TRX-SDR Switch Kit

This ham radio kit allows your SDR and Transceiver to share the same antenna. It also does switching between SDR and Transceiver audio with an adjustable delay.

Update: The kit is no longer in production. It is only available as an open hardware project. Navigate to the Documentation page to download the source files.

TRX-SDR audio and antenna switch by LZ1AQ

Table of Contents

Key Kit Features

  • With this T/R switch you can integrate your transceiver and SDR and have similar or better receive performance than a high grade  state of the art transceiver. The SDR frequencies and modes are synchronized  with your transceiver (with any popular SDR software) and with a simple mouse click you can go quickly to any frequency on the band.
  • While listening with your transceiver the transmit antenna is not disconnected from the SDR and the spectrum display on the PC is active.
  • SDR audio is muted during transmission and side tone is from the transceiver. The smooth and regulated delay to switch back the SDR audio at proper time avoids the disturbances from delayed signal from the SDR.
  • High power input RF limiter for high currents keeping your SDR safe.
  • No limits to the  power output – the kit sits between the transceiver  and the power amplifier.
  • Multilevel foolproof protection of the SDR input. PTT is used as a control signal for TX/RX switching.
  • No audio ground loops.
  • High quality connectors –especially reliable 6.35 mm phone plugs for all audio connections.
  • The 12 V transceiver power supply can be used to power the device.

The purpose of the Kit

The Kit manages the audio paths. During transmission the TRX audio is always selected, thus avoiding the effects of the audio delay from the SDR. During reception the user has the option to switch between TRX and SDR audio. The delay, with which the SDR audio is switched back, after the transmitting stops, can be regulated. This is important, since this delay depends on the specific SDR software and hardware being used.

The Kit allows your Transceiver (TRX) and Software Define Radio (SDR) to use the same antenna. During receiving both the SDR and the TRX are connected to the common antenna. When the TRX starts transmitting the SDR is disconnected and grounded.

For other possible use cases please check the technical documentation page.

Main features

  • Allows the SDR and the TRX to use simultaneously the same antenna. In receive mode the SDR is connected to the antenna, whereas during transmission it is disconnected and grounded. The TRX is permanently connected to the antenna.
  • In case a separate receive antenna is used, the SDR is fully protected during transmission.
  • The Kit consists of two separate boards:
    • Audio Switch – does audio path switching and also responsible for controlling the ANT Switch.
    • ANT Switch – It is RF shielded and contains the antenna relay and receive input limiter.
  • PTT from the TRX is used as a control signal*.
  • During transmission, the audio signal from the SDR is muted and the audio from the TRX is selected.
  • Has a variable audio delay circuit to avoid audio clicks when transmitting (due to SDR signal delay). With the potentiometer you can adjust the delay for the SDR audio after transmission ends.
  • Option to switch between TRX and SDR audio during receiving.
  • No AF ground loops. There is no common ground between TRX and SDR audio within the kit.
  • High quality connectors are used.
  • Double fault SDR input protection in case of antenna relay dysfunction. There is a high power input RF limiter for high currents keeping the input voltage of the SDR at safe levels. This protection can withstand full TRX power.
  • Protection against disconnected TRX PTT cable.
  • Protection from missing power supply.
  • Recommended to be powered from the 12 V power source used by the transceiver (i.e. common ground is needed).
  • Over current protection with resettable fuse.
  • Can control additional external relays (up to 200 mA current) synchronized by the TRX PTT signal – e.g. for PA control, TX antenna detuning, receive antenna switching, station automation etc. This approach avoids RF ground loops when multiple devices are used.
  • The kit boards are tested and ready for use.

* A standard “TX transmits” output from the TRX is used as a control signal. This signal has different names from transceiver manufacturers: SEND; TX-GND; T/R-LINE; Ground-on-Transmit. Further on we will refer to it as TRX PTT. This signal can be used also by other equipment connected to the TRX e.g. power amplifier (PA).