Cleveland RAYNET Group

Radio Amateurs can use various modes or types of transmission.  The most common is speech.  Not all modes can be used on all frequency bands.

Modulation

A radio frequency which doesn't change in any way is called a carrier, as it has the capability of carrying information.  No information (or message) is transferred by a carrier.  Once the carrier is changed, information can be sent which is represented by those changes.  The way in which the carrier is altered is called the modulation.

Amplitude modulation means that the instantaneous signal strength of the carrier varies according to the modulating signal e.g. speech.  Frequency modulation means that the instantaneous frequency of the carrier varies according to the modulating signal.  Phase modulation is very similar to frequency modulation, but the angle or phase of the carrier is changed.

Each mode or type of radio transmission can use any type of modulation, but due to some equipment standards or accepted practices, some modes may use only one type of modulation.

Bandwidth

This is the frequency space occupied by a single radio channel.  It is the frequency size necessary for adequate modulation plus some extra width, so it doesn't stray into an adjacent radio channel.  High quality speech could require a bandwidth of about 15 kHz, so a suitable radio frequency channel bandwidth could be 25 kHz.

If speech is filtered into a smaller channel,  it loses its high quality but is perfectly understandable. Speech can be filtered down to about 2.5 or 3 kHz and still sound fairly reasonable. In that way, radio channel bandwidths of 3, 4 or 5 kHz can be used, to allow more conversations to fit into a given frequency band or section of a band.

The bandwidth of a data signal e.g. from a computer, varies as the rate of information transfer.  If you are happy with sending a small amount of information in a given time, a small bandwidth is adequate.  If it is necessary to pass a lot of information in the same time, a large bandwidth is required for each channel.

Transmission Modes

Click the hyperlinks at the left for more details.

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Updated on 19 July 2009