English: Output tank circuit (LC circuit, tuned circuit) of power amplifier of shortwave amateur radiotelegraph transmitter from 1938. It consists of an inductor (coil, top) connected in parallel with a variable capacitor (bottom). It serves as a bandpass filter, removing any harmonic frequencies from the output so they will not be radiated by the antenna. Since the output power is 1 kilowatt the voltage on the circuit is several thousand volts, so to prevent arcing the capacitor plates are more widely spaced than a low voltage capacitor, and the coil is mounted on standoff insulators. The tuned circuit must have a narrow bandwidth (high Q) to filter the narrow CW signal, and the coil shows high Q construction: it is made of a single layer winding with turns separated to reduce losses from proximity effect, of thick silver-plated wire to reduce losses from skin effect, suspended in air without a coil form to reduce dielectric losses.