Advanced User Information

Technical documentation

Summary of the protection circuits

When TRX and SDR are using the same antenna we must avoid risks of damaging the SDR. We have taken several precautions to reduce the probability of such an event.

  • In transmit mode the SDR input is grounded (done by a relay inside the ANT Switch module). The maximal leakage signal input is around -30 dBm at TRX full power of 100w.
  • If the Audio Switch power supply is missing, the ANT Switch is always in transmit mode. That means that the SDR is disconnected and only the TRX is connected to the antenna.
  • If the operator forgets to connect the TRX PTT cable from the TRX to the Audio Switch, the ANT Switch is always in transmit mode. This protection works when the TRX PTT is the only line that is connected to the Audio Switch. If other equipment uses the same line (e.g. Power Amplifier) one must make sure that the TRX PTT output is connected to the Audio Switch by using a cable described here.
  • The ANT Switch is always in transmit mode, if the control cable is not connected.
  • If the TRX PTT cable and ANT Switch control cable are unintentionally swapped, the ANT Switch remains in transmit mode
  • If there is a failure in the Audio Switch which is causing both the SDR and the TRX to be connected to the antenna during transmission, a powerful limiter between the antenna and SDR will come into play. It limits the level to +12 dBm at 1 A RF current. This limiter can withstand 1 A current for a long time, and short peak RF currents up to 2 A. Besides, it is unlikely that in such a scenario the TRX can deliver similar current, since the SWR protection of the TRX is put into action and the output power is limited to very low levels. In such case the TRX has load of approximately 15 ohms. (At normal 50 ohms load the TRX current is 2 A peak at 100 W). In addition to that, there is a fuse (SMD R1 size 0402), located at the limiter input which blows up after several hundreds of milliseconds at currents above 1 A. When this happens, the SDR limiter is disconnected from the antenna terminal and the TRX normal antenna load is restored.
  • Since the Audio Switch is suggested to be powered by the TRX power supply which can be quite powerful (20 A and more), there is a resettable fuse which limits the supply current to 0.4A. This is done in order to avoid any problems caused by an unintentional short circuit.
  • There is a reverse polarity protection in the Audio Switch power supply line.
  • There is 12 V drive voltage at the central pin of the RCA male connector of the cable which drives the Ant Switch module when the antenna relay is energized. It is fused with limiting resistor R13. However, be careful not to touch the central pin to ground and always connect cables when the power is off.


The resistor R1, on the ANT Switch board, is designed to function as a fuse (Fig.1). The resistor has the following characteristic: 10 ohms, SMD size 0402, 60 mW. In the case of an unpredicted malfunction the resistor fuse will be blown. However, we don’t expect this to happen if the device is properly used and functioning as designed. If you suspect damage, check with ohmmeter the resistance between R1 terminals. It must be 10 ohms (+- 5%). If the resistor is damaged just remove it and don’t bother replacing it as there is a second spare fuse (R5) already mounted on the board. Once the damaged R1 is removed, connect R5 by soldering a jumper wire between JP1 terminals. This will insert the second fuse (R5) into the circuit and restore the connection of the SDR input to the antenna terminal.

Figure 1 – Simplified schematics of ANT Switch board with fuses.

Be careful not to touch the central pin of the ANT Switch control cable (RCA male connector) to ground. There is 12 V drive voltage (fused with resistor R13 on the Audio Switch) at the central pin of the RCA connector when the antenna relay is energized.

Omitting the antenna relay if second receive antenna is used

If a separate receive antenna is being used, the jumper JP2 of the ANT Switch might be set in 1-2 position (Fig.2). The receive antenna is connected to ANT terminal, while the TX connector must be left open. Warning! This will connect the SDR input directly to the antenna terminals omitting the fuse resistor and also the relay. In this case they are not needed, since the TRX output is not connected to the ANT Switch. The relay control cable is not needed anymore since the antenna relay is not used. The heavy duty protection limiter remains always connected which will limit the input power to no more than 10 dBm if the input RF current is less than 0.5 A. Currents above this limit are unlikely to happen even if the receiving antenna is extremely close to the TX antenna.

Figure 2 – ANT Switch board (back side). JP2 is shown in default 2-3 position.

This setting might be useful only in the case where the input signal level from the receive antenna is so low that losses of 1 or 2 dB are not desirable. However, if the SDR has permissible input levels lower than 10 dBm, do not do this and keep the jumper in 2-3 position.

Don’t forget to return JP2 to 2-3 shorted position if common antenna mode is used again. Not doing so might damage the SDR input.