1. Introduction
This manual provides detailed instructions for the assembly and operation of the experimental AM transmitter, available as both a DIY kit and a finished version. It is designed for radio enthusiasts and educational purposes, particularly for experimenting with and testing old medium wave radios.
1.1. Objectives
- Understand the principle of high-frequency signal oscillation of an AM transmitter.
- Understand the principle of high-frequency signal amplification of an AM transmitter.
- Understand the modulation principle of an AM transmitter.
2. Safety Warnings
- WARNING: Stay away from children and avoid misoperation.
- WARNING: Please do not disassemble or modify this product by yourself. Non-professional repairs may cause damage or safety accidents.
3. Specifications
| Feature | Specification |
|---|---|
| Type | AM transmitter |
| Model Number | experimental AM transmitter |
| DIY Supplies | Electrical |
| Origin | Mainland China |
| Use Voltage | 5V-6V (Power input base: 9V DC, internal positive and external negative) |
| Current | 60mA (Overall current: About 50mA) |
| Frequency Range | 530-1600kHz |
| Power Consumption | 0.3W |
| High Frequency Power | About 400MW |
| External Antenna | 6M (1/50 wavelength) or 75M (1/4 wavelength) copper wire (not included) |
| Package Dimensions | Length: 6cm, Width: 6cm, Height: 6cm |
| Package Weight | 0.145 kg |
| Data Record | No |
| High-concerned chemical | None |
| Electronic | No |
| Certification | NONE |
4. Package Contents (DIY KITS)
If you have purchased the DIY KITS version, your package should contain the following:
- Printed Circuit Board (PCB)
- Various electronic components (resistors, capacitors, inductors, transistors, variable capacitor, etc.)
- Connecting wires
- Other small parts necessary for assembly
Please check if all accessories are complete after unboxing.
5. Assembly Instructions (for DIY KITS)
This section provides guidance for assembling your experimental AM transmitter. Refer to the circuit diagrams and component layouts carefully.
5.1. Component Identification and Placement
Refer to Figure 5.2 for the component layout and Figure 5.3 for the full schematic. Ensure correct polarity for electrolytic capacitors, diodes, and transistors.
5.2. Wiring and Connections
The following points correspond to the numbered labels in Figure 5.2:
- Sound Source Input (1): Connect your audio source (e.g., computer, mobile phone, MP4) here.
- Volume Adjustment Knob (2): This knob controls the input audio volume.
- Frequency Point Adjustment Knob (3): Adjusts the transmission frequency within 530-1600kHz. The optimal frequency range is 1000-1600kHz.
- Amplitude Modulation Depth Adjustment Knob (4): Adjust this knob to optimize sound quality.
- Power Trim (5): Turn counter-clockwise to reduce output power.
- Impedance Matching (6): Used to improve sound quality and waveform.
- Power Input Base (7): Connect a 9V DC power supply. Ensure internal positive and external negative polarity.
- High Frequency Ground Wire (8): Connect to a suitable ground.
- Output Antenna (9): Connect a 6-meter (or 75-meter) 2.5 mm2 copper wire as your transmitting antenna.
5.3. Circuit Principles (Advanced)
The circuit is a common base modulated transformer-coupled oscillation circuit. The transmitting circuit's communication path is shown in Figure 5.3(b). Resistors Rb1, Rb2, and re are bias resistors. L5 and C form an intermediate frequency selection circuit, tuning the oscillation frequency with small impedance. C8 is a high-frequency bypass capacitor, effectively shorting the oscillation signal, placing the transistor in a common base configuration. L1 and L2 form an LC resonant circuit with C2, C3, and CA (oscillation capacitance). Adjusting CA changes the oscillation frequency. C2 and C3 are padding and compensation capacitors, ensuring low and high-end frequency tracking. L3 is the feedback coil for phase balance. The oscillation circuit in Figure 5.3(c) connects to the emitter via coupling capacitor C1. This design significantly weakens the influence of the transistor's input impedance on the oscillation circuit and maintains a high Q value, resulting in a better output waveform.
6. Operating Instructions
Once assembled (or if using the finished version), follow these steps to operate your AM transmitter:
- Connect Power: Connect a 9V DC power supply to the power input base (7). Ensure the power supply is a switching power supply with small ripple to reduce noise. The voltage should not exceed 9V.
- Connect Audio Source: Plug your audio source (e.g., computer, mobile phone, MP4) into the sound source input (1).
- Connect Antenna: Attach your transmitting antenna (e.g., 6m or 75m 2.5 mm2 copper wire) to the output antenna terminal (9).
- Tune Frequency: Use the frequency point adjustment knob (3) to select your desired transmission frequency between 530-1600kHz. The copper knob can also be used to adjust the frequency.
- Adjust Volume: Use the volume adjustment knob (2) to set an appropriate audio level. Avoid setting it too high to prevent distortion.
- Adjust Modulation Depth: Use the amplitude modulation depth adjustment knob (4) to optimize the sound quality.
- Adjust Power (Optional): If needed, use the power trim (5) to reduce the output power.
- Monitor Output: Use a medium wave radio to receive the signal. For advanced users, a spectrum analyzer can be used to verify the waveform and harmonic components.
7. Precautions
- Do not use the product in damp or dusty environments.
- Avoid severe impact or vibration on the product.
- When using this micro-power transmitter, avoid interfering with local radio stations and adhere to local radio management regulations.
- This device is intended for experimental and educational use by radio enthusiasts and should not be used for other purposes.
8. Maintenance
To ensure the longevity and proper functioning of your AM transmitter, keep it clean and dry. Avoid exposing it to extreme temperatures or direct sunlight. No specific routine maintenance is required beyond general care.
9. Troubleshooting
| Problem | Possible Cause | Solution |
|---|---|---|
| Audio distortion | Volume adjustment knob set too high; Power supply with high ripple. | Reduce the volume. Use a switching power supply with low ripple. |
| Low transmission power | Frequency set below 1000kHz; Power supply voltage too low; Antenna too short. | Adjust frequency to 1000-1600kHz. Ensure 9V DC power supply. Use recommended antenna length (6M or 75M). |
| Interference with reception | Proximity to household appliances. | Move the transmitter away from other electronic devices. |
| Transmitter not powering on | Incorrect power supply voltage or polarity; Loose connections. | Verify 9V DC power supply and correct polarity. Check all power connections. |
10. User Tips
- Power Source Flexibility: While the power input base specifies 9V DC, users have reported success powering the device using a USB cable or an external battery, suggesting a 5V-6V operating range is also viable. Ensure stable power delivery to avoid issues.
- Antenna Experimentation: The manual suggests 6M or 75M copper wire for the antenna. Experiment with different lengths and types of wire to optimize signal strength and clarity for your specific environment.
- Frequency Adjustment: For best performance, aim to operate the transmitter within the 1000-1600kHz range using the frequency adjustment knob.
11. Warranty and Support
For any issues or questions regarding your experimental AM transmitter, please contact the seller or manufacturer for support. Keep your purchase records for warranty claims, if applicable. Specific warranty terms may vary.
