BendixKing KHF 990 - Manual (Page 20)

WHY SINGLE SIDEBAND IS IMPORTANT IN HF
COMMUNICATIONS
As mentioned earlier, there are two characteristics of HF SSB com-
munications which allow long range capability. Skywave propagation
has been discussed in detail. The other characteristic is a transmis-
sion process known as single sideband. Single sideband (SSB) high
frequency (HF) communications was developed in the early 1950’s
as a means of increasing the effective range of HF signals. The KHF
950/990 is capable of both amplitude modulation (AM) operation,
such as is used in conventional VHF aircraft communications, and of
SSB operation.
AMPLITUDE MODULATION (AM)
In order to understand SSB operation, a discussion of AM operation
is helpful. Amplitude Modulation (AM) is a transmission process in
which the selected frequency (called the carrier frequency) and two
sidebands (which are frequencies above and below the carrier fre-
quency) are generated and transmitte
d. (Figure 1-2.) It takes about
two-thirds of the transmitter’s power just to transmit the carrier fre-
quency, yet the carrier does not contain any of the intelligence to be
communicated. Each of these sidebands contains all the intelligence
to be communicated. Standard broadcast stations (550-1600 kHz)
and short-wave broadcasts use AM since it allows simpler receivers.
NOTE: The use of lower sideband isn’t normally authorized for air-
borne HF use. It is normally disabled in the KHF 950/990, but can be
enabled for those who are authorized to use it.
Description
1-9 KHF 950/990 Pilots Guide
Rev. 0
Dec/96
AMPLITUDE MODULATION (AM)
fc = carrier frequency
fm = modulating frequency (voice)
fc-fm
LSB
fc fc+fm
USB
Figure 1-2 Amplitude Modulation