A high-temperature superconducting receiver for low-frequency radio waves

We have developed a receiver for low-frequency radio waves using high-temperature superconducting quantum interference devices (SQUID's). The primary application of such a receiver is to communicate in underground areas where the overburden results in significant losses at the usual radio frequencies. The receiver constructed consists of a SQUID, a small dewar, control electronics, and a battery pack. The SQUID was fabricated in our laboratory using an edge junction technology, discussed in previous publications. For this work we increased the effective area by including a standard galvanically coupled pickup loop. We have investigated background noise spectra and found that the low noise floor of the SQUID's can be fully utilized in the receiver, provided that the signals be encoded with a bandwidth narrower than the 60 Hz spacing between powerline harmonics. The most significant advantage of SQUID's for this application is that they allow the compact construction of three-axis receivers that are necessary to overcome a dominant source of vibrational or motional noise.