Patent GB-628955-A (GB)
Title: Improvements in method and apparatus for medium high frequency radio direction finders
IPC: G01S1/02
Filed: 1945-11-20
Abstract: 628,955. Radio direction-finders; pulse circuits. COLLINS RADIO CO. Nov. 20, 1945, No. 31230. Convention date, July 19, 1943. [Classes 40 (vi) and 40 (vii)] A radio direction-finder comprises three antenna pairs, each pair comprising two spaced dipole antennse, the pairs being set at 120 degrees with respect to each other, and connected to two receiver channels, one dipole of each pair feeding into one channel and the other dipole of each pair feeding into the other channel, the dipoles being connected sequentially by commutating means to their respective channels so that each dipole pair becomes effective in turn, and means being provided to integrate the signals obtained from each dipole pair and to suitably combine the integrated signals and apply the result to a cathode-ray tube indicator which indicates the direction from which signals are being received. It can be shown that the use of three symmetrically disposed antenna pairs greatly reduces error introduced by octant effect, diversity effect and non-linear circuits in the receiver. Means are also provided to. reduce polarization error introduced by current induced in the sheaths of the cables interconnecting the various dipole antenna, which currents cause re-radiation to the dipoles. " Pilot " signals are induced into the sheaths and, after reception, voltages are derived proportional to the amplitude of the pilot signals received to cancel out reception of signals which are re-radiated from the sheaths. It is necessary on account of the signals which are induced into one antenna pair by the currents flowing in the sheaths corresponding to other antenna pairs to supply pilot signals to all the sheaths at each instant in the commutation cycle, their relative signs being changed appropriately when the operative antenna pair is changed. Detailed description.-Fig. 7 shows the antenna arrangements. Units Al, A2 ... &c. comprise valve circuits with which the dipoles are associated. Thus A1 and A2, B1 and B2, C1 and C2 each comprise an antenna pair. Each dipole feeds directly into a push-pull amplifier which is normally biassed off, but this bias is removed sequentially for each pair by means of the commutator 11 which shortcircuits the source of bias potential for each antenna pair in turn. An injection voltage, derived by mixing a signal approximately at the intermediate frequency with the output of the common local oscillator to the two receiver circuits and which is consequently approximately at the frequency of the received signals, is applied to the push-pull amplifiers of Al, A2 ... &c. over line 53. The " pilot " signal, which is derived by mixing a signal 80 c/s removed from the signal approximately at I.F. mentioned above with the local oscillator output, is applied through the commutators 12A, 13, 14, which ensure the signal is applied in the appropriate sign, to the loops 12 adjacent the sheaths surrounding the feeders to the circuits A1, A2, &c., so that the pilot signals are induced in the sheaths. The output from the push-pull amplifiers of Al, B1, C1 comprising signals received from the distant station, injection voltage, and pilot signal received by radiation from the sheaths is fed to one receiver and the corresponding output from A2, B2, C2 is fed to the other receiver. A further amplifier in each unit A1 ... C2 amplifies the voltage between the corresponding dipole and the extremity of the corresponding sheath. These amplifiers are commutated in the same way as are the push-pull amplifiers and the signals therefrom are taken to the same receivers as those from the corresponding push-pull amplifiers. The difference between the two receiver output voltages is applied to a phase-splitting network whence are derived components of the 80 c/s signal phased 90 degrees apart. These are compared in two diode circuits with an 80 c/s reference voltage obtained by direct mixing of the injection and pilot voltages, these circuits delivering direct voltages having magnitude and sign depending upon the components of the two input signals in phase with the reference voltage. These voltages are each applied through a commutator to an appropriate one of three storage condensers and the outputs therefrom are applied as bias to two control circuits in each of the two receivers, one of which controls the " horizontal pick-up voltage.", i.e. the voltage between the sheath and the dipole and the other voltage derived therefrom and in quadrature therewith, so as to inject into the direction finding signal a signal which reduces the difference between the 80 c/s signals components at the output to zero, and so balances out the effects of pick-up in the horizontal sheaths. The outputs from the receivers are filtered to remove the 80 c/s components and applied to a rectifier circuit which delivers a direct voltage proportional to that component of the difference between the outputs which is in quadrature with the sum. This is applied to one of three integrating condenser circuits through a commutator synchronized with commutators 11, 12A, 13 and 14 so that the condensers have upon them voltages Ya, Yb, Yc due to the signals received by the three aerial pairs respectively. These voltages are applied to circuits which provide 60 c/s mains-frequency square waves of amplitude dependent upon them, which are then integrated to provide saw-tooth waves which are combined according to the expressions Yb+Yc-2Ya and #3(Yb - Yc). These voltages are applied to the Y and X plates of a cathode-ray tube respectively and since, as is shown in the Specification, the function Yb+Yc-2Ya/#3(Yb-Yc) is equal to the tangent of the direction angle of the received signal a straight line trace is drawn on the cathode-ray tube screen whose inclination indicates the direction from which signals are received. To provide a sense indication the 60 c/s mains voltage is caused to produce a defocusing pulse at a fixed epoch of its cycle which is applied to the cathoderay tube and so adjusted that a blur is formed at one extremity of the straight line trace and acts as a tail to an arrow indicating direction. Pulse circuits, Fig. 10.-To generate the 60 c/s square waves having amplitudes dependent upon the signals Ya, Yb, Yc, these signals Ya, Yb, Yc are applied to the control grids of three respective valves 111, 113 and 112. These valves are rendered alternately conducting and non-conducting by applying oppositely phased 60 c/s signals, derived from the supply mains, to the grids of the gasfilled valves 114, 115, so that the cathode potential of valve 114 becomes alternately positive and zero, thereby alternately preventing and allowing conduction in the valves 111, 112, 113. The valve 115 assists in obtaining a sharp cut-off in the valve 114. Square waves at 60 c/s are consequently produced at the anodes of valves 111, 112 and 113, the potentials at these anodes alternating between the H.T. supply potential and a value dependent upon the appropriate one of the signals Ya, Yb, Yc.