There happens to be a recent thread on the Community Radio discussion board of radio-info.com that deals speciffically with the use of the Talking House ATU with the SSTRAN. I don't think, however, that said ATU is off-topic on this thread, because the ATU is another way to tune the antenna with external loading coils.
Some time ago, Talking Sign posted on part15.us that the FCC stopped them from marketing their own version of the ATU. I don't know why the Talking House ATU is accepted by the FCC, but the Talking Sign ATU is not. My best guess is that the Talking Sign ATU was not certified along with the rest of the transmitter, while the Talking House ATU was.
talking to the SSTRAN designer, he said the SSTRAN likes to see about 800 ohm. So working into a 75 ohm load would not be useful.
Using a capacitor-input "L" network, which the SSTRAN was designed to use, the 800 ohms of load impedance at the collector of the output transistor can be matched to some lower impedance, such as 75 ohms. If this were not the case, a botton-loaded antenna could not be used with the SSTRAN.
The antenna that is recommended to be used with the SSTRAN has an inductor tapped at the input that serves as a combination of a loading coil and the small series inductor that is used in the "L" network. Using the prescribed tuning process, the appropriate loading coil and "L" network inductances are found for proper tuning. This kind of tuning of electrically-short mobile whip antennas for ham use was described in QST in the 1950s.
The output of the "L" network has a resistive load consisting of the sum of the ground loss resistance and the loading coil loss resistance. This load resistance is typically several tens of ohms--like coax cable. I took the liberty of explaining the SSTRAN output circuit because it is my preferred circuit for my own experimental transmitters.
I also do not think the ATU is off topic here. My previous response was intended to avoid confusion regarding certification issues and to clarify that experimentation with the SSTRAN or any home built unit is OK since certification does not apply. !5.219 or 15.209 do apply however.
Neil