Graph of the measured base feed resistance and reactance of a typical mast radiator antenna vs the height of the mast. A mast radiator antenna consists of a guyed lattice mast in which the steel mast structure itself is connected to the transmitter and functions as the antenna. It is mounted on a ceramic insulator which keeps it isolated from the ground, and fed between the bottom of the mast and the a buried copper wire ground system. They are the most widely used antennas for AM radio stations in the medium frequency band. The base impedancee of masts varies considerably with the construction of the mast and presence of nearby objects, this is a curve for a typical mast.

The mast is resonant at the lengths where the reactance goes to zero. In an ideal thin monopole antenna these would be at a length of ${\displaystyle 0.25\lambda }$ anf ${\displaystyle 0.5\lambda }$. but in actual mast radiators as shown here the thickness of the mast, and capacitance of guy wires and the base insulator causes resonance to occur at considerably lower frequencies. The most common mast lengths used are ${\displaystyle 0.625\lambda }$ (225°) at which horizontal (ground wave) radiation is maximum, and ${\displaystyle 0.53\lambda }$ (190°) at which fading is minimum