Why satellite links use different uplink and downlink frequencies In a bent-pipe communications satellite, why are separate frequency bands used for receiving (uplink) and transmitting (downlink)?

Introduction / Context:
Satellite transponders act as frequency translators and amplifiers, receiving a weak uplink from Earth and retransmitting at a different frequency back to Earth. Using distinct frequency bands for uplink and downlink is a foundational design choice that ensures reliable operation within the satellite and at ground terminals.

Given Data / Assumptions:

• Bent-pipe (transparent) repeaters translate between two separated bands (e.g., 6 GHz uplink and 4 GHz downlink).
• Onboard receivers handle micro- to milliwatt-level signals; transmitters radiate tens to hundreds of watts RF.
• Isolation between RX and TX chains is finite.

Concept / Approach:

RX and TX at the same frequency would cause self-interference: the high-power downlink would flood the front-end LNA, saturating or damaging it and burying the wanted uplink signal. Frequency separation allows filter and diplexer design to provide high isolation between the two chains, enabling simultaneous receive and transmit.

Step-by-Step Solution:

Verification / Alternative check:

Industry band plans (C, Ku, Ka) always define separated uplink and downlink allocations to ensure practical isolation and regulatory coordination.

Why Other Options Are Wrong:

Free-space loss and antenna gain are not improved by this choice. Terrestrial interference avoidance is secondary to the primary RX/TX isolation need. Doppler is negligible for GEO links.

Common Pitfalls:

Assuming frequency separation is about propagation differences; while atmospheric windows matter, isolation is the main reason.

Final Answer:

To prevent the powerful downlink transmitter from desensitizing the weak received uplink signal