This snap-on ferrite is a serious item - It's a 31-mix ferrite that is ideal for use as a mains filter and/or HF choke for the whole of HF. With increased amounts of domestic "QRM" such as switch-mode power-supplies, televisions, cable routers and LED lighting making HF reception in some situations a non-starter - how do you go about taming it?
Known as the Problem Solver in GM3SEK's In Practice articles (and a live demo in this RSGB "Clean Up Your Shack" video) the snap-on design enables you to wind several turns of cable in a non-invasive manner. 31-mix is perfect for use across HF and especially on the lower bands - and at 3x thicker than a typical FT240-31 toroid, it's a bit of an animal.
As a Main Filter: The picture above shows how the conductors from a 2.5mm twin&earth cable form 5 turns in total. You may wish to experiment with the placement of the filter as it's designed to stop the mains wiring acting as an aerial for any "nasty" electrical devices. See M0PZT's EMC/RFI Mains Filter article for more details.
It's also wise to test your house without mains power - Switch-off the main circuit-breaker and run your radio off a battery: How is the noise-floor now? If it has dropped, then that's good news - it means that you can switch things back on one at a time to discover what is causing the interference. With some time and a little patience, you may get a few S-points back and possibly a much improved HF experience.
As an RF Choke : 8 turns should be sufficient across the HF bands. This is a great temporary solution if you need to find the best place to fit a choke on your feeder before committing to one of our Balun/Choke boxes. We also sell 31-mix ferrites in FT240 (2.4") diameter for Baluns, Chokes etc. Smaller 5mm snap-on and 9mm snap-on ferrites for USB cables are also available.