File:Honeycomb coil vario-coupler.jpg

English: A triple honeycomb-coil or duo-lateral vario-coupler, an electronic component used in vacuum tube regenerative radio receivers, which were used from WW1 to the 1930s to listen to the first radio broadcasts. It was an air-core RF transformer, consisting of three coils of wire mounted on hinges so they could be swung closer or further from each other. The device was used in the common Armstrong regenerative (tickler) circuit to provide positive feedback to the detector tube. The center coil was connected to the grid of the detector tube. The lefthand coil was the input tuned circuit which was connected to the antenna and provided the input radio signal tor the tube. The righthand coil was the "tickler", connected in the detector's plate circuit, which fed energy from the plate output circuit back into the grid, greatly increasing the gain and selectivity of the receiver. The amount of feedback was controlled by moving the left coil closer or further from the center one, changing the coupling (mutual inductance) between the coils. The coils were wound in a crisscrossed "honeycomb" pattern so adjacent layers of wire were not parallel to each other, which reduced parasitic capacitance and losses due to proximity effect. The coils were often replaceable plug-ins so different radio bands could be received by plugging in different coils.