The simplest transmitter is an oscillator, an example of which is shown in Figure 1.  An oscillator is a circuit that generates a periodic signal of a defined frequency. This signal can be used as a carrier of information.  In this circuit, the carrier frequency is set by the crystal, which vibrates at its characteristic frequency when voltage is applied across it. 

Figure 1.  Circuit Diagram of a Simple Transmitter

The simplicity of this circuit lies in the fact that information is transmitted manually by the key operator using a special code of 'dots' and 'dashes' to represent alphanumeric characters. The signal generated by the oscillator is fed into the base of the transistor.  Whenever the key is pressed by the operator, this transistor drives the RF output transformer of the antenna at the crystal frequency. 

Thus, a burst of RF energy is emitted by the circuit every time the key is pressed by the operator. A 'dot' is a short burst of RF energy and a 'dash' is a long one. This method of transmission is known as continuous wave (CW) transmission.

The CW transmitter in Figure 1 is not a powerful one, typically being less than 1 watt.  Nonetheless, with proper selection of frequency and a good antenna, it can send information halfway around the world.

It is not difficult to improve this simple transmitter significantly. It can be enhanced by incorporating a power amplifier that boosts the power of the RF transmission. Such a set-up is depicted in Figure 2 below.

Figure 2.  An Enhanced CW Transmitter

See also:  Simplest Receiver

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