Micro FM FM wireless microphone transmitter circuit diagram

RF cable can be customized for other specifications

This simple single-tube FM wireless microphone circuit is known for its efficiency and stability. The key feature is that the oscillating coil also functions as the transmitting antenna, which ensures high transmission efficiency and minimizes interference. It's ideal for long-term operation and works best when placed in a fixed location. However, it’s not recommended for handheld use because the human body can easily affect the oscillation frequency, causing instability.

The transmitter is powered by a 1.5V battery and draws only about 2 to 3 mA, making it suitable for small button batteries and compact designs. The loop antenna L1 serves a dual purpose: it acts as both the oscillating coil and the transmitting element. This allows a high-frequency current to resonate at the same rate as the oscillator, ensuring optimal signal emission. The transmission frequency falls within the standard FM band (88–108 MHz), making it easy to receive with any FM radio.

In open spaces, the transmission range can exceed 100 meters. To build the internal loop antenna, you can use an enameled wire of 160 mm length and 1 mm diameter, forming it into a circular or square shape and embedding it inside the casing. When adjusting the circuit, make sure to fine-tune capacitor C3 so that the transmit frequency stays within the FM band for better reception. Avoid placing your hand near the antenna or bringing metal objects close during debugging, as this could disrupt the frequency and reduce the transmission distance.

For optimal performance, it's essential to use a UHF transistor, such as the 2SC3355, which has a characteristic frequency of 6.5 GHz and consumes 600 mW. A highly sensitive electret microphone is also recommended for the MIC input. You can test the microphone using a multimeter to ensure it meets the required sensitivity levels.