Communications: radio wave antennas – Antennas – Loop type
Patent
1987-09-15
1989-07-04
Sikes, William L.
Communications: radio wave antennas
Antennas
Loop type
343742, 340572, G08B 1324, H01Q 704
Patent
active
048455091
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to a receiving device for receiving a magnetic field in connection with a goods monitoring system.
In SE-PS 8404691-1 a system for detecting the presence of indicating devices is described which have the form, for example, of a narrow and thin, but relatively long strip of a highly permeable material, such as being commercially available under the name Permalloy. The presence of such strips is detected by means of coils, which emit and receive magnetic fields, and by means of devices for the detection of signals received.
When such a strip is exposed to a relatively weak external magnetic field, it assumes in its longitudinal direction a magnetic flux density, the magnitude of which can be twenty thousand times higher than in the external magnetic field. The presence of such strips is detected by a detector device, in which variations in voltage induced in a receiver coil are detected.
According to said patent, two coils are caused to emit a magnetic alterating field of high frequency, where a first one of the coils emits an alternating field with a frequency, which is different from the frequency of the field emitted by the second one of the coils, and where at least one difference and/or sum frequency n . f.sub.1 +m . f.sub.2 arising by intermodulation by the indicating device, where n and m are positive or negative integers, is caused to be received by one or several coil. According to the patent, a third magnetic alternating field of low frequency is caused to be emitted in said zone which has a frequency substantially lower than said frequencies f.sub.1,f.sub.2, where the low-frequency field is caused to have a field intensity sufficiently high for, independently of the fields with the frequencies f.sub.1 and f.sub.2, putting the indicating device into saturated condition in step with the frequency of the low frequency field. A field re-emitted from the indicated device is caused to be detected by means of a detector device as an intermodulation, occurring periodically with the frequency of the low-frequency field, between the frequencies f.sub.1 and f.sub.2.
One problem with known theft-prevention systems is that the orientation of the strip in the space is of great importance for whether or not the presence of the strip in the investigation zone will be detected and, thus, whether of not alarm will be sounded. This problem normally has been solved in such a way, that emitting and receiving coils were positioned in different places relative to the investigation zone.
Another problem is that a simple receiving coil is relatively sensitive to interferences of various kind, originating from other electric devices installed in shop premises.
SUMMARY OF THE INVENTION
The present invention provides a solution of the aforesaid problems. The present invention comprises receiving coils so arranged that indication of the presence of a strip in the investigation zone substantially takes place independently of the orientation of the strip in the space, and so arranged that they are insensitive to external interferences.
This implies that the number of expected false alarms is reduced, which is of utmost importance for a goods monitoring system of the kind here concerned.
The present invention, thus, relates to a receiving device for detecting the presence of an indicating device in a restricted investigation zone, where coils and associated feeding and receiving equipment are provided for emitting and receiving magnetic alternating fields, in order thereby to detect the presence of an indicating device in the investigation zone, which indicating device is capable to receive alternating fields and to re-emit an alternating field. The invention is characterized in that said receiving coils, which are provided to receive said re-emitted alternating fields, comprise at least two coil sets, where each coil set is a so-called balanced coil wound to a configuration corresponding to two substantially rectangular eights located to the side of each other in the same p
REFERENCES:
patent: 4118693 (1978-10-01), Novikoff
patent: 4243980 (1981-01-01), Lichtblau
patent: 4633250 (1986-12-01), Anderson, III et al.
Intermodulation and Safety System AB
Johnson Doris J.
Sikes William L.
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