Originally the simple vertical wire antenna, earthed at its lower end, used by Marconi. The term is now applied to vertical antennas one quarter wavelength long. Usually the antenna is connected to earth by a series reactance, which may be either an inductance or a capacitance, of such a value that together with the antenna it is electrically equivalent to one quarter wavelength. Usually the series reactance is variable, allowing the antenna to be tuned to any resonant frequency within a designated range.