Development of 350 kHz Analog Transceiver for Applying in Cave Communication

Abstract

This paper presents the development of a medium-frequency band 350 kHz analog transceiver for communication between inside and outside the cave. The proposed transceiver was developed from the SSB transceiver of HeyPhone 87 kHz of the British Cave Research Association (BCRA) by adding and creating the circuit of the transmitting and receiving parts of the transceiver. Part of the transmitter, the RF power amplifier, is developed for selectable transmitting power of 5 and 15 watts. Whereas a balun circuit is added to adjust the antenna impedance appropriate to the cave's environment. The receiver part is replaced by a superheterodyne circuit, while its front-end circuit is equipped with the low-noise amplifier to improve the sensitivity for receiving the selectable low signal at -80 dBm and -100 dBm. Moreover, it also includes the squelch circuit to eliminate the sound of noise when the radio is not receiving the desired transmission. From the testing result at Patihan cave Ubon Ratchathani province (total passage distance of 457 m), Ubon Ratchathani province, applying the modulated RF signal via the earth antenna from the mountain surface through the rock into the cave. Using the developed transceiver, we found that the cave communication system can provide a good signal quality between the outside and all the passages inside the cave.