GB831411A - Channel filter - Google Patents

Abstract

831,411. Channel filters. STANDARD TELEPHONES & CABLES Ltd. May 16, 1958 [May 17, 1957], No. 15799/58. Class 40 (8). In a branching arrangement comprising a group of three or more waveguide bandpass filters all having different pass bands, the said filters being each coupled at a given branching- point to a junction line common to all the filters by means of a corresponding coupling line, the lengths of the coupling lines are so chosen that at the midband frequency of the pass band of each filter, the sum of the admittances of the other filters, as they appear at the given branching point after transformation by the corresponding coupling lines, is substantially zero. In the arrangement shown, channel filters 3, 4 and 5 connected at 6, 7 and 8 to transmitters and/or receivers are connected through waveguides, or alternatively coaxial lines, of lengths l1, l2 and l3 to a branching point of a common waveguide 1 leading to an antenna. The lengths l1, l2 and l3 are so chosen that at the resonance frequency of filter 4 the input admittance of filter 3 transformed via l1 is equal to the conjugated complex value of the input admittance of filter 5 transformed via l3. The same condition is satisfied at the resonance frequency of filter 3 for the lengths l2 and l3 and at the resonance frequency of filter 5 for l1 and l2. It is stated that it can be shown that all these conditions can be met when each coupling line has a length of the order of half a wavelength at the corresponding mid-band frequency. The bandwidths of the individual filters are chosen smaller than the frequency spacing of the corresponding mid-band frequencies so that at frequencies in the pass-band of one filter the two other filters are at cut-off. The arrangement may be extended to six channels by connecting six filters in two groups (Fig. 2, not shown).