A number of different ‘modes’ co-exist to enable radio amateur transmissions to carry the required information because each has its own advantages and disadvantages. The most common types in use are summarised below:
- CW Continuous Wave – The transmitter output signal is unchanged except for being switched on and off to send Morse code. This uses the simplest type of transmitter but requires a receiver with the facility to convert steady radio frequency into an audible tone (Bandwidth 250Hz).
- AM Amplitude Modulation – The transmitter uses the audio signal to vary the strength of the transmitted signal (analogous to rapidly turning the volume control). The transmitter is moderately complex but the receiver uses the simplest type of demodulation to extract the sound at the receiver (Bandwidth 6kHz).
- NBFM Narrow Band Frequency Modulation – The transmitter uses the audio signal to vary the frequency (analogous to rapidly turning the tuning control). The transmitter and the receiver are moderately complex but can deliver noise free sound if the signal is sufficiently strong (Bandwidth 11kHz and 16kHz).
- LSB Lower Sideband – Amplitude modulation results in the information about the sound being carried in two duplicate narrow bands situated either side of the central transmitter frequency (carrier). Here the lower side band is selected for transmission and the carrier and upper sideband removed. This makes the best use of the transmitter power rating and also reduces the overall bandwidth. It requires complex transmitters and receivers and careful tuning of the receiver (Bandwidth 2.7kHz).
- USB Upper Sideband – Similar to the LSB mode except that the upper sideband is selected for transmission instead.
- FSK Frequency Shift Keying – This is a method of sending digitised information where a ‘0’ is represented by one tone and a ‘1’ by another. Some schemes use multiple tones so that bytes of information can be sent at the same time. This can give a warbling sound to the signal. Free software is available to encode and decode the information which may be voice, text or pictures.