File:J-pole antenna showing standing waves.svg

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 lines) and current (I, blue lines) 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 for efficiency, to prevent reflections, so the antenna absorbs all the power from the transmitter.