Editing Very low frequency (section)

=== 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.