These are simple wound common mode chokes consisting of a 5 meters RG58 with PL259 connectors wound 12 times around a 240-43 Toroid core 1m from the start of the coax.  Giving you just over 3m of remaining patch lead.
 
If placed between you tranceiver and antenna you will get sufficient choking effect to isolate your Tranceiver from currents or noise passing on the outside of your coax.
 
If you have purchased one of our EFHW antenna you should position the toroid at the receiver and and then have the remaining coax going to the antenna transformer.
 
NOTE
These are built to order and may take a day or so longer than quoted for dispatch.