A transmit/receive switch

Preparing the chassis with hardware and correct holes for the board, relay, and connectors.

Checking if the board fits before installing the relay.

The finished transmit/receive switch.

When putting a transmitter on the air with a separate receiver, something in the station needs to switch from transmit to receive and vice versa. In particular, when transmitting, the receiver should be disconnected from the transmitter and ideally muted, and the transmitter should be connected to the antenna system. When receiving, the receiver should be connected to the antenna system and not muted.

This device is called a transmit/receive switch, and is built into modern-day transceivers. A very simple design could consist of a switch that the operator manually throws when switching from transmit to receive and vice versa. However, such a design takes additional operator effort and does not permit full QSK operation.

I built the design presented on this page in two evenings in April 2022, and used it with a Drake 2-B receiver and Cricket 40 as a transmitter. It was keyed by an automatic keyer, and allowed for full QSK at my then CW speed of 15 wpm. Everything was built from parts I already had on hand.

This design uses a hamfest find coax relay hidden under a square board. 12 volts DC power is required, and the relay timing can be adjusted by changing one of the timing capacitors.

Some of the design was adapted from W1FB's Design Notebook, including the relay driver. Doug DeMaw's books are excellent if you can find them.

I have a schematic drawn somewhere, but need to find it. Until then, I only have pictures of the device.

References

[W1FB] Doug DeMaw, W1FB's Design Notebook, 1990, p. 44.