Electromagnetic intrusion detection system

Claim 2

An electromagnetic intrusion alarm system for detection of an intruder in a surveillance zone irrespective of intruder motioN, said system comprising: means for providing an electromagnetic field in a zone under surveillance and including: oscillator means for providing a signal of predetermined fixed frequency; transmitting antenna means for providing said electromagnetic field in response to said oscillator signal; and coupler means for conveying said oscillator signal to said antenna means and for providing a local oscillator signal; receiving antenna means for receiving electromagnetic energy returned from said zone; mixer means operative in response to the output signal from said receiving antenna means and to said local oscillator signal to provide a signal of substantially constant output level under static conditions in which no intruder is present within said zone, and of a different average output level in the presence of net amplitude and/or phase changes in received energy caused by intruder presence within said zone; means for establishing a reference threshold in accordance with the static positional characteristics of said zone and objects therein and of a level greater than said average output level of said mixer means; means having a bandwidth substantially less than the bandwidth necessary for processing Doppler signals and operative in response to said signal of substantially constant output level to provide an output signal having an amplitude less than said threshold level, and operative in response to said signal of different average output level to provide an output signal having an amplitude greater than said threshold level and which is representative of the net change in the amplitude and/or phase of said received energy to thereby represent irrespective of intruder motion the net change in the positional characteristics of said zone and objects therein; and means for providing an alarm indication of intruder presence only upon exceedance by said output signal of said reference threshold.

Claim 3

The system according to claim 1 wherein said output signal providing means exhibits a system bandwidth of less than 1 Hz.

Claim 4

The system according to claim 1 wherein said output signal providing means includes: amplifier means for amplifying said signal; means for AC coupling said signal to said amplifier means; limiter means for limiting the maximum amplitude of said amplified signal; and integrator means operative to integrate the signal from said limiter means to provide said output signal.

Claim 5

The system according to claim 3 wherein said alarm providing means provides a minimum time interval for which said output signal must exceed said threshold level to cause said alarm indication.

Claim 6

An electromagnetic intrusion alarm system for detection of an intruder in a surveillance zone irrespective of intruder motion, said system comprising: means for providing an electromagnetic field in a zone under surveillance and including: oscillator means for providing a signal of predetermined fixed frequency; transmitting antenna means for providing said electromagnetic field in response to said oscillator signal; and coupler means for conveying said oscillator signal to said antenna means and for providing a local oscillator signal; receiving antenna means operative in response to electromagnetic energy returned from said zone to provide at least one signal representative of said returned energy; mixer means operative in response to said at least one signal from said receiving antenna means and to said local oscillator signal to provide at least one mixer signal having a substantially constant output level under static conditions in which no intruder is present within said zone, and having a different average output level in the presence of net amplitude and/or phase changes in received energy caused by intruder presence within said zone; means for establishing a reference threshold in accordance with the static positional characteristics of said zone and objeCts therein and of a level greater than said average output level of said mixer means; means having a bandwidth substantially less than the bandwidth necessary for processing Doppler signals and operative in response to said at least one mixer signal of substantially constant output level to provide at least one output signal of amplitude less than said threshold level, and operative in response to said at least one mixer signal of different average output level to provide at least one output signal of amplitude greater than said threshold level and which is representative of the net change in the amplitude and/or phase of said received energy to thereby represent irrespective of intruder motion the net change in the positional characteristics of said zone and objects therein; and means for providing an alarm indication of intruder presence only upon exceedance by said at least one output signal of said reference threshold.

Claim 7

An electromagnetic intrusion alarm system for detection of an intruder in a surveillance zone irrespective of intruder motion, said system comprising: means for providing an electromagnetic field in a zone under surveillance and including: oscillator means for providing a signal of predetermined fixed frequency; transmitting antenna means for providing said electromagnetic field in response to said oscillator signal; and coupler means for conveying said oscillator signal to said antenna means and for providing a local oscillator signal; receiving antenna means operative in response to electromagnetic energy returned from said zone to provide at least one signal representative of said returned energy; mixer means operative in response to said at least one signal from said receiving antenna means and to said local oscillator signal to provide at least one mixer signal having a substantially constant output level under static conditions in which no intruder is present within said zone, and having a different average output level in the presence of net amplitude and/or phase changes in received energy caused by intruder presence within said zone; means for establishing a reference threshold in accordance with the static positional characteristics of said zone and objeCts therein and of a level greater than said average output level of said mixer means; means having a bandwidth substantially less than the bandwidth necessary for processing Doppler signals and operative in response to said at least one mixer signal of substantially constant output level to provide at least one output signal of amplitude less than said threshold level, and operative in response to said at least one mixer signal of different average output level to provide at least one output signal of amplitude greater than said threshold level and which is representative of the net change in the amplitude and/or phase of said received energy to thereby represent irrespective of intruder motion the net change in the positional characteristics of said zone and objects therein; and means for providing an alarm indication of intruder presence only upon exceedance by said at least one output signal of said reference threshold.

Claim 8

The system according to claim 5 wherein said receiving antenna means includes a power splitter operative to provide two signals representative of said returned energy; and wherein said mixer means includes a phase splitter operative in response to said local oscillator signal to provide quadrature phased local oscillator signals for said mixer means; said mixer means, threshold means and output signal providing means providing quadrature processing of said mixer signals.

Claim 9

The system according to claim 5 wherein said signal providing means includes: audio amplifier means for amplifying said at least one mixer signal; means for AC coupling said at least one mixer signal to said audio amplifier means; limiter means for limiting the maximum amplitude of said amplified at least one mixer signal; and narrowband means to integrate the signal from said limiter means, the signal from said integrator means being applied to said threshold means.

Claim 10

The system according to claim 5 wherein said output signal providing means includes narrowband means having a bandwidth of less than 1 Hz.