25 Metres Range, Short Wave AM Transmitter

Here the short wave AM Transmitter circuit design diagram. The circuit is quite simple and easy to build since it applies only a few electronic components. The primary feature of this transmitter is that it really is absolutely free from the LC (inductor, capacitor) tuned circuit and runs using a fixed frequency of 12 MHz that is very stable. An LC based tuned circuit is inherently unstable because of drift of resonant frequency due to temperature and humidity variations.

Resistors R1 and R2 are utilized for DC biasing of transistor T1. The capacitor C1 gives coupling in between the condenser microphone and the base of transistor T1. In the same way, resistors R3, R4 and R5 give DC biasing to transistor T2.

The oscillator segment is a combination of transistor T2, crystal XTAL, capacitor C2, C3 and resistors R3, R4 and R5. The crystal is excited by a portion of energy from the collector of transistor T2 via the feedback capacitor C2. The crystal vibrates at its essential frequency and the oscillations happening because of the crystal are placed to the base of transistor T2 across resistor R4. Using this method, continuous undamped oscillations are acquired. Any crystal having the frequency in short wave range could be substituted in this circuit, even though the operation was tried using a 12 MHz crystal.

The Transistor T1 has 3 capabilities:

1. The transistor features the DC path for extending +VCC source to transistor T2.
2. It amplifies the audio signals which is generated by condenser mic.
3. It injects the audio signal into the high frequency carrier signal for modulation.

The condenser microphone transforms the voice message into the electrical signal that is amplified by transistor T1. This amplified audio signal modulates the carrier frequency produced by transistor T2. The amplitude modulated output is acquired at the collector of transistor T2 and is transmitted by a loop antenna into space in the form of electromagnetic waves. The antenna could be tuned to a specific frequency by fine-tuning the trimmer C5 and also by modifying the length of ferrite rod into the coil.

The transmitted signals could be received on any short wave receiver without having distortion and noise. The range of this transmitter is 25 to 30 metres and may be expanded even more in case the length of the antenna wire is suitably extended together with good matching.