BendixKing PS440 System KHF 1050 - Manual (Page 25)

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.
SINGLE SIDEBAND OPERATION
By electronically eliminating the carrier wave and one sideband, a single
sideband transmitter manages to pack all of its power in transmitting the
remaining single sideband. (Figure 3A). Either the upper sideband
(USB) or the lower sideband (LSB) can be used since each sideband
contains all the required intelligence. However, from a practical stand-
point the USB is used almost exclusively in airborne HF SSB operations
and the LSB may be disabled. Upon receiving this SSB signal, the
receiver then generates the carrier frequency internally and combines it
with the one sideband in such a way that the intelligence (voice) can be
heard and understood by the pilot.
Revision 1 May/2003 KHF 1050/PS440 Pilot’s Guide
21
AMPLITUDE MODULATION (AM)
fc = carrier frequency
fm = modulating frequency (voice)
fc-fm
LSB
fc fc+fm
USB
Figure 2A - Amplitude Modulation
KHF1050 PG R1 4/29/03 3:14 PM Page 21