Low-Cost Powerful AM Transmitter Circuit (Approximately 13 km Range)

A simple AM transmitter circuit that can be built with low-cost components and is stated to be capable of shortwave broadcasting over an area of approximately 13 km in diameter. The circuit structure is not complex; nevertheless, it is stated to offer remarkable performance in terms of output power and coverage area.

Especially for those who want to examine an AM transmitter circuit on a low budget, this design is one of the examples that can help understand basic RF logic. The number of components used in the circuit is not high. This both makes assembly easier and offers a more accessible structure for hobby experiments.

As in the previously shared Long Distance FM Transmitter 5Km (Easy to Build, Powerful) article, a high effect with a simple structure is aimed here as well. However, this circuit operates with AM modulation, not FM. Therefore, its operating character, antenna behavior, and listening conditions should also be evaluated accordingly.

General Features of the Circuit

This AM transmitter circuit is designed to transmit signals in the shortwave (SW) band under suitable conditions. According to the given information, approximately 13 km of coverage can be obtained in an open area and with suitable antenna use. The actual range may vary depending on antenna length, environmental interference level, power supply quality, assembly layout, and the sensitivity of the receiver side.

One of the notable aspects of the circuit is that it can be built without requiring expensive RF components. Therefore, it may be useful both for electronics enthusiasts interested in RF circuits and for those who want to learn transmitter logic.

Supply Voltage and Power Supply Recommendation

It is recommended to use a 12V battery for supplying the circuit. The main reason for this is that a cleaner DC voltage is usually obtained with battery power. In RF circuits, noise on the power supply line can directly affect the output signal and listening quality.

If a 12V AC-DC adapter will be used for the power supply, its output must be very well filtered and stable. Otherwise, low-frequency hum or unwanted background noise may occur on the receiver side. Therefore, if adapter use is considered, special attention should be paid to the regulation and filtering section.

Coil Winding Information

L1 Coil

For the L1 coil, it is recommended to use 22 SWG copper wire. The coil should be wound with 10 turns, and a ferrite rod should be placed inside it. Using a ferrite rod can contribute to the operating stability of the circuit by affecting the inductive behavior of the coil.

It is important that the L1 coil is wound properly. If the windings are very irregular, frequency stability and overall performance may be negatively affected. Therefore, it is useful to make sure that the spacing between turns is as similar as possible.

L2 Coil

For the L2 coil, it is again stated that 22 SWG copper wire should be used and 10 turns should be wound. However, a ferrite rod should not be used in this coil. In addition, the coil diameter is given as 0.6 cm, approximately 1/4 inch.

The L2 coil plays an important role in the RF behavior of the circuit due to its physical dimensions. Therefore, attention should be paid not only to the number of turns but also to the coil diameter. Even small changes can affect the operating point of the transmitter.

Crystal Selection

A 4.4333 MHz crystal is used in the circuit. Crystal oscillator structures provide an advantage in terms of frequency stability. Therefore, it is important to use a crystal at the appropriate frequency and in good condition.

If the crystal lead connections are kept short, the possibility of picking up interference can be reduced. In RF circuits, the layout is often as important as the schematic. Therefore, the placement of the crystal, transistor, and coils should be made as orderly as possible.

Transistor and Cooling Warning

The BFY50 transistor is used in this circuit. However, the pinout of the transistor must be verified before assembly. Since the pin order may vary between different manufacturers of the same model or equivalent products, blind connection is not recommended.

In addition, using a heatsink for the BFY50 is strongly recommended. Otherwise, it is stated that the transistor may overheat and be damaged within a few minutes. This warning is especially important because in RF output stages, when suitable load conditions are not provided and heat dissipation is insufficient, the transistor can fail very quickly.

Why Is the Antenna Connection Important?

One of the most critical points to consider in this project is that the circuit should not be powered without an antenna connected. When the antenna is not connected, the transmitter stage may not see the proper load. This can create unwanted voltage and current stresses on the transistor and cause damage.

For the best results, it is recommended to use the suggested 75 ohm antenna cable. If the specified special cable is not available, copper wire at least 2 meters long can be used as an antenna. For better results, it may be useful to position the antenna high above the ground, in as open an area as possible, and away from sources of interference.

Points to Consider Before Assembly

Before building this type of AM transmitter circuit, it is useful to pay attention to the following points:

• The power supply line must be clean. Noisy adapters can cause hum.
• Coil dimensions must be preserved. Especially the number of turns and diameter should not be changed randomly.
• The crystal frequency must be selected correctly. A 4.4333 MHz crystal must be used.
• The BFY50 must be cooled. Use without a heatsink is risky.
• The antenna must definitely be connected. Applying power without an antenna may damage the transistor.
• Assembly should be done with short connections. Long wires can create unwanted effects in RF circuits.

For What Purpose Can This Circuit Be Examined?

Such projects can be instructive especially for learning RF transmitter logic, seeing how the coil-crystal-transistor relationship works in practice, and understanding the basic structure of shortwave transmission circuits.

In addition, for users interested in electronics, it can give ideas about the following topics:

• The basic structure of simple AM modulation circuits
• The effect of coil winding on frequency and behavior
• The importance of the load and antenna relationship in the RF output stage
• The effect of power supply line quality on the transmitted signal
• The stability advantage of crystal-controlled oscillators

Important Legal Warning

This project is for educational and examination purposes only. Situations such as long-term use, regular broadcasting, or transmitting signals over a certain coverage area may be subject to communication and broadcasting regulations that vary by country. Therefore, if real use is being considered, the relevant local regulations and permit requirements must definitely be researched.

Transmitter circuits in particular should be handled more carefully than receiver circuits. Unauthorized transmissions may cause interference problems and legal responsibilities.

I am providing the web page of the circuit source; there is currently no PCB drawing.