Leak Trough Line
The Trough Line tuner was developed to provide high quality FM reception. One of the difficulties of radio circuit design at the time was to design an oscillator that was stable (drifting oscillators required the listener to re-tune the radio, and resulted in distorted sound with FM reception).
Oscillator frequency is determined by the magnitude of inductance and capacitance in a tuned circuit. Temperature effects (particulary so with valve radios) made the inductance vary as the radio warmed up (primarily from changes in inductor dimensions). The Trough Line uses the inherent inductance in a single straight-wire conductor, placed concentrically inside an outer tubular conductor (a more detailed description is available at this web site). For ease of production, in the first Leak FM tuners the concentric outer conductor was in fact formed from a U-shaped trough. As a result, the tuner was christened the "Troughline" in reference to its very stable inductor (and hence stable oscillator). In this original model Trough Line the U-shaped trough had its open side placed facing downwards under the chassis.
Other details are as follows: