Shortwave Audion Receiver Introduction
The ideal entry point into shortwave receiver technology is a simple device employing only two transistors and a single 1.5 V battery. Enhanced performance can be achieved by adding an active PC loudspeaker. A unique feature of this setup is the audion circuitry utilizing a BC558C PNP transistor operating in emitter follower mode. This configuration capitalizes on the few picofarads of internal capacity between the transistor’s base and emitter, creating a capacitive voltage divider. This design allows the transistor to function as a three-point oscillator, commonly referred to as a Hartley oscillator. Minimal emitter current is necessary to initiate oscillation.
Adjusting the Audion Circuit
A trimpot (trimmer potentiometer) is utilized to fine-tune the audion for AM reception, ensuring it is just below the threshold of oscillation (immediately before oscillation begins). For CW (continuous wave) and SSB (single-sideband) reception, the trimpot is adjusted slightly higher to optimize the circuit’s performance.
Decoupling and Signal Amplification
The audio signal undergoes decoupling and amplification through the second transistor in the circuit. At the output connector K1, the signal is at line level and maintains an output impedance of approximately 1 kΩ.
Antenna Connections and Grounding
Either of the two antenna connections, ANT1 and ANT2, can be utilized based on the reception requirements. A robust ground connection is crucial for optimal circuit performance. For indoor use, a short wire antenna measuring less than a meter connected to ANT1 is adequate to capture numerous broadcast stations. For long-distance (DX) reception, an external antenna, such as a ten-meter-long aerial ‘long wire,’ is recommended. In this scenario, the ANT2 connection must be employed. The coupling to this input is intentionally weaker to minimize resonance and counteract any potential feedback. As a general guideline, the longer the antenna, the smaller the value of the coupling capacitor C1.
The BC558C transistor is housed in a plastic TO-92 case. When viewed with the flat side and leads facing downward, the three leads emerging from the transistor are, from left to right, the collector, base, and emitter leads.