Large high-power VLF transmitters used for submarine communication (typically about 15-30 kHz) generally transmit digital information by deliberately changing the phase and/or frequency of the carrier. Straight amplitude modulation is unattractive because these transmitters and their enormous antenna systems work most efficiently at nearly constant power.
MAIN MODULATION METHODS
- FSK – Frequency Shift Keying
The carrier moves between two (or sometimes more) closely spaced frequencies to represent digital data.
FSK is robust and relatively straightforward to receive.
- MSK – Minimum Shift Keying
MSK is a continuous-phase form of FSK.
It maintains a nearly constant signal amplitude and occupies relatively little bandwidth.
This makes it particularly useful at VLF.
- PSK – Phase Shift Keying
Information is encoded by changing the phase of the carrier.
Some military VLF systems use sophisticated forms of phase modulation.
- VERY NARROW-BAND CODED SIGNALS
Because the available bandwidth at VLF is extremely small, submarine messages are normally transmitted at very low data rates.
Error correction and coding can be used to improve reliability.
WHY IS THE BANDWIDTH SO SMALL?
Consider a transmitter operating at 20 kHz.
Wavelength:
wavelength = speed of light / frequency
wavelength = 300,000,000 / 20,000
wavelength = 15,000 metres
or about:
15 km
Even an enormous VLF transmitting antenna is therefore electrically very short compared with the wavelength.
The antenna system has a high Q and consequently a very narrow bandwidth.
Rapidly changing the carrier amplitude, or moving the transmitting frequency by a large amount, is therefore difficult and inefficient.
WHAT YOU MAY SEE WITH RADIO SKY PIPE II OR WITH SUPERSID SOFTWARE OR OTHER APPROPRIATE SOFTWARE.
When receiving submarine VLF transmitters with Spectrum Lab, the signal may not always appear as a single perfectly stationary line.
Depending upon the transmitter and its operating mode, you may see:
- Small frequency shifts
- Multiple closely spaced frequencies
- Sidebands around the carrier
- Changes in signal phase
- Regular patterns caused by digital modulation
- Periods of unmodulated carrier between transmissions
EXAMPLE VLF STATIONS
GBZ 19.58 kHz United Kingdom
GQD 22.10 kHz United Kingdom
DHO38 23.40 kHz Germany
NAA 24.00 kHz United States
These powerful transmitters are particularly useful for amateur VLF observations because they can also act as stable signal sources for studying changes in propagation through the ionosphere.