Figure shows the aerial in its diagrammatic form. Notice that the aerial is in the form of an elongated letter ‘J’; this shape gives rise to its nickname – the J-pole. The quarter-wave
RF transformer is the lower ‘U’ section below the half-wave element. At the bottom of the U section, the impedance is zero (this may become clearer later) and at the top of the U section it is high, thus matching the aerial impedance. The coaxial feeder cable is connected part-way up the U section, where the impedance is around 50 .