The invention relates to a system for the registration of the passage of articles through a priorly determined field of the kind that comprises a transmitter and a receiver with a common aerial for alternately transmittance and receiving as well as a marker fastened to the articles for receiving and transmittance of signals by the passage of a transmitter- and receiver bracket or -brackets.

Systems of the kind that is mentioned in the introduction are already known. They are, however, all complicated and expensive, sensitive to electric noise, which leads to false alarm, and they are difficult to set up, as experts are required to secure that the function is reliable enough.

Prom the specification to US Patent No. 3.500.373 a system is known which comprises a common aerial for the transmitter and the receiver as well as a marker, which consists of a tuned circuit. The transmitter transmits during each trans¬ mittance period a continuous, cyclically alternating fre¬ quency, and the registration of the marker passage takes place according to the grid-dip meter principle, as the resonant frequency of the marker is preferably in the middle of the frequency field of the transmitter. This system is rather complicated and vulnerable, because any energy absorb- tion will result in alarm. The aerial is also complicated, because it composed of three coils, which are placed round the passage that is to be controlled, and in such a way that they are at three planes, which are at right angles to each other.

The purpose of the invention is to provide a system for the registration of unauthorized taking of articles and the like, which system is simply built up by few components and consequently cheap, which is easily set up by non-experts

which is insensitive to electromagnetic distant fields a energy absorbing articles, and which is reliable so that false alarm can be excluded. This purpose is fulfilled b the fact that the transmittance and receiving function changes at a suitably high frequency, that the transmitt signals have a constant frequency during the complete transmittance period, that an analytical circuit monitor the number of received signals from the marker, and com¬ pares the frequency of these to that of the signals whic are transmitted by the transmitter, and that the marker consists of a tuned circuit, i.e. of an inductive and a capacitive component.

By means of this a quite unknown security against false is achieved. This claim, which must be fulfilled out of consideration for customers or visitors to shops or muse is not fulfilled by the known systems. This security is achieved by the fact that the analytical circuit firstly tors, whether a received signal has the same frequency a the signal that has just been transmitted, and secondly ther such a signal with the right frequency is received earlier fixed and large amount of times in an uninterrup ed succession.

Finally, according to claim 2, the system can be charact zed in that the transmitter- and receiver aerial is shap like a figure eight or a double loop.

By this you primarily achieve that distant fields can no be registered by the system, and secondly that the trans¬ mitted electromagnetic field gets a ring-shaped extensio which causes that the marker during the passage of the e tromagnetic field only at a certain orientation has a fl that is zero. This means that the risk for passage witho registration is very limited.

The invention will be further explained referring to the drawing, where

Fig. 1 shows the system according to the invention in the shape of a block diagram Fig. 2 shows a way of use of the system accord¬ ing to the invention perspectively and schematically, Fig. 3 shows another way of use with information of the construction of the aerial, and Fig. 4 shows a third way of use of the system according to the invention.

In Fig. 1 is shown how an aerial 1 is connected partly to a transmitter 2 and partly to a receiver 3, 4 through a switch 5. This switch is controlled by a synchronizing unit 6 with a suitably high frequency. By transmitting a brief signal from the transmitter 2 with a suitable frequency an alternating current is induced in the LC-circuit of a marker 18, if this marker is in the rather limited elec- tromagnetic field that is made by the transmitter aerial 1. When this signal transmittance stops, i.e. when the aerial 1 is connected to the receiver 3, 4 by means of the switch 5, the LC-circuit of the marker sends out a signal with the same frequency, until the oscillation in the LC- circuit dies out. This signal is received by the aerial 1 so that the signal is sent off via the receiver 3, ^ to a signal processing circuit 7. This circuit 7 receives at the same time impulses from the synchronizing circuit 6, so that it is able to count, how many signals are received from a marker in an uninterrupted succession. This can be achieved by zeroing the counter if no signal is received during the receiving period immediately after a transmitting periode. It has appeared that a claim for e.g. 100 received signals from a marker in an uninterrupted succession satis- factorily secures that it is a marker passage and not any electrical noise. The signal processing circuit 7 also moni¬ tors, whether the received signals have the same frequency

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as the signals which have been sent ^' out. by the transmitte 2. This secures furthermore from false alarm. If the alar is to be given, i.e. if 100 signals have been, received in an uninterrupted succession with a correct frequency the signal processing circuit 7 sends an larm signal to an auditory or a visual alarm system.

In Fig. 2 is shown how several aerials 8 can. be placed in a way that a broad passage can be controlled. The system is built-in in a frame stand. The aerial that is shaped l figure eight is placed in the frame parts 9,. 10, 11, 12, 13, 14, 15, and l6, ^■ whereas the transmitter 2, the receiv er 3, , and the monitoring ^' unit 7 are built-in in a box 17.

In Fig. 3 the system is extended by an extra aerial. One aerial 19 is here placed in a door, the other aerial bein placed in the floor by the door opening. As the two aeria are at right angles to each other the door passage is com pletely secured, the marker lδ being registered with what ever orientation it may have contrary to the placing in Figs. 2 and 4, where the marker may pass freely if it is oriented at right angles to the aerial as well as the flo

Finally a placing of the system is shown i Fig. ^" 4 by a cash register in a shop.

The construction according to the invention can be ^' used whereever you want to control the taking of articles. Thus you may use the system e.g. in libraries, as the marker can be fastened or placed invisibly in shape of thin films in the cover of the books.

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