Receiving VLF Radio Signals

Page banner graphic: sketch of a VLF base by Andrew Westcott, M0WAN

By Andrew Westcott 'M0WAN'

Introduction To Amateur Radio
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80m Antenna On A Postage Stamp
40 & 80m combined inverted V
• Receiving VLF Signals •
PLT Interference
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Introduction To VLF

I have many odd interests, but possibly none rival this particular subject for sheer 'oddness'. Few people are aware of the presence of Very Low Frequency (VLF) radio signals and earth currents, and probably even fewer could really care about it. However, there are those who have a serious interest in such things, and actively pursue the monitoring and recording of the effects, whilst theorising on how they might be caused, so at least I can rest reasonably reassured that I'm not completely alone here! My knowledge and experience of natural VLF phenomena is rather limited, but I enjoy the opportunity to set up my home made detection equipment somewhere away from built up areas and to just sit and listen to the variety of sounds, waiting for that really unusual one to come along. A bit like fishing perhaps...

I first became interested in these natural VLF signals when I accidentally stumbled across them whilst conducting experiments in earth current communication, after reading about it in an old copy of Practical Wireless. The article mentioned how bases were set up during the First World War for communication between the trenches, and I found the technique used very interesting to the point of attempting it myself.

Without going into too much detail, a signal is fed into the ground via two earth stakes set some 25 metres or so apart, and the signal can be received some distance away by using another pair of electrodes and some amplification. As it happened, I didn't manage to get much distance during my tests - perhaps a couple of hundred yards - partly due to a high level of mains hum and other noise masking the quiet signals, but it did give me the chance to hear for the first time the sound of atmospherics which I almost instantly decided to investigate at the expense of any further trials in earth current signaling.

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About Natural VLF Radio

Very low frequency radio waves, way below the reach of normal radio receivers and so low that they fall into our hearing range, are surrounding us all the time. Using the right apparatus, it is possible to detect these signals. These signals divide broadly into two classes:
  1) Those of a man-made origin,
  2) Those created by entirely natural phenomena.

Those signals falling into the man-made class consist mainly of two types:
Firstly, mains 'hum' caused by fields radiated from the mains electricity supply lines, known in the U.K. as the National Grid. These alternating currents have a fundamental frequency of 50Hz here in the U.K., and unless steps are taken to site the receiving equipment in a remote location, these currents represent the bulk of what is detectable and indeed can present a major obstacle to detecting the fainter, natural signals. It is relatively easy to filter out the fundamental, but the higher harmonics can still present problems, extending as they do well into the audible range.

Secondly, radio transmissions in the VLF or ELF part of the electromagnetic spectrum manifest themselves as high pitched whistles of varying frequencies as well as signals beyond our hearing range. Removing these, assuming they are of no interest, is important as at times they may be so powerful as to saturate the front end of the equipment so a good measure of filtering should be applied to combat this. Nowadays most of the navigational aids which used to occupy these frequencies have been closed down in favour of modern navigational aids, but the presence of some may still cause a problem.

A sonograph displaying a whistler graphically, image created by Andrew Westcott, M0WAN.

A sonograph of a whistler, showing the characteristic falling frequency, represented by the sweeping curve on the graph.

Signals resulting from natural phenomena are many and varied, with various theories devised to explain them. These signals, in which I am particularly interested, conveniently fall into the audio range and are audible in appropriate equipment as various odd noises such as crackles and crunches, set in a background of noise which has been well described as being not totally unlike the sound of frying bacon.

Occasionally, eerie noises consisting of a whistle falling in frequency, such as you might imagine a passing military shell to make, occur, these interestingly enough being known as 'whistlers'. No doubt lightening strikes at various locations around the globe account for most of the noises and studies have shown that whistlers are probably initiated by such strikes, and a look at the sonograph on the right seems to verify this, with the thick dark vertical band on the left representing a lightning event, and the sweeping curve following it is the sound of the resultant whistler, the graph showing nicely the characteristic falling frequency. The many narrow vertical bands are caused by various minor crackles, known as 'sferics', an abbreviation of 'atmospherics'. It has been suggested that mechanical stress on certain types of rock could also produce electric currents by the process of piezoelectric generation, and the sudden fracturing of a deep layer of piezo-reactive rock could also produce sudden electrical noise, although these signals would mainly be confined to electric currents within the earth rather than launching as radio waves.

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The Magnetosphere And Lightening

A diagram of the Earth's magnetosphere, placed here by Andrew Westcott, M0WAN, with permission of NASA

A diagram of the Earth's magnetosphere.
Reproduced with the permission of NASA

Our planet, Earth, generates a magnetic field around it known as the magnetosphere, and this is known to extend for a minimum of around a quarter of a million miles out into space, but as the diagram shows, the solar wind distorts the shape of this field in such a way that it is dragged out behind the Earth in a direction away from the Sun, and in this direction extends for well over two million miles. This 'force field' surrounding the earth represents one of the ingredients for some interesting effects.

Now, it has been estimated that as many as one thousand thunderstorms are active in the atmosphere at any one time, and each of the lightening strikes releases a huge amount of electromagnetic energy which ranges in frequency from almost zero right up to light and beyond. It is now assumed that these lightning-initiated radio bursts may propagate around the magnetosphere by various means, returning some seconds later after travelling a vast distance, exhibiting a degree of frequency separation in much the same way as light passing through a prism is split into an ordered spread of frequencies, or colours. This frequency splitting is thought to be what gives rise to the falling tone of a whistler, the initial wideband burst being split into its range of frequencies by the long passage around the magnetosphere.

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The Equipment

There are two relatively easy ways to detect these VLF signals: Either by a pair of earth probes to intercept the current induced in the ground by the passing radio wave, or directly from the air using an antenna. My first experiments used the probe method as this involved simpler circuitry but I discovered a high level of mains interference which seemed almost impossible to get away from so I turned my attention to trying to detect the electrostatic field of the radio waves using an antenna which offered a far reduced level of mains hum. I've purposely decided to go into some detail here as some may wish to re-create the circuitry used, although at this point I'd like to make a little disclaimer about my electronics knowledge: Although I do have a good understanding of the basics of electronics, the circuitry shown here is basic and could probably be improved, and some components could possibly even be removed, but it works, and I doubt much real improvement could be realised by making any modifications to what I've built.

A photo of my completed pre-amplifier board, by Andrew Westcott, M0WAN

A photo of the front end of my detector

All that is required for receiving signals via a pair of earth probes is an audio amplifier, but the situation is a little more complicated when designing for reception via an antenna, simply because of the need for a far higher input impedance. In practice, with both methods there is the mains hum which needs to be reduced to a sensible level, and higher frequency radio signals need to be filtered out to prevent intermodulation products finding their way down into the audio spectrum and producing phantom signals which aren't really there.

The photo on the left shows my receiver's 'front end'. It has been constructed on a piece of copper laminate board which was cut to fit inside an air rifle pellet tin, to provide overall shielding against interference and feedback from later stages of amplification which proved necessary due to the high 10 megohm input impedance, and the high gain. I have used FETs all round in this section for their linearity, ease of biasing and low noise performance. Oh, and I happened to have several 2N3819 devices kicking around...

The input to the board is at the top side and a hole (hidden behind the screen) is provided to fit onto the rear of a terminal post which will protrude into the tin from the equipment box front panel, this providing the support for the board. The metal screen was fitted to isolate the input of this first stage from the rest, and proved beneficial in improving the stability of the circuit as a whole although it did initially produce some microphonic effects which were cured by soldering a thick wire between the sheild and the case to mechanically brace it.

The circuit consists of a high impedance buffer stage forming the input, feeding two stages of conventional FET amplification, with a final buffer stage to provide a low impedance output to drive the following passive filter stages. I later fitted some basic RF filtering to control some slight RF breakthrough I experienced, but it wasn't essential as linearity seemed good and little RF demodulation was taking place.

Preamplifier circuit diagram by Andrew Westcott, M0WAN
Circuit diagram of the front end of the receiver.

The whole receiver was built using the 'bird's nest' principle, enabling me to avoid all that tedious messing about with etching fluid, and this approach also enabled me to easily modify the circuitry, which did in fact prove useful after some field tests.

Photo of the interior of the receiver by M0WAN - Andrew Westcott
Photo of the interior of the receiver, showing the separate modules.

Looking at the photo above, the preamp module can be seen in the lower right, mounted in its screening tin minus the lid; To the left of that is the output amplifier, a standard circuit built around a TDA2003 audio chip; To the left again can be seen a small jumble of components including the orange capacitors - this is the record output stage built around the output level control. Of the two tag boards at the top, the left one is a low pass filter with a following buffer stage having a -6dB point around 15KHz to aid in attenuating ultrasonic frequencies, and the right hand board is a switchable high pass filter with following buffer stage with a -6dB point at around 300Hz to assist in reducing mains hum and its lower order harmonics.

I have plenty more to add to this page, so please consider it incomplete, as is the case with most of my pages, but it all takes time! I intend to make available here some recordings of the phenomena I have detected along with some more info on the subject, and will also be placing some useful links here for those who want to pursue this further.

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