In earlier days for reception of VLF signals you either had to buy or to build an appropriate receiver or converter. Today it is much easier; you only need a standard PC with a sound card (Pentium processor and clock speed of 100 MHz or more is also required) and a few additional devices.
Every PC (with Pentium processor and a beat frequency of 100 MHz and more) with a built-in sound card can be converted with the usage of only a little additional equipment into a powerful receiver for frequencies below 24 kHz! As hardware only an appropriate aerial, as software a spectral analysis program, which is available on the internet as freeware, is required.
The use of an inductive aerial is recommended for VLF reception. This aerial should consist of a coil with as many windings and as large a diameter as possible.
The electrical and mechanical properties of the coil, its inductance, resistance in ohms, number of turns, etc. are of low importance and may vary in a widely. Therefore its manufacture is very easy. You only need one roll of insulated wire, which is connected to the LINE or MIC input of the sound card with a cable at least 2 metres long. As cable you can use any type with two conductors (although shielded or coaxial cable should be quieter).
In many shops ready-soldered plugs can be bought for all types of PC sound card inputs and you may need not even have to solder the necessary plug for the sound card input by yourself.
For good reception the coil should be placed as far away as possible from the PC (if possible at least 2 metres) and from other electrical devices, with the axis being parallel with the ground.
In buildings made of steel and concrete a place near the window is the best choice.
It is possible, if you want to achieve a higher sensitivity, to connect two or more coils in series. You have to mind the sense (clockwise or counterclockwise) of the windings, otherwise the induced signals cancel each other out.
Setting of sound card inputEdit
After the aerial was connected to the sound card input, the signal input to be used must be selected at the PC. Therefore click with the right mouse button on the loudspeaker symbol at the right end of the Windows task bar.
A popup menu appears, on which you have to choose the option for "loudness" or "Open Volume Controls (w2k)". On the dialog, which is now shown, click on the menu point for "options" and then choose from the sub menu the point "properties". You will see a second dialog on the screen, click the button for "recording" and click afterwards on the "OK" button. Now you can set the sound card entrance you want to use. For VLF reception, the balance control of this entrance should be put in the middle and the loudness regulator to maximal loudness. All these settings can be changed of course later, if e.g. the spectral analysis of a sound signal shall be done, which is fed via an other soundcard entrance in the PC.
General considerations about the reception technique and analysis softwareEdit
There are many programmes available for the spectral analysis of signals, which come via the sound card in the PC (look http://www.vlf.it/harald/strangerec.htm). My favourite software is "SpecPlus", since this extremely powerful software offers the possibility to save the received signals as frequency/time/intensity diagrams (such called spectrograms) automatically in a disk space saving way as jpeg files. Besides this, "SpecPlus" runs very stable, when a Windows95, Windows98 or Windows ME operating system is used. (I do not know how well it runs under Windows NT 4.0, Windows 2000 or Windows XP. It does not work under Windows3.x!) Monitors of PCs generate strong noise in the VLF range. For this reason it is very sensible not to view the spectrograms directly on the screen, but saving them as file on hard disk and viewing them at a later point of time. Therefore automatically working with monitor switched off is the best operation mode. Between midnight and 6o'clock local standard time is usually the best time for VLF reception, because most electric devices are switched off at that time and the level of disturbing signals is therefore lowest. As a corollary, the PC is probably not being used for other tasks at these hours, so it can be used for VLF reception!
The Software "SpecLab"Edit
"SpecLab" is downloadable as zip file from http://www.qsl.net/dl4yhf/spectra1.html. After successful download and extraction with WinZip in an installation directory, the software can be installed, experience shows that no troubles occur. The program runs under Windows 98, 2000, ME, XP, Linux/WINE, but not under Windows Vista.
After successful installation some settings have to be done. First you should set the colour palette. There are many false colour display modes available. Although false colour displays can look like somewhat pop, I prefer for good reason a mode which shows the signals black and the background white. First, such a display mode can be easily understood without knowledge of the colour palette used and you can, if you want, compress the received jpeg files further with other programmes without loosing information (if you use a display in false colours the colours can easily changed by this process. This effect is not desired at all, because the colours show the intensity of the signal, so the information is lost in compression. For the setting of the colour palette a file containing an appropriate colour palette is load via the menu point "Option" and the submenu point "Load Color Palette". Now the sampling rate is adjusted. Choose therefore the menu point "Option" and then the submenu point "Audio Settings". The sampling rate determines the value of the highest frequency you can receive. Its value is: F = sampling rate / (2 * Input Sample Rate Divisor). For a maximum reception frequency of 24 kHz (customary sound cards cannot process input signals with higher frequencies), you have to set the sampling rate on 48000 Hertz and the Input Sample Rate Divisor on 1. Then you have to fix the FFT properties. Choose therefore under "options" the menu point "FFT Settings". For VLF reception you must set FFT Output on "Logarithmic" and for FFT type you should choose "Real Number FFT Starting at 0". At last the display mode is adjusted. Therefore, select "Display Spectrum display Settings" submenu via the "Option" menu.
Set the amplitude range on the dialog which will now appear for VLF reception between -130 dB and -60dB. Experience shows that this range is very sensitive for the display of VLF signals, because stronger signals do normally not occur and the noise of the sound card has a value in the order of -110dB to -120dB. There are some further adjustments possible in this window (look at the picture).
Finally you have to adjust in the main dialog with the controls "B" (brightness) and "C" (contrast) below "Color Palette" the best value for the sensitivity. Disconnect or short-circuit your aerial for this purpose and move the control "B" so that the background just appears white. Set the contrast of the display with the control "C". You can set it in such way, that small differences in signal intensity appear or not. Because of the fact that the settings of the regulator "B" and "C" influence each other, you have perhaps to do these steps several times, until you are content with the display. Afterwards your PC is ready for VLF reception!
For automatic operation you have to choose under the menu point "File", the submenu point "Periodic actions".
With the settings shown here every 40 seconds a graphic file showing the spectrogram with the name "ns" + running number(starting with 1) + ".jpg" is written into the directory g:\vlf\07032002. (I assign with the two letters at the beginning of the file name the direction, in which the axis of the reception coil shows: 'ns' is for north/south direction, 'ow' for east/west direction'. This is very sensible, because the used aerial shows, as does every inductive aerial, a strong directional characteristic.) Keep in mind, that the directory you mention, must exist and that there is a toggle at "active". Otherwise no recordings are done!