GB2274953A - Navigation system incorporating screened two-loop antenna - Google Patents

Abstract

A radio-position finding system includes a receiver having an antenna 1 comprising at least one pair of antenna loops 2, 3, 4 in positional and electrical quadrature. The loops 2, 3, 4 are enclosed in an electrical screen. Either by phase inversion of one of the signals received by the coils or by means of two antennas a receiving system can develop signals of which the phase difference represents twice the angle subtended by a pair of transmitting stations that radiate signals of similar or harmonically related frequencies. The arrangement provides a positional indication with reference to circular position lines passing through the locations of the transmitting stations. <IMAGE>

Description

This invention relates to radio position-finding systems including antenna systems which provide at least in one plane substantially omni-directional reception and which have a high degree of immunity to locally reradiated signals. Such attributes are particularly useful in phase comparison radio navigation position location wherein signals having a fixed phase or time relationship are radiated from fixed transmitting stations so that a measurement of phase difference at a receiver provides an indication of position relative to the transmitting stations. The indication normally identifies a hyperbolic or circular position line or one in a family of such position lines.

It has been known for many years to provide partial screening of a single loop antenna employed as a direction finder. Such an antenna has been screened by an earthed tubular shield having a non-conductive spacer preventing the creation of a short-circuited loop in the vicinity of the antenna loop. It has also been known for many years to provide a pair of crossed antenna loops in conjunction with a goniometer in order to facilitate direction finding; such an arrangement is known as a Bellini-Tosi direction finder.

It has also been known, from DE-3016951-A to provide a multiplicity of crossed loop antenna coils in conjunction with a phase shift network to allow omnidirectional reception.

In one aspect of the invention, at least one pair of the antenna loops are disposed physically in quadrature, that is to say in mutually intersecting orthogonal planes and are electrically connected or connectible in quadrature. There may be two such units, each consisting of two screened coils, positionally and electrically in quadrature. In effect, each unit forms a circularly polarized vertical antenna, one being adapted for clockwise rotation and the other to anti-clockwise rotation.Two adjacent units, which may be located on a vehicle or ship, are useful in conjunction with a radio navigation or radio position-fixing system comprising at least two transmitting stations which radiate signals of similar or harmonically related frequency so that for example a phase difference between these signals at a mobile receiver prov.ides an indication of position relative to the stations. The received signal from each antenna unit is modified by a phase magnitude dependent on the angle subtended to the transmitting stations and the rotation sense of the feeding antenna. The second unit provides the opposite modification to the signal phase so that two sets of phase difference readings can be obtained.

The mean of these two sets provides a normal positional indication reading but with the advantage of a screen reducing the effect of locally reradiated voltage fields.

The difference of the two readings provides a phase proportional to twice the angle subtended by the associated transmitting stations and can provide a position line indication in addition to the position lines normally obtained in phase comparison radio navigation systems.

The effect of two units may alternatively be obtained using a single unit and providing periodic phase inversion of the signals from one of the antenna loops.

Reference will hereinafter be made to the accompanying drawings, of which: Figure 1 illustrates the physical arrangement of a tubular screen for an antenna unit according to the invention; Figure 2 illustrates the physical configuration of two antenna loops and the associated electrical connections according to one embodiment of the invention; Figures 3 and 4 illustrate families of position lines characteristic of a preferred system according to the invention; and Figure 5 illustrates an alternative form of antenna.

An antenna unit 1 of the kind which may be employed in the present invention is shown in Figure 1. It has the general shape of a ball and consists of at least two and preferably three tubular rings 2, 3 and 4 respectively, supported and connected by radial tubular sections 5, there being cruciform pieces 5 between the various sections. A tubular post 6 extends downwardly from the tubular framework. At each join one of the parts longitudinally overlaps the other but the overlapping parts are radially separated by insulating material. In this manner the shielding against electrical fields is rendered substantially complete but the shielding is interrupted so as to prevent the flow of electric current round any closed figure formed by the framework.At the centre of the tubular framework a tubular central spider 7 may be similarly interconnected with the radial arms that extend to and support the peripheral tubular parts of the screen, which is preferably earthed.

This form of construction provides electrical shielding for antenna coils which can be disposed inside each of the tubular rings formed by the tubular network. The screened antenna system thereby formed is immune to a considerable degree to locally reradiated signals and can provide effective reception since the magnetic component of the received signals is substantially unaffected by the electrical screening.

Alternatively the screening may be provided by means of ferrite or iron dust elements with small pickup coils.

A coil may be disposed within each ring. Any two coils, which are preferably spatially orthogonal, provide substantially omnidirectional reception in a respective plane. The frame work may be simplified if only two coils are provided. The provision of a third coil is useful for providing an indication of skywave by means of reception in a vertical plane in conjunction with one or other of the coils employed for reception in the azimuthal plane.

Figure 2 illustrates two loops 8 and 9, which may be disposed in the tubular rings 2 and 3. One side of each loop is connected to a common earth to which the other side of each loop is connected by a respective tuning capacitor 10, 11. These sides of the loops are connected through respective amplifiers 12, 13 to two phase shift networks 14, 15, which provide 90 electrical degrees of phase shift between the input signals before they are combined in a combiner 16, which is coupled to a balanced RF output line 17.

An antenna system comprising an entire tubular screening network with antenna coils within the tubular network has substantial utility as an omnidirectional antenna. However, the invention particularly concerns the use of two such antenna systems or a single, switched antenna system on a mobile vehicle or other carrier and co-operating with signals radiated in fixed phase relationship from two or more transmitting stations. Such phase comparison radio navigation systems can be used to provide at a mobile receiver an indication of phase difference between signals received from at least two of the stations and thereby provide an indication of the position of the receiver with reference to hyperbolic or circular position lines (depending on the configuration of the system). Such navigation systems have for many years provided considerable benefits in position fixing.

Figure 3 illustrates in dashed lines the pattern of hyperbolic position lines each corresponding to a particular phase difference between signals received from the two stations at the foci of the hyperbolae. Figure 4 illustrates the same two stations, operated in conjunction with a receiver in a different mode which provides intersecting families of intersecting circular position lines, each family being centred on a respective station.

The determination of a position line is normally ambiguous and requires resolution by techniques which are well established.

In one development according to the invention, there may be a mobile receiver equipped for example with either two antenna units as shown in Figure 2 or with a single such unit wherein one of the phase shift networks includes a switchable phase inverter.

Each of the antenna systems or units thereby provided constitutes a circularly polarized vertical antenna, one being set to clockwise and the other to anti-clockwise rotation of the axis of polarization. The received signal obtained from each unit modifies the normal phase angle detected by the receiving apparatus by a magnitude dependent on the angle subtended to the pair of transmitting stations which provide the signals of which the phases are compared.

The second unit provides the opposite modification to the signal phases so that two sets of phase difference readings will be obtained. The mean of these two sets of readings provides the normal phase difference obtained by a conventional antenna though one has the advantage of a screen reducing the effect of locally reradiated electric fields. An additional facility provided by the present invention is that the difference of the two readings provides a phase proportional to twice the angle subtended by the appropriate transmitting stations. This in effect provides a family of coarse position lines which can be used to resolve ambiguities in the position fixing obtained by the normal reception of signals. The position lines obtainable are circles each passing through the locations of the transmitting stations, as shown in solid lines in Figures 3 and 4.

Figure 5 illustrates an alternative form of screening which is suitable for use when reception is required only in a horizontal plane. In this form an antenna 50 comprises top and bottom metal shells 51 and 52 defining a ball which encloses the coils (not shown). The top and bottom shells are separated by insulation 53 around the common equator and the shells are interconnected by a metal tube 54. In this form of the antenna there are effectively two interruptions of the screen by the insulation for each coil.

Claims (3)

1. A radio position-finding system comprising at least two transmitting stations adapted for radiating respective radio signals enabling a phase comparison to be made between them, and a mobile receiver having an antenna system comprising at least one antenna is omni-directional in at least a horizontal plane and which comprises a pair of orthogonal screened loops which are coupled electrically in quadrature, the antenna system being adapted to act as two antennas for receiving circularly polarised waves of opposite rotational sense and thereby to provide two corresponding received signals of which the phase difference is proportional to twice the angle subtended at the receiver by the said transmitting stations.

2. A system according to claim 1 wherein the antenna system comprises a single antenna having a pair of said loops and is arranged to provide said received signals in a time sequence.

3. A system according to claim 1 wherein the antenna system comprises two antennas each comprising a pair of said loops.