MFJ-993B Intellituner Automatic Antenna Tuner arrived this week
Too much reflective power coming back from a badly tuned antenna can not only in extreme cases cause damage to a transmitter but it is also an indicator of bad resonance which means power loss from the actual transmission.
This auto tuner cannot correct for actual resonance of the antenna (you have to, for example, adjust the antenna itself) but it does improve power transfer between the transmitter and the antenna by matching the specified load impedance of the radio to the combined input impedance of the feedline (the feedline is the coax cable between the transmitter and the antenna).
The device will also the SWR (amount of reflective power as a ratio), the power output in Watts from the transmitter, the frequency in MHz, and it has an L-Network display too showing inductance value and the capacitance value both on the antenna and the transmitter side.
The tuner operates on the HF band between 1.8 MHz and 30 MHz and will handle output power up to 300 Watts. Two other nice features are it can switch between two different antennas, and it has a built-in 4:1 balun which allows a balanced line antenna to be directly connected to it without the need to buy a balun. I also have a connector directly to the radio which allows tuning to be done from the radio itself to the auto tuner.