SDR Protection Circuit V2 with free worldwide postage

SDR Protection Circuit V2   

By  Peter Munn ZL2P (Oct 2025)

We have seen a number of Kiwisdr radios that have suffered serious damage due to one of the following causes;

1. Nearby lightning storms
2. Transmitting into an antenna close the to the Kiwisdr antenna
3. Accidentally transmitting directly into the Kiwisdr

This is an improved version of the V1 protection circuit which worked well protecting a receiver against nearby lightning and static but didn't always protect against accidentally transmitting high power into the protection unit. 

While this has been designed for the Kiwisdr radios it will work with any receiver.

The V2 unit adds a replaceable 100mA fast blow fuse as an extra level of protection.

The circuit has a 3D printed cover held in place with a 1.4mm screw. 

There is a spare fuse supplied with each protection circuit in a plastic bag. 

Don’t lose the spare fuse as 100mA 1808 fast blow fuses are not easy to buy.

If you blow the fuse please try to figure out what went wrong and fix things so it doesn’t happen again!  Check the fuse with a multimeter on Ohms range. A good fuse will be about 2 Ohms. To replace the fuse, pry the old one out of the fuse holder  with a tiny screwdriver.

In testing, the fuse blew in 2 seconds with 1 Watt RF power and 1 sec with 10 Watts. With 100 Watts the fuse blew instantaneously. 

The signal diodes also clamp the voltage from the RF transformer into the SDR.

It is important that the SDR is grounded as the back to back switching diodes may not operate if the input voltage is the same on both the RF and ground terminals (ie common mode voltage). This could happen when the antenna is not grounded eg a dipole.

The Kiwisdr 2 enclosure connects to the internal radio ground so adding a ground wire under one of the enclosure screw terminals is sufficient. A ring solder terminal has been supplied with recent radios for this purpose.