In the early years of radio technology spark transmissions ruled the (air) waves. They occupied a relatively wide bandwidth, in what came to be known as the long waveband. The receivers used had a corresponding band-width, as ‘wide open’ as the proverbial ‘barn door’. Most were simple detectors without an amplifier stage.
The requirements for this kind of receiver are totally different from normal radio reception: the receiver must have bandwidth as wide as possible, with maximum sensitivity in the long wave region. A further special request: since the wave packets of a single spark are often extremely short, the receiver should integrate them into a longer pulse whose spectrum should lie well inside the audible range.
As for the circuit, the audion stage in the collector circuitry detunes the input circuit. To prevent self-oscillation we need to add a 10 kΩ resistor. Using an oscilloscope you can see extremely short pulses on the emitter of the BC557 being broadened. The amplitude is frequently sufficient to drive the final amplifier into limiting. A1 μs long input pulse results in a circa 1 ms long audio pulse in the loudspeaker.
Author : Burkhard Kainka Copyright : Elektor