Tikalon Blog by Dev Gualtieri

Very Low Frequencies

May 27, 2019

I've been interested in electronics since my childhood. One of my favorite "toys" was the battery/push-button/doorbell kit that my parents gave me when I was in fifth grade. I used the large battery to make a variety of electromagnets for other experiments. Thin strands of wire made good electrical detonators for the percussion caps used in toy cap guns.

Although the transistor and I were born at nearly the same time, all my early electronic circuits were made from vacuum tubes and components donated by a neighbor who repaired televisions. Guitar amplifiers were popular items in the 1960s, so I built several for my friends using components harvested from derelict televisions.

Symbols for vacuum tubes. From left to right, a diode, tridode, tetrode, and pentode. The 12AX7 dual triode was my favorite tube for low-noise microphone preamplifiers. (Click for larger image.)

Useful information was hard to obtain in those days before the Internet. My electronics education came from reading the hobby electronics magazine, Popular Electronics, that had a monthly printing of several hundred thousand copies, and two other magazines, Radio-Electronics and Electronics World. Don Lancaster was a prolific author of articles in these magazines, and he was also the author of two of my favorite books, the TTL Cookbook,[1] and the CMOS Cookbook.[2] Don Lancaster published an article on construction of a color organ, that era's version of a laser light show, in April, 1963. I built a variation of that circuit a few years later, and I published a more modern version last year.[3]

Eventually, audio frequency transistors became available to electronics hobbyists at reasonable prices. One of the first transistor circuits that I built was a radio receiver for frequencies in the 100-15,000 hertz frequency range. That was not because there were any broadcast stations on those frequencies in the ELF (extremely low frequency, <3 kHz) and VLF (very low frequency, 3 - 30 kHz) bands, but because there are natural electromagnetic emissions called radio atmospherics.

These radio atmospherics, often called sferics or spherics, are generated by lightning discharges, and they propagate over long distances by channeling in a natural waveguide formed between the Earth's surface and the ionosphere. It's easy to see how such low frequencies are generated, since the current path in a lightning discharge is several miles in length. One interesting VLF phenomenon is the dawn chorus, radio emissions at audio frequencies that occur near dawn at local time and resemble bird chirps. The dawn chorus is supposed to arise through interactions between electrons in Earth's magnetosphere and the VLF background radiation.

Another VLF phenomenon is the whistler, a Doppler-shifted signal of descending frequency caused by dispersion of the radio signals as they travel through ionospheric and magnetospheric plasma. Such whistlers persist for a few seconds, and their detection on Jupiter by the Voyager spacecraft indicated that Jupiter has lightning, also.

Anyone who listens to AM radio during a thunderstorm knows that individual lightning strikes are readily detected using radio. This is especially true for VLF. It's been proposed that 13.6 kHz, a frequency at the edge of a radionavigation band, should be reserved for this purpose. Lightning detection technology has progressed considerably in the half century since I built my VLF radio. Blitzortung.org is an Internet-connected network community of about 500 people with advanced VLF receivers. Detected lightning strikes are timestamped using GPS receivers, and this allows an accurate triangulation of the lightning strike location.

Lightning strikes along the Eastern United States near 4:00 AM on April 15, 2019.

This map was generated by the VLF receivers of the blitzortung.org project.

(Lightning data by blitzortung.org and contributors, licensed under the Creative Commons Attribution-ShareAlike 4.0 License. Click for larger image.)

Natural radio sources are not the only signals detected at VLF. When I built my VLF receiver, I detected signals from every electric motor in my neighborhood and the ignition systems of passing automobiles. All these devices generate electrical sparks and simulate lightning discharges. One VLF pioneer, Robert A. Helliwell (1920-2011), who discovered whistlers with one of his students, Jack Mallinckrodt, decided that locating a remote VLF station in Antarctica would be a way to eliminate such man-made interference.[4] I wrote about Helliwell in an earlier article (Robert A. Helliwell, June 16, 2011).

VLF pioneer, Robert A. Helliwell

Helliwell's discovery of VLF whistlers was accidental.

Karl Jansky observed lightning noise in his experiments that resulted in the discovery of the first extraterrestrial radio source in 1933.

(Stanford University Image.)

This West Antarctic VLF outpost, called Siple Station, was active from 1971-1988, and it had an antenna that was thirteen miles long. The reason for this huge length is the long wavelength of VLF radiation. A frequency of 10 kHz corresponds to a wavelength of 30 kilometers, or 18.64 miles. The antenna was used to inject VLF radio signals into Earth's magnetosphere, to be detected half a world away in Canada. This was possible, since the ionosphere is transparent to such low frequencies, a principle that Grote Reber used in his Tasmanian radio astronomical observations.

There's an adage that "You can't fool Mother Nature," made memorable in a 1974 margarine commercial.[5] Conventional antennas are efficient only when their size is near the target wavelength. That's because the electric field intensity in antenna conductors increases in value to about half a wavelength. That's why the commonly used dipole antennas are a half wavelength wide, and you would expect a VLF dipole antenna to be huge. A team of scientists from the SLAC National Accelerator Laboratory (Menlo Park, CA), Gooch and Housego, LLC (Highland Heights,Ohio), a photonics technology firm, and SRI International (Menlo Park, California) have shown that it is possible to fool Mother Nature by creating efficient VLF antennas of much smaller size using a piezoelectric antenna. Their research is published in an open access article in Nature Communications.[6-7]

VLF signals are desirable for communication, since they penetrate tens of meters into soil or seawater, and the signal attenuation is ≤6 dB/1000 km within the Earth-ionosphere waveguide.[6] This is in contrast to shorter wavelength signals that are easily blocked by water, layers of rock and building structures, don't bend well over the horizon, and are sometimes limited in range to line-of-sight.[7] The piezoelectric lithium niobate electric dipole antennas in the present study are about 10 cm in size, and they are driven at an acoustical resonance that allows radiation at more than 300 times the expected efficiency for their size.[6]

Conceptual diagram of the piezoelectric VLF antenna.

The antenna consists of a rod-shaped crystal of lithium niobate. An oscillating voltage applied to the bottom of the rod makes it vibrate.

This mechanical stress triggers an oscillating electric current (shown by the arrows), its electromagnetic energy being emitted as VLF radiation (blue waves).

(Image by Greg Stewart/SLAC National Accelerator Laboratory (modified). An animated version can be found here.)

When an oscillating electric voltage is applied to the lithium niobate rod, it vibrates because of the piezoelectric effect, alternately shrinking and expanding. This mechanical stress creates an oscillating electric current whose electromagnetic energy is emitted as VLF radiation.[7] In conventional antennas, the electric current arises from the motion of electric charges moving the length of the rod, and these motions are of the same scale as the wavelength. The piezoelectric antenna efficiently excites electromagnetic waves with wavelengths that are much larger than the motions along the crystal.[7] By adjusting the piezoelectric resonance, the wavelength can be changed to transmit signals in a wider bandwidth that allowed data transfer rates of more than 100 bits per second.[7]

The new antenna's compact size will enable VLF transmitters that weigh just a few pounds. Experimental communication over a hundred foot distance showed that the VLF radiation was produced at 300 times greater efficiency than conventional antennas and with nearly a hundred times greater bandwidth.[7] Says Mark Kemp of SLAC, the principal investigator of this project,

"Our device is also hundreds of times more efficient and can transmit data faster than previous devices of comparable size... Its performance pushes the limits of what's technologically possible and puts portable VLF applications, like sending short text messages in challenging situations, within reach."[7]

Piezoelectric VLF antenna during testing

The piezoelectric VLF antenna, the 4-inch length clear rod at the center of the assembly, during testing.

(Photo by Dawn Harmer/SLAC National Accelerator Laboratory.)

Says Kemp, "There are many exciting potential applications for the technology... Our device is optimized for long-range communication through air, and our research is looking at the fundamental science behind the method to find ways to further enhance its capabilities."[7] This research was funded by the Defense Advanced Research Projects Agency (DARPA), and a patent is pending on this invention.[7]

Very Low Frequencies

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