3.3 Necessity of modulation

1.1 Terminology used in electronic communication system
1.2 Bandwidth of signals
1.3 Bandwidth of transmission medium
1.4 Earth's atmosphere

2.1 Propagation of electromagnetic waves in the atmosphere

3.1 Amplitude modulation
3.2 Frequency modulation
3.3 Necessity of modulation

The purpose of communication system is to transmit information or message signals. Message signals are also called baseband signals, which essentially designate the band of frequencies representing the original signal as delivered by the source of information.

No signal in general is a single frequency sinusoid but it spreads over a range of frequencies called the signal bandwidth.

Suppose we wish to transmit an electronic signal in the audio frequency (AF) range (baseband signal frequency less than 20 $kHz$) over a long distance directly.

Factors affecting transmission of signals are,

1. Size of the antenna or aerial : For transmitting a signal, we need an antenna or aerial. This antenna should have a size comparable to the wavelength of the signal at least $\left( {\frac{\lambda }{4}} \right)$ in dimension. So, that the antenna properly senses the time variation of the signal.

For an electromagnetic wave of frequency 20 $kHz$, the wavelength $\left( \lambda \right)$ is 15 $km$ and such long antenna is not possible to construct and operate.

Therefore, there is a need of translating the information contained in our original low-frequency baseband signal into high or radio frequencies before transmission.

2. Effective power radiated by an antenna: A theoretical study of radiation from a linear antenna (length $l$) shows that the power radiated is proportional to ${\left( {\frac{l}{\lambda }} \right)^2}$.

This implies that for the same antenna length, the power radiated increases with decreasing $\left( \lambda \right)$ i.e. increasing frequency.

Hence, the effective power radiated by a long wavelength baseband signal would be small. For a good transmission, we need high powers and hence this also points out to the need of using high-frequency transmission.