Total posts : 45366
You asked some reasonable questions for which I will attempt answers.
[quote]Could the difference be that yours has been modified for the external antenna, and 12vman’s has not been modified?[/quote]
Yes, and I asked that. I’ll have to wait for 12vman to reply.
[quote]would it make any sense to increase the operating voltage of the final RF stage?[/quote]
No. My unit runs at 89 to 93 mW. input depending on the load and an increase of a few milliwatts output power that may result won’t be noticable. This circuit was carefully designed to operate as it is and I wouldn’t try to change it.
[quote]Are there any high effeciency transistors that could be substituted for the stock units?[/quote]
At these frequencies and power levels, I doubt it. Generally the operation of a well designed circuit depends primarily on the choice of biasing and passive components. If that were not the case, then it would be necessary to configure each built circuit for each individual transistor since the gain (beta) spread of small signal transistors can vary 4 to 1 and still be in specification. By making circuit gain and biasing (and efficiency ) independent of transistor gain (beta) we don’t have to be concerned about the beta spread.
Since the circuit operation depends on the passive components, there would be little or no change with a transistor substitution. Changing resistors will change the circuit operation but since the SSTRAN unit runs at near the legal limit with very clean modulation I don’t see this as an option since it is already optimized.
I recommend that you build and adjust the unit as instructed in the manual. If you don’t then you might have problems which would not be helpful in making a good first impression when you go on the air.
Though I discourage modifying this device I am glad that your are thinking about options and hope my answers are useful.