Purpose of high-directivity antennas in satellite communications Why are highly directional antennas employed in satellite links for both spacecraft and ground terminals?

Introduction / Context:
Antennas determine link budgets and coverage patterns in satellite systems. High directivity concentrates power where it is needed and reduces interference to and from other systems, enabling efficient spectrum use and smaller terminals.

Given Data / Assumptions:

• Parabolic reflectors or phased arrays on satellites and earth stations.
• Spot beams and shaped coverage are common at C, Ku, and Ka bands.
• Goal: increase gain, SNR, and enable frequency reuse via spatial separation.

Concept / Approach:

Directivity and gain are proportional; by narrowing the beam, the same transmit power yields higher effective isotropic radiated power (EIRP) in the service area, and ground stations receive stronger signals (higher G/T). This also allows reuse of the same frequency in geographically separated beams, increasing capacity.

Step-by-Step Solution:

Verification / Alternative check:

Operator coverage maps show multiple spot beams; link budgets show improved EIRP and G/T with narrow beams.

Why Other Options Are Wrong:

Polarization corrections are not the primary purpose. Channel selection happens in baseband/RF filtering, not by antenna pattern alone. Free-space loss cannot be eliminated by antennas, only mitigated via gain.

Common Pitfalls:

Assuming high gain solves all impairments; rain fade, interference coordination, and regulatory limits still apply.

Final Answer:

To focus a spot beam onto a specific region on Earth for higher gain and frequency reuse