disclosed in the earlier Italian application.