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#1
July 16th 03, 09:23 PM
 Frank Gilliland Posts: n/a
Power vs Mode (The fundamentals)

Too many people are preoccupied with RF power; i.e, WATTS. What they don't
realize is that the MODE of operation is far more important than power...

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 is usually somewhere around 30%, so the average RF power that is
transmitted is closer to 4.6 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 more like 30%, which means that a 0.3 watt
SSB transmission has the same effect as using 4.6 watts to transmit that very
same sideband using AM. Therefore, with normal speech, SSB is closer to 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 about
30%. So using voice on SSB the average power will be about 4 watts. Now since we
already know that SSB modulated with normal speech is 15 times more efficient
than AM. Therefore, 4 watts of SSB is equivalent to 60 watts of AM power, or 52
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 half
the bandwidth of AM (6 KHz for AM vs 3 KHz for SSB). That means it receives half
the noise of AM, thereby doubling the all-important signal-to-noise ratio, and
effectively doubling the power of the transmitted signal.

All summed up, a stock CB with SSB has the same range and talk-power as the same
CB using AM with a 100 watt linear -- and it's LEGAL!

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#2
July 16th 03, 11:23 PM
 Posts: n/a

Frank Gilliland wrote:
All summed up, a stock CB with SSB has the same range and talk-power as
the same CB using AM with a 100 watt linear -- and it's LEGAL!

True.

--
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-------------------------------------------------------------
50 GB/Month
#3
July 17th 03, 02:20 AM
 jim Posts: n/a

#4
July 17th 03, 02:35 AM
 [email protected] Posts: n/a

#5
July 17th 03, 03:08 AM
 [email protected] Posts: n/a

snip
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 more like 30%, which means that a 0.3 watt
SSB transmission has the same effect as using 4.6 watts to transmit that very
same sideband using AM. Therefore, with normal speech, SSB is closer to 15 times
more efficient!

snip

You are wrong about SSB being 15 times more efficient. Your reasoning
is flawed in that.............................................. ......

If speech modulates a AM signal to a average of 30% then the same
speech will modulate a SSB to a similar reduced potential.
**********************************
On A.M. , with a 4 watt carrier at 100% modulation , we have 2 watts
of audio power used for the sidebands. One watt on each sideband.
This duplication of sidebands is not necessary to convey intelligence.

If we use the same transmitter and convert it to DSB ( double
sideband ) by removing the carrier , we can now have 2 watts per
sideband.

If we now remove the other sideband , and concentrate all of
the power into one sideband , we have a 4 watt sideband. With
this method of removing the carrier and one sideband we can
put 4 watts of intelligence out on SSB as compared to 1 watt on
A.M.. This makes a SSB transmission 4 times as powerful as its
A.M. counterpart.

transmitted , only 1/2 the bandwidth is needed. This means that twice
the number of stations could operate in the same bandspace as A.M.. In
addition to this , because the bandwidth needed is only 1/2 of A.M. ,
only 1/2 of the atmospheric noise is picked up with the signal. This

So when you add it all up you have 6db gain on transmit , and
3db gain on receive. That's effectively 9db of total gain.

#8
July 17th 03, 03:45 AM
 Landshark Posts: n/a

wrote in message ...

snip
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 more like 30%, which means that a 0.3 watt
SSB transmission has the same effect as using 4.6 watts to transmit that very
same sideband using AM. Therefore, with normal speech, SSB is closer to 15 times
more efficient!

snip

You are wrong about SSB being 15 times more efficient. Your reasoning
is flawed in that.............................................. ......

If speech modulates a AM signal to a average of 30% then the same
speech will modulate a SSB to a similar reduced potential.
**********************************
On A.M. , with a 4 watt carrier at 100% modulation , we have 2 watts
of audio power used for the sidebands. One watt on each sideband.
This duplication of sidebands is not necessary to convey intelligence.

If we use the same transmitter and convert it to DSB ( double
sideband ) by removing the carrier , we can now have 2 watts per
sideband.

If we now remove the other sideband , and concentrate all of
the power into one sideband , we have a 4 watt sideband. With
this method of removing the carrier and one sideband we can
put 4 watts of intelligence out on SSB as compared to 1 watt on
A.M.. This makes a SSB transmission 4 times as powerful as its
A.M. counterpart.

transmitted , only 1/2 the bandwidth is needed. This means that twice
the number of stations could operate in the same bandspace as A.M.. In
addition to this , because the bandwidth needed is only 1/2 of A.M. ,
only 1/2 of the atmospheric noise is picked up with the signal. This

So when you add it all up you have 6db gain on transmit , and
3db gain on receive. That's effectively 9db of total gain.

While I can disagree with Frank when he's being
a troll, even though is exact figures are a little off, basically
he's correct.

Landshark

--
The happy people are those who are producing something;
the bored people are those who are consuming much and
producing nothing.

#9
July 17th 03, 04:20 AM
 sheik yerbooti Posts: n/a

"Landshark" . wrote in message
...

wrote in message

...

snip
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 more like 30%, which means that a

0.3 watt
SSB transmission has the same effect as using 4.6 watts to transmit

that very
same sideband using AM. Therefore, with normal speech, SSB is closer to

15 times
more efficient!

snip

You are wrong about SSB being 15 times more efficient. Your reasoning
is flawed in that.............................................. ......

If speech modulates a AM signal to a average of 30% then the same
speech will modulate a SSB to a similar reduced potential.
**********************************
On A.M. , with a 4 watt carrier at 100% modulation , we have 2 watts
of audio power used for the sidebands. One watt on each sideband.
This duplication of sidebands is not necessary to convey intelligence.

If we use the same transmitter and convert it to DSB ( double
sideband ) by removing the carrier , we can now have 2 watts per
sideband.

If we now remove the other sideband , and concentrate all of
the power into one sideband , we have a 4 watt sideband. With
this method of removing the carrier and one sideband we can
put 4 watts of intelligence out on SSB as compared to 1 watt on
A.M.. This makes a SSB transmission 4 times as powerful as its
A.M. counterpart.

transmitted , only 1/2 the bandwidth is needed. This means that twice
the number of stations could operate in the same bandspace as A.M.. In
addition to this , because the bandwidth needed is only 1/2 of A.M. ,
only 1/2 of the atmospheric noise is picked up with the signal. This

So when you add it all up you have 6db gain on transmit , and
3db gain on receive. That's effectively 9db of total gain.

While I can disagree with Frank when he's being
a troll, even though is exact figures are a little off, basically
he's correct.

Landshark

How the hell wouldyou know if he was off a little or not? you say this

#10
July 17th 03, 04:58 AM
 Frank Gilliland Posts: n/a

wrote:

snip
Frank;
Statistical studies of the distribution of signals on the air
versus the signal strength shows that the probability of successful
communication will be the same if the SSB power is equal to one-half
the power of one of the two AM sidebands.

That's exactly what I said.

pretty close to what the studies show.

Whats the Peak Envelope Power of a Legal AM radio?

Where did I say anything about PEP? I know what you are trolling for (if you're
not trolling then somebody else will), and yes, the PEP of a 4 watt AM carrier
that is 100% modulated is 16 watts. But that number is misleading because you
are forgetting that an AM signal has a lot of excess baggage. This is one
instance where less is more....

PEP is used when determining the maximum power handling capability of the final
(output) amplifier. In other words, if the AM carrier is going to be 4 watts,
the final must be capable of providing 16 watts. It means nearly the same with
SSB, because the PEP rating of the output amplifier is the peak power level at
which the signal will begin to distort, and since the peak-to-average modulation
ratio is about 3 to 1, a transmitter capable of 12 watts should safely handle
about 4 watts average power in SSB.

But if a transmitter is capable of 16 watts, it seems foolish to use AM with
it's 1 watt sidebands when you can transmit SSB with an effective power of
almost 200 watts AM or better, depending on your average modulation. The FCC
limits the radios to 12 watts PEP, but that's still much better than 16 watts
PEP in AM. In other words, PEP does not represent "talk-power".

For those who don't already know, Peak Envelope Power (PEP) is the RF power at
the brief instant an audio cycle peaks the modulation. PEP is used to describe
SSB power because the standard wattmeter can't measure the average power of an
power will be somewhere around 4 watts, but you won't be able to tell because
your wattmeter will be bouncing around with your modulation. So SSB is measured
with a 'peak' value (PEP) instead of a 'real' value (RMS). So even though the
needle is bouncing around, you just need to keep it below the maximum PEP rating

OTOH, AM power is measured in RMS (true) watts, and is a measurement of carrier
power only. The modulation is detected (demodulated) and measured seperately as
a percentage. Modulation -must- be measured seperate from the carrier because
carrier power should remain steady while under modulation; and modulation is
read directly so the operator doesn't have to perform carrier subtractions and
square-root calculations on a PEP reading in order to find the modulation
percentage. Of course you can always add a PEP meter if you really want, but
what's the point of making things difficult and expensive when you already have
all the information you need?

Why is 12 watts the limit for a legal SSB radio?

Because the FCC says so.

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