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If you put the antenna at the transmitter with a very short cable, it should be the same as using any other antenna.
The trick is to get the antenna to load right depending on what the SST output is designed to work into.
If it already has a loading coil internally, not so good. I’m sure this has been discussed here somewhere.
But, to separate the SST from the ATU with a long cable, no, the SST with the ATU is not certified as a system as the TH with ATU is.