Natural radio is a term given to very low frequency (VLF), ultra low frequency (ULF), and extremely low frequency (ELF) radio waves emitted by charged particles in Earth's atmosphere and magnetosphere. These emissions can have frequencies in the range of tens to thousands of hertz, as opposed to the megahertz and gigahertz frequencies we normally associate with man-made broadcasts. Because of its low frequencies, natural radio can be translated directly into sound, revealing its complex and beautiful nature. Natural radio signals are being generated all the time, but go unnoticed because they are difficult to detect. They are frequently overwhelmed by man-made VLF noise sources, primarily the noise from alternating current power lines. Detection of natural radio signals requires special electronic equipment capable of detecting VLF signals.
Despite the term natural radio has been used to describe VLF/ULF/ELF radio emissions, electromagnetic waves produced by natural sources in our universe, do not only occur in this region but in almost every frequency in the electromagnetic spectrum. In this case, these frequencies cannot be translated directly into sound and other means of equipment is needed to listen to them. But they are also parts of the natural radio.
From Plate Tectonics to Particle Physics, Natural Radio is any electromagnetic energy which occurs in Nature. Such phenomena have been around since before we had the technology to detect them, before life existed in earth or even before our solar system was born. Here are some examples:
Sferics - Mainly the ubiquitous snap, crackle and pop of lightning all over the earth, this can be heard anytime, anywhere, on frequencies from 1 Hz to beyond 100 MHz. Just tune your AM radio to an empty frequency. Sferics below deserve special mention.
Tweeks - A modified form of lightning sferic where the electromagnetic impulse undergoes a process of dispersion, and its frequency components become 'smeared out' in time. Commonly sounds like a 'chirp' or 'tweek'. Dispersion results from interaction between the energy wave, the magnetic field of the earth, and the ionosphere (ozone layer). Can be received from 100 Hz to 30 kHz with homebuilt gear, best in an electrically quiet location.
Whistlers - A sferic which has become dispersed in time to the point of becoming a descending tone. Whistlers are less common, and may be only associated with recently discovered 'upward lightning'. Whistlers can 'echo' back and forth between the northern and southern hemispheres of the earth between magnetic conjugate points, sometimes blending into an eerie and beautiful harmony. Receive techniques same as tweeks.
Chorus - A sferic normally associated with dawn or aurora especially during magnetic storms. Chorus can sound like whispery voices, or like distant singing of birds, barking animals, surf crashing on a shore, or almost anything. Receive techniques same as tweeks.
Earthquake waves and precursors
- Not sferics, occur at lower frequencies. Gaining credibility in mainstream
science as a possible early warning for major earthquakes since the landmark
work of Antony Fraser-Smith. Equipment, which can be home built, usually
includes a computer to record signals 0.001 to 10 Hz, below human hearing.
In the range of 0 to 10 Hz, we 'hear' or sense waves from the internal groans
and creaks of the Earth, the solar wind howling around the Earth's
magnetosphere, magnetic storms, and a host of unknown phenomena. Although these
are infrasonic, below human hearing, you can speed up a recording to bring them
within range of human hearing.
Schumann resonance - A resonant cavity is formed between the ionosphere and the earth. Energy from sferics or other sources may excite this natural resonator to ring at about 8 Hz. Special purpose receiver, can be home built.
Other Planetary sources - Earth is not the only planet to exhibit a radio personality. Venus is quite similar to Earth, and the four gas giants, Jupiter, Saturn, Uranus and Neptune are natural emitters of HF radio signals. Jupiter's powerful voice may be received on Earth using simple hobby equipment on frequencies around 10 to 40 MHz. The FCC has set aside a 'quiet zone' from 25.55 to 25.67 MHz as one of the radio astronomy listening windows.
Solar phenomena - The sun is by far the most powerful electromagnetic emitter in our solar system, generating broadband radio emissions, flares, magnetic waves, and storms of electrically charged nuclear particles and ions. These directly cause and/or influence the propagation of electromagnetic waves throughout the known spectrum. The lowest documented electromagnetic solar cycle is about 22 years, or 0.0000000014 Hz. High end frequencies above 100000000000000000000 Hz have been detected.
Pulsars - Some stars collapse into rapidly spinning cores of extremely dense matter, their intense magnatic fields sweep out charged particles forming an electromagnetic pulse. Typical pulsars rotate about once per second, compared to the earths rotation of once per 24 hours. A faster spinning pulsar is PSR J0835-4510 ( psrb 0833-45 ) is a remnant of the Vela supernova, and rotates at about 11 Hz (period=0.089308556629 s) and sounds to me like something getting slapped by a ceileing fan. The fastest spinning pulsar PSR J1939+2134 ( psrb 1937+21 ) rotates at 642 Hz, (period=0.001557806468819794 s) and its surface at the equator is moving about 14% of the speed of light.
Gamma Ray Bursts - Satellites designed to monitor nuclear tests received unexpected gamma ray bursts (GRBs) not of earthly origin. In average week, several of these distant, mysterious cosmic events will occur, which for a few seconds will illuminate our planet with gamma rays brighter thatn our own sun. At such great distances the energy at the source must be immense, the entire energy output of our sun over its 10 billion year lifetime is emitted in a few seconds. Even more surprising, the pulse time indicates these distant sources are also much smaller than our solar system.
Other Cosmic sources - Our universe is filled with many other strange and mysterious electromagnetic voices. Main sequence stars, neutron stars, pulsars, quasars, novae, gas clouds, and even the afterglow of the birth of the universe itself all have their part in the electromagnetic music of the spheres. We don't really know the full range of frequencies, but two high energy (high frequency) events above 10^20 eV have been recorded so far, and the lowest frequency cycles are so slow our technology has not been around long enough to see a complete cycle.
Human biological sources - The human nervous system uses electricity, and thus creates electromagnetic waves. These waves are usually very weak, but researchers are exploring possible connections with parapsychology in humans, and effects of external sources on living beings. Brainwave radiation can be recorded by instruments at close range in a shielded room between 0 Hz and 100 Hz.
Marine biological sources
- Many species of marine life form use high levels of electricity and
electromagnetic waves for sensing, location, communication and defense.
Electroreceptive life can sense but not create electromagnetic energy. Dogfish
and sharks have receptors called ampullae of Lorenzini, small innervated bulbs
attached to a long jelly-filled canal running to an opening in the skin.
Detection threshold is only 0.005 microvolt/cm = may detect a flounder at 30 cm,
even if buried in the sand. They do not recognize cut up fish buried in sand but
will attack a buried cable or electrode. Electrogenic life can generate
electromagnetic energy, generally in the frequency band of 1 Hz to 10 kHz. Most
Electrogenic fish generate up to 1 volt or so for sensing, location and
communication. Here is an Electrical Organ Discharge (EOD) of the Elephant-Nose
fish (Gnathonemus Petersi). Jamming of the signal appears when several species
of weakly electric fish coexist in the same area, or individuals of the same
species. Eigenmannia produces pulses at the frequency 250 - 700 Hz, when another
fish interferes, it shifts their frequency (jamming-avoidance reflex). Each fish
then has its own private channel. Individuals may be recognized.
In this section, the philosophy about listening to natural radio will be presented.
In this section, we will focus on the equipment that is needed in order to listen natural radio in audio frequencies. Below, such a receiver is presented (click the image for a clearer version). Its frequency range is about 100Hz - 11kHz. In fact the receiver can receive from 50Hz at least. This is a variation of the famous BBB-4 VLF receiver that is easy to construct, it has been constructed and it works perfectly ok for this range of audio frequencies. This version sports a 2nd audio amplifier stage at Q3. The output level with this version is sufficient to drive a crystal headphone to a comfortable volume. The "crystal" headphone is like those used on the old crystal radios. A "Walkman" type headphone is NOT the same thing! Check out Mouser #25CR060. Or use as input to your amplifier. This simple receiver works so well that it remains a basis for a good VLF receiver for audio-frequency natural radio phenomena listening in a simple, straightforward design!
D1/D2 Optional - protects Q1 from nearby lightning. A second plug has been added to give the ability to connect the recording equipment and the phones at the same time. One of the two levels can be precisely adjusted using a multi-turn potentiometer. The main reason I used the multi-turn potentiometer was its resistance to humidity. Most carbon potentiometers will induce noise to the audio signal due to humidity after a long period of being unused, but wire potentiometers will not. Output on both output plugs is microphone level audio. I tried to cascade another 2n3904 class-A amplifier section but it gave me more microphonics rather than audio amplification, so I used only what is displayed in the schematic above. T1 is a Radio Shack 1K CT primary to 8 ohm secondary. Only 1/2 of primary is used, secondary is not used.
The receiver has been designed for high mobility. If you are far away from home you may not be able to find batteries. The consumption of the receiver is very low and the receiver will operate for months before the battery gets consumed. The battery configuration was chosen so that this will be a highly portable receiver. If you switch to battery 1, the receiver will operate from battery 1 and you can charge battery 2, without affecting the operation of the receiver. Also, if battery 2 is charged you can use its power to power other devices external to the receiver too, without affecting the operation of the receiver. If you switch to battery 2 the opposite scenario is taking place. I used two 9v rechargeable batteries and I embedded them into the receiver, to create a high mobility device.
BBB-4 receiver notes and other information
The "BBB-4" is a broadband 0.2 to 12+ kHz V.L.F. receiver with a passband peak at approximately 1.5 - 2 kHz, and is designed to receive naturally-occurring V.L.F. phenomena (such as "whistlers") that occur as electromagnetic (radio) waves at audio-frequencies. This receiver was designed to be hand-held or tripod mounted, and its output patched to a MICROPHONE-LEVEL input such as a tape-recorder or speaker-amplifier (such as the one available at Radio Shack ("Mini Audio Amplifier/Speaker" cat. # 277-1008). "Radio Shack" in the US and Canada is also known as "Tandy" overseas, such as in the U.K.
Dissatisfied with more complicated and cumbersome multi-turn loop receiver schemes back in 1991, the designer of the BBB-4 receiver opted to design a whistler receiver that was simple to build and use by newcomers to this realm of radio. He also desired the BBB-4 to be as sensitive and low-noise as possible using a small whip antenna while being highly immune to broadcast and utility station overload.
The BBB-4 V.L.F. receiver circuit ("BBB-4" standing for a fourth version of my "Bare-Bones-Basic" designs) is remarkably sensitive and works very well with short whip antennas between 1-3 metres in length, since it operates on the same principle as high-impedance "active antennas" designed for other frequency ranges (such as long, medium or shortwaves).
The BBB-4, due to its F.E.T. "front-end" being a high-performance J-FET, has an input impedance of about 10 megohms, which is why the short whip antenna--more correctly called an "electrical-field probe"--works fantastically for being such a tiny fraction of the received-frequencies' wavelengths in size (the ultimate isotropic antenna). R1 and C2 act as a roll-off to frequencies above about 20-30 kHz, efficiently eliminating potential receiver overload/intermod from Loran-C (100 kHz), strong AM-BCB signals or SWBC signals, and frequencies up into the VHF ranges. R2 sets the gate impedance for the J-FET. R4 and R3 set the optimum bias on the FET for maximum dynamic range and minimum susceptibility to overload and intermod. C3 and C6 slightly helps roll-off low-frequencies such as powerline "hum."
The "pi-filter" consisting of C4, L1 and C5 roll-off frequencies beginning at about 7 kHz, so there are not excessive levels of 10.2 - 13.6 kHz "Omega" signals (AND Russian "ALPHA" in the Eastern hemisphere) or military communication signals in the 18 to 25 kHz range, which can create problems with the recording system (particularly cassette-type recorders) connected to the output of the receiver. Mini-Disc or other recording systems capable of broadband recording past 20 kHz will suffer far less problems from ALPHA and mil. comms., and the pi-filter caps can be further reduced to .002 uF - experiment here if you like. R5 to R8 , Q2 and C7 form a fairly low-noise Class-A audio amplifier which boosts the output from Q1 to a level plenty for all microphone-level recorder inputs and even some more "sensitive" line- level inputs. Bypassing R3 (220 ohm) with a 2.2 uF cap will boost FET gain somewhat, esp. the higher frequencies--depending on your location and listening conditions, this may or may not be desireable.
The circuit can be built on perfboard such as IC-LSI boards or even wired point-to-point, as layout is not very critical. However, the parts' values ARE critical for optimum passband shape and sensitivity. It is recommended that this circuit be installed into a metal enclosure for maximum RF shielding.
A note about R1: experiments in early 2001 have shown that lower receiver J-FET temp. noise results with the substitution of the 1 megohm resistor with (first) a 200K folowed by a 150 milliHenri inductor (choke) in series.
L1, the inductor, can either be a 160-200 milliHenry choke or the Radio-Shack 1K to 8 ohm audio transformer available at Radio-Shack (cat. # 273-1380). Use the black and green or black and blue wires (the center-tap and one end of the primary winding).
The audio transformer was used as an inductor, since it is easily available at Radio Shack stores. In fact, ALL the parts are available at Radio Shack. Capacitors C4 and C5 work with L1 to reduce Omega to tolerable levels. Solder the J-FET into the circuit LAST, and take measures to protect the FET from static electricity. The total cost for parts (not including an enclosure) for the BBB-4 are in the neighbourhood of $15-20 U.S.
E-field-probe receivers of this type need to be operated at locations away from trees, buildings, or other obstacles by about 100 feet/30 metres. This is because received signal levels (due to E-field attenuation) will be poor if the receiver is operated too near (or under) such obstructions.
The greatest nemesis to monitoring and recording naturally occurring VLF phenomena are electric AC powerlines, which emit annoying hum at 50/60 Hertz and also harmonics beyond 1 or 2 kHz. The only cure for this "hum" problem it to locate monitoring sites well away from AC powerlines. Locations at least 1 km/0.6 mi. or so away from a.c. powerlines will begin to be acceptable, though the farther you can get from powerlines, the better. Hilly or mountainous terrain (with open areas free of trees) offer larger areas away from powerlines, though large fields and meadows where the powerlines are shielded by trees, etc. may be surprisingly hum-free. (On the other hand, "low-hum" areas can exist remarkably close to or witiin towns and cities if most AC lines are below ground.)
Remote locations such as deep into desert and wilderness regions offer the most rewarding locations, both aesthetically and electrically, to listen, and you may be able to get over 10 miles from the nearest powerline. If so, you can make the receiver's antenna several metres in length (keeping it vertical) for maximum sensitivity. Longer vertical antennas or horizontal wires may either overload the receiver, or in the case of long/low wires, will create a mismatch which will actually reduce output. Experiment here.
NOTE: A 100 milliHenry choke across C5 (the second .0047 cap in the audio filter) will greatly reduce the below 1 kHz frequencies, including pesky power- line hum. This may enable you to listen far closer to AC power-lines including even some backyard locations! Grounding is non-critical. High-impedance FET receivers of this type need only minimal grounding to work well--even just the body of the listener holding the metal enclosure of the receiver will be adequate in most cases. If you wish for really low noise performance in the input stage, replace R1 with a 100 milliHenry choke.
If recording, it is best to stick a 8-10 inch ground rod into the soil to reduce the possibility of feedback with some tape recorders. Also, a small ground-rod (8-10 inches long) will cut noise from body or foot movements (due to capacitive interaction with the ground). If you ground the receiver to objects such as fences, beware that certain grounds may couple AC powerline noise to the receiver, which is why I recommend a simple Earth ground. Better quality tape recorders, with adjustable input level controls, are desirable, as "cheapie" portable recorders with auto-level control will often have annoying variations in record level due to lightning-sferics. And, these cheap recorders also put noise of their own onto the tape. A shielded 600 ohm patch-cord will suffice between the output of the BBB-4 and microphone input of a tape recorder or speaker amplifier.
The most common naturally occurring V.L.F. emissions to be heard are the myriad "crackling and popping" sounds of lightning-stroke electromagnetic impulses (static/sferics) from lightning storms within a couple thousand-km/mile radius of the listener. Since there are nearly 100 lightning storms in progress anywhere on the Earth at any given time, and that millions of lightning strokes happen daily, there is NEVER a moment when these lightning "sferics" will not be heard. However, the density and strength of lightning sferics can vary day-to-day and hour-to-hour. Mid-winter offers the lowest density of sferics, and summer evenings can be full of a dense barrage of strong sferics masking everything else.
The other most common (and most awesome) sounds are "whistlers"--eerie descending tones caused when the lightning electromagnetic energy gets "ducted" along Earth's magnetic lines-of-force (magnetosphere) to the opposite polar hemisphere, then gets rebounded back to the vicinity of the originating lightning stroke impulse. However, there doesn't have to be lightning within sight or even a few hundred miles of your listening location--lightning from storms up to thousands of km/miles away, particularly if more to the north of your location, can generate large whistlers which are heard continent-wide.
On the other hand, it's quite spectacular to watch safely distant lightning storms generate whistlers in the receiver's output--you hear the huge "crack" of the lightning impulse sferic, then, if the conditions to support whistlers are occurring, a whistler may follow from 1 to 2 seconds after the lightning stroke. Optimum times to listen for natural V.L.F. phenomena, such as whistlers, are between sunset and sunrise, with the midnight to sunrise period generally being the best.
Statistically, the greatest activity to be heard is from 2 a.m. to first-light (dawn) - sferics tend to be fairly low as compared to the sunset period. Dawn Chorus can occur during magnetic- storms, and will peak anywhere from an hour before sunrise to 2 hours past sunrise.
Whistlers can occur at anytime, but the period of minimum frequency is midday. Sometimes, activity can also occur just before and after sunset, but sferics will be fiercer. Lightning sferics will be most fierce during summer afternoons and minimum (generally) an hour or so after sunrise until thunderstorm activity picks up later on. Winter can present delightfully low lightning sferics_other activity will be more "in the clear."
Tweeks, the "ringing/pinging" sounds of sferics caused by the Earth- surface/ionosphere "waveguide," will be best from an hour after sunset to 2-3 a.m. local time, gradually tapering off toward sunrise. Their number and intensity of "pinginess" can vary from night to night_some nights they can sound rather "pale," but other nights they can ring in a variety of beautiful mixtures and pitches. Whistlers, which may or may not be heard on some days or even weeks, can range in sound from quite pure notes to very diffuse "breathy" sounds. They can swoop in frequency from very high to low, or abruptly cut-off as they descend in pitch.
If you live north of the about latitude 45 in the US and Canada, or in northern Europe, or extreme southern Australia and New Zealand (closer to the auroral zones), you will likely be able to hear interesting natural radio activity about 50% of the time, especially during magnetically disturbed/storming periods. For those farther south, don't be discouraged if you listen for several hours, or several sessions on different days, without hearing whistlers or other natural radio phenomena. When you DO hear them, it will make up for the "dry" times, as there is nothing like "live" listening! Listeners located north between 40-55 degrees north or south latitude are in the optimum latitudes for monitoring natural V.L.F. phenomena.
If you can see visible Aurora (Northern/Southern Lights) from your location, you are at a great location for natural V.L.F phenomena monitoring! Latitudes between 20-30 degrees north and south will hear less, but at times, still loud phenomena. I've heard whistlers just fine in Hawaii_presumably those whistlers were louder farther north, but still, they were heard! DO NOT operate this receiver (or any other) when nearby lightning threatens! Take appropriate lightning precautions when lightning is occurring nearby (within 5-10 miles).
Nearby lightning will cause excessively loud sferics in the receiver's output, and whistlers will not be louder just because lightning is close-by. Reserve listening for fair weather periods_most often, the best and loudest natural V.L.F. phenomena will happen during clear weather, since lightning can be quite distant, as mentioned above, and still spawn loud whistlers.
1) A 100-200 milliHenry inductor connected across C5 the will act as a high-pass filter, nicely attenuating 60-360 Hz powerline emissions (hum). Experiment here.
2) If you want less gain from the Q2 stage (and slightly lower noise), reduce R8 to 4.7K.
3) Listen to WWV-shortwave (2.5, 5, 10, 15, 20 MHz) for geo- magnetic indices at 18 min. past each hour (WWVH-Hawaii at :45). A K- index at or above 3 indicates enhanced conditions for natural phenomena, especially chorus.
4) The U.S. NOAA/USAF/Space Environment Center (SEC) runs a fantastic Website with geomagnetic indices and other information available at:http://sec.noaa.gov. Lists the past 30+ days' geomagnetic conditions, proton and electron fluences, and so forth. There seems a rough correlation between high electron fluences, low K-indices (especially following a magnetic-storm period) and louder whistlers at middle latitudes-- look for fluences at 1.0E + 07 or higher. This seems to folllow best during equinoctal periods (autumn, spring). Periods when the solar flux is rising also seem to be times when whistlers occur more often. Nights of fabulous whistlers can be unpredictable and can't be reliably predicted via "indices," (but it seems lower K indices are better if following a period of higherK indices) so if you have a chance to listen and it's past sunset, do so! Again, high K-indices over 3-4 mean probable chorus (auroral chorus at higher latitudes and dawn chrous at mid. latitudes around dawn and local sunrise. Conditions which hinder HF propagation make Natural Radio come alive!
FINAL NOTE: This E-field VLF receiver will not work with ham or SWL antennas, nor loop antennas, nor longwires. As it is presented herein, it will not receive man-made broadcasts/transmissions such as military VLF stations in the 16-25 kHz spectrum, nor earthquake (ULF) signals below 5 Hz, as some have previously asked me.
If you want to record your samples and process them on the computer, the recording equipment and software is as important as the receiver itself. Depending on the frequency spectrum you want to receive and the quality of recording, you have to choose the appropriate recorder. Battery life is important for mobile applications too. Have in mind that tape recorders may induce their own noise into the recording in such low signal levels, so it is better to choose a better quality recorder.
Mini Disk is one of the best quality recorders, if you record without compression. It can record signals beyond the human earring (you cannot listen to them but you can see them on the computer spectrogram). Even when using some sort of low compression it will be ok, since the bandwidth of the receiver is not beyond the human earring. The only problem with mini disk is that it has mechanical moving parts (motor), so the battery life is limited and that it can record for a limited time. Also, the medium allows for limited recording time. By using a mini disk, you cannot leave your equipment outside in the night recording for more than 1 - 2 hours (stereo/mono rec)
To overcome this problem in my approach, I used a digital dictation recorder, the cheaper that I could find, the Olympus VN-5500. These recorders consume very little power they are easy to operate and they can quickly erase and write tracks. You can use any digital recorder you can find but take care of the recording qualities. My recorder has 3 recording qualities. This basically specifies the bandwidth of the recording. This bandwidth must not be too narrower than the bandwidth of the receiver because you could loose valuable audio signals. In fact you will find the best recording quality by trial and error. For example my recorder has three recording qualities (the finest has 300Hz - 7KHz bandwidth) but I found in many cases the medium quality better for tweeks, as it induces less noise from the background.
Specifications of the VN-5500:
512Mb built-in memory, 300 hours recording time in LP mode, about 22 hours in HQ mode, 4 folders, Voice activated, Scheduling function, LCD display, Headphone and microphone sockets, Built-in microphone and speaker, Requires 2 x AAA batteries, Size (H)10.2, (W)3.6, (D)2.05cm, Weight 63g, 512Mb built-in memory.
The only drawback of this recorder is that it has not got a USB port to write the samples directly to the computer, so you need to connect it to the line input of your sound blaster and re-record the samples using an appropriate recording software. USB connectivity is available in more expensive recorders.
There are a lot of software packages which will help you record your samples in your computer. I have found a freeware program that lets you record samples from your recording equipment to your computer, edit them and convert them to mp3. The software is called WavePad and you can download it here.
This is a series of recordings that I have made through the years. Usually, each recording was done in a specific place a specific day, so I have sorted out these recordings based on the dates and places.