GB1518604A - Radio direction-finding apparatus - Google Patents

Abstract

1518604 Radio direction finding DEFENCE SECRETARY OF STATE FOR 29 April 1977 [4 May 1976 (2)] 18051/76 and 18052/76 Heading H4D A radio direction finder comprises a pair of crossed loops A, B, Fig. 1, the outputs of which are combined in antiphase in a common amplification channel 7 via individual modulators 5, 6 wherein one output is variably attenuated so as to be increasing with time during a given period when the other output is synchronously variably attenuated so as to be decreasing with time. The combined output of the channel contains a zero cross-over, the timing of which within the period is indicative of the radio bearing. Referring to Figs. 5. 6 wherein four possible bearings are shown for orthogonally crossed loops, and modulator 5 increases its output during periods I and III and decreases during periods II and IV, whilst modulator 6 decreases during periods I and III and increases during periods II and IV (i.e. a phase reversal occurs at the end of period II), the outputs of modulators 5 and 6 for the four bearings are as shown at (a) and (b) respectively. (c) and (d) showing the combined outputs indicate that a zero crossing will be detected in period I for bearings # 1 and # 3 and in period II for bearings # 2 and # 4 . This 180‹ ambiguity is resolved using the output of an omnidirectional antenna C, made at 10 in phase with one of the possible phases of the combined output. The zero crossing detector 12 gives a pulse indicating by its polarity, the direction of the crossing. The transient producing phase reversal is avoided by amplitude modulating the loop signals by sine and -cosine signals respectively the loops being centre earth balanced. Using such modulation the position of the zero crossing, expressed as an angle is equal to the bearing.