communications subject..please help me to answer this questions.. thanks so much!

1. Why is it desirable to have the modulation index of an AM signal as large as possible without overmodulating?

2. Describe what happens when a typical AM modulator is overmodulated, and explain why overmodulation is undesirable.

3. Describe three different ways in which the modulation index of an AM signal can be measured.

4. WHY and by HOW MUCH does suppressing the carrier improve the efficiency of an AM signal?

5. WHY and by HOW MUCH does eliminating one sideband improve the efficiency of an AM signal?

1. If the modulation index is greater than 1 (more than 100% modulation) the modulated carrier signal is periodically reduced to zero. This does not effect the transmitter but at the receiver these "dead carrier" intervals result in dropouts in the audio stream from the loudspeaker producing a distorted output that is difficult to understand.

http://www.complextoreal.com/chapters/mod1.pdf

http://www.ece.uvic.ca/~peter/35005/350nc7.pdf

http://profile.iiita.ac.in/IEC200806...Modulation.pdf

Once you remove a carrier we call the resulting signal a sideband signal

2 examples .... DSB ... Double Sideband) suppressed carrier
or SSB ... Single Sideband

Efficiency is gained mainly due to the reduction of the bandwidth of the transmitted signal ... for example an AM transmission that could be ~ 25kHz is only 3kHz on SSB

Time for you to hit the books or google.... check out various books like the ARRL Amateur Radio handbook for excellent understandable discussions on the differences between AM and SSB and you will find out why us amateur operators love SSB for very weak signal work :)

Dave
VK2TDN