Albrecht AE7500 Guide D'utilisation page 27

The different modes for CB radios have historical reasons, as CB radio is already,
since the 1950s in existence. AM mode (amplitude modulation) corresponds with
the technical process of medium and short wave radio and is today, except for CB
radio, only used for aircraft radio. When speaking the transmission power is varied in
the rhythm of the voice. AM is traditionally the radio mode which is used by truck
drivers worldwide on CB radio. The low residual noise is advantageous if for instance
in diesel trucks with little interference the squelch stays open. The receiving
characteristics in AM corresponds rather with the human ear: distant stations seem
lower than stations nearby; still you can perceive if other persons are speaking on
your channel, even if a local station seems to blanket everything: at least the
blanketing tone can be heard and you can check back at later stage! One of the
disadvantages of AM is shorter range compared to FM and SSB and the higher
sensibility of AM for ignition interference and board computers.
In FM the transmission the power is constant, even when speaking, hence hardly any
interferences of other devices are to be expected in this mode. Instead, when
speaking the frequency of the radio varies, hence it is called frequency modulation.
Due to expected interferences in AM and SSB modes some countries for a long time
permitted only FM. Advantage is the greater range compared to AM and the same
volume of nearby and distant stations as well as a clear modulation sound. Loud
noise, which can be heard with weak or missing signals, is a disadvantage. In F M
mode it is virtually impossible to leave the squelch open.
SSB (Single Sideband Modulation) marks the highlight in the development of long-
distance voice transmission. The transmission mode SSB avoids the disadvantage of
the low range of AM and focuses the entire transmission power on a minimum
frequency width, where during a speech pause the power is nearly shut down to
zero. During receiving a special circuit makes sure that the "side bands" and the
"carrier", which were cut off during transmission, are restored. This allows an
enormous improvement in range compared to AM and FM. Not only the transmitter
has a greater range, but also the receiver has, due to the halved bandwidth, better
sensitivity. The price for this is the disadvantage that for receiving very accurate
tuning is necessary, and that during transmission interferences of neighboring
devices is greater. Due to the necessary fine tuning of the receiver with the Clarifier,
the use of SSB while driving a vehicle is a problem and can – such as making a
phone call – distract from the traffic. SSB is rather an operating mode for base
station. This is the reason why SSB today is only used by skilled radio operators –
you virtually have to train and develop some intuition for the correct setting.
USB and LSB
In SSB mode a conventional channel is separated in two halves: the so called upper
side band (Upper Side Band) and the lower side band (Lower Side Band). Both can
be used independently. Example: you transmit on channel 9. In reality you only use
the frequencies from 27.065 up to approx. 27.070 MHz, while in LSB you are using
the frequencies below 27.065 to approx. 27.000 MHz. Both side bands are mirrored;
hence transmitter and receiver must be set to the same side band.
How to Receive SSB
As the receiver station you cannot know, which side band your counter station is
transmitting. The fact that the counter station is transmitting SSB is easily detected,
since the "squawking sound" is very different from FM or AM signals. Receive first in
USB. Very carefully turn the Clarifier. If you can hear a "Mickey Mouse" voice, try to
fine tune to a recognizable pitch. If this does not work, then try the other side band
LSB. You need some routine until you can clearly hear your dialog partner in SSB
mode.
English 27