| 摘要 |
An apparatus is desiged for underground use in order to facilitate a two-way communication. The apparatus is operated within the frequency band 0,1-2 MHz, in order to make the electromagnetic waves propagate advantageously by utilizing the metal structures installed in the underground facilities. The transmitter/receiver unit of the apparatus is provided with an analog to digital converter 5 and respectively a digital to analog converter 20 in order to transmit the intermediate signal coded in digital form in between the two units of the apparatus. An antenna 12 is switchable from series resonance for transmission to parallel resonance for reception by mean of switches (26, Fig 2) controlled by a control member 25. <IMAGE>
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