SW transmitter circuit based on IC BEL1895

Here is the SW transmitter circuit based on IC BEL1895. This particular transmitter circuit operates in the shortwave HF band (6 MHz to 15 MHz) and can be used for short-range communications and educational purposes.

The circuit consists of a microphone amplifier circuit, a variable frequency oscillator and a modulation amplifier stage. Transistor T1 (BF195) is used as a simple RF oscillator. Resistors R6 and R7 determine the base bias, while resistor R9 is used for stabilization. Feedback is provided by 150pF capacitor C11 to maintain oscillation. The primary shortwave oscillator coil and variable capacitor VC1 (365pF, 1/2J gang) form a frequency determining network.

By changing the coil inductance or capacitance of the bank capacitor, the oscillation frequency can be changed. The carrier RF signal from the oscillator is coupled through the secondary inductance of transformer X1 to the next RF amplifier cum modulation stage, which is assembled around transistor T2 operating in class "A" mode. The audio signal from the audio amplifier assembled around IC BEL1895 is coupled to the emitter of transistor 2N2222 (T2) for radio frequency modulation.

IC BEL1895 is a monolithic audio power amplifier suitable for sensitive AM radio applications. It can deliver 1W of power into 4 ohms from a 9V supply, with low distortion and low noise characteristics. Since the amplifier has a voltage gain of approximately 600, the signal from the condenser microphone can be linked directly to its input without any amplification.

The stability of a shortwave transmitter is governed by the quality of the tuned circuit components and the degree to which the supply voltage is regulated. A 9V regulated power supply is required. The RF output of the antenna consists of harmonics because transistor T2 does not have a tuning coil in its collector circuit. However, for short distance communication, it does not cause any trouble. The harmonic content of the output can be reduced by a high-Q LC filter or a resonant LC trap tuned to each prominent harmonic. The transmitter has a power output of approximately 100 milliwatts.