While researchers are finding ways to make communications at 60 GHz practical (see "Researchers Develop Innovative Solutions for 60 GHz 5G Networks"), experimenter Dex McIntyre, amateur station call sign W4DEX, transmitted signals from North Carolina on 8.971 kHz that were received by Paul Nicholson in the United Kingdom in early June.
McIntyre didn't need an FCC license to transmit the signal across the Atlantic as the FCC hasn't designated any allocations below 9 kHz.
It’s very difficult to build an antenna that's efficient at such a low frequency. The effective radiated power (ERP) of the 8.971 kHz signal was on the order of 150 microwatts. Nicholson used sophisticated digital signal processing (DSP) software to receive the signal over both daylight and nighttime paths over a distance of about 6,194 kilometers or 3840 miles. The carrier was GPS-locked, allowing nighttime reception with a bandwidth of 46 microhertz with an SNR of around 12.5 dB. A 23 microhertz bandwidth was used during daylight hours.
To confirm that Nicholson was actually receiving the signal, McIntyre shifted the frequency slightly. Reception was confirmed at 8.971100 kHz at the same signal strength.
McIntyre, located in North Carolina, used a transmitter consisting of a Hewlett Packard HP 3586B selective level meter along with a tracking generator. The low-level generator output was amplified by a Wandel & Golterman A-160 level regulator feeding a Hafler P3000 stereo audio amplifier, which had been strapped for mono output, resulting in about 400 watts of audio into an 8 Ohm load. The antenna, which was used in the 29 kHz and other VLF experiments previously reported, is the same one McIntyre uses for 160 meters, but is equipped with a gigantic base-loading coil containing nearly a mile of wire.
"The vertical wire is spaced 1.5 meters from the tower, hanging from an insulator 29 meters above ground," said McIntyre. "[The] ‘top hat’ consists of about 170 meters of number 18 Copperweld. Most of the top hat wires run about seven to 20 meters over the top of a combination of oak and pine trees. Total antenna capacitance is close to 1200 pF."
More information about the experiment and the experimenters is available in the American Radio Relay League(ARRL) news release Radio Amateur’s Sub-9 kHz VLF Signal Detected Across the Atlantic. McIntyre told ARRL, "I’ll probably make more transmissions on 8.9 kHz when there is no chance of thunderstorms. Then, maybe sliding down in frequency to see how low I can go for Paul to detect the signal."