Calculation of modulation index

Calculate the modulation index when the un modulated carrier power is 15KW, and after modulation, carrier power is 17KW.

a.) 68%
b.) 51.63%
c.) 82.58%
d.) 34.66%

Correct answer: b.) 51.63%
Explanation:

The total power in an AM is given by
P_t= P_c ( 1 + m²/2)
17= 15(1 + m²/2)
m²/2= 0.134
m=0.5163
= 51.63%

Calculation of modulation index using antenna current

An AM transmitter has an antenna current changing from 5 A un modulated to 5.8 A. What is the percentage of modulation?

a.) 38.8%
b.) 83.14 %
c.) 46.8%
d.) 25.2%

Correct Answer: 83.14 %

Explanation:

Modulation index m is given by
m= √ (2{I_t/I_c}²-1)
= √ (2 (5.8/5)² -1)
= √ (2 (5.8/5)² -1)
= 0.8314
= 83.14%