This Shortwave Transmitter circuit operates in shortwave HF band (6 MHz to15 MHz), and can be used for short range communication and for educational purposes.
The Shortwave Transmitter circuit consists of a mic amplifier, a variable frequency oscillator, and modulation amplifier stages. Transistor T1 (BF195) is used as a simple RF oscillator. Resistors R6 and R7 determine base bias, while resistor R9 is used for stability. Feedback is provided by 150pF capacitor C11 to sustain oscillations. The primary of shortwave oscillator coil and variable condenser VC1 (365pF, 1/2J gang) form the frequency determining network.
The BEL1895 operational amplifier (IC1) serves as the microphone preamplifier and modulation amplifier. The condenser microphone connects through C4, and the op-amp provides the necessary gain to boost the weak microphone signal to levels suitable for modulating the RF stages. R3 provides feedback control, while VR1 acts as a gain/modulation depth control.
The audio output from the BEL1895 drives the modulation circuit that connects to the emitter of the final amplifier transistor.
Shortwave transmitter – RF Oscillator Section (Center)
Transistor T1 forms the master oscillator operating on 40 meters (approximately 7.0-7.3 MHz). This appears to be a Colpitts oscillator configuration with the frequency determined by the tank circuit formed by RFC1 (the inductor) and the capacitive voltage divider network of C10 and C11.
The ganged variable capacitor (1/2 SJ GANG 365P) provides frequency tuning across the 40-meter band. The “ganged” design means multiple capacitor sections are mechanically linked to a single tuning control, allowing simultaneous tuning of multiple circuits.
RF Power Amplifier and Modulation (Right Side)
Transistor T2 serves as the RF power amplifier and is where the amplitude modulation actually occurs. The key to understanding this circuit is recognizing that the audio signal from the BEL1895 is applied to T2’s emitter circuit.
How Emitter Modulation Works
In emitter modulation, the audio signal varies the DC bias voltage on the emitter of T2. When the audio signal goes positive, it increases the emitter voltage, which increases the transistor’s forward bias and gain. When the audio goes negative, it decreases the emitter voltage, reducing the gain. This varying gain directly translates to varying RF output amplitude – which is amplitude modulation.
The beauty of emitter modulation is that it provides high modulation efficiency while maintaining relatively good linearity. The modulation occurs in the final amplifier stage, so the full output power can be modulated effectively.
Further reading: