Land based security warning system - uses HF line beneath surface and coupled to oscillator with frequency changes on area intrusion

摘要

An automatic security warning system for restricted open areas of ground uses an unshielded h.f. line located some 20 cm beneath the surface. The operating frequency is in the long wave range and is selected for the specific installation. The entry of a person, animal or vehicle into the restricted area is detected by a frequency change in an oscillator coupled to the h.f. line. Compensation may be included for variations due to external affects e.g. temp. humidity. A number of parallel lines with oscillators operating at different frequency provide an accurate indication of the point of entry.