Total posts : 45366
I have not seen the SSTRAN circuit, but I know that matching a low-power transmitter to a short antenna is a big circuit design problem. All of the circuit designs I have seen are very poor, and nobody, as far as I know, has come up with a good one. This is something I am working on, and this is why I am interested in this thread, which originally dealt with using vacuum tubes for Part 15 AM. I like the low capacitance of tubes, and the high output impedance of pentodes. Pentodes, however, are unsuitable for Part 15 AM, because the screen power counts toward the 100 mW input power allowed for the final stage. This leaves triodes, which might work allright if the conduction angle can be made small enough. I will probably stick with the JFET, which, however, has high output capacitance. I have mentioned in this thread that capacitance in the coupling between the transmitter and antenna is bad for efficiency.
In the design of the coupling circuit, there are several bad choices to choose from. Using a toroid looks good on paper, but there are unpredictable core losses to deal with. I like a large diameter multiturn loop, which has no core losses, low copper loss, and has a high coefficient of coupling for effective impedance transformation between the output of the final stage and the antenna. Wire made up of a bundle of fine, insulsted, strands is supposed to reduce skin effect, but I am not completely sure about that. The proximity effect in a wire bundle causes something like skin effect, where most of the current flows in the outer strands. The coil capacitance of the multiturn loop is high.
As for the specific problem you wrote about, it seems to be a design problem that has to be beaten to death on the test bench. As you probably know, most of circuit design is not all that scientific. It mostly consists of fiddling around until you get something to work. I know I have added resistors or capacitors to circuits without any idea why they were there, or how they worked.