Need of modulation; antenna parameters

Need of modulation

Modulation is the process of varying a property (amplitude, frequency or phase) of a high-frequency carrier by the low-frequency message signal. It is needed for the following reasons:

1. Practical antenna size: an antenna needs a length of about λ/4 or λ/2. For a 3 kHz audio signal, λ = c/f = 3 × 10⁸/(3 × 10³) = 100 km, so even a λ/4 antenna would be 25 km long, which is impractical. At 100 MHz, λ = 3 m, so the antenna is only about 0.75 m.
2. Multiplexing: many signals can share one channel using different carrier frequencies (FDM), without mixing.
3. Reduced noise and interference: some modulation methods (such as FM) improve the signal-to-noise ratio.
4. Longer range: high-frequency signals radiate efficiently and travel farther.
5. Avoids mixing of signals: all audio signals lie in the range 20 Hz to 20 kHz; if they were sent directly, they would overlap.
6. Practical equipment: it allows the use of smaller, standard-sized, low-cost circuits.

Three parameters of an antenna

1. Gain: the ratio of the power radiated in the preferred direction to that of a reference (isotropic) antenna, including losses.
2. Radiation pattern: the plot of field strength against direction; it shows the main lobe and the side lobes.
3. Bandwidth: the range of frequencies over which the antenna works satisfactorily.

Others: directivity, polarization, input impedance, beamwidth and efficiency.