GB2057807A - Improvements in or relating to direction finders - Google Patents

Abstract

A radio direction finder of the kind used aboard an aircraft or ship to determine its direction relative to a radio beacon includes an antenna consisting of a number of individual loops 21-24 which are spaced at regular angular intervals around a vertical axis. Each loop is switched into circuit in turn so that a signal of cyclically varying amplitude is received. The direction of the beacon can be derived from the waveform generated by the switch antenna. <IMAGE>

Description

SPECIFICATION Improvements in or relating to direction finders This invention relates to radio direction finders of the kind which are commonly used aboard aircraft or ships to enable their direction to be determined relative to a radio beacon. If directions relative to two or more beacons can be established the position of the aircraft, or ship as the case may be, can also be established.

To enable the direction of a radio beacon to be determined, radio direction finders incorporate some form of antenna arrangement in which the strength of a received signal is made to vary by rotating the direction of greatest sensitivity of reception of the antenna. By noting the position of greatest or least signal strength, the direction of the beacon can be deduced. The present invention seeks to provide a radio direction finder which is capable of operating in conjunction with a radio receiving antenna of the kind disclosed in our co-pending patent application no. 7929348.

According to this invention a radio direction finder includes an antenna with a plurality of antenna loops, each loop lying in a single plane and with the loops being spaced at regular angular intervals around an axis; means for switching each loop in turn to a receiver so that the receiver receives a radio signal from a beacon, with a waveform corresponding to the switching of each loop superimposed on the received radio signal; and means responsive to said waveform for determining the direction of the beacon relative to the radio direction finder.

Normally the radio signal which is broadcast by a radio beacon includes an audio tone which modulates the radio frequency carrier signal. In practice, it is the frequency of the audio tone which enables the identity of the beacon to be determined.

Preferably each antenna loop is repetitively coupled to the radio signal at a first frequency which is below the audio tone, and preferably again the different antenna loops are sequentially coupled to the receiver at a second frequency, which is above the audio tone.

Since the beacons will generally broadcast audio tones lying in a defined audio band, conveniently the first frequency is substantially below this band and the second frequency is substantially above this band. This enables the first and second frequencies to be readily removed at the receiver by means of simple filters before the received signal is applied to an audio tone detector or other device which indicates the identity of the beacon.

Preferably the second frequency signal is arranged to clock a shift register having a number of stages equal to the number of antenna loops.

Preferably again the shift register is arranged to recycle at a rate equal to said first frequency.

The signal generated within the antenna thus includes four separately identifiable frequency components, since in addition to the first and second frequencies, it contains the radio frequency carrier signal which is broadcast by the beacon and also the audio tone which identifies the particular beacon which is broadcasting. The amplitude of the carrier signal varies cyclically at a rate corresponding to said first frequency, and contains amplitude steps corresponding to the switching of the antenna loops at said second frequency.

The invention is further described by way of example with reference to the accompanying drawings in which, Figure 1 illustrates an antenna forming part of a direction finder in accordance with the present invention, Figure 2 shows the way in which the antenna is coupled to a receiver, and Figure 3 illustrates in diagrammatic form a radio direction finder in accordance with the present invention.

Referring to Fig. 1, the antenna consists of a large number of individual conductive loops 1, with each loop being arranged to surround a ferrite ring 2. The loops 1 do not form a continuous spiral, but instead each end of each loop is connected to a switching matrix 3. Each loop lies in a single plane which is perpendicular to the plane of the paper as drawn and the loops are regularly angularly spaced around the ring. In practice, a very large number of individual loops would be provided, but in the drawing only sixteen are illustrated for the sake of clarity. Thus the angular spacing between adjacent loops is 223%. One way in which the antenna could be used in conjunction with a radio direction finding circuit is illustrated in Fig. 2.

In Fig. 2 four loops 21, 22, 23 and 24 are illustrated in a very diagrammatic manner. A section of the ferrite ring is illustrated at 27.

Both ends of each of loops 21, 22, 23 and 24 are connected via separate diodes 28 and 29 to respective common lines 30 and 3'1, which form an input to the radio direction finding receiver which is purely diagrammatic.

The mid points of each loop are connected to leads 32 to 35 as shown, so that these centre taps are shown and each of these points connected to a separate output of a multistage shift register 36.

The purpose of the shift register 36 is to provide a signal which corrects each loop in turn to a receiver by forward biasing the diodes 28 and 29. The strongest signal which is obtained in response to a signal radiated by a radio beacon will correspond to that loop which is aligned with the direction of the beacon. As is known radio direction finders contain a 180 ambiguity and to resolve this ambiguity, it is usual to provide an omnidirectional antenna. In Fig. 2, the omni-directional antenna 37 is connected to the input stages of the receiver circuit 38 as shown.

The receiver circuit 38 is shown in a very diagrammatic manner and consists of coupled inductors 39, which couple the signals obtained from the loop antennas with those obtained from the omni-directional antenna 37. An amplifier 40 is provided with a tuned input circuit 41, which is tuned to the carrier frequency at which the radio beacon is transmitting.

Fig. 3 illustrates the radio direction finder in greater detail. An antenna unit 42 is arranged to drive a receiver 43, from which an indication is provided by an indictor 44 of the angular direction of a beacon, and from which an audio tone is provided at an output terminal 45 to enable the identity of the beacon to be determined. An antenna 46 of the kind illustrated in Fig. 1, but shown here in side elevation, is coupled via an inductor 39 to a tuned input circuit 41 and an amplifier 40.

An omni-directional antenna 37 which in practice can take the form of an inverted dish as shown is also coupled to the inductor 39.

Those parts of the drawing which correspond to Fig. 1 are given like reference numerals where possible. The output from the amplifier 40 is passed to the receiver 43.

The receiver 43 contains an oscillator 47 having a frequency of 110 Hz, which is below the normal audio band. The output of this oscillator is fed to the antenna unit 42, where it is passed through a pulse squarer 48. The output from the pulse squarer is passed via a frequency multiplier 49 and a pulse shaper 50 to the counter input of a thirty six stage shift register 51. It is assumed here that the antenna 46 contains thirty six individual antenna loops and in consequence of this, the frequency multiplier 49 is arranged to have a multiplication factor of 36 also. The pulse squarer 48 also is connected directly to the re-set input of the register 51 over line 52.

The effect of this is that the different antenna loops are energised in turn within one period of the 110 Hz oscillation and each antenna loops is respectively energised at intervals equal to this period. A predetermined one of the antenna loops is energised each time the register 51 is re-set so that the direction of greatest sensitivity of the antenna can be determined relative to the orientation of this loop.

Thus the signal obtained from amplifier 40 includes four frequency components. It includes the carrier frequency of the signal transmitted by the radio beacon, the audio tone which is modulated on to the carrier signal, a pulsed signal having a frequency of 3,960 Hz (which is 110 Hz multiplied by thirty six) and the oscillator frequency of 110 Hz itself. The amplitude of the received signal varies cyclically at the frequency of 110 Hz.

This signal is applied to one input of a mixer 53 where, it is mixed with the output from a local oscillator 54, which is tuned to the required carrier frequency less a required I.F.

value by frequency selection means (not shown). The mixer therefore provides a low frequency I.F. signal which contains the remaining three signals and this is passed via an amplifier 55 to a synchronous detector 56.

The synchronous detector 56 also receives the 110 Hz signal from the oscillator 57 and its function is to produce from the l.F. signal an output signal having a nominal 110 Hz frequency but which is in phase with the signal obtained from the amplifier 55. This signal is passed through a narrow bandpass filter 57 tuned to 110 Hz and is phase compared at a phae comparator 58 with the output of the oscillator 47. The phase comparator 58 therefore provides as an output a signal representative of the phase difference between the 110 Hz signal generated by the oscillator 47 and the corresponding signal obtained from the antenna unit 42. This gives the direction of the radio beacon which is indicated by the indicator 44.

The output of the amplifier 55 is applied to an audio detector 59. The output of this detector is passed through a bandpass filter which removes the unwanted frequencies of 110 Hz and 3,960 Hz to leave the required audio tone. This tone is amplified by the amplifier 61 and provided at the output terminal 45. This tone can either be recognised by a trained operator, or its frequency accurately measured by a frequency detector so as to enable the radio beacon to be correctly identified.

Claims (7)

1. A direction finder including an antenna with a plurality of antenna loops, each loop lying in a single plane and with the loops being spaced at regular angular intervals around an axis; means for switching each loop in turn to a receiver so that the receiver receives a radio signal from a beacon, with a waveform corresponding to the switch of each loop superimposed on the received radio signal; and means responsive to said waveform for determining the direction of the beacon relative to the radio direction finder.

2. A direction finder as claimed in claim 1 and wherein it is arranged to receive a radio signal in which a radio frequency carrier signal is modulated by an audio tone.

3. A direction finder as claimed in claim 2 and wherein each antenna loop is repetitively coupled to the receiver at a first frequency which is below the audio tone.

4. A direction finder as claimed in claim 3 and wherein the different antenna loops are sequentially coupled to the receiver at a sec ond frequency, which is above the audio tone.

5. A direction finder as claimed in claim 4 and wherein the second frequency signal is arranged to clock a shift register having a number of stages equal to the number of antenna loops.

6. A direction finder as claimed in claim 5 and wherein the shift register is arranged to recycle at a rate equal to said first frequency.

7. A direction finder substantially as illustrated in and described with reference to Fig.

3 of the accompanying drawings.