In , "Landshark"
. wrote:
snip
While I can disagree with Frank when he's being
a troll, even though is exact figures are a little off.....
Here's a corrected version just for you, Hypocrite Landshark:
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AM (Amplitude Modulation) is composed of three parts: The carrier, the lower
sideband and the upper sideband. The carrier stays constant while the sidebands
vary in power according to the modulation. When a 4 watt carrier is modulated to
100%, there will be 1 watt transmitted in each sideband, for a total of 6 watts
of RF power that is being transmitted. But the voice can't modulate the carrier
to 100% all the time -- speech does not have a constant amplitude. Average
modulation of speech is generally accepted to be 33% (a peak-to-average ratio of
3 to 1), so under that standard the average RF power that is transmitted would
be 4.67 watts.
Now AM works fine if you don't mind wasting power. This is because the audio is
carried only in the sidebands, not the carrier. And because the two sidebands
are mirror images of themselves, only one sideband is needed and the other is
wasted. If we eliminate the carrier and one sideband (resulting in the mode
called Single Sideband, or SSB), we are left with a 1 watt sideband that will
work just as well as if we burned 6 watts to transmit two sidebands and a
carrier. In other words, SSB is -AT LEAST- 6 times more efficient than AM. But
remember that average modulation is 33%, which means that a 0.33 watt
SSB transmission has the same effect as using 4.67 watts to transmit that very
same sideband using AM. Therefore, with normal speech, SSB is 14.15 times
more efficient!
Let's translate all this into watts. CB permits 12 watts for SSB. For speech
communication, the average power is the same as the average modulation, or 33%.
So using voice on SSB the average power will be 4 watts. Now since we
already know that SSB modulated with normal speech is 14.15 times more efficient
than AM. Therefore, 4 watts of SSB is equivalent to 56.61 watts of AM power, or
48.61 watts of carrier power with 4 watts in each sideband. And under 100%
modulation the SSB power will be 12 watts, while it takes 72 watts to do the
same job on AM (48 watts of carrier with 12 watts in each sideband).
But SSB has another advantage: Because it only uses one sideband, it uses less
than half the bandwidth of AM (6 KHz for AM vs 2.7 KHz for SSB). That means it
receives 45% less noise than AM, thereby increasing the effective transmitted
power by a factor of 2.22.
All summed up, a CB radio capable of 12 watts PEP on SSB has the same range and
talk-power as AM from an amplifier capable of 426.24 watts PEP (12 watts PEP is
the power of one sideband from a 100% modulated AM signal with a carrier power
of 48 watts RMS: Therefore, 48 watts RMS x 2.22 = 106.56 watts RMS (effective);
106.56 watts RMS x 4 = 426.24 watts PEP) -- and it's LEGAL!
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