A phase measuring system between two underground antennas in the depth of about 500 meters is developed for a frequency band from 10MHz to 80MHz. Underground medium and the coaxial cables of 500 meters at this frequency range are very lossy and the receiving signal power is very low. The direct coupling between the transmitting and receiving cable drums is significant (about-70dB isolation) compared with the receiving power, large isolation is needed. One may avoid this direct coupling by converting the received frequency by using the heterodyne system. In order to measure the phase of the received signal, frequency-converted signal is reconverted into the original frequency by using the same coherent local oscillator.
Whole receiver system is designed and the low noise amplifier, mixer, and various filters are designed and fabricated. The minimum detectable signal is -145dBm and the typical operating error is the ±0.5dB in amplitude at -90dBm level and ±5 degrees in phase at -70dBm.
