File:Pancake vario-coupler 1922.jpg

English: An electronic component called a "pancake" or "spiderweb" vario-coupler, used in early vacuum tube radio receivers, manufactured by Herrold Laboratories, San Jose. It is a type of air-core RF transformer. It consists of three spiral coils of wire suspended vertically on hinges so they can be swung toward or away from each other. When a coil is facing its neighbor, with their magnetic axes collinear, the coupling between the coils is maximum. As a coil is swung away at an angle, the coupling decreases. The price is $5.50. These coupled coils were used as Hartley tickler coils in Armstrong regenerative radio receivers. The center coil would be the secondary, connected to the input grid circuit of the triode vacuum tube. One of the outside coils would be connected to the antenna and serve to input the radio signal into the circuit. The other outside coil, called the "tickler" coil, would be connected to the plate circuit of the tube, feeding back energy from the plate circuit to the grid, providing positive feedback which increased the receiver's gain. The spiral construction of the coils reduces their parasitic capacitance and losses at radio frequencies