RESCUE DEVICE BACKGROUND OF THE INVENTION The present invention relates to rescue apparatus relying on radio transmission, for example for people stranded in the sea. For such applications it is important to be able to provide a personal radio alarm generator that is of very low cost and that is compact and light in weight, so that it can be worn or carried about by the person without discomfort. Furthermore the alarm should be able to function continuously for up to 24 hours or more.

SUMMARY OF THE INVENTION A rescue system according to the present invention includes a device comprising a housing holding one or more batteries, an aerial, an RF generator connected to the aerial, and timing means arranged to provide a train of enabling pulses that turns the RF generator on and off repeatedly.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a rescue device according to the invention.

FIG. 2 is a sectional view showing parts of the rescue device.

FIG. 3 is a schematic diagram of electrical parts of the rescue device.

FIG. 4 is a schematic diagram of electrical parts of a receiver for detecting signals from the rescue device.

DESCRIPTION OF PREFERRED EMBODIMENTS Rescue device 1 comprises a hollow watertight cylinder 2 containing two AA size batteries 3 and a printed circuit 4.

Cylinder 2 may be of plastic. An aerial 5 is attached to cylinder 2. When rescue device 1 is not in use, aerial 5 is wound around cylinder 2 and held in this coiled state by rotatable lever 6 of a switch 7 as illustrated in FIG. 2. When lever 6 is rotated to take position 8 switch 7 is turned on and aerial 5 automatically unfurls to take a substantially straight shape as shown in FIG. 1. Aerial 5 has a curved cross section, like a steel tape measure, causing it to straighten when it is in a free state. Aerial 5 is preferably coated with insulating material at least at its lower part. Metallic screw cap 10 is removable for battery replacement and has a water-tight seal not shown. When screwed in, cap 10 joins battery anode 11 to printed circuit 4 via threaded bushing 12 and wire 13.

By placement of batteries 3 below the longitudinal axis of cylinder 2 the center of gravity of rescue device 1 is well below the axis. Consequently when rescue device 1 is floated in water with aerial 5 released the aerial is automatically positioned above the water and substantially vertical. Rescue device 1 includes an arrangement for attaching it to a person.

The arrangement includes an eyelet 15 that is part of casing 2 and to which is linked an attachment cord 16. Attachment cord 16 terminates at its other end, not shown. in an adjustable noose that can be worn round the neck. The noose has a stop for ensuring that it will always be loose round the neck.

FIG. 3 shows the electrical arrangement of the rescue device. On turn on of switch 7 capacitor 20 is charged. LED lamp 21 is turned on, and oscillator 22 is energised. Oscillator 22 has a low frequency compared with the RF transmission frequency of the rescue device, for example oscillator 22 can have a frequency of 2000 cycles per second, and the RF can be of 500 megacycles per second. Connected to the output of oscillator 22 is a single- shot circuit having an output pulse width less than the the period of oscillator 22, for example the pulse width can be 50 microseconds or less. Thus transistor 24 is turned on, applying DC power to RF power oscillator 25 for 50 microseconds once every 500 microseconds. The RF power from oscillator 25 is applied to aerial 5 via step-up transformer 26. Power oscillator 25 may include a crystal to give precise frequency. The RF power applied to aerial 5 can be up to five watts or more, even though battery 3 may be too weak to provide 5 watts of DC power. This is because RF transmission occurs for only 10% of the time and capacitor 20 can draw and store electric charge at the rate, for example 0.5 amperes from the low power batteries 3 during the intervals between the RF transmission bursts.

LED lamp 21 serves to assist in night-time location of the person wearing the rescue device, and also serves to reassure the person that the rescue device is active.

Alkaline size-AA batteries have a capacity of about 2500 ma hours. Thus for 48 hours of operation the average current drawn from batteries 3 should not exceed 50 ma, corresponding to 0.15 watts drawn from the batteries. Because the RF transmission is intermittent the power delivered by power oscillator 25 to the aerial can be up to 1.5 watts for the case where the duty ratio is 10 %. It can be up to 15 watts if the duty ratio is reduced to 1 %.

FIG. 4 illustrates a receiver for detecting signals from a receiver aerial 30. Aerial 30 is directional, for example by having a reflector, and movable. Signals received by aerial 30 are fed to a frequency selective amplifier 31 tuned to the frequency of oscillator 25. The output of amplifier 31 is fed to a demodulator 32 the output of which is fed to a detector 33 selective to signals having the frequency of oscillator 22. The output of detector 33 is fed to an indicator 34 which provides an audio alarm and a visual indication of the strength of the detected signal. Detection is possible even if the signal received by aerial 30 is extremely weak, because detector 33 is sensitive only to the frequency of oscillator 22 and rejects noise that accompanies very faint signals. Once an operator has been alerted to the alarm he can move the aerial to detect the direction from which the RF transmission is emanating and proceed with rescue operation accordingly.

Rescue device 1 may include an additional battery tester, not shown, that enables the battery to be checked without actuating switch 6.