JEE · Physics
Explain, for background and conceptual learning only, how signals are transmitted using elements of a communication system, propagation modes and amplitude modulation, while stating clearly that this topic is not confirmed as part of the current official JEE Main or JEE Advanced syllabus.
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In short
This page therefore does not present exam relevance, weightage, question-paper evidence or preparation priority for Communication Systems. It exists only as a plain conceptual explainer, in case a candidate wants background understanding of the topic or is checking whether it still appears in their own official syllabus document for a given year.
A communication system carries a message from a sender to a receiver over a channel. It uses a transmitter to prepare the signal, a medium or channel to carry it, and a receiver to recover the original message, allowing for some noise and distortion along the way.
Always confirm current scope directly from the official syllabus document linked below before treating any topic here as examinable.
This table records what was and was not found in the official documents checked. It does not assert removal or inclusion beyond what could be directly verified.
Sources: NTA JEE Main syllabus page and the JEE (Advanced) 2026 syllabus PDF, both linked in the sources section below.
Prerequisite
Alternating Current
Sinusoidal signals, frequency and amplitude are used again when describing modulation.
Prerequisite
Waves
Wave frequency, wavelength and superposition underlie signal and carrier wave behaviour.
Parent
JEE Physics
Subject hub for the Physics chapter set.
Parent
JEE syllabus
Check the current official scope directly before treating any chapter as examinable.
This is a readiness check for the concepts on this page, not a claim about exam scope.
A message signal is converted by a transmitter into a form suited to a channel, then recovered by a receiver at the other end.
A basic communication system has a transmitter, a channel and a receiver. The transmitter processes the original message signal into a signal suited to the channel. The channel carries the signal, and unavoidably adds some noise. The receiver extracts the message signal from what actually arrives.
Bandwidth is the range of frequencies a signal occupies, or the range of frequencies a transmission medium can carry without excessive loss or distortion. A medium can only carry a signal faithfully if its own bandwidth covers the frequency range the signal needs.
Different frequency ranges travel through the atmosphere by different dominant paths.
A ground wave travels along the surface of the earth and is used at lower frequencies, but it loses energy over distance as it follows the curved surface. A sky wave reaches the receiver after reflecting from an ionised layer of the upper atmosphere, which works only for a range of frequencies the ionised layer can reflect. A space wave travels in a nearly straight line between transmitter and receiver, so its range is limited by the curvature of the earth and by the heights of the transmitting and receiving antennas.
A low-frequency message signal, such as an audio signal, is not sent directly over long distances because it needs an impractically long antenna and cannot be separated easily from other similar signals. Modulation superimposes the message signal onto a higher-frequency carrier wave, which allows practical antenna sizes and lets multiple signals share the same medium on different carrier frequencies.
In amplitude modulation, the carrier's amplitude is varied in step with the message signal, while its frequency stays fixed.
In amplitude modulation, the instantaneous amplitude of a fixed-frequency carrier wave is varied in proportion to the instantaneous value of the message signal. The carrier frequency itself is not changed. This produces a modulated wave whose envelope traces the shape of the message signal, provided the modulation index stays within a valid range.
The modulation index compares the amplitude of the message signal to the amplitude of the unmodulated carrier. When the message amplitude does not exceed the carrier amplitude, the modulation index stays at or below one and the envelope of the modulated wave follows the message without distortion. When the message amplitude exceeds the carrier amplitude, the modulation index exceeds one and the demodulated output becomes distorted.
An amplitude modulated wave built from a single-tone message signal contains the carrier frequency together with two additional frequencies, one just above and one just below the carrier, called sidebands. The transmitted power is distributed between the carrier and the two sidebands, and this split depends on the modulation index.
Demodulation recovers the original message signal from the received modulated wave. For amplitude modulation with a modulation index at or below one, this is done by tracing the envelope of the received wave and removing the leftover carrier-frequency ripple, which requires the receiver circuit to follow envelope changes at the message frequency but not at the much higher carrier frequency.
A simple way to classify which propagation description fits a given frequency range, for conceptual understanding only.
Modulation index equals message amplitude divided by carrier amplitude.
Modulation index compares message amplitude to carrier amplitude in amplitude modulation.
Use when — A single-tone message signal amplitude modulates a fixed-frequency carrier, and the intent is to check whether the envelope stays undistorted.
Common trap — Treating a modulation index above one as still giving a clean, undistorted envelope.
The amplitude modulated wave contains three frequencies: the carrier frequency, the carrier frequency minus the message frequency, and the carrier frequency plus the message frequency.
A single-tone amplitude modulated wave contains the carrier frequency and two sideband frequencies placed symmetrically around it.
Use when — The message signal is a single sinusoidal tone amplitude modulating a carrier.
Common trap — Assuming only the carrier frequency is transmitted and ignoring the sidebands that actually carry the message information.
Answer: Modulation index = 0.4; envelope stays undistorted.
The modulation index is the message amplitude divided by the carrier amplitude: mu = 4 / 10 = 0.4.
Since 0.4 is at or below one, the envelope of the modulated wave follows the message signal without distortion under this idealised condition.
Assuming Communication Systems is examinable for the current attempt because an older syllabus, a coaching PDF or a previous year's paper included it.
Knowledge gapWhy it happens
Syllabus scope changes between cycles, and secondary sources can lag or misreport the current official document.
How it is corrected
Check the official syllabus PDF for the exact session and year being attempted before allocating study time to this chapter.
Using the modulation index formula without checking whether it stays at or below one before assuming the envelope is undistorted.
Execution errorWhy it happens
A modulation index above one changes the shape of the demodulated output, so the same formula does not guarantee an undistorted envelope in every case.
How it is corrected
Compute the modulation index first, then state explicitly whether it is at or below one before describing the envelope shape.
Confusing ground wave, sky wave and space wave propagation without checking which frequency range and distance the situation describes.
Recall gapWhy it happens
Each propagation mode depends on frequency range, distance and the presence of a reflecting atmospheric layer, so the description in the question needs to be matched to the right mode rather than guessed.
How it is corrected
Identify the frequency range and distance stated in the problem, then match it to ground, sky or space wave behaviour using the method selector table above.
FAQ
Straight answers about how Rank Sarthi fits into serious exam preparation.
Not confirmed as of this page's last review. It was not found as a listed unit in the JEE Main syllabus document currently published on the official NTA JEE Main site. Check the official document for your specific session before deciding whether to prepare it.
Not confirmed. It was not found in the JEE (Advanced) 2026 syllabus PDF, which is stated to carry forward the JEE (Advanced) 2025 physics unit list. Check the official JEE Advanced syllabus document for your specific year.
The page is written as a plain conceptual explainer for background understanding, and to give a place to check current status, not as exam preparation content. It deliberately avoids weightage, PYQ and trend claims for this reason.
Confirm the current official syllabus for your specific JEE Main session and JEE Advanced year first. If the topic is absent from the official document you are checking, prioritise chapters that are confirmed as part of the current scope instead.
Scope status was checked against the JEE Main syllabus document published on the official NTA JEE Main site and against the JEE (Advanced) 2026 syllabus PDF, in which Communication Systems could not be located as a listed physics unit. [VERIFY BEFORE INDEXATION] No official notification explicitly stating that Communication Systems has been removed from the JEE Main syllabus was directly located in this review; the absence from the current document was the basis for the non-examinable framing on this page. [VERIFY BEFORE INDEXATION] The JEE (Advanced) 2025 physics unit list should be independently rechecked to confirm the same absence, since the 2026 document only states it repeats the 2025 scope rather than reprinting the full list in this review. This page carries no PYQ, weightage or trend claims because exam relevance is unconfirmed.
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