GB514778A - Improvements in radio signalling systems - Google Patents

Abstract

514,778. Electric resonators ; duplex wireless signalling. BRITISH THOMSON-HOUSTON CO., Ltd. May 9, 1938, No. 13838. Convention date, May 7, 1937. [Classes 40 (iii) and 40 (v)] Relates to a system in which an undesired frequency (such as the 35.6 Mc./sec. emission from a police-car transmitter 25) is excluded from a receiver R tuned to an adjacent frequency (such as 34.8 Mc./sec.) by connecting the receiver at a voltage node 31 of the undesired (transmitter) frequency in a coaxial resonator .23. According to this invention the impedance of the receiver is made equal to the surge impedance of the line, and the line may be tune 1 by means of an inductance or condenser 27 so as to present a non-reactive input impedance. The length of an open-ended resonator 23 may be #/2, where X is the undesired wavelength (Fig. 1, not shown) or may be # (Fig. 2, not shown), or any multiple of #/2. If the resonator 23 is short-circuited at the right-hand end, its length may be any odd multiple of #/4 (Figs. 3, 4, not shown). The impedance of the resonator at the input terminals 33 is inductive and capacitive respectively, for desired frequencies lower and higher than the undesired frequency, and may be neutralized by the insertion at 27 of a condenser or an inductance, as the case may be (Figs. 5, 7, not shown). To facilitate tuning and adjustment the resonator may be made slightly shorter than the required fraction of the undesired wave-length, compensation being effected by a shunt condenser 29, Fig. 11, and Figs. 8, 9 (not shown). A similar arrangement may be used at the idle end of the resonator as at 34, Fig. 11, and Fig. 10 (not shown). The impedance of the antenna 24, being low, is mismatched to the input impedance of the resonator and this mismatching helps to eliminate the transmitter frequency.