As can be seen from the table the power that is in the AF is divided equally between the two sidebands, furthermore the information in the AF is contained in both sidebands. It should also be noted that only one third of the signal is carrying the information.

Single Sideband (SSB)

There is redundancy in double sideband transmissions in that the information is contained in both the upper and lower sidebands. Additionally, the original RF carrier wave having served its purpose to get the audio information into radio frequencies is now redundant. So it is possible to remove one of the sidebands and the carrier wave because the remaining sideband contains all the information. This is known as single sideband (SSB) operation.

When using sky wave propagation for communication, the differing refraction occurring at different frequencies leads to an increase in distortion if the bandwidth is too large. The ionosphere comprises electrically charged particles which cause high levels of static interference on radio waves, the use of SSB significantly reduces the effect of this interference. The MF & HF frequencies used for long range communication are in great demand, hence the use of SSB transmissions increases the number of channels available. The use of SSB also reduces the amount of power required.

Thus the main advantages of SSB are:

•Double the number of channels available with double sideband

•Better signal/noise ratio (less interference)

•Less power required hence lighter equipment

Modulation 3

Phase Modulation

In phase modulation the phase of the carrier wave is modified by the input signal. There are two cases: the first is where the input is an analogue signal when the phase of the carrier wave is modified by the amplitude of the signal; secondly, with a digital signal it is known as phase shift keying, the phase change reflects a 0 or 1; e.g. 0° phase shift indicates a zero and 180° phase shift represents a 1. (Note: this is the simplest case as multiple data can be represented by using many degrees of phase shift.)

There are two cases used in navigation systems, MLS and GPS. GPS uses binary phase shift keying, MLS uses differential phase shift keying.

Modulation 3

48

EMISSION CHARACTERISTICS

Modulation 3

For example, VHF radio telephony communications have the designation A3E. Reference to the table gives the following breakdown:

A - Amplitude modulation - Double sideband

3 - Single channel containing analogue information E - Telephony, including sound broadcasting

This means an RF carrier wave is being amplitude modulated with speech.

HF radio telephony communications have the designation J3E, this gives:

J - Amplitude modulation – single sideband with suppressed carrier 3 - Single channel containing analogue information

E - Telephony, including sound broadcasting

This means an RF carrier wave is being amplitude modulated with speech then the RF carrier wave is being removed along with one of the sidebands.

It is not necessary to know the details of the table.

Other designators relevant to the equipments discussed in phase 2 are:

With the exception of ADF it is unlikely that knowledge of these designators will be examined.

Modulation 3