Editing Very low frequency (section)

=== 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.