The M0CVO Antennas 1:1 Line Isolator operates from 1.8 - 30MHz. It will handle up to 400W CW or 450W PEP. This is used to prevent unwanted stray RF feedback coming into the shack via your feed line. Banish the interference to the other items nearby radios, telephones, etc. To use, fix the isolator to a convenient point outside  this could be a tree trunk, fence post, wall, pole or whatever. Attach the coax from your antenna to one connector on the isolator and then attach a second piece of coax to the other connector and use this to connect to your transceiver or ATU. The insertion loss of this unit is approx 0.02dB so is therefore negligible. It does not affect the SWR / tuning of the antenna at all if you were using an ATU before, it will still be the same.