English: High Q plug-in inductor from 1937 used in tuned circuits of shortwave radio transmitters. Amateur radio transmitters of this era used tuned circuits, not quartz crystals, to control the radiated frequency. The coil plugged into a jack in the transmitter amplifier, so the frequency band could be changed by exchanging the coil with one of a different inductance. It shows high Q construction: a single layer winding is used with the turns spread apart, to reduce proximity effect losses; it is made of thick silver plated wire to reduce skin effect losses, and instead of a coil form is suspended by thin insulating strips to reduce dielectric losses.