Have you ever wanted to build your own radio transmitter? Well, this Steve Quest AM transmitter circuit is perfect for beginners. Moreover, it uses common parts that are easy to find. An AM (Amplitude Modulation) transmitter takes an audio signal and overlays it onto a radio frequency (RF) carrier wave. This allows the signal to travel wirelessly.
The Heart of the Circuit
This AM Transmitter circuit operates on a 12V DC power supply. Right away, you’ll notice a crystal, CR1, which is crucial. So, this 7 MHz crystal acts as the frequency reference for the oscillator, ensuring a stable output frequency. Transistor Q3 (a 2N2222) works with the crystal to generate these oscillations.
Next, the signal moves to the amplification stage. Transistor Q2 (a BD139) amplifies the radio frequency signal. This boosted signal is then ready for modulation.
Adding Your Audio
Here’s where the magic of AM happens! The U1 transformer is key for modulation. Yes, you can use a standard 6V 500mA power transformer for this purpose. An audio source, like a microphone or music player, connects to the “MOD” input. This audio signal then varies the amplitude of the RF carrier, creating the modulated signal.
AM Transmitter – Output and Antenna
After modulation, the signal passes through an output matching network. This includes components like L1, C4, L2, C5, and C6. These parts help to efficiently transfer the power to your antenna. Finally, the “ANT” connection is where you’ll connect your antenna for broadcasting.
AM Transmitter – Important Notes for Builders
The diagram provides specific details for winding two essential components:
- U2 Transformer: This requires 28 turns for the primary coil and 8 turns for the secondary, wound on a 1/4 inch ferrite core.
- L1 Inductor: This needs 28 turns on a 1/4 inch ferrite core.
Additionally, L2 is a 1.5uH choke, which helps to filter and stabilize the output
Safety Notes
Remember that this AM transmitter operates on amateur radio frequencies. Therefore, you need a proper license to use it. Also, keep power levels low during testing. Furthermore, always use an appropriate antenna to prevent interference.
Similar Circuits
| OXO QRP HF transmitter |
| Simple series modulated AM QRP HF transmitter |
| 7MHz CW / AM QRP Transmitter |