Editing Very low frequency (section)

== Modulation ==
Because of the small [[bandwidth (signal processing)|bandwidth]] of the band, and the extremely narrow bandwidth of the antennas used, it is impractical to transmit [[audio signal]]s ([[amplitude modulation|AM]] or [[frequency modulation|FM]] [[radiotelephony]]).<ref name="Holtet">{{Cite conference |date=17–27 April 1974 |editor-last=Holtet |editor-first=J.A. |title=ELF-VLF Radio Wave Propagation |url=https://books.google.com/books?id=K3HyCAAAQBAJ&pg=PA373 |conference=NATO Advanced Study Institute |publisher=Springer Science and Business Media |pages=372–373 |isbn=9789401022651 |place=Spåtind, Norway}}</ref> A typical AM radio signal with a bandwidth of 10&nbsp;kHz would occupy one third of the VLF band. More significantly, it would be difficult to transmit any distance because it would require an antenna with 100 times the bandwidth of current VLF antennas, which due to the [[Chu-Harrington limit]] would be enormous in size. Therefore, only text data can be transmitted, at low [[bit rate]]s. In military networks   [[frequency-shift keying]] (FSK) [[modulation]] is used to transmit [[radioteletype]] data using 5&nbsp;bit [[ITA2]] or 8&nbsp;bit [[ASCII]] character codes. A small frequency shift of 30–50&nbsp;hertz is used due to the small bandwidth of the antenna.

In high power VLF transmitters, to increase the allowable data rate, a special form of FSK called [[minimum-shift keying]] (MSK) is used. This is required due to the high {{mvar|[[Q factor|Q]]}} of the antenna.<ref name=NAVELEX-0101-113/>{{rp|pages=&nbsp;3.2–3.4, §3.1.1}} The huge capacitively-loaded antenna and [[loading coil]] form a high {{mvar|Q}} [[tuned circuit]], which stores oscillating electrical energy. The {{mvar|Q}} of large VLF antennas is typically over 200; this means the antenna stores far more energy (200&nbsp;times as much) than is supplied or radiated in any single cycle of the transmitter current. The energy is stored alternately as [[electrostatic energy]] in the topload and ground system, and magnetic energy in the vertical wires and loading coil. VLF antennas typically operate "voltage-limited", with the voltage on the antenna close to the limit that the insulation will stand, so they will not tolerate any abrupt change in the voltage or current from the transmitter without arcing or other insulation problems. As described below, MSK is able to modulate the transmitted wave at higher data rates without causing voltage spikes on the antenna.

The three types of [[modulation]] that have been used in VLF transmitters are:
;[[Continuous Wave]] (CW), Interrupted Continuous Wave (ICW), or On-Off Keying: [[Morse code]] [[radiotelegraphy]] transmission with unmodulated carrier. The carrier is turned on and off, with carrier on representing the Morse code "dots" and "dashes" and carrier off representing spaces. The simplest and earliest form of radio data transmission, this was used from the beginning of the 20th&nbsp;century to the 1960s in commercial and military VLF stations. Because of the high [[Q factor|antenna {{mvar|Q}}]] the carrier cannot be switched abruptly on and off but requires a long time constant, many cycles, to build up the oscillating energy in the antenna when the carrier turns on, and many cycles to dissipate the stored energy when the carrier turns off. This limits the data rate that can be transmitted to 15–20&nbsp;words/minute. CW is now only used in small hand-keyed transmitters, and for testing large transmitters.
;[[Frequency-shift keying]] (FSK): FSK is the second oldest and second simplest form of digital radio data modulation, after CW. For FSK, the carrier shifted between two frequencies, one representing the binary digit '1' and the other representing binary '0'. For example, a frequency of 9070&nbsp;Hz might be used to indicate a '1' and the frequency 9020&nbsp;Hz, 50&nbsp;Hz lower, to indicate a '0'. The two frequencies are generated by a continuously running [[frequency synthesizer]]. The transmitter is periodically switched between these frequencies to represent 8&nbsp;bit [[ASCII]] codes for the characters of the message. A problem at VLF is that when the frequency is switched the two [[sine wave]]s usually have different [[phase (waves)|phase]]s, which creates a sudden phase-shift transient which can cause arcing on the antenna. To avoid arcing, FSK can only be used at slow rates of 50–75&nbsp;bit/s.
;[[Minimum-shift keying]] (MSK): A [[continuous phase modulation|continuous phase]] version of FSK designed specifically for small bandwidths, this was adopted by naval VLF stations in the 1970s to increase the data rate and is now the standard mode used in military VLF transmitters. If the two frequencies representing '1' and '0' are 50&nbsp;Hz apart, the standard frequency shift used in military VLF stations, their phases coincide every 20&nbsp;ms. In MSK the frequency of the transmitter is switched only when the two sine waves have the same phase, at the point both sine waves cross zero in the same direction. This creates a smooth continuous transition between the waves, avoiding transients which can cause stress and arcing on the antenna. MSK can be used at data rates up to 300&nbsp;bit/s, or about 35&nbsp;[[ASCII]] characters (8&nbsp;bits each) per second, approximately 450&nbsp;words per minute.