Building a Custom T/R Relay for Hallicrafters SX-117 & HT-44 Twins
If you’re running vintage "twins" like the Hallicrafters SX-117 receiver and HT-44 transmitter , you know the challenge of managing antenna switching and receiver muting. While these rigs were designed to work together, a robust external T/R (Transmit/Receive) relay is the best way to protect your receiver’s front-end and provide a seamless operating experience. This project covers a "fail-safe" design that handles RF switching and receiver muting in one shielded enclosure. The Design Philosophy: Fail-Safe First The relay is wired so that the Normally Closed (NC) position connects the Antenna to the Receiver. If the relay loses power or a cable fails, your receiver stays connected to the antenna, and your transmitter is isolated. The Schematic Logic We utilize a DPDT (Double Pole Double Throw) relay to handle two independent tasks simultaneously: Pole 1 (RF Path): Switches the Antenna between the SX-117 and the HT-44. Pole 2 (Control/Mute): Shorts the SX-117 M...