Editing Very low frequency (section)

==Antennas==
{{multiple image
| align             = center
| direction         = horizontal
| image1            = Cutler VLF antenna array.png
| caption1          = "Trideco" antenna tower array at the US Navy's [[VLF Transmitter Cutler|Naval Radio Station Cutler]] in Cutler, Maine, USA. The central mast is the radiating element, while the star-shaped horizontal wire array is the capacitive top load. About {{cvt|1.2|mi|km|0|order=flip}} in diameter, it communicates with submerged submarines at 24&nbsp;kHz (12,500&nbsp;meter wavelength) at a power of 1.8&nbsp;megawatts, one of the most powerful radio stations in the world.
| width1            = 370
| image2            = VLF umbrella antenna - Anthorn Radio Station UK - central mast.jpg
| caption2          = Central mast of a similar "trideco" antenna of the NATO VLF transmitter at [[Anthorn radio station]], UK, showing six insulator strings attaching the toploads to the six vertical radiator wires
| width2            = 223
| image3            = Jim Creek VLF antenna.png
| caption3          = Another type of large VLF antenna: the "valley-span" antenna, consisting of one or more long horizontal topload cables spanning a valley, fed in the center by vertical radiator cables. This example is at the US Navy [[Jim Creek Naval Radio Station|Jim Creek station]] near [[Seattle]], which transmits on 24.8&nbsp;kHz at a power of 1.2&nbsp;MW.
| width3            = 265
| image4            = Tsushima Omega Tower 1977 2.jpg
| caption4          = [[Umbrella antenna]] of the [[Omega (navigation system)|Omega navigation system]] beacon on [[Tsushima Island]], Japan, which transmitted at 10–14&nbsp;kHz; 389&nbsp;meters high, [[:ja:対馬オメガ局|it was dismantled in 1998]].
| width4            = 220
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}}

A major practical drawback to the VLF band is that because of the length of the waves, full size resonant antennas ([[half wave dipole]] or [[monopole antenna|quarter wave monopole]] antennas) cannot be built because of their physical height.<ref name="Watt" />{{rp|pages=14}}  Vertical antennas must be used because VLF waves propagate in vertical polarization, but a quarter-wave vertical antenna at 30&nbsp;kHz (10&nbsp;km wavelength) would be {{convert|2.5|km|ft|abbr=off}} high. So practical transmitting antennas are [[electrical length|electrically short]], a small fraction of the length at which they would be self-resonant.<ref name="Seybold">{{Cite book |last=Seybold |first=John S. |url=https://books.google.com/books?id=4LtmjGNwOPIC&q=cross+polarization+discrimination&pg=PA57 |title=Introduction to RF Propagation |publisher=John Wiley and Sons |year=2005 |isbn=978-0471743682 |pages=55–58}}</ref><ref name="Johnson-1993">{{Cite book |url=http://seklad69associates.com/seklad69associates.com/EEG_808_and_815_files/Antenna%20Engineering%20Handbook.pdf |title=Antenna Engineering Handbook |publisher=McGraw-Hill |year=1993 |isbn=007032381X |editor-last=Johnson |editor-first=Richard C. |edition=3rd}}</ref>{{rp|pages=&nbsp;24.5–24.6}} Due to their low [[radiation resistance]] (often less than one ohm) they are inefficient, radiating only 10% to 50% of the transmitter power at most,<ref name="Hunsucker" /><ref name="Watt" />{{rp|page=14}} with the rest of the power dissipated in the antenna/ground system resistances. Very high power transmitters (~1&nbsp;megawatt) are required for long-distance communication, so the efficiency of the antenna is an important factor.

[[File:Flattop antenna 1912.png|thumb|A "triatic" or "[[Inverted-L antenna|flattop]]" antenna, another common VLF transmitting antenna. It consists of vertical radiator wires each connected at top to parallel horizontal capacitive topload wires stretching up to a kilometer, supported on tall towers. The transverse support cables suspending the horizontal wires are called "triatics".]]

=== VLF transmitting antennas ===
High power VLF transmitting stations use capacitively-toploaded [[monopole antenna]]s. These are very large wire antennas, up to several kilometers long.<ref name="NAVELEX-0101-113">{{Cite book |url=http://www.navy-radio.com/manuals/0101-1xx/0101_113-03.pdf |title=Naval Shore Electronics Criteria - VLF, LF, and MF communications systems |date=August 1972 |publisher=U.S. Navy |location=Washington, DC |id=Manual NAVELEX 0101-113 |department=Naval Electronics Systems Command}}</ref>{{rp|pages=&nbsp;3.9–3.21}}<ref name=Johnson-1993/>{{rp|pages=&nbsp;24.8–24.12}} They consist of a series of steel [[radio mast]]s, linked at the top with a network of cables, often shaped like an umbrella or clotheslines.<ref name="Watt" />{{rp|page=p.14}} Either the towers themselves or vertical wires serve as [[monopole antenna|monopole]] radiators, and the horizontal cables form a ''capacitive top-load'' to increase the current in the vertical wires, increasing the radiated power and efficiency of the antenna. High-power stations use variations on the [[umbrella antenna]] such as the "delta" and "[[Umbrella antenna#Trideco antenna|trideco]]" antennas, or multiwire [[T-aerial|flattop]] (triatic) antennas.<ref name="Watt">{{Cite book |last=Watt |first=Arthur D. |url=https://archive.org/details/VLFRadioEngineering/page/n143/mode/2up |title=VLF Radio Engineering |publisher=Pergamon Press |year=1967}}</ref>{{rp|pages=p.129–162}}  For low-power transmitters, inverted-L and [[T-aerial|T antennas]] are used.

Due to the low radiation resistance, to minimize power dissipated in the ground these antennas require extremely low resistance [[ground (electricity)|ground]] (Earthing) systems, consisting of radial networks of buried copper wires under the antenna. To minimize [[dielectric loss]]es in the soil, the ground conductors are buried shallowly, only a few inches in the ground, and the ground surface near the antenna is sometimes protected by copper ground screens. [[Counterpoise (ground system)|Counterpoise]] systems have also been used, consisting of radial networks of copper cables supported several feet above the ground under the antenna.

A large [[loading coil]] is required at the antenna feed point to cancel the [[capacitive reactance]] of the antenna to make it [[resonant]]. At VLF the design of this coil is challenging; it must have low resistance at the operating RF frequency, [[Q factor|high {{mvar|Q}}]], must handle very high currents, and must withstand the extremely high voltage on the antenna. These are usually huge air core coils 2–4 meters high wound on a nonconductive frame, with RF resistance reduced by using thick [[litz wire]] several centimeters in diameter, consisting of thousands of insulated strands of fine wire braided together.<ref name="Watt" />{{rp|page=p.95}}

The high capacitance and inductance and low resistance of the antenna-loading coil combination makes it act electrically like a [[Q factor|high {{mvar|Q}}]] [[tuned circuit]]. VLF antennas have very narrow [[bandwidth (signal processing)|bandwidth]] and to change the transmitting frequency requires a variable inductor ([[Transformer types#Variometer and variocoupler|variometer]]) to tune the antenna. The large VLF antennas used for high-power transmitters usually have bandwidths of only 50–100&nbsp;hertz. The high {{mvar|Q}} results in very high voltages (up to 250&nbsp;kV)<ref name="Watt" />{{rp|page=p.58}} on the antenna and very good insulation is required.<ref name="Watt" />{{rp|page=p.14,19}} Large VLF antennas usually operate in 'voltage limited' mode: the maximum power of the transmitter is limited by the voltage the antenna can accept without [[electrical breakdown|air breakdown]], [[corona discharge|corona]], and arcing from the antenna.

=== Dynamic antenna tuning ===
The bandwidth of large capacitively loaded VLF antennas is so narrow (50–100&nbsp;Hz) that even the small frequency shifts of FSK and MSK modulation may exceed it, throwing the antenna out of [[resonance]], causing the antenna to reflect some power back down the feedline. The traditional solution is to use a "bandwidth resistor" in the antenna which reduces the {{mvar|Q}}, increasing the bandwidth; however this also reduces the power output. A recent alternative used in some military VLF transmitters is a circuit which dynamically shifts the antenna's [[resonant frequency]] between the two output frequencies with the modulation.<ref name=Johnson-1993/>{{rp|page=&nbsp;24.7}}<ref name=NAVELEX-0101-113/>{{rp|page=&nbsp;3.36}} This is accomplished with a [[saturable reactor]] in series with the antenna [[loading coil]]. This is a [[magnetic core|ferromagnetic core]] [[inductor]] with a second control winding through which a DC current flows, which controls the inductance by magnetizing the core, changing its [[magnetic permeability|permeability]]. The keying datastream is applied to the control winding. So when the frequency of the transmitter is shifted between the '1' and '0' frequencies, the saturable reactor changes the inductance in the antenna resonant circuit to shift the antenna resonant frequency to follow the transmitter's frequency.

=== VLF receiving antennas ===
The requirements for receiving antennas are less stringent, because of the high level of natural [[atmospheric noise]] in the band. At VLF frequencies atmospheric [[radio noise]] is far above the [[Noise figure|receiver noise]] introduced by the receiver circuit and determines the receiver [[signal-to-noise ratio]]. So small inefficient receiving antennas can be used, and the low voltage signal from the antenna can simply be amplified by the receiver without introducing significant noise. Ferrite [[loop antenna]]s are usually used for reception.