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 will work as an effective Current Balun. This means that you won't need any further choking (loops of coax, ferrite rings etc).
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 availabe in either 100w or 400w versions.