Transmit-receive tube

From Glossary of Meteorology

transmit–receive tube

(Abbreviated TR tube.) A gas-filled waveguide cavity that acts as a short circuit when ionized by high-power energy but is transparent to low-power emission when not ionized.

It is used as a switch to protect the receiver of a radar from the high power of the transmitter while passing the low-power signals received at the antenna.
See recovery time.