First, letís clarify that 1 S-Unit is 6 Db. and that to raise your signal on some distant receiver, 1 S unit requires a 4 times increase in power.
Decibel's Watt's and Headroom
Since we are talking about Amplitude Modulation, there is more than just the carrier. The ratio must remain the same. This means that for a 100% modulated carrier, the peaks will be roughly 4 times greater than the carrier.
Letís assume for the sake of easy numbers, that a station has a carrier of 400 Watts (375 Watts in a perfect world). The fully modulated carrier will reach peaks of 1,600 Watts.
If you were to run asymmetrical modulation, you would have to drop the carrier even more. If you ran a 200 Watt carrier, you could have 200% positive peaks. Though 200% is too high, and begins to distort as far as the receiver is concerned, it gives a pretty good idea as to how much headroom you would need. 150% positive peaks are what you should shoot for.
Once the Oscilloscope shows the envelope growth that you want, and increasing carrier reduces the positive peaks, you have reached the limit of the amplifier. It doesn't matter what the specifications say, or a Technician, or and Engineer. It is what it is.
Think of the amplifier as a bucket of water. The bucket being 1/8 full, assuming that you had enough extra water to put in it, (positive peak modulation) you could have the kind of growth you are looking for. If you tune the amp on a dead carrier, or if you drive it with too much power, it will be like adding more watter to the bucket. Once the bucket is 1/4 full, your maximum positive modulation can only be 100%.
If you continue to fill it, and watch the Watt meter go up, then the carrier is increased, but there is less or no envelope growth. Hence the thing we are trying to achieve (AMPLITUDE MODULATION) or a CHANGE in AMPLITUDE can't occur. The only change is from a big carrier to zero carrier. As long as it is not pinched or flat topped, it can sound good, but will be very quiet.
Lets say that the carrier was up to 800 Watts. You pick up 1 full S Unit, but your audio looses almost 6 db in volume from where it started. That is almost a 12 db down from where it should be. Moving up 1 S Unit should have in itself made you louder, and you got quieter than you were.
Combined, this is a bad deal. As for thinking youíre going to get over another station by doing this... Skip plays a bigger role in this game than we do. The smartest thing to do is to take control of how you sound.

Decibels Watts and Headroom

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