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Simple-TR-Switch

This is a non-RF-sensing, MCU controlled Simple T/R Switch with RX Grounding feature which can be built for less than 2 dollars!

It was designed, and tested for https://github.com/kholia/Easy-Transceiver project.

Relay Module Looks Relay Module Schematic

Various approaches

TX power is 10W from IC-705 operating in FM mode.

  • With a 2-channel relay as a T/R switch, when both RX Ground and Center are disconnected and left floating, a voltage of around ~2.2v is induced across RX connections which is unsafe for typical SDRs.

    ~2.2 Vpp is 10.93 dBm!

    https://www.random-science-tools.com/electronics/dBm-Watts-volts.htm

  • With a 1-channel relay as a T/R switch which is switching the Center connection only, and with all Grounds (TX, RX, Antenna) shorted, a voltage of around ~1.66v is induced across RX connections which is theoretically safe for common SDRs.

  • With a 2-channel relay as a T/R switch, with all Grounds (TX, RX, Antenna) shorted, with Center switching using Channel 1, and RX Center Grounding using Channel 2 during TX, a voltage of around ~500mV is induced across RX connections which should be fine for most common SDRs.

Beyond Limits

SDRs are sensitive devices

Comparisons

No external 12v power is needed unlike Easy TR Switch from https://www.qrpkits.com/ezseries.html.

The ~4000 INR SDR Transceiver and Receiver Switch Antenna Sharer TR Switch Box 160 MHz from Banggood doesn't work in Automatic Mode if there is Broadcast/SW Interference in your area. The Banggood TR switch switches on the RTX (red) and disconnects the SDR (RX) as soon as the antenna is connected under BCI conditions, when operating in Automatic Mode.

Relay Connections

We use a 2-channel relay as a T/R switch.

Short all Grounds (TX, RX, Antenna).

Channel 1 (In 1):

- COM -> Antenna +ve
- NC -> RX +ve
- NO -> TX +ve

Channel 2 (In 2)

- COM -> Ground
- NC -> Not Connected
- NO -> RX +ve

In TX mode, turn on (i.e. make NO active) both Channel 1 and Channel 2, and you should be good to go!

The following is a rough-schematic, only to show the connections involved. Please use the relay module in practice. The flyback diodes across the relay coils are important.

Schematic

Added Protection

Here is another additional mechanism to protect sensitive SDRs.

RF Limiter

This is OK for HF (shortwave).

Extender PCB

Use this for easy connections ;)

Extender PCB

Use 0.5 sq. mm. wire with bootlace ferrules to ensure good contact. Keep the wires short.

Alternate (but riskier) Option

Solid switching using BS170 could be another option.

The info folder has more information on this.

Results (Feb-2022)

This TR switch affects SWR in the range of 0.10 to 0.15 for the worse, which is quite acceptable in practice.

The TR switch is not affected by BCI.

After RX grounding, the RX line doesn't have any appreciable induced ("picked up") voltage on it. The actual value couldn't be exactly determined due to the measurement noise but it was less than 400mV as an rough approximation. This was measured via an oscope using the differential probing technique (CH1 - CH2 in MATH mode).

The uSDX RX continued functioning just fine when connected to this TR switch, after TX'ing 10W from IC-705 transceiver which was also connected to the same TR switch.

BOM Sources

References

Credits

Thanks to VU2ASH (Ashok Joshi), K.P.S. Kang, and LW1ECP (Daniel Alonso) for all the help.