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SYLVANIA 721A Gas Switching Tube (Transmit-Receive Switch)
Tube alternat
Sperröhre, TR-Zelle

The 721A (1942) and 721B (1946) are double-gap gas switching diodes used in pulsed radar systems at S-band frequencies near 3000 MHz. Used in radars including SCR-545 and SL.

Two inches long, have disc seal with keep-alive electrode. Contain water vapor and hydrogen. To reduce arcing, a matrix cathode consisting of nickel powder and carbonates was used. The mixture was sintered onto the surface of a machined nickel cathode base. The matrix cathode extended the life of the tube by a factor of 10.

The tube was used in shunt branching circuits. Three tubes were designed specially for use in a TR box (transmit-receive switch): the 721A, 724B, and 1B23. The tubes were generally applied in series with a receiver line. For transmitting, the TR tube prevents power from reaching the receiver. For receiving, the TR position is adjusted for an even number of quarter wavelenghts so that maximum voltage is induced across the receiver input.

In 1944, 400,000 of the above three types were made. The 721A eliminated the ATR (anti-transmit-receive) tube. The 721A was an essential part of the radar equipment on almost every major ship in the U.S. fleet.

Fiche technique • Data sheet • Datenblatt: 721A.pdf


SYLVANIA 721A Transmit-Receive Switch

Hauteur • Height • Höhe : 78 mm • 3" 1/8
Diamètre • Diameter • Durchmesser : 28 mm • 1" 1/8

SYLVANIA 721A Transmit-Receive Switch

SYLVANIA 721A Transmit-Receive Switch


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