Communication Systems

Modulation index:

UHF (Ultra High Frequency) range lies between and . These frequencies cannot reflect from the ionosphere and are not suitable for ground wave propagation due to high attenuation.
They are best suited for:
- Line-of-sight communication
- Television broadcast
- Satellite communication
- Mobile phones
Hence, they propagate via space waves — which travel directly from the transmitter to receiver.

Energy of one photon: Number of photons per second:

According to the bandwidth capacity and channel spacing in standard communication, a co-axial cable can support up to 125 TV signals using frequency division multiplexing.

Sky wave propagation (also called ionospheric propagation) is used for long-distance radio communication by reflecting signals off the ionosphere.
- The ionosphere reflects frequencies typically in the range of: - Frequencies below 30 MHz are reflected, but above that, signals penetrate the ionosphere.
Hence, is suitable.

Modulation index in amplitude modulation is given by: Where:
- (peak voltage of message signal)
- (peak voltage of carrier signal)

An oscillator is essentially an amplifier that produces self-sustained oscillations without an external input signal. This is achieved by feeding a portion of the output back to the input in phase.
This is called positive feedback, and it ensures that oscillations are maintained.
- Negative feedback is used in amplifiers to stabilize gain.
- Positive feedback is required for oscillators to build and sustain the waveform.

The process of retrieving the original message signal from a modulated carrier wave is called demodulation. In communication systems, modulation is used to encode the information on the carrier wave, and demodulation is the process of decoding that information from the carrier. Amplification refers to increasing the signal strength, while attenuation refers to signal weakening. Thus, demodulation is the correct process for retrieving information. Thus, the correct answer is .

Step 1: Understand Amplitude Modulation and Define Amplitudes
In amplitude modulation (AM), the amplitude of the carrier wave varies in accordance with the amplitude of the modulating signal.
Let be the maximum amplitude of the modulated wave.
Let be the minimum amplitude of the modulated wave.
The problem states that the maximum amplitude of a modulated wave is three times its minimum amplitude. So, we can write: Step 2: Recall the Formula for Modulation Index
The modulation index ( ) for an amplitude-modulated wave is defined as the ratio of the change in amplitude of the carrier wave to the unmodulated carrier wave amplitude. It can also be expressed in terms of the maximum and minimum amplitudes of the modulated wave as: Step 3: Substitute the Given Relationship into the Modulation Index Formula
Substitute from equation (1) into the modulation index formula: Simplify the numerator and the denominator: Cancel out from the numerator and denominator: Step 4: Analyze Options
\begin{itemize} \item Option (1): 0.55. Incorrect. \item Option (2): 0.25. Incorrect. \item Option (3): 0.75. Incorrect. \item Option (4): 0.5. Correct, as it matches our calculated modulation index. \end{itemize}

The modulation index ( ) is given by: $ A_{\text{max}} : A_{\text{min}} = 7:3 A_{\text{max}} = 7k A_{\text{min}} = 3k k $

Let’s break this down step by step to calculate the maximum distance between the antennas and determine why option (2) is the correct answer.
Step 1: Understand the line-of-sight communication formula
For line-of-sight communication, the maximum distance between a transmitting antenna (height ) and a receiving antenna (height ) over a spherical Earth is:

where:

• is the radius of the Earth,
• is the height of the transmitting antenna,
• is the height of the receiving antenna.

Step 2: Identify the given values and calculate the heights

• Radius of the Earth,
• Height of the transmitting antenna,
• Height of the receiving antenna,

Step 3: Calculate the maximum distance


Total distance:

This is close to option (2) 96 km, suggesting slight rounding in the problem. Adjusting for approximation:

Step 4: Confirm the correct answer
The calculated distance, with slight rounding, matches option (2) 96 km.
Thus, the correct answer is (2) 96 km.

The minimum length of a transmitting antenna is given by: Given: