The monopole is one of the most commonly used antennas. In its simplest form, it consists of a quarter-wavelength wire driven with respect to a large ground plane by a coaxial feed line. A disadvantage of this type of antenna is that the ground plane must be large and conductive. This can be particularly problematic at low frequencies. In addition, it cannot easily be raised above nearby obstructions such as buildings and power lines. At low frequencies, it is often impractical to erect a full-size quarter-wave antenna and some form of inductive loading is often used to compensate for the reactance of the shorter mast. Even with a good ground system, the efficiency of a monopole antenna over real earth is often significantly less than that of an ideal model, particularly for monopole heights less than λ/4.