Audio signal cannot be transmitted because

Options

A. the signal has more noise.
B. the signal cannot be amplified for distance communication.
C. the transmitting antenna length is very small to design.
D. the transmitting antenna length is very large and impracticable.

Explanation

**Step-by-Step Solution:**

1. **Understanding Audio Signals**: Audio signals are sound waves that have a certain frequency range, typically from about 20 Hz to 20 kHz. The wavelength of these signals can be calculated using the formula:
\[ \text{Wavelength} (\lambda) = \frac{v}{f} \]
where \(v\) is the speed of sound (approximately 343 m/s in air) and \(f\) is the frequency of the audio signal.

2. **Calculating Wavelength**: For example, for a frequency of 20 kHz:
\[ \lambda = \frac{343 \text{ m/s}}{20000 \text{ Hz}} = 0.01715 \text{ m} \text{ (or 1.715 cm)} \]
This shows that audio signals have relatively short wavelengths compared to radio waves.

3. **Antenna Size Requirement**: The length of a transmitting antenna is typically related to the wavelength of the signal it transmits. For efficient transmission, the antenna length is usually a fraction (like 1/4 or 1/2) of the wavelength. For audio signals, this would mean constructing antennas that are impractically large.

4. **Practical Limitations**: The impracticality arises because as the frequency decreases (and thus the wavelength increases), the size of the antenna required for effective transmission becomes larger than what can be feasibly constructed or used.

5. **Conclusion**: Therefore, audio signals cannot be transmitted effectively using conventional antennas due to the large size required for the antenna relative to the wavelength of the audio signals.

**Final Answer**: Audio signals cannot be transmitted effectively because the required transmitting antenna length is very large and impractical.

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