Modulation index ,mf = \(\frac { Maximum\ frequency\ deviation }{ minimum\ signal\ freqency } \)

Ps = 1/2 ma² Pc
1.246 = 1 + \(\frac { ma^{ 2 } }{ 2 } \)
ma²/2 = 0.246
m_a = (2 x 0.246)^1/2 = 0.701 = 70.1%

(i) The AM wave has sideband frequency of (fc + fs) and (fc - fs). Sideband frequency = (500+1) kHz and (500-1) kHz 501 kHz and 499 kHz
(ii) Bandwidth required = 499 kHz to 501 kHz = 2 kHz

The needs of modulation for transmission of a signal are given as below:
(i) The transmission of low frequency signal needs antenna of height 4-5 km which is impossible to construct. So, there is a: need to modulate wave in order to reduce the height of antenna to a reasonable height.
(ii) Effective power radiated by antenna for low wavelength or high frequency wave as
P∝\(\frac { 1 }{ { \lambda }^{ 2 } } \)
So, for effective radiation by antenna, there is a need to modulate the wave.

Transducer is a device which converts one form of energy into another form. Microphone is a transducer because it converts sound energy into electrical energy. Repeater is a device which picks up electromagnetic signal, amplifies it and then again transmits it to the receiver. It is used  when transmitted signal is attenuated beyond a certain limit or distance between a transmitter and a receiver is very large.

For the transmission of signals, the following two types of transmission media are used.
(i) Skywave propagation or short wave Propagation.
(ii) Space wave communication or line of sight communication.
Range of frequencies
(i) Sky wave propagation 3 MHz > f> 300 MHz
(ii) Space wave communication 30 MHz < f < 300 MHz or more.

(ii) Modulator A modulator is a device. Which does the modulation of low frequency message signal.

(i) When the radio waves from the transmitting antenna reach the receiving antenna directly, the wave propagation is called line of sight communication.
(ii) Sky waves cannot be used for transmission of TV signals because the reflecting layer that is ionosphere becomes transparent for high frequency range of TV signal. They go to the sky but do not come back to surface. Radio waves for radio based communication are used as sky waves. An antenna of height h can transmit signal upto a maximum distance of d, where d =\(\sqrt{2hR}\) where, R is the radius of the earth.

(a) Here
P = Intermediate Frequency Stage
Q = Detector 
(b) IF Stage-Used to change the carrier frequency to lower frequency.
Detector-Used to recover the modulating signal from the modulated carrier wave.

1. Analog signals are continuous variations of voltage or current.
2. Digital signals are those which can take only discrete (stepwise) values.

The maximum range of ground wave propagation depends upon (i) the frequency of wave (ii) the power of the transmitter.
The maximum range of ground wave propagation can be increased by increasing  the power of transmitter which is so onlyfor very low frequency band (VLF) and not for medium frequency band (MF).                 

Our earth's atmosphere is transparent to the visibleradiations coming from sun,stars etc. but reflects back the infrared radiations and hence it does not allow the infrared radiations since the earth gets heated to much lower temperature than the temperature of sun,the radiations emitted by earth arew mostly in the infrared region,according to Planck's law.These radiations emitted by earth are reflected back by earth's atmosphere Due to ot,the earth's surface remains warm at night.This phenomenon is called Green House effect.