Why Single Sideband Is Important In Hf Communications; Amplitude Modulation (Am) - Honeywell KHF 1050 Pilot's Manual

WHY SINGLE SIDEBAND IS IMPORTANT IN HF
COMMUNICATIONS
As mentioned earlier, there are two characteristics of HF SSB communi-
cations which allow long range capability. Skywave propagation has
been discussed in detail. The other characteristic is a transmission
process known as single sideband. Single sideband (SSB) high frequen-
cy (HF) communications was developed in the early 1950's as a means
of increasing the effective range of HF signals. The KHF 1050 is capa-
ble of both amplitude modulation (AM) operation, such as is used in con-
ventional 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 frequency) are gen-
erated and transmitted. (Figure 2A.) It takes about two-thirds of the
transmitter's power just to transmit the carrier frequency, 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 airborne
HF use. It is normally used in the KHF 1050, but can be selected for
those who are authorized to use it.
Revision 0 Mar/2003
AMPLITUDE MODULATION (AM)
fc = carrier frequency
fm = modulating frequency (voice)
fc-fm
LSB
Figure 2A - Amplitude Modulation
fc fc+fm
USB
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KHF 1050/PS440 Pilot's Guide