Building a Custom T/R Relay for Hallicrafters SX-117 & HT-44 Twins

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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:

  1. Pole 1 (RF Path): Switches the Antenna between the SX-117 and the HT-44.

  2. Pole 2 (Control/Mute): Shorts the SX-117 Mute line to ground during transmission to prevent "thumps" and feedback.

Bill of Materials (BOM)

ComponentQuantityNotes
Relay1DPDT 12V DC Power Relay (e.g., Omron G2R-2-DC12)
Enclosure1Die-cast Aluminum Box (Hammond 1590B or 1590BB)
RF Connectors3SO-239 Bulkhead Jacks (Antenna, TX, RX)
Control Jacks2RCA Bulkhead Jacks (PTT Input, SX-117 Mute)
Power Jack12.1mm DC Barrel Jack
Protection Diode11N4007 (Flyback diode for coil protection)
Internal Wire—RG-316 or RG-142 Teflon Coax for RF; 18AWG for DC

Wiring & Pinout Guide

1. The RF Path (Pole 1)

  • Antenna Jack (J1): Connect to Pole 1: COM.

  • SX-117 RX Jack (J2): Connect to Pole 1: NC (Normally Closed).

  • HT-44 TX Jack (J3): Connect to Pole 1: NO (Normally Open).

2. The Mute Line (Pole 2)

  • SX-117 Mute Jack (J5): Connect center pin to Pole 2: NO.

  • Ground: Connect Pole 2: COM directly to the metal chassis.

  • Result: When the relay energizes, the Mute line is grounded, silencing the receiver.

3. The Control Circuit (Relay Coil)

  • Connect +12V DC to one side of the relay coil.

  • Connect the PTT Input (J4) center pin to the other side of the coil.

  • CRITICAL: Solder the 1N4007 diode across the coil pins with the stripe (cathode) facing the +12V side. This protects the HT-44 keying circuit from voltage spikes.

Wiring Diagram Summary

ConnectorConnection PointRelay PinState
AntennaStation AntennaPole 1: COMShared
ReceiverSX-117 ANTPole 1: NCDefault (RX)
TransmitterHT-44 RF OUTPole 1: NOKeyed (TX)
MuteSX-117 MUTEPole 2: NOGrounded on TX
PTT InHT-44 KeyingCoil (-)Pulls to Ground

Build Tips for Success

  • Short Leads: Keep the coaxial jumpers inside the box as short as possible to minimize SWR impact and RF leakage.

  • Shielding: Use a metal box. Plastic enclosures will allow RF to "leak" into the shack and can cause the SX-117 to oscillate or pick up noise.

  • Grounding: Ensure the SO-239 jacks have a "bite" into the aluminum to provide a common ground plane for the whole system.

73 and good luck with the build!

— WX7V




ASCII Schematic & Terminal Layout

       +---------------------------------------------------+
       |              T/R RELAY PROJECT BOX                |
       |                                                   |
       |   [J1] ANTENNA (SO-239)                           |
       |         |                                         |
       |         |                                         |
       |     +---|---+             +-------+               |
       |     |  COM  |             |  COM  |               |
       |     |       |             |       |               |
       |     |   o   |             |   o   |               |
       |     |  /    |             |    \  |               |
       |     | o   o |             |   o  o|               |
       |     | NC  NO|             |  NC  NO               |
       |     +-|---|-+             +------|+               |
       |       |   |                      |                |
       |       |   |                      |                |
       |       |   +-----------+          +-----+          |
       |       |               |                |          |
       | [J2] RX OUT     [J3] TX IN       [J5] MUTE OUT    |
       |   (SO-239)        (SO-239)          (RCA)         |
       | (To SX-117)     (To HT-44)       (To SX-117)      |
       |                                                   |
       +-----------------------+---------------------------+
                               |
                               |
              +----------------V----------------+
              |          RELAY COIL             |
              |                                 |
              |   +12V DC ----+-----|<|----- [J4] PTT IN
              |               |  (Diode)        (RCA)  |
              |               +-------------+ (HT-44)  |
              +----------------------------------------+

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