FM Transmitter from David Sayles

Description: This small transmitter employs a Hartley-type oscillator. Typically, the capacitor in the tank circuit connects to the base of the transistor; however, at VHF frequencies, the base-emitter capacitance of the transistor behaves like a short circuit, maintaining effective operation. The coil consists of four turns of 18 SWG wire wound around a quarter-inch former. The aerial tap is located approximately one and a half turns from the supply end. Audio sensitivity is notably high when utilized with an ECM-type microphone insert.

The described transmitter circuit is characterized by its simple yet effective design, utilizing a Hartley oscillator configuration to generate VHF signals. The Hartley oscillator is known for its stability and ease of tuning, making it a popular choice for small transmitter applications. In this circuit, the oscillation frequency is determined by the inductor and capacitor values within the tank circuit.

The choice of using a quarter-inch former for the coil winding ensures a compact design while maintaining adequate inductance for VHF operation. The four turns of 18 SWG wire provide a balance between resistance and inductance, optimizing the circuit's performance. The aerial tap, positioned one and a half turns from the supply end, allows for efficient coupling of the RF signal to an external antenna, enhancing transmission range and clarity.

The integration of an ECM (Electret Condenser Microphone) type microphone insert significantly improves audio sensitivity. This type of microphone is known for its low noise and high output, making it ideal for capturing audio signals with clarity. The combination of the Hartley oscillator and the ECM microphone results in a transmitter capable of delivering high-quality audio transmission over VHF frequencies.

Overall, this small transmitter design exemplifies a practical approach to VHF signal generation and audio transmission, suitable for various applications such as amateur radio, educational projects, or audio broadcasting. Proper tuning and component selection are essential to ensure optimal performance and compliance with relevant frequency regulations.This small transmitter uses a hartley type oscillator. Normally the capacitor in the tank circuit would connect at the base of the transistor, but at VHF the base emitter capacitance of the transistor acts as a short circuit, so in effect, it still is. The coil is four turns of 18swg wire wound around a quarter inch former. The aerial tap is about one and a half turns from the supply end. Audio sensitivity is very good when used with an ECM type microphone insert.