A 4:1 Current Balun (Guanella) for use with OCFD, Loops and other situations where your aerial is around 200ohms. Do not be fooled by cheap (and poor design) single-core versions - only a twin-core such as this will work as an effective Current Balun. This means that you won't need any further choking (loops of coax, ferrite rings etc).
Picture 2 shows the ferrite toroids with 12-turns of cable each.
Best use for this is at the end of your 300/450-ohm feeder before a short run of coax to your antenna tuner. This 4:1 balun is more robust than the flimsy ones found in many low-end tuners and certainly doesn't use iron-powder cores (see note below).
If you're using a Doublet or G5RV, a balun with a 1:1: ratio is advised unless you know the impedance of your aerial will benefit from a 4:1 ratio at the frequencies of interest. Our 1:1 Choke/Balun is available in either 100w or 400w versions. We also offer a 2:1 Voltage Balun.