A high isolation T/R switch is essential to protect the receiver from excessive RF during transmit mode. This relay system provides more isolation than a single relay can.

Recognizing this problem, even the Dow Key Relay series later introduced a model with high isolation contacts. More information on this item will be provided in a separate article.

Relay K1 switches the antenna port to the receiver or transmitter. It also grounds the transmitter or receiver ports when they are not active. Relay K2 is a backup that also grounds the receiver input again, to provide even more isolation. The second set of contacts on K2 can be connected to MUTE the receiver for voice operation. Depending on your particular receiver (consult its manual), you will need to wire it for normally open (NO) or normally closed (NC). Beware the presence of high voltage in some radios, and many do not want the relay contacts connected to ground, since BOTH contacts are floating at higher voltage. For CW operation, you can arrange to not mute the receiver and it may provide a CW monitor for your sending, if the receiver can work with the overload signal level. A separate toggle switch with appropriate voltage rating can be used in parallel or series with the relay contacts, as needed.

Note that relays and switches with adequate voltage and current ratings are necessary for this project. I used relays with a 24 VDC coil, since that is what I had in my junk box. Use relays and a wall wart that are compatible with each other. I do not recommend AC relays, unless you are experienced with such devices and UL standards of wiring. You can use the switched 110 VAC from a vintage transmitter to send AC to the primary input of a wall wart to run a low voltage relay system. It is much safer for you and the equipment.

The lamp and diodes protective circuit goes between the T/R relay output port and the actual receiver antenna terminals. There is always a risk of damage to a solid state receiver when interfaced with a transmitter; there is no substitute for careful voltage checks with appropriate test equipment before using it with a receiver. But it is possible to use solid state receivers with transmitters, such as the SB303 solid state receiver as part of the last version of the Heathkit Twins (SB401 transmitter). You alone must decide whether the system you build from this information is suitable for your particular application.

You also will need diodes and a small lamp used as a fuse to provide further protection. I will show this circuit separately later. Service monitors often include this style of protection, or more sophisticated methods. When testing a receiver part of a transceiver with a service monitor, it is good lab practice to unplug the microphone or otherwise disable push to talk to avoid activation of a transmitter. Attention to this detail is essential to avoid damage. This is why my article Johnson 250-39 shows the test setup with a lamp indicator for testing isolation before connecting a receiver.

Before connecting this relay box to your transmitter and receiver, I recommend that you test everything with an ohm meter between the ports. First, test the receiver port with no AC on the wall wart primary AC input. It should connect to only the antenna port when not energized in receive mode. The transmitter port also should be connected to ground. For the second test, plug the wall wart into an AC outlet to simulate transmit mode and test the receiver port is connected to ground. The transmitter port should now be connected to the antenna. The mute circuit should be isolated from ground at all times and be compatible with your receiver system, as described in the receiver manual.

IMPORTANT: Keep all leads very short for RF wiring. Keep all grounding leads very short. Be sure connections to the chassis are very secure and use multi tooth lock washers. It is best to use separate screws for ground lugs and mechanical mounting of components.

The metal box shown is from Radio Shack. You can also get nice enclosures from Ten Tec.