The high capacitance and inductance and low resistance of the antenna-loading coil combination makes it act electrically like a high tuned circuit. VLF antennas have very narrow bandwidth and to change the transmitting frequency requires a variable inductor (variometer) to tune the antenna. The large VLF antennas used for high power transmitters usually have bandwidths of only 50-100 Hertz, and when transmitting frequency shift keying (FSK), the usual mode, the resonant frequency of the antenna must sometimes be dynamically shifted with the modulation, between the two FSK frequencies. The high results in very high voltages (up to 200 kV) on the antenna and very good insulation is required. 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 air breakdown, corona, and arcing from the antenna.