Chris wrote:
I will probably get some flames from this but here it goes. I have been into
CB radio for a number of years but don't agree with most of what I hear.
That's just one of the reasons I'm looking to amateur radio. One of the
things I often hear in CB circles is that one should turn a 4 watt AM radio
down to 1 1/2 watts and let it "SWING". How is this possible? What really
happens when you do this? I think I know. So, how much carrier should you
have for an amp or final stage with a known max output. In other words, if
it can produce 8 watts max unmodulated carrier, is a 4 watt carrier ideal?
If it produces 100 watts, is 50 watts ideal? How much "space"does it need
for proper modulation? Is there a website that explains this well? I'm an
electronics tech thirsting for knowledge.

In amplitude modulation, the strength - "amplitude" - of the carrier is
adjusted by the audio you wish to transmit.

The degree to which this strength is adjusted is the "modulation
percentage". If the carrier is cut completely at negative voice peaks,
and strengthened to twice its normal level at positive peaks, then the
signal is said to be "100% modulated".

The receiver at the other end can only detect the *changes* in carrier
strength - not the carrier itself. If you reduce the modulation
percentage, you reduce the strength of the changes - the strength of the
signal the other guy can hear.

On the other hand, the laws of physics prohibit negative power. Once
you've modulated 100% - and reduced the carrier to zero at negative
peaks - you CAN'T go any further. It's physically impossible. If you
try, you'll generate sharp cutoffs that result in "splatter" - strong
interfering noises in adjacent channels. (and your signal on the
channel you're meaning to transmit on will become seriously distorted
and difficult to understand) [0]

So the point is, you want to modulate as close to 100% as practical
while ensuring you never *exceed* 100%.

For normal "high-level" modulation the amount of audio power required to
achieve 100% modulation is half the RF power. A 4-watt carrier requires
two watts of audio to modulate it 100%.
--
Doug Smith W9WI
Pleasant View (Nashville), TN EM66
http://www.w9wi.com

[0] It is possible to exceed 100% in the *positive* direction -
increasing the carrier beyond twice its normal level - without causing
distortion and interference. Such schemes are common at AM broadcast
stations. I'm not aware of any CB radio that contains such a circuit.