The need for transmitting and receiving microwave energy at different frequencies and with different polarizations is now often encountered in telecommunications systems, such as ground based antenna systems which communicate with a satellite. In a typical installation, a given frequency band is dedicated to a preselected number of frequency separated channels each having a carrier of designated frequency. For more effective use of the band, separate information carrying signals at each frequency are propagated with horizontal and vertical polarizations, enabling two programs to occupy the same part of the spectrum. Various techniques are known for separation of signals in accordance with polarization, including dual ports of orthogonal orientation, mechanically rotatable frequency selective elements and the like. However, it is desirable concurrently to transmit and receive in both frequency bands and both polarizations. It is also often desirable to use the same or a like arrangement to separate received signals of different polarizations in two frequency bands from a common port, or to combine transmissions from four different sources at a common port.
For the typical transmit and receive application, a broadcast band often uses 24 channels, with 12 different frequencies and both horizontally and vertically polarized signals at each frequency. Where it is desired both to transmit and receive concurrently, dynamic switching and circulating devices cannot be used and a diplexer must be employed. With four discrete information bands, however, the problems of achieving efficient signal separation in a passive manner are considerable. Where a diplexer can function with needed efficiency, however, it can couple an existing antenna system to transmit/receive electronics and provide capabilities for doubling or quadrupling the capacity o an existing system. However, to do so economically requires overcoming a number of interrelated problems.
When electromagnetic wave energy is transferred along a waveguide, the waveguide is usually configured so as to propagate energy stably in a preferred mode. Thus, the broad and narrow walls of a rectangular waveguide are so dimensioned as to propagate most efficiently at a given frequency. In this mode of propagation the electric field vector, in effect the wave polarization, is perpendicular to the broad walls, and the waveguide will not propagate the orthogonal polarization. A square waveguide can adequately, but somewhat less stably and efficiently, propagate two orthogonally polarized signals at the chosen wavelength. However, difficulties quickly arise if it is desired to propagate additional signals at a second wavelength, shorter than the first. Here the square waveguide does not act as a cutoff, as it might if the second wavelength were too long, but tends to introduce multi-mode operation, internal reflections and inherent losses. Nonetheless, a common port and waveguide section are needed for diplexers used in conjunction with a common antenna. In this combination the high power transmitted signals provided to the antenna feed cannot significantly interact with the much lower power level received signals. Likewise the signals of different polarization should be kept distinct, and spurious modes cannot be introduced.
Specifically, it is necessary to maintain a very low VSWR and a minimum interchannel isolation that is greater than 30 dB, while also having very low insertion loss and a high degree (in excess of 35 dB) polarization purity. Prior art systems have recognized the problems of internal signal reflections, and the creation of undesirable modes of propagation, and have accordingly adopted sophisticated expedients for achieving the needed levels of performance. These have usually ben based on the premise that symmetrical couplings and complex configurations are needed, with the result that the systems have been both cumbersome and expensive, and have often resulted in lower performance than is desired.
With existing systems, moreover, both price and performance present substantial problems and it is desirable to have a passive diplexer system that not only is lower in cost but improved in performance.