First time operation

Technical documentation

There are 3 LEDs – two red, one on each board, and one green on the Audio Switch board.

The red LEDs are active when the TRX is in receiving mode. The green LED is also active in receiving mode, but it is synchronized with the Audio Switch. The green LED is switched on only after audio delay completes. Both of the LEDs, located on the Audio Switch, have an additional connector which can be used to connect externally mounted LEDs.

  • Check that the JP2 jumper on the ANT Switch is in default 2-3 position (Fig.1).
  • Make sure that all the cable connections are correct.
  • If the Audio Switch is powered by a separate power supply, make sure that its negative terminal is connected to TRX power supply negative terminal (grounds must be connected).
  • Connect the power supply to the Audio Switch. The red LEDs on both boards must be on. This is the receive mode. The green LED should be also on, signaling that the SDR audio is connected to the headphones. If optional TRX/SDR switch is used it must be in off position.
  • If the red LED does not lit when the power supply is switched on, check the cables or ground connections as described earlier since most probably the protection circuit is activated.
  • The next step is to check the audio paths. You must get audio signals from the TRX or from the SDR when using the optional external TRX/SDR switch. In TRX audio mode the green LED must be off.
  • Set the TRX in SSB mode, reduce the power output to minimum and push the microphone button to enter transmit mode. All LEDs now must be off, meaning the device is in transmit mode. When back into receiving mode, the green LED must be switched on with some delay which depends on the potentiometer settings.
  • Once you are confident in the operation of the device, increase the power to nominal level. Your setup is ready!
  • Tune to some station and listen on the SDR audio. Toggle several times between transmit and receive mode. Rotate the potentiometer clockwise, to decrease the delay time, until clicks or SDR audio are being heard when transmission ends. Then turn slightly counterclockwise to eliminate these effects. Transitions from transmit to receive mode must be smooth, without any clicks and delayed signals from the transmission. This is the optimal position. Now the audio muting delay is slightly greater than the SDR delay and your hearing will not be disturbed.
Close view of the ANT Switch board
Figure 1 – Close view of the ANT Switch board

Be aware that some SDRs have very long delay times (i.e. 500 to 800 ms) which is not very convenient for contest QSOs. Delays up to 200 ms are acceptable for contest contacts. Very often the SDR delay time will depend on the receiving settings (e.g. bandwidth) [3], [4].

The inputs of the both receivers are connected to the same antenna and there will be some signal level reduction since the signal from the antenna is distributed between two devices (see the specification).


  • Never open the ANT Switch shield during transmission to avoid unintentional touch of RF active parts.
  • In the case of multiple devices are connected to the TRX PTT line, use a TRX PTT cable as described here.
  • Do not connect cables when the boards are powered.
  • JP2 jumper in ANT Switch board must be in 2-3 shorted position except for the case described in the section “Omitting the antenna relay when separate receive antenna is used”.
  • Do not insert the ANT Switch between the power amplifier and the antenna. It must be always inserted between the TRX and the PA.