English: Diagram of a J-pole antenna, a vertical end-fed half-wave omnidirectional transmitting antenna used in the shortwave frequency bands. The diagram shows the standing waves of voltage (V, red bands) and current (I, blue bands) on the antenna elements. It consists of a vertical element a half wavelength (${\displaystyle \lambda /2}$) long, fed in series with a quarter wavelength (${\displaystyle \lambda /4}$) transmission line stub, with the parallel wire feedline (F) from the transmitter attached along the stub. The stub serves as an impedance matching device to transform the high feedpoint impedance of the half-wave antenna down to match the impedance of the feedline, so the antenna will absorb and radiate all the power from the transmitter without reflections.