RadioBanter - Power vs Mode (The fundamentals)

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- - Power vs Mode (The fundamentals) (https://www.radiobanter.com/cb/29386-power-vs-mode-fundamentals.html)

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:

===========
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!
==================

Feel better?

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If speech modulates a AM signal to a average of 30% then the same
speech will modulate a SSB to a similar reduced potential.

The efficiency of SSB over AM increases as the modulation decreases, approaching
infinity as the modulation and output approach zero. That's because even when
there is 0% modulation in AM you still have carrier power.

I stand corrected when running a mode below its maximum capability,
but if you are horse racing with the transmitters power rating being
equal between modes then this massive advantage disappears.
This would occur in a hors race, and when it does a SSB transmission
only has a 6db advantage over AM

On Wed, 16 Jul 2003 20:58:17 -0700, Frank Gilliland
wrote:

In , Swan Radioman
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.

Your 100 watt figure is
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
SSB signal. For example, if your radio is capable of 12 watts PEP, your average
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
of the radio.

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.

Thanks for being such an asshole, You never said anything about
PEP. I figured someone else was going to bring it up so I asked.
My post in now way flamed or attacked you, in fact I backed you
up. Whats you damn problem?

"Swan Radioman" wrote in message
...

Thanks for being such an asshole, You never said anything about
PEP. I figured someone else was going to bring it up so I asked.
My post in now way flamed or attacked you, in fact I backed you
up. Whats you damn problem?

here is a hankie, douche bag

On Wed, 16 Jul 2003 20:58:17 -0700, Frank Gilliland
wrote:

In , Swan Radioman
wrote:

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

Because the FCC says so.

Why?

In , Swan Radioman wrote:

snip

Thanks for being such an asshole, You never said anything about
PEP. I figured someone else was going to bring it up so I asked.

And that's why I said, "if you're not trolling then somebody else will".

My post in now way flamed or attacked you, in fact I backed you
up. Whats you damn problem?

The PEP question raised a point that I thought should be addressed. If you took
it personally then that's your problem.

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On Thu, 17 Jul 2003 08:58:27 -0700, Frank Gilliland
wrote:

In , Swan Radioman wrote:

snip

Thanks for being such an asshole, You never said anything about
PEP. I figured someone else was going to bring it up so I asked.

And that's why I said, "if you're not trolling then somebody else will".

My post in now way flamed or attacked you, in fact I backed you
up. Whats you damn problem?

The PEP question raised a point that I thought should be addressed. If you took
it personally then that's your problem.

Ok, I apologize for my response to you.

In , Swan Radioman wrote:

On Wed, 16 Jul 2003 20:58:17 -0700, Frank Gilliland
wrote:

In , Swan Radioman
wrote:

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

Because the FCC says so.

Why?

I can only speculate. Any CB radio capable of both AM and SSB would probably use
the same final, which must be linear to allow SSB operation, and be capable of
16 watts PEP (4 watts AM, 100% mod). But the -average- power of SSB, if limited
to 16 watts PEP, would be higher than that of AM with a carrier of 4 watts RMS
(assuming a peak-to-average modulation ratio of 3 to 1), and could require a
final with higher power dissipation. So my guess would be that the FCC adopted
the lower limit of 12 watts PEP to put the average RF power of SSB -below- that
of AM, therefore preventing the need for a CB design that would allow AM power
greater than 4 watts carrier RMS.

If you have a better explanation, please share it.

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Swan Radioman wrote
On Wed, 16 Jul 2003 20:58:17 -0700, Frank Gilliland
wrote:

Swan Radioman
wrote:

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

Because the FCC says so.

Why?

The short answer; to facilitate communication. To allow the greatest
number of people to have equal access to the CB frequencies. If
everyone decided their own power limit, high power stations would
overwhelm low power stations.

-Aaron-