Patent GB-596611-A (GB)
Title: Improvements in or relating to radio direction finders
IPC: G01S3/08
Filed: 1945-06-29
Abstract: 596,611. Radio diredtion-finding; frequency modulation. TENCH, F. (Automatic Electric Laboratories, Inc.). June 29, 1945, No. 16573. [Class 40 (v)] A radio direction-finder comprises two spaced parallel loop aerials connected differentially with a parallel loop aerial spaced midway between them. The output from the spaced aerials is combined with the output from the central aerial, the output from the spaced aerials being periodically reversed in sign, while the combined outputs are connected alternately, in synchronism with the reversal, to two condensers, the charges on which are compared by a centre-zero current meter. The spaced loop output voltage obtained for any angular setting of the system is illustrated at A, Fig. 8, the 0-180 degrees direction lying in the plane of the loops and thus compressing a pair of null positions free from night effect. Curve B shows the voltage output from :the central.loop, and curve C the combined output. The spaced loop aerial output is periodically reversed in sign by a set of switches driven from a motor, Fig. 1 (not shown). The combined output, when curve A is reversed, is as shown at D, Fig. 9, curve E showing the corresponding receiver output after amplification and detection when the sign of the output is independent of the phase of the radio frequency signal. The motor also operates, synchronously with the first-mentioned set of switches,. a further set in the meter circuit to switch the output of the receiver from one condenser to the other so that the charges on the two condensers correspond with the respective directivities of the aerial system. One plate of each condenser is earthed, a centre zero current meter being connected between the other two, together with an alternating current electrolytic condenser to cause the meter to be dead-beat. Thus, the signals applied to each side of the meter are dependent upon angular setting as shown at E and F in Fig. 10, the meter response being as shown at G. It can thus be seen that null positions are obtained at 0, 90, 180, 270 degrees, but that the true direction can be determined without any ambiguity by a study of the motion of the needle close to the null position. The separation between the spaced loops giving the greatest sensitivity is onequarter wavelength at any particular frequency, a greater separation being less desirable. Consequently, the spacing between the loops is chosen to be one-quarter wavelength at the highest operating frequency of the system. Receiver.-The signals from the antennas are amplified and detected in a superheterodyne receiver which is adapted to receive either amplitude or frequency modulated signals. The signals from the central antenna are amplified to an extent which may be varied and are then applied to a radio frequency stage together with the periodically reversed signals from the spaced loops. Two output channels which divide in the intermediate frequency stages are provided ; one comprises a stage of intermediate frequency amplification, a detector, a stage of audio-frequency amplification and an output jack for use with phones, and the other comprises a stage of intermediate frequency amplification, a detector, a stage of audio-frequency amplification and the switched condensers and indicating meter circuits. The second channel also comprises a signal strength meter and an output jack for use with phones. Signals may be received on four bands which are grouped in pairs, each pair having a different intermediate frequency, separate interstage transformers being supplied in the intermediate frequency stages. For reception of frequency modulated signals, the intermediate frequency transformers are damped to broaden the frequency response and, in the listening channel, the intermediate frequency amplifier is converted to a limiter by altering the grid bias while the detector is converted to a discriminator by suitable switching. When unmodulated signals are being received, the output from a beat-frequency oscillator is injected into one of the I.F. stages so that an audible beat note is produced.