How does the energy content of atmospheric noise vary with frequency?
Electronics and Communication Engineering
Communication Systems
Difficulty: Easy
Choose an option
-
Adoes not depend on frequency
-
Bdecreases as frequency is increased
-
Cincreases as frequency is increased
-
Deither (a) or (c) depending on the temperature
-
Eremains constant across spectrum
Answer
Correct Answer: decreases as frequency is increased
Explanation
Introduction / Context:Atmospheric noise arises due to lightning discharges and other natural processes. It plays a role in determining radio link design, particularly in HF and VHF bands.
Given Data / Assumptions:
- Noise sources: natural (lightning, cosmic noise).
- Observed across radio frequencies.
Concept / Approach:Atmospheric noise power is inversely related to frequency. At lower frequencies, noise is dominant; at higher frequencies, thermal and man-made noise dominate.
Step-by-Step Solution:
At LF/MF (~30 kHz–3 MHz): atmospheric noise dominates.At HF/VHF and above: atmospheric noise decreases rapidly with frequency.Thus, energy content decreases as frequency increases.Verification / Alternative check:ITU noise curves (P.372) confirm steep decline of atmospheric noise beyond a few MHz.
Why Other Options Are Wrong:
Independence or increase with frequency contradicts experimental evidence.Temperature has minor effect, not a deciding factor here.Common Pitfalls:
Confusing atmospheric noise with thermal noise (which is flat).Final Answer:
decreases as frequency is increased