GB2163029A

GB2163029A - Inductive communication system

Info

Publication number: GB2163029A
Application number: GB08420017A
Authority: GB
Inventors: Peter James Raynor
Original assignee: Individual
Current assignee: Individual
Priority date: 1984-08-06
Filing date: 1984-08-06
Publication date: 1986-02-12
Also published as: GB2163029B

Abstract

A magnetic induction field is modulated in accordance with information to be transmitted and the resulting field detected at a remote receiver 2 where the information is extracted and suitably displayed or amplified. By utilising sufficiently sensitive apparatus, the invention may be used underwater over greater distances than electromagnetic communication. Furthermore the rate of data transfer possible and the usable frequency range is greater than with acoustic communication methods. <IMAGE>

Description

SPECIFICATION Inductive communication system This invention relates to an inductive communication system. More particularly, it relates to a wire-less communication system which is equally effective in air, water or any other nonmagnetic medium and will also operate across the interface between media.

Communication systems which use various ranges of the electromagnetic spectrum are well known, but are in general unsuitable for use when one or both of the transmitting and receiving stations are under water. Acoustic methods have been developed for under water communication but these methods are subject to interference and noise.

Furthermore, the rate of data transfer and the type of data which can be transmitted by acoustic radiation is limited.

It is an object of the present invention to provide a method of communication which is applicable to the transmission of signals at any chosen frequency and bandwidth and can therefore be used for the transmission of a variety of signals, including speech and high frequency video signals.

According to the present invention there is provided a method of communicating information between two stations comprising the steps of imparting to an electrical current a modulation in accordance with the information to be transmitted, utilising the modulated current to generate a varying magnetic induction field at transmitting means, detecting said induction field at receiving means remote from said transmitting means, and demodulating the detected signal to extract said information, wherein at least part of the path between said transmitting and receiving means comprises a medium which substantially absorbs any electromagnetic radiation produced with and in synchronisation with said induction field, and said transmitting and receiving means are separated by a distance over which said electromagnetic radiation field would, in free space, be of greater magnitude than said induction field.

In a preferred embodiment of the invention, an RF carrier signal is modulated by audio frequency signals from a microphone, and used to generate an RF induction field which is detected by a loop aerial the received signal being demodulated and the audio frequency component amplified using conventional radio circuitry.

The receiving or transmitting means, or both, may be placed underwater so that the water absorbs electromagnetic radiation, the induction field being substantially unaffected by any non-magnetic medium.

The receiving and transmitting means may be tuned to the same frequency, so that several transmitting and receiving pairs can operate in close proximity at different frequencies, without any substantial interference.

Apparatus in accordance with the invention may operate at any chosen radio frequency, depending upon the type of signals to be transmitted.

When a current is caused to flow in a conductor in free space, magnetic and electric fields having several components are producted. The magnetic field includes a component which varies inversely with distance from the conductor, and a component which varies inversely with the cube of the distance from the conductor. Similarly varying electric fields are also produced. The former magnetic field is 180 out of phase with its equivalent electric field and the two fields form the magnetic and electric components of an electromagnetic field known as the radiation field. The magnetic and electric fields which vary inversely with the cube of the distance form independent magnetic and electric inductance fields respectively.

At positions close to the conductor, the inductance fields are found to predominate over the radiation field, but, since the induction fields decrease more rapidly with distance, the radiation field usually predominates at greater distances. It is the radiation field which therefore forms the basis of radio communications. At a distance of 211 (about one sixth) of the wavelength the induction and radiation fields are found to be equal. By using a coil or inductor to produce the fields the magnetic induction field produced is of considerably greater magnitude than the electric induction field. If an electric dipole is used the converse is true.

The magnetic induction field is often used for communication within the range where the induction field predominates over the radiation field, for instance in inductive loop systems whereby signals are inductively transferred from a surrounding loop to portable receivers with magnetic pick ups inside the area defined by the loop.

In a medium other than free space, some absorption of electromagnetic radiation occurs, the degree of absorption depending on the medium and the frequency of radiation. Losses also occur at interfaces between media. The magnetic induction field is, however, substantially free from absorption, or reflections at interfaces. Hence in a medium such as water, or even through the earth, only the radiation field is attenuated. The residual field at a distance from a transmitting coil therefore only contains the induction component which, although small, is capable of being detected by a sensitive receiver since interference caused by magnetic 'noise' is negligible in such media.

Embodiments of the invention will now be described, by way of example only, with reference to'the accompanying drawings. In the drawings: Figure 1 is a schematic block diagram showing the essential elements of apparatus in accordance with the invention.

Figure 2 is a schematic block diagram showing apparatus in accordance with the invention adapted to operate with alternating fields at radio frequencies.

Figure 3 is a schematic explanatory diagram showing the magnetic field lines produced by a current-carrying coil.

Referring to Figure 1, a communication system comprises a transmitter-1 and a receiver 2. The transmitter comprises a microphone 2 connected to the input of an audio frequency amplifier4, the output of which is connected to a loop aerial 5. The receiver comprises a further loop aerial 6 connected to a receiving audio frequency amplifier 7, the output of which is connected to a loudspeaker 8.

Speech signals from the microphone3 are amplified by the amplifier 4 and used to modulate a DC current which is passed through loop 5. The current in the loop causes a magnetic induction field to be generated by the loop, which varies with the current. This varying field induces a varying voltage in the receiving loop 6, which is remote from the transmitting loop 5. The received signal is then amplified by the amplifier 6 and used to drive the loudspeaker 8. In this way, signals are transmitted between two stations by means of magnetic coupling.

The simple DC system described above is limited in application, however, and it is preferable to work with alternating currents which are modulated by the intelligence signals to be transmitted. By using radio frequency carriers the transmitter and receiver can be tuned for optimum efficiency and several transmitters and receivers can operate independently at different frequencies without substantial interference. Further benefits of inductive field communication at radio frequencies will be described.

A communication system operating art a radio frequency (RF) is shown in Figure 2. The system includes standard radio circuits and comprises a transmitter 100 and receiver 200. The transmitter comprises a microphone 3 connected to the input of an audio frequency amplifier 4, the output of which is fed to a modulator 9. The output of the modulator is connected to one input of a power amplifier 11.

An RF oscillator supplies another input of amplifier 11. The output 10 is connected to a transmitting loop aerial or coil 5 via a filter 12.

Receiver 200 comprises a receiving loop 6 connected to a standard detection circuit which comprises a filter 13, connected to an RF amplification stage 14; the output of which is passed to an audio frequency detector 15. The output of the detectoris passed, via an audio frequency amplifier7 to a loudspeaker 8.

It will be appreciated that the-apparatus comprises standard radio circuits and therefore operation of the receiver and transmitter will not be described in detail. Briefly, speech signals picked up by the microphone are exemplified and used to modulate an RF signal of any chosen frequency generated by the RF oscillator 10. The resulting modulated waveform is filtered and supplied to the loop aerial 5. Current flowing in the aerial causes an induction field and a radiation field to be produced, as described above.

Depending on whether the receiving unit 200 is underwater or out of water, the receiving loop aerial 6 will respond respectively to the induction field or the radiation field, by means of electromagnetic induction. In either case, a voltage is induced in loop 6 which varies in proportion to the received signal.

The electrical signal induced in the receiver is then filtered and amplified before being supplied to a detection stage 15, which can be a conventional diode detector, for separating the audio frequency intelligence signal from the RF carrier. The resulting audio frequency signal is then further amplified and supplied to the loudspeaker 8, or to headphones.

If neither the transmitter nor the receiver are underwater, then the apparatus will function as a conventional radio system, since at distances of more than Y6th of the wavelength the radiation field swamps the induction field and an electromagnetic wave is receivable.

If both the transmitter and receiver are underwater, then although both the radiation and induction fields are generated, the radiation field is rapidly attenuated and the induction field predominates. The magnetic induction field does, of course, decrease in magnitude as a function of the radial distance from the transmitting aerial 5.

However, since water provides a substantially noise free environment, the receiver 200 can be made sufficiently sensitive to detect the magnetic field at a considerable distance from the transmitter 100. A range of about 1 Omlwatt can be achieved. The communication method is equally effective when operating across an air-water interface and can transmit signals through any non-magnetic or slightly magnetic material.

Hence, the method is particularly suited to communication between an aircraft or a ship and a diver operating within the range of the system from the ship, or between two divers. However, the path between the transmitting and receiving aerials should preferably be free from magnetic materials, such as metal, to avoid losses due to magnetic shielding. Localised communications, such as those from diver to diver, wil only require transmitters with an output power of a few watts, but more powerful transmitters can easily be obtained which are suitable for longer distance communication, such as from an aircraft to a submarine, or even between two submarines.

Since the invention is not greatly dependent upon carrier frequency, the intelligence which can be transmitted is not limited to speech or similar signals. By increasing the carrier frequency, and hence the bandwidth, video signals or digital information requiring a high rate of data transfer can be transmitted.

Communication between a transmitter and a receiver is at its most efficient when the transmitter and receiver are tuned to the same frequency.

Tuning is simple to achieve using well-known radio techniques. By tuning several transmitter-receiver pairs at different frequencies, they can operate independently without significant interference between pairs. Alternatively, a system can have several independent transmitters, transmitting at different frequencies, and one or more tunable receivers, the operators of which can tune their receiver to receive any desired transmission.

Two-way communication is possible using apparatus in accordance with the invention, by providing each station with a transmitter and receiver operating at different frequencies.

Continuous two-way operation is therefore possible without the need for an operator to switch from a 'receive' to a 'transmit' mode. Continuous operation of this type is particularly useful for a diver, for example, who would wish to keep both hands free at all times.

Referring to Figure 3 now, the magnetic field 16 close to a current-carrying coil 5 is highly directional. A receiving coil 6A placed close to coil 5 must therefore be aligned such that the two coils are co-axial. In this way optimum coupling may be achieved. Displacement of receiving coil 6A will reduce the density of field lines cutting the coil, hence reducing the mutual coupling.

Further from the transmitting coil, i.e. at positions of the receiving coil such as 6B, it is seen from the figure that the coil can be considerably displaced perpendicularly to its axis without significantly affecting the degree of coupling.

Since the system described above uses magnetic coupling as a means of communication, it is unaffected by acoustic noise, such as that from ships, unlike existing ultrasonic communication techniques. The magnetic field will, however, be disturbed by the pressure of a large mass of magnetic material. This effect can be use to detect objects such as submarines or ships. It is also selective in that entities such as shoals of fish will not affect the magnetic field and so only metal or other such like materials are detected. Acoustic methods generaliy need complicated detection circuits to differentiate between relevant and irrelevant signals.

Since a magnetic field is not easily screened, a system operating in accordance with the method of the present invention can be used as a radio jamming device since any radio receiver is, to some degree, receptive to a magnetic field. Hence, a powerful magnetic field can adversely affect the reception of radio signals.

Claims

1.A A method of communicating information between two stations comprising the steps of imparting to an electrical carrier signal a modulation in accordance with the information to be transmitted, utilising the modulated current to generate a varying magnetic induction field at transmitting means, detecting said induction field at receiving means remote from said transmitting means, and demodulating the detected signal to extract said information; wherein at least part of the path between said transmitting and receiving means comprises a medium which attenuates any electromagnetic radiation field produced synchronously with said induction field such that the induction field at the receiving means is of greater magnitude than that of the electromagnetic field, and said transmitting and receiving means are separated by a distance over which said electromagnetic radiation field would, in free space, be of greater magnitude than said induction field.

2. A method as claimed in Claim 1 wherein the transmitting and receiving means are underwater.

3. A method as claimed in Claim 1 wherein communication is across an air-water interface.

4. A method as claimed in any one of the preceding claims wherein after demodulation, the extracted signal is amplified and fed to a loudspeaker.

5. A method as claimed in any one of the preceding claims wherein the electrical carrier signal is an AC current.

6. A method as claimed in any one of the preceding claims wherein the information comprises speech signals.

7. A method as claimed in any one of claims 1 to 5 wherein the information comprises a scanned visual image.

8. A method as claimed in Claim 5 wherein the transmitting and receiving means are tuned to a common frequency.

9. A two-way communications system wherein two stations each include receiving means and transmitting means, the receiving means on one station being tuned to a common frequency with the transmitting means of the other station, the common frequency being different to that of the remaining pair of transmitting and receiving means, wherein each pair of transmitting and receiving means is operative to communicate information in accordance with the method of claim 5.

10. A method of communicating information between two stations substantially as hereinbefore described with reference to and as illustrated by the accompanying drawings.

11. Apparatus for communicating between two stations substantially as hereinbefore described with reference to and as illustrated by the accompanying drawings.