A multi-ratio UnUn for use on HF (1.8-30MHz) with up to 250w RF power. Sometimes, a single ratio just isn't enough if you're changing bands or out /P with different lengths of wire. The 3 ratios allow you to easily find a better match to your radio/tuner - thus bringing the impedance down to a more suitable level for your ATU.
The 31-mix ferrite is more suited to the lower HF bands (below 15MHz). 43-mix is best for 5-30MHz.
Use of counterpoise/radials is advised.