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JOTA : Jamboree On The Air - an annual event in which scouts worldwide can communicate with each other and radio amateurs using ham radio equipment.

J-pole antenna : Vertical antenna composed of two parallel elements of differing widths. The longer element is connected to signal and the shorter to ground at an intermediate point; the elements are connected to each other at the base.

JT65 : A weak signal digital mode developed for use in the VHF and  UHF bands for  EME and other "reflected path" modes. The software for this mode, WSJT was written by Joe Taylor K1JT.

JT65-HF : An adaptation of JT65, for use on the HF bands.