In , Skipp peaks into this mess
wrote:
: Frank Gilliland wrote:
: 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...
What many of the technical heads don't realize is the simplicity of AM
mode for the average joe. The average arm chair CB person doesn't want to
crank a clarifier knob in SSB operation. AM remains popular for the
average CB operation regardless of efficieny issues.
I don't think it's a matter of the adjustment of one knob, since people seem to
get their kicks out of having lots of knobs on their radios. I think it's more
about the cost of the radios. You can pick up an AM rig at any flea market for
pennies; but a working SSB rig, new -or- used (that hasn't been butchered), will
cost you a few bills.
: 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.
Modulation percentage & duty cycle.
Duty cycle is irrelevant since there is no modulation percentage to be measured
when the radio isn't transmitting.
In the real world, one must consider the radio service, operator and
equipment. Speech processing (power mics into proper mic limiter
circuits as an example) and background noise can lead to modulation
percentages greater than 30%. You should qualify your statement to say
something like the typical human voice might modulate an unprocessed AM CB
about 20 to 30% on average. Throw in a typical power mic or some type of
speech processor box (like the Heil type of of radio equipment) and that
all goes out the window.
I took the peak-to-average modulation ratio from the ARRL handbook (as well as
several other textbooks). Take up your argument with it's author.
[cut and paste a little bit of good theory]
With 100-percent sine-wave modulation, a transmitter produces 1.5
units of RF power.
....."units"?
The additional 0.5 unit of power is furnished by the
modulator and is distributed equally between the two sidebands. This AM
transmitter is compared with an SSB transmitter rated at 0.5 unit of
peak-envelope power (PEP).
If the "additional 0.5 unit of power" is distributed equally between the two
sidebands, don't you mean 0.25 unit for a single sideband?
Peak-envelope power is defined as the RMS power
developed at the crest of the modulation envelope.
Many of these rec radio cb technical posts fail to mention the source of
the additional power which is furnished by the modulator.
I didn't. Read my post again.
: Now AM works fine if you don't mind wasting power.
Most people prefer to trade the "wasted power" for the simplicity of AM
operation. Kind of the SUV of radio thing... just lacking the dam cell
phone planted in your ear as you drive along.
And I prefer to believe that "most people" are uneducated as to the benefits of
SSB, which is why I wrote that post.
: 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.
Lets add some more real information.
When the RF signal is demodulated in the AM receiver an audio voltage
develops which is equivalent to the sum of the upper- and lower-sideband
voltages, in this case 1 unit of voltage. This voltage represents the
output from a diode detector as normally used for AM reception. Such
detection is called coherent detection because the voltages of the two
sidebands are added in the detector.
Holy Smoked Oysters, Skip! That's called "envelope detection" and has nothing to
do with sidebands! And the "coherer detector" was an ancient method of detection
that was used long before tubes, even before galena crystals! It used iron
filings that magnetized and 'cohered' to each other under modulated RF currents,
changing the overall resistance with the modulation. You are WAY, WAY out in
yonder pasture with THAT one, Skip.
When the RF signal is demodulated in the SSB receiver, an audio voltage
of 0.7 unit develops which is equivalent to the transmitted
upper-sideband signal. If a broadband noise level is chosen as 0.1 unit
of voltage per 6 kc bandwidth, the AM bandwidth, the same noise level is
equal to 0.07 unit of voltage per 3 kc bandwidth, the SSB bandwidth.
A bit obtuse, but ok....
These values represent the same noise power level per kc of bandwidth,
Wrong. Noise voltage level is NOT noise power level, the latter being the sum of
all the noise within the bandwidth. Narrowing the bandwidth does not reduce the
noise voltage level but it DOES reduce the noise power level, and it does so in
direct proportion to the bandwidth. IOW, cut the bandwidth in half and you cut
the noise power level in half.
snip faulty explanation based on your lack of understanding
: 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!
I don't agree, but you be your own judge. Also those of you nit pick
types notice the mention just above "stock CB". It's a big deal when
assuming modulation percentages and duty cycle. Throw a typical power mic
into the mix and the big picture quickly changes.
Many CB'ers use power mics, most AM (and FM) Broadcasters use heavy Audio
processing. A science unto itself which is also an it's own industry.
Overmodulation is next week's lesson.
Your results will probably vary...
cheers
skipp
http://sonic.ucdavis.edu
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