Choosing an ATU
Manual ATUs are the traditional type used for years and give you total control over adjustment and tend to give the widest range of adjustable values. This means they will be able to cope with almost any situation.
Automatic ATUs are now becoming very popular - and rightly so. Although many transceivers now have built-in automatic ATUs, these ATUs are only able to cope with modest VSWRs and can only be used for coax systems. For the most part they should be regarded as a means of handling VSWRs up to around 4 or 5:1. Dedicated automatic external ATUs have a much wider coverage and capability.
Some ATUs will only hand unbalanced coax systems, but most offer the ability to handle balanced feeders and end fed wires. Many automatic ATUs offer a similar range of facilities, but check out the spec. to make sure they will do the job you need.
The power rating of ATUs is somewhat complicated because when feeding coax systems the internal voltages are not nearly as high as they are for end fed wire systems or even balanced feeders. As a guide we recommend that when using balanced feeder you divide the power rating by a factor of 2 and for end fed wire divide by a factor of 3. Thus a 300W ATU will be OK for coax at that power level, but only cope with 200W for balanced feeder and just a 100W for end fed wires.