Why Is Modulation Needed in Communication Systems? 7 Reasons, Antenna Size, FDM & Demodulation (with MCQs)

Your voice contains frequencies of only a few kilohertz. Yet radio stations broadcast at around 1 MHz (AM) or 100 MHz (FM), and mobile phones use 900 MHz to several GHz. Why don’t we just send the voice signal directly? The answer is modulation. “Need for modulation” is one of the most frequently asked short questions in BCS (EEE), Telecom, power-sector and university exams, and it also appears regularly as MCQs. This post explains every reason with numbers, animations and practice questions.

⚡ Key takeaways

• Modulation = varying a property (amplitude, frequency or phase) of a high-frequency carrier according to the low-frequency message signal.
• Demodulation = recovering the original message from the modulated wave at the receiver.
• Main reasons: (1) practical antenna size (λ/4), (2) multiplexing of many signals, (3) longer range / efficient radiation, (4) reduction of noise & interference, (5) narrowbanding, (6) frequency assignment and matching the channel.
• Antenna length ∝ 1/f — a 15 kHz signal needs a 5 km antenna; a 1 MHz carrier needs only 75 m.

📑 Contents

1. Elements of a communication system
2. Baseband vs passband transmission
3. What is modulation?
4. Why modulation is needed (7 reasons)
5. Demodulation
6. Types of modulation
7. Solved examples
8. Practice MCQs
9. বাংলায় দ্রুত রিভিশন

1. Elements of a Communication System

Communication is the process of transferring information from one place (source) to another (destination). Every electronic communication system has the same basic blocks:

Fig. 1 – Block diagram of a communication system. Modulation happens in the transmitter and demodulation in the receiver.

• Input transducer converts the physical message (sound, light, temperature) into an electrical signal, e.g. a microphone.
• Transmitter processes the signal — amplification, filtering and above all modulation.
• Channel is the medium: free space, coaxial cable, twisted pair, optical fibre. Here the signal is attenuated and corrupted by noise.
• Receiver amplifies, filters and demodulates; the output transducer (e.g. loudspeaker) gives back the message.

2. Baseband vs Passband Transmission

The baseband signal is the original band of frequencies produced by the source or input transducer. Examples: telephone voice 300 Hz – 3.4 kHz, high-fidelity audio 20 Hz – 20 kHz, analog TV video 0 – 4.2 MHz (NTSC) / up to 5 MHz (PAL).

• Baseband communication: the baseband signal is sent without modulation. Possible only over short wired links — e.g. intercoms, a computer’s Ethernet LAN cable, the local telephone loop.
• Passband (carrier) communication: the baseband spectrum is shifted to a higher frequency using a carrier. This is required for radio, TV, satellite, mobile and most long-distance links.

3. What Is Modulation?

Modulation is the process in which some characteristic of a high-frequency carrier wave is varied in accordance with the instantaneous value of the modulating (message) signal.

c(t) = A_c cos(2πf_ct + φ)

The carrier has three parameters we can change: amplitude A_c → Amplitude Modulation (AM); frequency f_c → Frequency Modulation (FM); phase φ → Phase Modulation (PM). The carrier itself carries no information — it is just a “vehicle”; the message rides on it.

Animation 1 – The same message signal (green) carried by AM (amplitude changes) and FM (frequency changes).

4. Why Is Modulation Needed? (Need for Modulation)

4.1 Practical antenna length

An antenna radiates efficiently only when its length is comparable to the wavelength — typically λ/4 (quarter-wave monopole) or λ/2 (half-wave dipole). Since λ = c/f, low frequencies need enormous antennas:

Signal / carrier frequency	Wavelength λ = c/f	Antenna length λ/4
3 kHz (voice, unmodulated)	100 km	25 km ❌
15 kHz (audio, unmodulated)	20 km	5 km ❌
1 MHz (AM broadcast carrier)	300 m	75 m
100 MHz (FM broadcast carrier)	3 m	75 cm
900 MHz (GSM mobile)	33.3 cm	8.3 cm
2.4 GHz (Wi-Fi)	12.5 cm	3.1 cm

A 5 km tall antenna is impossible to build. By shifting the message to a high carrier frequency, the antenna becomes small enough to fit on a tower, a car or inside a phone. This is the most important reason for modulation.

4.2 Multiplexing – many signals on one channel

All radio stations produce audio in the same 20 Hz – 20 kHz range. If they transmitted baseband signals simultaneously, they would overlap and nobody could separate them. Modulation shifts each station to a different carrier frequency, so their spectra sit side by side without overlap. This is Frequency Division Multiplexing (FDM); the receiver selects one station by tuning to its carrier.

Animation 2 (simplified spectra) – Modulation moves each baseband spectrum to its own carrier frequency so many stations share one medium.

4.3 Increase the range of communication

Power radiated by an antenna rises sharply with frequency for a given antenna size (for a short dipole, radiation resistance R_r = 80π²(l/λ)², so radiated power ∝ (l/λ)²). Low-frequency signals radiate very weakly and are absorbed quickly. High-frequency carriers radiate efficiently and can use sky-wave, line-of-sight or satellite propagation to travel long distances.

4.4 Reduce noise and interference

Noise cannot be eliminated, but some modulation schemes resist it. FM trades extra bandwidth for a much better signal-to-noise ratio, and digital modulation (PCM, PSK, QAM) with error-correcting codes can regenerate the signal almost perfectly. Choosing a carrier frequency away from strong noise sources also reduces interference.

4.5 Narrowbanding

Audio from 20 Hz to 20 kHz has a highest-to-lowest frequency ratio of 1000 : 1. An antenna designed for one end of this range would be useless at the other. After modulating a 1 MHz carrier, the band becomes about 0.98 – 1.02 MHz — a ratio of only about 1.04 : 1. Now one antenna and one set of tuned circuits work well over the whole band.

4.6 Frequency assignment and channel separation

Regulators (the ITU internationally; BTRC in Bangladesh) assign each service its own band — AM, FM, TV, mobile, satellite, aviation, defence. Modulation lets each user place its signal exactly in its allotted band.

4.7 Match the signal to the channel

Every channel passes certain frequencies best: telephone lines pass about 300 – 3400 Hz, optical fibre works at about 193 THz (1550 nm), the ionosphere reflects 3 – 30 MHz. Modulation places the information exactly where the channel performs best (for example, a dial-up modem modulates data onto audio tones so it can pass through a telephone line).

🧠 Memory trick: “A MR. NFC”Antenna size · Multiplexing · Range · Noise reduction · Narrowbanding · Frequency assignment · Channel matching.

5. Demodulation

Demodulation (detection) is the reverse process carried out in the receiver: it extracts the original message from the modulated carrier. Examples: an envelope detector (diode + RC filter) for AM, a discriminator or PLL for FM, and a coherent (synchronous) detector for DSB-SC/SSB and PSK.

6. Types of Modulation

Category	Carrier	Types	Typical use
Analog (continuous-wave)	Sinusoid	AM (DSB-FC, DSB-SC, SSB, VSB), FM, PM	AM/FM radio, analog TV
Pulse – analog	Pulse train	PAM, PWM (PDM), PPM	Sampling, motor control, TDM
Pulse – digital	Pulse train	PCM, DM, ADM, DPCM	Digital telephony, audio CD
Digital (keying)	Sinusoid	ASK, FSK, PSK (BPSK, QPSK), QAM	Mobile (GSM, 4G, 5G), Wi-Fi, satellite, modems

For a detailed comparison see AM vs FM and PCM vs DM vs ADM.

7. Solved Examples

Example 1. Find the minimum (λ/4) antenna height needed to radiate (a) a 10 kHz audio signal directly and (b) the same signal after modulating a 1 MHz carrier.

1. (a) λ = c/f = (3 × 10⁸)/(10 × 10³) = 30 000 m ⇒ λ/4 = 7.5 km (impractical)
2. (b) λ = (3 × 10⁸)/(1 × 10⁶) = 300 m ⇒ λ/4 = 75 m (practical)

Example 2. A 1 MHz carrier is amplitude-modulated by audio of 50 Hz – 10 kHz. Find the frequency range and the highest-to-lowest ratio of the modulated signal.

1. Range = (1000 − 10) kHz to (1000 + 10) kHz = 990 kHz – 1010 kHz (bandwidth 20 kHz)
2. Ratio = 1010/990 ≈ 1.02, compared with 10 000/50 = 200 before modulation.

8. Practice MCQs (click to see answer)

1. The main reason for modulation in radio communication is to: (a) Increase bandwidth (b) Reduce antenna height (c) Increase noise (d) Reduce carrier power

✔ (b) Reduce antenna height to a practical size.

2. Antenna length for efficient radiation is generally of the order of: (a) λ/4 (b) 4λ (c) 10λ (d) λ/100

✔ (a) λ/4 (or λ/2).

3. The quarter-wave antenna length for a 1 MHz carrier is: (a) 7.5 m (b) 75 m (c) 300 m (d) 750 m

✔ (b) 75 m (λ = 300 m).

4. Sending many signals over one channel by assigning different carrier frequencies is called: (a) TDM (b) FDM (c) CDM (d) PCM

✔ (b) Frequency Division Multiplexing.

5. In amplitude modulation, which parameter of the carrier varies with the message? (a) Frequency (b) Phase (c) Amplitude (d) Wavelength

✔ (c) Amplitude.

6. The process of recovering the message from a modulated wave is: (a) Multiplexing (b) Demodulation (c) Sampling (d) Encoding

✔ (b) Demodulation (detection).

7. Transmission of a signal without modulation is called: (a) Passband (b) Broadband (c) Baseband (d) Wideband

✔ (c) Baseband transmission.

8. Which modulation offers better noise immunity at the cost of more bandwidth? (a) AM (b) FM (c) DSB-SC (d) SSB

✔ (b) FM.

9. A carrier wave is usually a: (a) Low-frequency square wave (b) High-frequency sinusoidal wave (c) DC signal (d) Random noise

✔ (b) High-frequency sinusoid (for analog and keying modulation).

10. QPSK is an example of: (a) Analog modulation (b) Pulse analog modulation (c) Digital modulation (d) No modulation

✔ (c) Digital (phase-shift keying) modulation.

9. বাংলায় দ্রুত রিভিশন

📝 পরীক্ষার আগে এক নজরে

• মডুলেশন: উচ্চ কম্পাঙ্কের বাহক তরঙ্গের (carrier) বিস্তার, কম্পাঙ্ক বা দশা বার্তা সংকেত অনুযায়ী পরিবর্তন করার প্রক্রিয়া। ডিমডুলেশন: রিসিভারে মূল বার্তা ফিরে পাওয়ার প্রক্রিয়া।
• প্রয়োজনীয়তা: (১) অ্যান্টেনার দৈর্ঘ্য ব্যবহারযোগ্য করা — ১৫ kHz সংকেতের জন্য ৫ কিমি অ্যান্টেনা লাগত, ১ MHz বাহকে মাত্র ৭৫ মিটার; (২) মাল্টিপ্লেক্সিং (FDM) — একই মাধ্যমে অনেক স্টেশন; (৩) দূরে প্রেরণ; (৪) নয়েজ ও ব্যতিচার কমানো; (৫) ন্যারোব্যান্ডিং; (৬) ফ্রিকোয়েন্সি বরাদ্দ; (৭) চ্যানেলের সাথে মিলানো।
• অ্যান্টেনার দৈর্ঘ্য ≈ λ/4 এবং λ = c/f, তাই কম্পাঙ্ক বাড়লে অ্যান্টেনা ছোট হয়।
• মডুলেশন ছাড়া প্রেরণকে বেসব্যান্ড ট্রান্সমিশন বলে।
• প্রকারভেদ: অ্যানালগ (AM, FM, PM), পালস (PAM, PWM, PPM, PCM), ডিজিটাল (ASK, FSK, PSK, QAM)।

Related Posts

• AM vs FM: Difference Between Amplitude and Frequency Modulation
• Advantages of FM over AM
• Advantages and Disadvantages of Analog Modulation
• Most Asked Communication Systems MCQs

References: Kennedy & Davis, Electronic Communication Systems; B. P. Lathi, Modern Digital and Analog Communication Systems; Simon Haykin, Communication Systems. Images and animations: original work by Totalecer.