How to Build a Low-Power FM Transmitter Circuit

_Building an FM transmitter produces a moment of genuine disbelief the first time it works. You tune a radio to an empty frequency, your circuit hums to life, and audio comes out of the speaker from something you built with five components._

_The circuit here is intentionally simple — a single 2N3904 transistor oscillator. Short range, well within legal limits for unlicensed transmission in most countries, and a genuine working circuit rather than a theoretical exercise._

How FM transmission works

FM — frequency modulation — varies the carrier wave's frequency slightly in proportion to the audio signal, rather than varying its amplitude like AM.

At rest, the transmitter oscillates at a fixed frequency, say 88.5 MHz. When audio is applied, the frequency shifts slightly above and below that centre in proportion to the audio waveform. The FM receiver detects these frequency variations and converts them back to audio.

The oscillation frequency is set by the LC tank circuit — inductor and capacitor in combination:

f = 1 / (2π × √(L × C))

For the 88–108 MHz FM band, this means very small values — a few turns of wire and picofarad capacitors.

Components

• 2N3904 NPN transistor (or BC547)
• 10nF capacitor (C1, audio coupling)
• 4.7pF capacitor (C2, tank circuit)
• 22pF trimmer capacitor (C3, frequency tuning)
• Air core inductor: 5 turns 22 AWG wire, 5mm diameter (≈100–150nH)
• 470Ω resistor (R1, base bias upper)
• 10kΩ resistor (R2, base bias lower)
• Electret microphone or 3.5mm audio input
• 9V battery

How the circuit oscillates

R1 and R2 bias the 2N3904's base to its active region. The inductor and C2/C3 form the tank circuit connected from collector to VCC. The transistor's own collector-base capacitance provides the feedback needed for self-oscillation.

Audio enters through C1 (DC blocking) to the base, slightly modulating the transistor's operating point, which slightly shifts the resonant frequency of the tank. That shift is FM.

The antenna — a 30–40cm wire from the collector through the inductor — radiates the modulated signal. A full quarter-wave antenna (75cm) radiates more but isn't necessary for a few metres of range.

Winding the inductor

This is where most first-time builds go wrong. Wrap 5 turns of 22 AWG bare copper wire tightly around a 5mm former (a pencil works). Remove the former carefully — the coil should hold its shape.

Spread the turns slightly so they don't touch. Compressed or stretched turns change inductance and shift your frequency. The trimmer capacitor (C3) handles final tuning — set to mid-position, power up, scan the FM band on a radio nearby. When you find your carrier, adjust C3 to move it to a clear frequency.

Getting a clean signal

Decoupling is critical. A 10nF ceramic cap directly between VCC and GND, as close to the transistor as physically possible. Without it, supply noise modulates the signal and gives a buzzy output.

Keep the layout compact. At 100 MHz, a centimetre of wire is significant inductance. Long leads between components detune the tank circuit. Build on copper-clad board if possible, components tight.

Shielding — even a small metal enclosure around the circuit — dramatically reduces interference pickup and signal leakage.

Legal note

Low-power FM transmitters are legal in most countries for personal use within strict power limits. This circuit from a 9V battery and a single 2N3904 is well within those limits — a few metres of range at most.

Don't add amplifier stages to increase range. Higher power requires licensing in essentially every jurisdiction and interferes with other users. The point of the project is the circuit, not the broadcast range.

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• [2N3904 component page](/component/2n3904)
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