Monitoring utility transmissions in the ELF and VLF range, are gaining increased popularity. And contrary to common belief, it is also quite easy. Unlike the enormous antennas needed on the transmitter side, the VLF monitor, will get very good results with an E-field probe, a simple wideband loop antenna or just a piece of antenna-wire.
Detecting ELF transmissions are a bit more tricky. To do so, you need either a special ELF E-field probe, an induction coil with 1k++ turn, - or a large piece of land, where it is possible to install a ground dipol.
The receivers used for VLF monitoring, reflects human creativity. As 2/3 of the VLF signals, actually are within the range of audio equipment, numerous mic and RIAA preamps, has been converted for VLF radio-reception. Others has taken the more technical step and are using Frequency Selective Level Meters as “longwave” receivers, with excellent results. And of course we have the various commercial receivers with tuning range down to 10 kHz. But if your main interests are in the VLF range, you will have much better value for money investing in DSP filters and a good loop antenna.
The use of soundcards and PC’s, make it even easier to monitor, the various utility transmissions below 30 kHz. For details on how to make yourself a “soft” VLF receiver, see the chapter; A SOFT RECEIVER FOR VLF USE, at the end of this paper.
The majority of the transmissions, you can pick up in the VLF range, are of military origin. Most of them are intended for surface and subsurface naval forces. As the radio-signals in the VLF range penetrate water, they are very useful as one-way control and command links, for submarine forces. US NAVY stations like NAA NCTS Cutler and NWC NCS Harold E. Holt, as well as VTX3, the Indian Navy station INS Katabomman, has almost global surface coverage.
As most of these stations use encrypted transmissions, it can be hard to identify what you have tuned into. Almost no ID’s are given and in most cases, you must identify the station, by its transmission pattern mode and use of frequency.
To help with this identification, also non-active stations are listed. At least you know what you NOT has tuned in on. Joke aside, during periods with high naval activity, it is not uncommon to receive transmissions at frequencies, not normally carrying traffic. Several of the VLF stations around the globe, are also at irregular times performing tests, on other frequencies, then their normal frequency of operation.