TR (transmit–receive) protection tube in radar duplexers: how does its loss behave for weak echo signals versus the strong transmit pulse?

Introduction:
In pulsed radar, a duplexer must protect the sensitive receiver during the high-power transmit pulse and then admit very weak echoes a short time later. Gas-discharge TR tubes (or modern limiter diodes) provide level-dependent isolation.

Given Data / Assumptions:

• Monostatic radar with a single antenna shared by transmitter and receiver.
• TR device placed in the receiver path.
• Transmit pulse is orders of magnitude stronger than echoes.

Concept / Approach:

During the transmit pulse, the TR device ionizes or otherwise switches into a high-loss/reflective state, isolating the receiver front end. Between pulses, with only weak echo levels present, the device remains non-conducting (or unswitched), presenting low insertion loss so echoes reach the receiver with minimal attenuation.

Step-by-Step Solution:

Verification / Alternative check:

Limiter transfer curves show a knee beyond which insertion loss increases rapidly with input power; below the knee, loss is minimal.

Why Other Options Are Wrong:

• Reversing which level gets high loss would either damage the receiver or block echoes.
• Both high or both low loss contradicts the protective function.
• Level-independent loss fails to protect during transmit.

Common Pitfalls:

Assuming all duplexers are purely passive hybrids; TR limiters provide the level-dependent isolation crucial for radar.

Final Answer:

Low loss for weak (low-level) echo signals and high loss for the strong transmit pulse