The question of 'tuner' and 'coupler' and the distinction between them is a point on which you will find quite a bit of confusion and contention. All for no reason. The words are often used interchangeably, so many people equate them in their minds. People who use the words to represent different things make the distinction based on what each thing does.
If you want to use the terms distinctly, then a TUNER is something which TUNES just after the output of your transceiver. It's purpose is to match the feedline to the transceiver. What it tunes is the system composed of the feedline and the antenna to provide a 50 Ohm connection (or whatever impedance is needed for the match) for the transceiver. A COUPLER is put at the feed point of the antenna. It's purpose is to match the antenna to the 50 ohm feedline (or whatever other impedance is needed, but usually it's 50 ohms). A coupler keeps the SWR down along the whole feedline, while a tuner does not.
Which is better? There you'll find lots of opinion, but in general, most experts would consider the antenna feed point as the proper place to insert some sort of coupling device. Books like the Antenna Handbook or others usually have collections of matching sections for antennas run at a specific frequency. A coupler does the same thing, but it does it across a wide range of frequencies.
SO which do you want? A tuner built into your rig is there to protect the transceiver, not to give you the best signal. It's convenient, simple, and at most HF frequencies, losses will be fairly minimal on a practical level. A coupler at the antenna feed point will maximize your energy flow into the antenna, giving you the best signal possible with that physical antenna. Most matching sections though, are only for a single frequency and a small band around that frequency. The Smartuner (SGC's product), automates the tuning by selecting the best match at the feed point to maximize energy flow. This minimizes SWR across the whole of the feed line which also minimizes energy loss. Since the Smartuner is completely independent with its own internal intelligence, it operates completely without attention once it's properly installed. Many of our customers have had one for 5, 10, or more years and never even bother to look at it since it just sits there and does its job.
Should you use both? Generally not, simply because they're usually not needed. It shouldn't make any difference if you do, though. The transceiver might change power levels or do something else to confuse the remote coupler. Most people simply turn off the internal tuner and rely on the external one, since they can usually get demonstrably better signal reports when using the remote coupler.
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