Measurements of electrical impedance at frequencies ranging from a few tens of kilohertz to about 1 gigahertz. This broad radio-frequency (rf) range encompasses frequencies from the extralow-frequency band, through the long-, medium-, and short-wave ranges, and then through the very high frequency (VHF) and the ultrahigh-frequency (UHF) bands. In the electrical context, impedance is defined as the ratio of voltage to current (or electrical field strength to magnetic field strength), and it is measured in units of ohms (Ω).