As radio frequency increases, how does the absorption (attenuation) of a ground wave by the Earth’s surface typically change?
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ADecreases with increasing frequency
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BIncreases with increasing frequency
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CRemains the same
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DEither (a) or (c)
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EBecomes independent above 1 MHz
Answer
Correct Answer: Increases with increasing frequency
Explanation
Introduction / Context:Ground-wave propagation dominates at low and medium frequencies. Understanding frequency-dependent ground losses helps in selecting operating bands and predicting coverage for AM broadcast and LF/MF communication systems.
Given Data / Assumptions:
- Ground wave hugs the Earth's surface and interacts with lossy ground.
- Soil has finite conductivity and permittivity, causing frequency-dependent attenuation.
Concept / Approach:
As frequency rises, the ground's effective loss tangent and surface impedance lead to greater attenuation of waves traveling along the surface. Hence, ground-wave range shortens with higher frequency, motivating MF/LF for long-range ground-wave services.
Step-by-Step Solution:
Consider the surface-wave attenuation factor which increases with frequency for typical earth conductivities.Higher f → smaller skin depth in soil → more loss → shorter ground-wave range.Therefore, absorption increases with frequency.Verification / Alternative check:
Coverage charts for AM broadcast show reduced ground-wave range at higher MF compared to lower MF/LF bands for comparable power and soil conditions.
Why Other Options Are Wrong:
- Decreases/Remains same: contradict empirical data and surface-wave theory.
- Independence above a threshold is not accurate for real soil parameters.
Common Pitfalls:
Confusing sky-wave behavior (ionospheric) with ground-wave; assuming free-space loss applies unchanged to surface waves.
Final Answer:
Increases with increasing frequency