Communication Systems JEE Main: Complete Guide

Communication systems is a small chapter in JEE Main Physics, but it consistently delivers one to two questions per session and is almost entirely conceptual — making it one of the most mark-efficient chapters to prepare. Unlike chapters that demand heavy calculation, communication systems rewards students who understand the terminology and principles clearly.

Elements of a Communication System

Every communication system has three fundamental elements: the transmitter, the channel, and the receiver. The transmitter converts the information signal into a form suitable for transmission. The channel carries the signal — this could be free space, a wire, or an optical fiber. Noise is the unwanted signal added to the information signal during transmission. The bandwidth of an AM signal is twice the message bandwidth — a key fact JEE Main tests numerically. For the physics underlying wave concepts, see our electromagnetic waves guide.

Modulation: AM and FM

In amplitude modulation (AM), the amplitude of the carrier varies in proportion to the message signal. In frequency modulation (FM), the frequency of the carrier varies in proportion to the message signal. The modulation index in AM is the ratio of the message amplitude to the carrier amplitude. It must be between 0 and 1 for linear modulation without distortion. FM is less susceptible to amplitude noise than AM and provides better sound quality. Take a free mock test on communication systems to test your conceptual grasp.

Propagation of Electromagnetic Waves

Radio waves propagate by three modes. Ground waves travel along the Earth's surface. Sky waves are reflected by the ionosphere and can travel over very long distances. Space waves travel in straight lines and are limited by the horizon — used for TV, FM radio, and line-of-sight microwave links. Frequencies above the critical frequency pass through the ionosphere and require space wave propagation with tall towers or satellite relay. JEE Main tests the range of a TV tower and the relationship between tower height and coverage distance.

Bandwidth Requirements of Different Signals

Speech requires approximately 4 kHz, music approximately 20 kHz, and a TV signal approximately 4 MHz bandwidth. Multiplexing techniques allow multiple signals to share the same channel. The channel bandwidth limits the maximum number of simultaneous users.

Satellite Communication and Optical Fiber

Geostationary satellites relay signals between locations separated by thousands of kilometers. They orbit at approximately 36,000 km altitude in equatorial orbits with a 24-hour period. One satellite can cover about one-third of the Earth's surface. Optical fibers use total internal reflection — the principle directly from our ray and wave optics guide. Follow our physics score strategy and upgrade for ₹149/month to access our full question bank.