Microwave Link Planning: Why Are Terrestrial Microwave Repeaters Typically Spaced About 50 km Apart? Consider the main physical and system-level reasons that set practical hop length limits for point-to-point microwave links operating over the Earth's surface.

Introduction / Context:
Terrestrial microwave links are designed as line-of-sight hops between towers. A common rule of thumb is that hops are roughly a few tens of kilometers (often near 30–60 km), with 50 km frequently cited. This question probes the dominant limiting factor for long, ground-based paths.

Given Data / Assumptions:

• Frequency is in the microwave band where propagation is essentially line-of-sight with modest diffraction.
• Earth is curved, so the radio horizon is finite even with tall towers.
• Standard refractivity (k-factor around 4/3) is assumed for effective Earth radius in path design.

Concept / Approach:

The maximum path length is fundamentally constrained by geometry: the Earth's curvature hides distant terrain and reduces clearance through the Fresnel zone. Even if transmit power and receiver sensitivity are favorable, insufficient geometric clearance causes strong diffraction loss and fading. Atmospheric attenuation and equipment limits matter, but geometry dominates the first-order spacing choice.

Step-by-Step Solution:

Verification / Alternative check:

Link budgets that ignore curvature predict longer ranges, but path profiles including terrain and bulge show clearance loss. Raising tower heights or adding passive/active repeaters is used to extend range, confirming the geometric limit.

Why Other Options Are Wrong:

Atmospheric attenuation: Significant at high rain rates or very high bands, but not the first-order limiter at ~50 km. Output tube power: Modern solid-state radios and dishes mitigate this; geometry still dominates. Excessive dc voltage: Not relevant to hop distance selection.

Common Pitfalls:

Confusing free-space path loss with geometric blocking; neglecting Fresnel zone clearance and k-factor variability.

Final Answer:

because of earth's curvature