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Many posts here and on other boards, and also linked from the SSTRAN website, recommend using wire radials for the transmitter ground system. It seems that ground rods, while good for lightning discharge (not protection) do not in general work well for part15 AM.
Most folks recommend 10 radials 10 feet long buried just beneath the soil.
As far as your 8.5 volt reading is concerned, last fall I built a base loaded antenna similar to Antenna Guy Carl’s design (SSTRAN site) and tested it in my driveway with 10 10 foot radials on the surface of the drive. I achieved a nice resonant peak, but the test point readings were in the 8 volt range. The weather became bad and the project was shelved so I don’t know if a better result would happen if I took more care in installing and tuning this system.
Another thought is check that your SSTRAN is wired for a loaded antenna with ground. Also check that the tuning cap is not at the end of its range. SSTRANs manual describes this. If it is at the end of range, changing C23 may help. When I built mine, I used mini jacks so I can easily change C23 while experimenting. This saves the PC board from damage as I swap parts.
Hope this helps.