Question

Ionosphere acts as a reflector for the frequency range of

• A. \(3 - 30 \operatorname{kHz} \)
• B. \(3 - 30 \operatorname{MHz} \)
• C. \(3 - 30 \operatorname{Hz} \)
• D. \(3 - 30 \operatorname{GHz} \)

Hint:

Remember the general behavior of different frequency bands with respect to the ionosphere: Lower Frequencies (kHz range): Mostly absorbed or attenuated, especially during the day. High Frequencies (3-30 MHz): Best for ionospheric reflection (skywave propagation), enabling long-distance communication. Higher Frequencies (above 30 MHz, into GHz range): Generally pass through the ionosphere and are used for satellite communication or line-of-sight terrestrial links.

Answer 1

The Correct Option is B

Step 1: Understand the role of the ionosphere in radio communication.
The ionosphere is a part of Earth's upper atmosphere, approximately from 60 km to 1000 km above the surface, where solar and cosmic radiation ionize atoms and molecules, creating a plasma containing free electrons and ions. These ionized layers have the property of reflecting certain frequencies of electromagnetic waves (radio waves) back towards Earth. This phenomenon is crucial for long-distance radio communication, known as skywave propagation, as it allows signals to travel beyond the line of sight.
Step 2: Identify the typical frequency range for ionospheric reflection.
The ability of the ionosphere to reflect radio waves is frequency-dependent.
Very Low Frequencies (VLF) and Low Frequencies (LF) (e.g., kHz range): These waves are largely absorbed by the lower layers of the ionosphere (D-layer) during the day, though some reflection can occur at night. They are less efficiently reflected for typical long-distance communication compared to higher frequencies.
Medium Frequencies (MF) (e.g., hundreds of kHz to a few MHz): These waves also experience significant absorption, especially in the D-layer during the day, but can be reflected by the E and F layers, particularly at night.
High Frequencies (HF) (3 MHz to 30 MHz): This is the primary frequency range for which the ionosphere acts as a reliable reflector. These waves can penetrate the lower D and E layers and are efficiently reflected by the higher F-layer, enabling worldwide radio communication (shortwave radio). This is the "skywave" propagation that allows signals to bounce between the ionosphere and the Earth's surface over long distances.
Very High Frequencies (VHF) and Ultra High Frequencies (UHF) (above 30 MHz, into GHz range): These frequencies are generally too high to be reflected by the ionosphere. They typically pass through the ionosphere into space, making them suitable for satellite communication and line-of-sight terrestrial communication.
Step 3: Evaluate the given options.
(1) 3 - 30 kHz: This is in the VLF/LF range. While some interaction occurs, it's not the primary range for effective reflection for general communication.
(2) 3 - 30 MHz: This is the High Frequency (HF) band, which is precisely the range known for efficient ionospheric reflection and skywave propagation.
(3) 3 - 30 Hz: This is an extremely low frequency (ELF) range, used for very specialized applications (like communication with submarines) due to its unique propagation characteristics, not ionospheric reflection for typical radio.
(4) 3 - 30 GHz: This is in the microwave range (UHF and higher). Waves at these frequencies generally penetrate the ionosphere.
Step 4: Conclusion.
Based on the characteristics of radio wave propagation through the ionosphere, the frequency range of 3 - 30 MHz (High Frequency band) is where the ionosphere most effectively acts as a reflector for long-distance communication.
The final answer is \( \boxed{3 - 30 \, \text{MHz}} \).