Simple Shortwave Transmitter: Circuit & Tuning (10-15MHz)

Description

This economical short-wave radio transmitter operates across a range of 10 to 15 MHz, utilizing a ½J gang condenser VC1 to establish the carrier frequency, alongside inductor L1. Fine adjustments to the frequency are achieved through modulation with VC2. The carrier signal undergoes amplification courtesy of transistor T4, subsequently coupled to the RF amplifier transistor T1 (BD677) via transformer X1*. Notably, this design does not incorporate a modulator transformer.

The audio signal originating from a condenser microphone is initially boosted by transistor T3 (BC548). The amplified audio output from T3 is further processed by transistor T2 (BD139), which modulates the operating current of the RF amplifier transistor T1, adapting it dynamically to the amplitude fluctuations of the audio signal. RFC1 serves as a blocking capacitor, effectively preventing the carrier RF signal from transistor T2 and the power supply from affecting the circuit. The modulated RF signal is then coupled to the antenna through capacitor C9.

More about components

Transistors play a crucial role in this radio transmitter design. Specifically, the BD677 and BD139 are NPN bipolar junction transistors. These semiconductors act as amplifiers, controlling current flow within the circuit. Bipolar junction transistors are typically used for switching and amplifying analog signals. They are characterized by their ability to switch between conducting and non-conducting states based on the applied voltage. Furthermore, the BC548 is a silicon NPN transistor commonly utilized in low-power audio applications, providing a stable amplification stage for the microphone signal.

Circuit diagram:

For the antenna, a 0.5m long telescopic aerial can be employed. Details concerning the RF choke, inductor L1 and coupling RFC1 are implemented to prevent the carrier RF signal from transistor T2 and the power supply from interfering with the circuit operation. The modulated RF signal is coupled to the antenna via capacitor C9. For antenna, one can use a 0.5m long telescopic aerial. Details of RF choke, inductor L1 and coupling transformer X1, we utilized a prefabricated short-wave antenna coil featuring a tuning slug (Jawahar make), which exhibited satisfactory performance. We assessed the transmitter’s reception capabilities up to 75 metres and observed robust signal strength.