Shortwave AM transmitter circuit design diagram

Here is the shortwave AM transmitter circuit design diagram. This circuit is very simple and easy to build as it only applies a few electronic components. The main features of this transmitter are that it does have no LC (inductor, capacitor) tuning circuitry at all and operates at a very stable 12MHz fixed frequency. LC-based tuned circuits are inherently unstable due to resonant frequency drift caused by temperature and humidity changes.

Resistors R1 and R2 are used for DC biasing transistor T1. Capacitor C1 provides coupling between the condenser microphone and the base of transistor T1. Likewise, resistors R3, R4, and R5 provide DC bias for transistor T2.

The oscillator section is a combination of transistor T2, crystal XTAL, capacitors C2, C3 and resistors R3, R4 and R5. The crystal is excited by a portion of the energy from the collector of transistor T2 through feedback capacitor C2. The crystal vibrates at its fundamental frequency and the oscillations that occur due to the crystal are placed on the base of transistor T2 through resistor R4. Using this approach, continuous undamped oscillations can be obtained. Any crystal with a frequency in the shortwave range can be substituted in this circuit, even though this operation has been attempted with a 12MHz crystal.

Transistor T1 has 3 functions:

This transistor has a DC path for extending the +VCC source to transistor T2.

It amplifies the audio signal produced by a condenser microphone.

It injects audio signals into high-frequency carrier signals for modulation.

The condenser microphone converts the voice information into an electrical signal that is amplified by transistor T1. This amplified audio signal modulates the carrier frequency generated by transistor T2. The AM output is obtained at the collector of transistor T2 and transmitted into space in the form of electromagnetic waves through the loop antenna. The antenna can be tuned to a specific frequency by fine-tuning trimmer C5 and modifying the length of the ferrite rod into the coil.

The transmitted signal can be received on any shortwave receiver without distortion and noise. The range of this transmitter is 25 to 30 meters, which can be extended further if the length of the antenna wire is properly extended and well matched.