Latest 2014 Sub-9kHz News for the up-to-date information.
There is growing interest in amateur radio operation below 9kHz in the so-called "Dreamer's Band". Before 2010 few expected any real distances could be achieved by radio amateurs at VLF. Radiated powers are low, antennas highly inefficient and everything was stacked against success. However, with the advent of powerful weak signal processing software and dedicated work by a few enthusiasts, this has all changed. Some spectacular results have been, and continue to be, achieved. These pages bring together this work. I hope you find the pages both interesting and inspiring.
The Dreamer's Band covers 8.7-9.1kHz in the UK. This is the frequency band allocated for experimental use by Notice of Variation to the Amateur Radio licence. At the time of writing only a handful of UK stations are active on VLF. In Europe, where licence restrictions are less onerous, it seems more stations are becoming active every week. There is also interest in other parts of the world including Alaska, mainland USA, Australia and Japan. A number of stations have set up so-called "grabbers", receivers connected to the internet so that transmitting stations and others interested can see who is being copied and where.
At VLF, the receiving system can be very simple. It is quite different from traditional HF radio because reception is at near audio frequencies and external noise limits reception not receiver noise figure. A suitable small antenna, such as a voltage probe or loop, is fed via a pre-amplifier (with high dynamic range) into the sound card of a PC. This is all that is required as all detection and signal processing is then done in the PC using software. Locking the software-based receiver to a VLF MSK station or a GPS reference signal ensures great frequency accuracy and stability. These are essential requirements when looking for very long duration, and weak, signals buried in noise. Bandwidths may be measured in micro-Hertz, so it is vital to know precisely where a station is transmitting to a fraction of a Hertz. The image above shows a typical VLF screen grab using Spectrum Lab. Signals are too weak to hear, but are visible over a period of minutes or hours as lines on the screen at distinct frequencies. Notice how closely the signals are spaced: the figures on the right are Hz, not MHz.
To radiate a VLF signal is another story: it is no mean feat. Several carefully engineered pieces have to come together to ensure success including (a) as large an antenna system as possible, (b) a very stable frequency source that is stable to a few uHz over many hours, (c) high transmit power, (d) efficient antenna matching systems with very low losses, and (e) as effective a ground system as possible. Few radio amateurs are able to achieve this, especially from a home QTH. However some have and it is these pioneers that are creating new opportunities for amateurs in this exciting part of the spectrum. Among these pioneers is Stefan DK7FC whose kite supported Marconi vertical is shown on the right.
At the moment little traditional amateur radio communications is taking place at VLF. Signals are too weak, in most cases, for anything other than a continuous carrier or slow QRSS or DFCW to be detected and identified by time, frequency and possibly bearing. One challenge at VLF is to find a way of exchanging real 2-way QSO information in a sensible time when durations of hours, or possibly days, may be needed for a signal to be integrated sufficiently to confirm correct reception at all. Over shorter ranges, and in some exceptional DX cases, QRSS3 or QRSS30 or 60 may be possible. Only a couple of German stations have managed to exchange real QSO data and this was over a distance of around 20km.
For local tests, a small stand-alone receiver can be useful and suitable circuits are described elsewhere on this site. For some applications, up-conversion to an HF receiver can be useful but then frequency errors may be compounded. Most people use preamps directly feeding a PC soundcard.
Another aspect of the VLF scene is earth mode communication using the soil or rock beneath our feet as a communications medium. Everyone can do this as equipment is simple and results are encouraging over ranges of a few kilometres. Moreover, the equipment is almost identical to that needed to communicate by radiated signals at VLF except that a large antenna in the air is not required.
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