G - Physics
08
B
337/58, 337/62,
G08B 13/24 (2006.01) H01Q 13/20 (2006.01)
Patent
CA 1332185
ABSTRACT The present invention is a leaky cable which can be used as an antenna or as an intruder detector sensor either buried in a single trench or above ground and which substantially reduces sensitivity variations due to the environment. This is effected by substantially blocking egression of the electric field from the cable but allowing magnetic fields to escape, and by substantially slowing the velocity of and attenuating the externally propagating electromagnetic field. In the preferred embodiment, the leaky coaxial cable is comprised of an inner conductor, a dielectric surrounding the inner conductor, a first external shield having low series impedance at VHF frequencies surrounding the dielectric, means for coupling a magnetic field through the first external shield, a second external shield surrounding the first external shield having high series impedance relative to series impedance of the first external shield and means for limiting VHF conduction current between the shields, which effectively causes separation of the internal and external propagation fields of the cables. The external shields are arranged so that the first external low series impedance shield does not short circuit the second external high series impedance shield, thus separating the internal and external propagating fields of the cable. One way to achieve this result is to place a thin semiconductive or insulating sheath between the two shields. A second way is to ensure that the skin depths at VHF in the two shields are adequate to effectively separate the two signals. The external signal, propagating on the outside of the second external shield and the internal signal propagating on the first external shield are effectively separated thereby.
580343
Harman R. Keith
Hill Charles Richard
Rich Brian G.
Smith Kenneth L.
Ridout & Maybee Llp
Senstar Corporation
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