--- title: "Need of modulation; antenna parameters" subject: "Basic Electronics Engineering" chapter: 5 key: "semester_2/basic_electronics_engineering:modulation-need-antenna-parameters" source: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/modulation-need-antenna-parameters site: BCT Engineering --- # Need of modulation; antenna parameters Basic Electronics Engineering · Chapter 5: Communication System - Asked in: 2070 Magh, 3+3 marks - Video: [What is Modulation ? Why Modulation is Required ? Types of Modulation Explained.](https://www.youtube.com/watch?v=mHvV_Tv8HDQ) — ALL ABOUT ELECTRONICS - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/modulation-need-antenna-parameters - Whole chapter: https://www.bctengineer.com/md/semester_2/basic_electronics_engineering/chapter-5 ## Question Explain why modulation is needed in Communication System. Mention any three parameters of antenna. ## Short answer Modulation is needed mainly to reduce antenna size to practical lengths (high-frequency carriers allow λ/4 antennas) and to enable frequency-division multiplexing. Three antenna parameters are gain (power radiated in preferred direction relative to isotropic), radiation pattern (field strength vs. direction plot), and bandwidth (usable frequency range). ## Answer #### 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is modulation needed for practical antenna size? A. Antennas need length of about lambda/4 or lambda/2, and audio frequencies give impractically huge wavelengths B. Audio signals cannot travel through wires C. Antennas must be longer than 100 km D. Modulation reduces the frequency of the signal Answer: A. The answer states this: Antennas need length of about lambda/4 or lambda/2, and audio frequencies give impractically huge wavelengths. (Key point: Antenna size) 2. For a 3 kHz audio signal, lambda = c/f is about: A. 100 m B. 100 km C. 3 km D. 0.75 m Answer: B. The answer states this: 100 km. (Key point: Wavelength calculation) 3. How does modulation allow many signals to share one channel? A. All signals are sent on the same 20 Hz to 20 kHz band B. Signals are sent one bit at a time only C. Each signal uses a different carrier frequency (FDM) D. Each signal uses a different antenna size Answer: C. The answer states this: Each signal uses a different carrier frequency (FDM). (Key point: Multiplexing) 4. Antenna gain is the ratio of: A. The range of frequencies over which the antenna works B. The antenna length to its width C. Field strength to wavelength D. Power radiated in the preferred direction to that of an isotropic antenna, including losses Answer: D. The answer states this: Power radiated in the preferred direction to that of an isotropic antenna, including losses. 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