Apparatus for detecting a body in motion on the ground of a protected area

#1. Field of the invention

#The present invention relates to an apparatus for detecting a body in motion on the ground of a protected area, particularly suited for military requirements.

#More precisely, the invention relates to an apparatus capable of perceiving and signalling the presence of a body in motion on ground of any type and structure, covered with any kind of vegetation. This apparatus can be advantageously used as a part of an alarm system designed to prevent introduction of persons or transport means in areas of military interest, as well as in particularly dangerous areas due to the presence of noxious chemicals or ionizing radiation. The apparatus can also be used for carrying out a perimeter alarm system for civilian needs around residential or factory buildings.

#The apparatus according to the invention is illustrated hereinafter in greater detail by reference to the following description of an exemplificative and not limitative embodiment, with reference to the accompanying drawing, in which

#FIG. 1 illustrates the distribution of the intensity of the radio frequency along a given direction; and

#FIG. 2 shows a block diagram of the apparatus according to the present invention.

#The apparatus according to the present invention is based on the observation of a physical effect as follows. When electromagnetic waves having a certain frequency, particularly in the radio frequency range, are radiated into the ground, by using a buried antenna and a shielded transmitter in such a manner as not to radiate out of the ground, the radio frequency is distributed on the surface of the ground. In particular, with an omnidirectional antenna the radio frequency is uniformly distributed in all directions. Moreover it should be noted that such distribution is stationary, namely the intensity of wave is not uniformly distributed on the ground, but it shows a sinusoidal distribution (standing wave state). The distribution of intensity along a given direction is generally shown in FIG. 1. The same distribution is obviously obtained along the other directions so that in a fully homogeneous ground, the maxima and minima succeed each other as concentric waves with respect to the transmitting antenna. Such concentric waves are affected with a greater or lesser amount of distortion depending on irregularities present in the ground. It should be noted in this connection that in a steady state, the maxima which are denominated " peaks" and the minima, which are denominated "valleys", follow each other in space as a function of the transmitted frequency, namely the wave length. More precisely, the distance of a peak from a valley is equal to .lambda./4 whereas the distance between two peaks (or between two valleys) is equal to .lambda./2. Moreover the amplitude of a peak (or valley) follows a decreasing exponential state on going away from the antenna.

#Such distribution in a steady state of the radio waves can be explained by the fact that the ground is a high impedence medium for radio frequency and consequently it has a behaviour similar to an unmatched impedence transmission line for radio frequency. Moreover the fact that radio frequency continues to progress in the ground and does not leave it completely at a short distance from the transmitting antenna, can be explained on the basis of the same principle by which a light ray propagating perpendicularly to the thickness of a flat sheet of glass, does not leave the glass itself.

#As to such distribution of the intensity of the radio frequency wave on the ground, a field measurement device has been provided comprising a particular radio receiver tuned to the transmitter and having an instrument for measuring the intensity of the wave to which it is tuned. The measurement device is provided moreover with a particular antenna formed with a dipole contained in a metal box which is closed excepting for one face, namely that which has to be laid on the ground. In such a condition, the antenna can perceive only the radio wave coming from the surface of the ground. By the above apparatus it has been shown by experimental tests that when the transmitting and receiving antennas have been placed at a certain distance one from the other, rather than detecting a constant value (depending on the distance) of the intensity of the electromagnetic waves transmitted into the ground, a variation of intensity directly proportional to the speed of motion of a body in contact with the ground is perceived, even at a range of one hundred meters from the receiving antenna.