GB845957A - Improvements in or relating to electric wave filters - Google Patents

Abstract

845,957. Waveguide filters. STANDARD TELEPHONES & CABLES Ltd. Dec. 15, 1958, No. 40290/58. Class 40(8). [Also in Group XL(c)] A waveband-elimination filter comprises a main W.G.I of rectangular cross-section transmitting waves of frequencies lying in a first predetermined range, two tuning screws 2, 6 each extending centrally through a wide wall of the W.G. and spaced apart by a distance which lies between one half and one quarter of the W.G. wavelength corresponding to the maximum frequency of the first predetermined range, and first and second absorbing means 4, and 11 for absorbing substantially without reflection waves having frequencies lying in a second predetermined range, each of the absorbing means 4, 11 being placed at a point in the W.G. at a distance of half the W.G. wavelength at the mean frequency of the second predetermined range from the corresponding tuning screw 2, 6. A third absorbing means 8 can be placed at a point in the W.G. at a distance of half the W.G. wavelength at the said mean frequency from one of the tuning screws 2,6. A third tuning screw can extend through a wide wall of the W.G. at a point halfway between the tuning screws 2 and 6. Each of the absorbing means 4, 8 and 11 comprises a rectangular slot 3, 7 and 10 cut through a wide wall of the W.G.1, with its axis normal to the longitudinal axis of the W.G.1, each slot being resonant at the mean frequency of the second predetermined range and communicating with a stub consisting of a length of subsidiary W.G. of rectangular cross-section with a reflectionless termination 5, and if such dimensions as to transmit waves of frequencies in the second predetermined range but not to transmit frequencies in the first predetermined range. Each stub can be provided with one or more tuning screws 27 extending through the wide wall of the subsidiary W.G. Each reflectionless termination 5 is a resistive element presenting a resistance equal to the characteristic impedance of the subsidiary W.G. Each of the tuning screws 2, 6 and 9 can be replaced by two or more screws arranged in a line normal to the axis of the W.G. 1. One or more further tuning screws can be provided in an extension to the right of the W.G.1. The screws 2, 6, 9 and the stubs 4, 8, 11 may be all attached to the same wide wall of the W.G. 1 or there could be one or more stubs and one or more screws attached to each wide wall. A frequency-changing repeater is shown in Fig. 3 in which the elements and the connections between them are constructed from W.G.s. Modulated carrier waves of a frequency 4,000 m/cs. per sec. are received by an antenna 14 and delivered after amplification to a transmitting antenna 15 with the frequency changed to 3,960 m/cs. per sec. The antenna 14 is connected through a B.P. filter 16 to a B.E. filter 18 as described above and to a mixer 19 in which the frequency is changed to 70 m/cs. per second. The output of the mixer 19 is connected through an amplifier 20 to the mixer 21, and the antenna 15. A local oscillator 22 supplies waves of frequency 4,030 m/cs. per sec. to the mixer 21, the lower sideband at a frequency of 3,960 m/cs. per sec. being selected and supplied to the antenna 15. The output of the oscillator 22 is also supplied through a B.P. filter 26 to a shift mixer 23 to which is connected a shift oscillator 24 supplying waves at a frequency of 40 mc/s. per sec. The upper sideband at 4,070 mc/s. per sec. is selected by the B.P. filter 25 and supplied to the B.E. filter 18. The tuning screws 2 and 6 of the B.E. filter 18 are adjusted to resonate at 8,060 m/cs. per sec. and the tuning screw 9 is adjusted for compensation of the reflections at 4,000 m/cs. per sec., the compensation for the frequency of 4,070 m/cs. per sec.: being also substantially complete. The microwave system of which the repeater of Fig. 3 forms a part is one in which the respective channels are allotted frequencies in the band 3,600 to 4,200 m/cs. per sec.