English: Homemade single tube Armstrong regenerative shortwave radio receiver using construction apparently dating from 1930s-1940s. The regenerative receiver design was abandoned by commercial radio manufacturers in the 1920s due to its tendency to radiate interference, but due to its simplicity and low cost continued to be built by radio amateurs for shortwave listening until World War 2. In the Armstrong circuit, the feedback is provided by a "tickler" coil in the plate circuit of the vacuum tube which is coupled to the tuning coil in the grid circuit. The tickler coil is visible as a variometer winding within the vertical tuning coil, mounted on a shaft which could be rotated by a knob on the front panel to adjust the feedback.