Listener's Guide

Very Low Frequency Radio

A world of radio listening on the longest of long-waves.

VLF, SFERICS, Lightning And A Funny Story

The clever folk at the UK Met Office devised an amazing system for plotting the location of thunderstorms by detecting the E-field generated by lightning strokes in the frequency range around 10kHz.

These VLF signals travel for thousands of kilometres, and by using a network of receivers and accurate timing measurements it is possible to locate the position of a thunderstorm from as far away as West Africa or the Amazon basin.

So far, so good, and all hail the cleverness. However, as part of my work load a few years back I had to run the EMC ruler over the Met Office ATD/NOS receiver in order to fulfil the EU regulations on CE marking, and as the twelve-foot long vertical antenna rod was brought into the test facility my thoughts lightly turned to my work on comparing active rod antennas with loop antennas.

This had proved quite conclusively that, certainly for low frequency listening, a loop antenna was far and away the best thing to use because of its insensitivity to E-field interference. Of course this interference was the very signal that the Met Office were trying to detect, hence their very proper use of an active vertical rod antenna.

However, (another however. Have as many as you like, John! - Editor) if I was in the vicinity of a thunderstorm, the very last thing I would want around is a twelve foot rod inviting Thor to cast down a mighty thunderbolt and strike me dead – but on to the EMC testing. One of the standard tests (EN 61000-4-5 for those brave enough to look it up) is known as the Surge test and its main function is to simulate the effects on equipment under test of a nearby lightning strike. Such events can generate huge transient currents in cables and structures and resultant high voltage discharges in and around the poor item being tested.

So, here we had a rod antenna with a high impedance active amplifier mounted at its base, connected to the receiver box by up to 50 metres of screened cable and inviting Thor to do his worst. Well, standing in for Thor, I applied the surge test to the screen of the connecting cable, pressed the start button, and stood back (always a wise thing to do when surge testing). There was an almighty bang and the first surge blew the sh*t out of the active amplifier at the base of the twelve foot rod. OOPS!

It's an interesting problem when the very device designed to detect lightning destroyed itself when lightning was around, and I'm glad that the Met office persevered with the ATD system and placed it into successful service.

As a listener, the episode may make those VLF crunches more interesting when you realise that they may well come from a thunderstorm in India or Brazil. As for me, I will never forget the look of horror on the face of the Met Office man who watched his antenna amplifier disappear in a cloud of smoke.

Happy Days!

John Wilson

Long-Wave Radio - The Long Goodbye

We just heard the news that Germany will be pulling out of long-wave broadcasting. Services on 153, 177 and 207kHz will close, leaving just a few happy DX fans who will be able to see what distance stations break the radio silence.

We old-timers have affection for long-wave. We know about its dependability, the distances it can cover and the comfort it brings on long pan-European car journeys as your voice from home stays with you as you cross border after border. We also know that running a long-wave transmitter is not cheap, so the accountants will be more than happy to see them go.

We got an education from long-wave. We heard our first classical music via Deutschlandradio Kultur on 177kHz, fading in on long winter afternoons. If 153kHz was in the clear, other distant stations could be heard.

We made our first crystal sets to listen to the Light Programme on 1500 metres. We saved our money for a gold-bonded OA47 germanium detector diode and felt very superior. Good to know a precious few still experiment with Schottky barrier diodes, keeping that sense of wonder alive. A radio without batteries?

We built a one-valve regenerative radio for headphones. By feeding back some of the signal allowing it to be amplified again, the results amazed this young listener and still do. Add a second valve and you could drive a loudspeaker. Ours was an EF86 and a 6BW6, in truth nothing more than an audio amplifier, detection taking place at the grid of the first valve.

We still love our long-wave. About 90,000 people still listen to Radio 4 this way, complaining bitterly when Test Match Special takes over in the summer. Even with so many other ways to listen, we listen to TMS on an old Roberts portable radio on long-wave with perhaps a small Pimms, my dear old thing.

The BBC has said that Droitwich is on its last set of valves. They can last fifteen years, they could fail tomorrow but when they go, that will be that. We wonder if other users of 198kHz would save it. Embedded in Radio 4LW's signal is a time and frequency standard, data to help the electricity companies share the load around the country and the iconic Shipping Forecast. Nobody did it better than Charlotte Green, who found nothing amusing at all about winds light to variable.

Technology has moved on and left long-wave behind. Try buying a radio with AM and that will mean medium-wave reception only. Chances are the chap in the shop will neither know nor care about long-wave and wonder why you are asking. We hope that there are old radios up in lofts that can be resurrected for that last long-wave listen.

For us, that's early mornings listening to RTE on 252kHz. Some great music on a waveband with a wonderful history.

VLF - Very Long Wave Radio

Below the good old long-wave is a range of frequencies used as National Standards for frequency accuracy and time.

Nuclear technology has made these very accurate indeed so to convey the pure engineering of these stations, no mathematical shorthand has been used. Down here, we are talking real numbers. Sad to say, these stations are closing down.

The ELF ranges, where the frequencies are so low we could hear them if they were vibrations in air, contain submarine navigation signals. These require antennas so vast that an entire geological feature such as an atoll is used, soaking up many megawatts of power to get a signal through the Earth, not over its surface.

Geostationary satellites are todays more economical solution.

The Dawn Chorus

Occasionally, shortly after sunrise and even extending into the mid-morning, a phenomenon called Dawn Chorus may occur. Dawn chorus can resemble the sound of a flock of birds singing and squawking, dogs barking, or sound like whistlers raining down by the hundreds per minute

Dawn Chorus results from hundreds of overlapping, rapidly upward rising tones that can be continuous or appear in bursts, called chorus trains.

Chorus trains sound fascinating. The bursts of chirps and squawks (risers) seem to suddenly commence, and over the course of two to five seconds, weaken and fade away, then repeat over again, often in different pitches.

Bursts of chorus trains happening at different octaves can overlap in a beautiful cacophony.

Dawn chorus occurred several times a month during years of high sunspot activity after solar flares and/or coronal mass ejections on the Sun send a barrage of charged particles into the Earth's magnetic field, causing a geomagnetic storm and also producing Aurora.

In years of low-sunspot counts and few solar flares, coronal mass ejections from the Sun can still cause magnetic storms once or twice a month. Chorus doesn't always only occur at dawn, especially for listeners located at higher latitudes, particularly in southern and central Canada, Alaska, and in northern Europe.

This auroral zone is source to a vast amount of natural VLF phenomena.

During auroral displays, chorus is often heard, as well as hiss of various pitches, sliding-tone emission which eerily and weirdly rise in pitch slowly over one to several seconds duration.

The chorus which occurs during displays of Aurora is called Auroral Chorus.

SFERICS

The snap, crackle and pop of lightning-stroke electromagnetic impulses from lightning storms within a couple thousand miles of the receiver; the more powerful the lightning stroke or the closer it is to the VLF receiver's location, the louder the pops and crashes of sferics will sound in the headphones.

Several million lightning strokes occur daily from an estimated 2000 storms worldwide, and the Earth is struck 100 times a second by lightning. At times the receiver's output is a cacophony of crackling and popping sferics from lightning strokes originating in storms near and far.

These huge sparks of lightning strokes are powerful sources of electromagnetic (radio) emission throughout the radio frequency spectrum - from the very lowest of radio frequencies up to the microwave frequency ranges and the visible light spectrum.

However, most of the emitted electromagnetic energy from lightning is in the very lowest part of the radio spectrum, from 0.1 to 10 kHz.

The radio pulses produced by lightning strokes travel enormous distances at these very-low radio frequencies, following the surface of the Earth as ground waves.

It is interesting how generally quiet and lightning sferic-free the hours are from just after sunrise to mid- morning, when thunderstorms tend to be at their minimum.

Later, the crackling and popping of lightning sferic activity picks up as afternoon thunderstorms build in numbers and intensity.

Weather monitoring agencies employ special receivers and direction-finding equipment in order to determine where lightning strikes are occurring and the potential for wildfire ignition, hazards, to aviation and damage to electric power utilities from those lightning strikes.

The Curious World Of Very Low Frequency Radio

A whistler, as heard in the audio output from a VLF whistler receiver, generally falls lower in pitch, from as high as the middle-to-upper frequency range of our hearing downward to a low pitch of a couple hundred cycles-per-second (Hz).

Measured in frequency terms, a whistler can begin at over 10,000 Hz and fall to less than 200 Hz, though the majority are heard from 6,000 down to 500 Hz.

Whistlers can tell scientists a great deal of the space environment between the Sun and the Earth and also about Earth’s magnetosphere.

The causes of whistlers are generally well known today though not yet completely understood. What is clear is that whistlers owe their existence to lightning storms. Lightning stroke energy happens at all electromagnetic frequencies simultaneously that is, from DC to Light.

Indeed, the Earth is literally bathed in lightning-stroke radio energy from an estimated 1,500 to 2,000 lightning storms in progress at any given time, triggering over a million lightning strikes daily.

The total energy output of lightning storms far exceeds the combined power output of all man-made radio signals and electric power generated from power plants.

Whistlers also owe their existence to Earth’s magnetic field (magnetosphere), which surrounds the planet like an enormous glove, and also to the Sun.

Streaming from the Sun is the Solar Wind, which consists of energy and charged particles, called ions. And so, the combination of the Sun’s Solar Wind, the Earth’s magnetic field surrounding the entire Planet (magnetosphere), and lightning storms all interact to create the intriguing sounds of whistlers.

The writer remembers a Practical Wireless project in the late Sixties for an ELF radio. The new Ferroxcube transformer cores formed the base of the untuned coils working somewhere below 9kHz.

The gain came from three stages of OC71's, an OA47 detector and my last OC71 to drive the headphones. The thing was alive.

Screaming whistles and whale-like howls tracked the course of electrical storms across entire continents - they showed me the magnetic changes brought on by the movement of the Earth's tectonic plates could be heard, but by now I was too scared to listen.

The Thing was assigned to The Twilight Zone in the attic.